The slipper snail, Crepidula: an emerging lophotrochozoan model system

A Small Change With a Twist Ending: A Single Residue in EGF-CFC Drives Bilaterian Asymmetry

Asymmetries are essential for proper organization and function of organ systems. Genetic studies in bilaterians have shown signaling through the Nodal/Smad2 pathway plays a key, conserved role in the establishment of body asymmetries. Although the main molecular players in the network for the establishment of left-right asymmetry (LRA) have been deeply described in deuterostomes, little is known about the regulation of Nodal signaling in spiralians. Here, we identified orthologs of the egf-cfc gene, a master regulator of the Nodal pathway in vertebrates, in several invertebrate species, which includes the first evidence of its presence in non-deuterostomes. Our functional experiments indicate that despite being present, egf-cfc does not play a role in the establishment of LRA in gastropods. However, experiments in zebrafish suggest that a single amino acid mutation in the egf-cfc gene in at least the common ancestor of chordates was the necessary step to induce a gain of function in LRA regulation. This study shows that the egf-cfc gene likely appeared in the ancestors of deuterostomes and "protostomes", before being adopted as a mechanism to regulate the Nodal pathway and the establishment of LRA in some lineages of deuterostomes.

Proteomic and Transcriptomic Analyses in the Slipper Snail Crepidula fornicata Uncover Shell Matrix Genes Expressed During Adult and Larval Biomineralization

The gastropod shell is a composite composed of minerals and shell matrix proteins (SMPs). SMPs have been identified by proteomics in many molluscs, but few have been studied in detail. Open questions include (1) what gene regulatory networks regulate SMP expression, (2) what roles individual SMPs play in biomineralization, and (3) how the complement of SMPs changes over development. These questions are best addressed in a species in which gene perturbation studies are available; one such species is the slipper snail, Crepidula fornicata. Here, SEM and pXRD analysis demonstrated that the adult shell of C. fornicata exhibits crossed lamellar microstructure and is composed of aragonite. Using high-throughput proteomics we identified 185 SMPs occluded within the adult shell. Over half of the proteins in the shell proteome have known biomineralization domains, while at least 10% have no homologs in public databases. Differential gene expression analysis identified 20 SMP genes that are up-regulated in the shell-producing mantle tissue. Over half of these 20 SMPs are expressed during development with two, CfSMP1 and CfSMP2, expressed exclusively in the shell gland. Together, the description of the shell microstructure and a list of SMPs now sets the stage for studying the consequences of SMP gene knockdowns in molluscs.

Slipper snail tales: How Crepidula fornicata and Crepidula atrasolea became model molluscs

Despite the great abundance and diversity of molluscs, only a few have attained "model research organism" status. One of those species is the slipper snail Crepidula fornicata. Its embryos were first used for classical lineage tracing studies in the late 19th century, and over a 100 years later they were "re-discovered" by our labs and used for modern fate mapping, gene perturbation, in vivo imaging, transcriptomics, and the first application of CRISPR/Cas9-mediated genome editing among the Spiralia/Lophotrochozoa. Simultaneously, other labs made extensive examinations of taxonomy, phylogeny, ecology, life-history, mode of development, larval feeding behavior, and responses to the environment in members of the family Calyptraeidae, which includes the genus Crepidula. Recently, we developed tools, resources, and husbandry protocols for another, direct-developing species, Crepidula atrasolea. This species is an ideal "lab rat" among molluscs. Together these species will be valuable for probing the cellular and molecular mechanisms underlying molluscan biology and evolution.

Blastocoel morphogenesis: A biophysics perspective

The blastocoel is a fluid-filled cavity characteristic of animal embryos at the blastula stage. Its emergence is commonly described as the result of cleavage patterning, but this historical view conceals a large diversity of mechanisms and overlooks many unsolved questions from a biophysics perspective. In this review, we describe generic mechanisms for blastocoel morphogenesis, rooted in biological literature and simple physical principles. We propose novel directions of study and emphasize the importance to study blastocoel morphogenesis as an evolutionary and physical continuum.

Retinoids promote penis development in sequentially hermaphroditic snails

Sexual systems are surprisingly diverse, considering the ubiquity of sexual reproduction. Sequential hermaphroditism, the ability of an individual to change sex, has emerged multiple times independently across the animal kingdom. In molluscs, repeated shifts between ancestrally separate sexes and hermaphroditism are generally found at the level of family and above, suggesting recruitment of deeply conserved mechanisms. Despite this, molecular mechanisms of sexual development are poorly known. In molluscs with separate sexes, endocrine disrupting toxins bind the retinoid X receptor (RXR), activating ectopic male development in females, suggesting the retinoid pathway as a candidate controlling sexual transitions in sequential hermaphrodites. We therefore tested the role of retinoic acid signaling in sequentially hermaphroditic Crepidula snails, which develop first into males, then change sex, maturing into females. We show that retinoid agonists induce precocious penis growth in juveniles and superimposition of male development in females. Combining RXR antagonists with retinoid agonists significantly reduces penis length in induced juveniles, while similar treatments using retinoic acid receptor (RAR) antagonists increase penis length. Transcripts of both receptors are expressed in the induced penis. Our findings therefore show that retinoid signaling can initiate molluscan male genital development, and regulate penis length. Further, we show that retinoids induce ectopic male development in multiple Crepidula species. Species-specific influence of conspecific induction of sexual transitions correlates with responsiveness to retinoids. We propose that retinoid signaling plays a conserved role in molluscan male development, and that shifts in the timing of retinoid signaling may have been important for the origins of sequential hermaphroditism within molluscs.

Expression Pattern of Nitric Oxide Synthase during Development of the Marine Gastropod Mollusc, Crepidula fornicata

Nitric Oxide (NO) plays a key role in the induction of larval metamorphosis in several invertebrate phyla. The inhibition of the NO synthase in Crepidula fornicata, a molluscan model for evolutionary, developmental, and ecological research, has been demonstrated to block the initiation of metamorphosis highlighting that endogenous NO is crucial in the control of this developmental and morphological process. Nitric Oxide Synthase contributes to the development of shell gland, digestive gland and kidney, being expressed in cells that presumably correspond to FMRF-amide, serotoninergic and catecolaminergic neurons. Here we identified a single Nos gene in embryonic and larval transcriptomes of C. fornicata and studied its localization during development, through whole-mount in situ hybridization, in order to compare its expression pattern with that of other marine invertebrate animal models.

Departures from isotropy: the kinematics of a larval snail in response to food

The swimming behavior of invertebrate larvae can affect their dispersal, survival and settlement in the ocean. Modeling this behavior accurately poses unique challenges as behavior is controlled by both physiology and environmental cues. Some larvae use cilia to both swim and create feeding currents, resulting in potential trade-offs between the two functions. Food availability is naturally patchy and often occurs in shallow horizontal layers in the ocean. Also, larval swimming motions generally differ in the horizontal and vertical directions. In order to investigate behavioral response to food by ciliated larvae, we measured their behavioral anisotropy by quantifying deviations from a model based on isotropic diffusion. We hypothesized that larvae would increase horizontal swimming and decrease vertical swimming after encountering food, which could lead to aggregation at food layers. We considered Crepidula fornicata larvae, which are specifically of interest as they exhibit unsteady and variable swimming behaviors that are difficult to categorize. We tracked the larvae in still water with and without food, with a portion of the larvae starved beforehand. On average, larvae in the presence of food were observed higher in the water column, with higher swimming speeds and higher horizontal swimming velocities when compared with larvae without food. Starved larvae also exhibited higher vertical velocities in food, suggesting no aggregation behavior. Although most treatments showed strong anisotropy in larval behavior, we found that starved larvae without food exhibited approximately isotropic kinematics, indicating that behavioral anisotropy can vary with environmental history and conditions to enhance foraging success or mitigate food-poor environments.

Integrated mRNA and Small RNA Sequencing Reveals Regulatory Expression of Larval Metamorphosis of the Razor Clam

The razor clam, Sinonovacula constricta, is an important economic marine shellfish, and its larval development involves obvious morphological and physiological changes. MicroRNA plays a key role in the physiological changes of the organism through regulating targeted mRNA. This study performed miRNA-mRNA sequencing for eight different developmental stages of S. constricta using Illumina sequencing. A total of 2156 miRNAs were obtained, including 2069 known miRNAs and 87 novel miRNAs. In addition, target genes were predicted for key miRNAs differentially expressed between adjacent development samples by integrating the mRNA transcriptome. Further analysis revealed that the differentially expressed genes were enriched in complement activation, alternative pathways, translation, and negative regulation of monocyte molecular protein-1 production. KEGG pathway annotation showed significant enrichment in the regulation of the ribosome, phagosome, tuberculosis and fluid shear stress, and atherosclerosis. Ten mRNAs and ten miRNAs that are related to larval metamorphosis were identified using real-time PCR. Furthermore, the double luciferase experiment validated the negative regulatory relationship between miR-133 and peroxisome proliferator-activated receptor-γ (PPAR-γ). These results indicated that the target genes regulated by these differentially expressed miRNAs may play an important regulatory role in the metamorphosis development of S. constricta.

BMP signaling plays a role in anterior-neural/head development, but not organizer activity, in the gastropod Crepidula fornicata

BMP signaling is involved in many aspects of metazoan development, with two of the most conserved functions being to pattern the dorsal-ventral axis and to specify neural versus epidermal fates. An active area of research within developmental biology asks how BMP signaling was modified over evolution to build disparate body plans. Animals belonging to the superclade Spiralia/Lophotrochozoa are excellent experimental subjects for studying the evolution of BMP signaling because a highly conserved, stereotyped early cleavage program precedes the emergence of distinct body plans. In this study we examine the role of BMP signaling in one representative, the slipper snail Crepidula fornicata. We find that mRNAs encoding BMP pathway components (including the BMP ligand decapentaplegic, and BMP antagonists chordin and noggin-like proteins) are not asymmetrically localized along the dorsal-ventral axis in the early embryo, as they are in other species. Furthermore, when BMP signaling is perturbed by adding ectopic recombinant BMP4 protein, or by treating embryos with the selective Activin receptor-like kinase-2 (ALK-2) inhibitor Dorsomorphin Homolog 1 (DMH1), we observe no obvious effects on dorsal-ventral patterning within the posterior (post-trochal) region of the embryo. Instead, we see effects on head development and the balance between neural and epidermal fates specifically within the anterior, pre-trochal tissue derived from the 1q1 lineage. Our experiments define a window of BMP signaling sensitivity that ends at approximately 44-48 ​hours post fertilization, which occurs well after organizer activity has ended and after the dorsal-ventral axis has been determined. When embryos were exposed to BMP4 protein during this window, we observed morphogenetic defects leading to the separation of the anterior, 1q lineage from the rest of the embryo. The 1q-derived organoid remained largely undifferentiated and was radialized, while the post-trochal portion of the embryo developed relatively normally and exhibited clear signs of dorsal-ventral patterning. When embryos were exposed to DMH1 during the same time interval, we observed defects in the head, including protrusion of the apical plate, enlarged cerebral ganglia and ectopic ocelli, but otherwise the larvae appeared normal. No defects in shell development were noted following DMH1 treatments. The varied roles of BMP signaling in the development of several other spiralians have recently been examined. We discuss our results in this context, and highlight the diversity of developmental mechanisms within spiral-cleaving animals.

An automated aquatic rack system for rearing marine invertebrates

BACKGROUND

One hundred years ago, marine organisms were the dominant systems for the study of developmental biology. The challenges in rearing these organisms outside of a marine setting ultimately contributed to a shift towards work on a smaller number of so-called model systems. Those animals are typically non-marine organisms with advantages afforded by short life cycles, high fecundity, and relative ease in laboratory culture. However, a full understanding of biodiversity, evolution, and anthropogenic effects on biological systems requires a broader survey of development in the animal kingdom. To this day, marine organisms remain relatively understudied, particularly the members of the Lophotrochozoa (Spiralia), which include well over one third of the metazoan phyla (such as the annelids, mollusks, flatworms) and exhibit a tremendous diversity of body plans and developmental modes. To facilitate studies of this group, we have previously described the development and culture of one lophotrochozoan representative, the slipper snail Crepidula atrasolea, which is easy to rear in recirculating marine aquaria. Lab-based culture and rearing of larger populations of animals remain a general challenge for many marine organisms, particularly for inland laboratories.

RESULTS

Here, we describe the development of an automated marine aquatic rack system for the high-density culture of marine species, which is particularly well suited for rearing filter-feeding animals. Based on existing freshwater recirculating aquatic rack systems, our system is specific to the needs of marine organisms and incorporates robust filtration measures to eliminate wastes, reducing the need for regular water changes. In addition, this system incorporates sensors and associated equipment for automated assessment and adjustment of water quality. An automated feeding system permits precise delivery of liquid food (e.g., phytoplankton) throughout the day, mimicking real-life feeding conditions that contribute to increased growth rates and fecundity.

CONCLUSION

This automated system makes laboratory culture of marine animals feasible for both large and small research groups, significantly reducing the time, labor, and overall costs needed to rear these organisms.

Natural Heteroplasmy and Mitochondrial Inheritance in Bivalve Molluscs

Heteroplasmy is the presence of more than one type of mitochondrial genome within an individual, a condition commonly reported as unfavorable and affecting mitonuclear interactions. So far, no study has investigated heteroplasmy at protein level, and whether it occurs within tissues, cells, or even organelles. The only known evolutionarily stable and natural heteroplasmic system in Metazoa is the Doubly Uniparental Inheritance (DUI)-reported so far in ∼100 bivalve species-in which two mitochondrial lineages are present: one transmitted through eggs (F-type) and the other through sperm (M-type). Because of such segregation, mitochondrial oxidative phosphorylation proteins reach a high amino acid sequence divergence (up to 52%) between the two lineages in the same species. Natural heteroplasmy coupled with high sequence divergence between F- and M-type proteins provides a unique opportunity to study their expression and assess the level and extent of heteroplasmy. Here, for the first time, we immunolocalized F- and M-type variants of three mitochondrially-encoded proteins in the DUI species Ruditapes philippinarum, in germline and somatic tissues at different developmental stages. We found heteroplasmy at organelle level in undifferentiated germ cells of both sexes, and in male soma, whereas gametes were homoplasmic: eggs for the F-type and sperm for the M-type. Thus, during gametogenesis, only the sex-specific mitochondrial variant is maintained, likely due to a process of meiotic drive. We examine the implications of our results for DUI proposing a revised model, and we discuss interactions of mitochondria with germ plasm and their role in germline development. Molecular and phylogenetic evidence suggests that DUI evolved from the common Strictly Maternal Inheritance, so the two systems likely share the same underlying molecular mechanism, making DUI a useful system for studying mitochondrial biology.

Combined effects of temperature, salinity, and diet simulating upwelling and nonupwelling seasons alter life-history characteristics of a tropical invertebrate

Upwelling is known to affect the ecology and life history of temperate nearshore organisms, and these effects are thought to be mediated by changes in temperature and food supply. However, little information is available for tropical systems. To understand how changes in the intensity of upwelling might impact marine invertebrates, we tested how factorial combinations of temperature, salinity, and phytoplankton availability affected growth and reproduction of a common intertidal snail, Crepidula cf. marginalis. We used temperatures typical of nonupwelling (29°C), moderate (26°C) and severe (23°C) upwelling, salinities typical of nonupwelling (30 ppt) and upwelling (34 ppt) and a good diet (Isochrysis) and a better diet (Isochrysis and Tetraselmis) as a proxy for increased productivity during upwelling. Overall, temperature and diet had consistent effects on body size, with better food and lower temperatures promoting larger size, as well as promoting shorter time to first reproduction. Diet had the largest effects on clutch size, with clutch size increasing with better diet. Temperature had the largest effect on offspring size and the frequency of discarded broods; offspring size decreased with increasing temperature and the frequency of discarded broods also decreased with increasing temperatures. We found no significant 3rd order interactions and few significant strong 2nd order interactions, which have often been found in similar experimental studies using stressful treatments. For this tropical slipper limpet, the effect of higher food and cooler temperatures during upwelling appears to be positive, promoting higher growth rates, larger clutch sizes, and larger offspring size suggesting that both factors likely play an important role underlying reproductive responses to upwelling. Climatic changes, like El Niño, which suppress upwelling in the Bay of Panama, appear likely to negatively impact this species.

Instant Ocean Versus Natural Seawater: Impacts on Aspects of Reproduction and Development in Three Marine Invertebrates

Marine invertebrate larvae have often been reared in artificial rather than natural seawater, either for convenience or to avoid potentially confounding effects of unknown contaminants. This study sought to determine the impact of artificial seawater on various aspects of development for three marine invertebrate species. We examined the impact of Instant Ocean on growth, survival, and fecundity of the deposit-feeding polychaete Capitella teleta at 2 salinities: 24 and 34 ppt; the impact on survival, growth rate, and time to metamorphic competence for the slipper limpet Crepidula fornicata; and the impact on larval growth for the sea star Asterias forbesi. Juveniles of C. teleta survived better in natural seawater than in Instant Ocean at both salinities but at the higher salinity grew more quickly in Instant Ocean; fecundity was not significantly affected by the type of seawater used at either salinity. Using Instant Ocean in place of natural seawater had no pronounced impact on the survival of C. fornicata larvae or on how long it took them to become competent to metamorphose; however, larvae grew somewhat more quickly in Instant Ocean than in natural seawater for the first 4 days of development, but by day 7 they were about 4.5% larger if they had been reared in seawater. The type of seawater used affected the growth of A. forbesi larvae, with larvae growing significantly more slowly in Instant Ocean than in natural seawater, no matter how growth was measured. In conclusion, our results suggest that although Instant Ocean may be a reasonable substitute for natural seawater for work with some species, using it may affect experimental outcomes in some aspects of work with other species.

The natural history of Calyptraeaaurita (Reeve, 1859) from Southern Chile (Gastropoda, Calyptraeidae)

Hard bottom communities of the Reloncaví Estuary and adjacent areas, Region de los Lagos, Chile (42°S), were studied between 2008 and 2011. All hard substrates between the lower intertidal and 25 m depth were dominated by the calyptraeid gastropods, Crepipatelladilatata and C.fecunda. Epibenthic coverage of the hard bottoms markedly decreased further down with the exception of vertical cliffs. In a depth range between 26 to 48 m repeatedly dense patches of another calyptraeid species, Calyptraeaaurita (Reeve, 1859), were observed. Densities reached up to 1475 individuals m^-2 and covered up to 50 % of the rock surfaces. In shallower depths C.aurita was not present. However, despite its huge abundance, C.aurita has not been documented for more than 150 years in the southeastern Pacific, being described superficially by Reeve, through only shell characteristics. Here, we redescribe and compare it with other members of the family Calyptraeidae through characteristics of shell, radula, and soft parts, including also details of the egg mass and intracapsular development of their embryos. Males were mobile and females sessile. Shell size ranged from 6.6 to 12.4 mm for immature individuals, from 10.6 to 24.9 mm for males, 15.1 to 25.9 mm for intersex individuals, and from 21.0 to 39.6 mm for females. Up to three individuals stacked together were found, always presenting a female at the base with up to a maximum of two male individuals above. Laboratory studies demonstrated that C.aurita has an indirect larval development, liberating planktotrophic larvae with a bilobed ciliated velum into the water column. A transplantation experiment demonstrated that survival, growth, and reproduction of C.aurita is also possible in depths shallower than its normal distribution. The geographic distribution of C.aurita, was previously only known as being from Valparaíso (33°S) and is now extended down to the Reloncaví Sound (41°S).

Ectomesoderm and epithelial-mesenchymal transition-related genes in spiralian development

BACKGROUND

Spiralians (e.g., annelids, molluscs, and flatworms) possess two sources of mesoderm. One is from endodermal precursors (endomesoderm), which is considered to be the ancestral source in metazoans. The second is from ectoderm (ectomesoderm) and may represent a novel cell type in the Spiralia. In the mollusc Crepidula fornicata, ectomesoderm is derived from micromere daughters within the A and B cell quadrants. Their progeny lie along the anterolateral edges of the blastopore. There they undergo epithelial-mesenchymal transition (EMT), become rounded and undergo delamination/ingression. Subsequently, they assume the mesenchymal phenotype, and migrate beneath the surface ectoderm to differentiate various cell types, including muscles and pigment cells.

RESULTS

We examined expression of several genes whose homologs are known to regulate Type 1 EMT in other metazoans. Most of these genes were expressed within spiralian ectomesoderm during EMT.

CONCLUSIONS

We propose that spiralian ectomesoderm, which exhibits analogous cellular behaviors to other populations of mesenchymal cells, may be controlled by the same genes that drive EMT in other metazoans. Perhaps these genes comprise a conserved metazoan EMT gene regulatory network (GRN). This study represents the first step in elucidating the GRN controlling the development of a novel spiralian cell type (ectomesoderm). Developmental Dynamics 247:1097-1120, 2018. © 2018 Wiley Periodicals, Inc.

Molecular, phylogenetic and developmental analyses of Sall proteins in bilaterians

Background

Sall (Spalt-like) proteins are zinc-finger transcription factors involved in a number of biological processes. They have only been studied in a few model organisms, such as Drosophila melanogaster, Caenorhabditis elegans, Schmidtea mediterranea and some vertebrates. Further taxon sampling is critical to understand the evolution and diversification of this protein and its functional roles in animals.

Results

Using genome and transcriptome mining, we confirmed the presence of sall genes in a range of additional animal taxa, for which their presence had not yet been described. We show that sall genes are broadly conserved across the Bilateria, and likely appeared in the bilaterian stem lineage. Our analysis of the protein domains shows that the characteristic arrangement of the multiple zinc-finger domains is conserved in bilaterians and may represent the ancient arrangement of this family of transcription factors. We also show the existence of a previously unknown zinc-finger domain. In situ hybridization was used to describe the gene expression patterns in embryonic and larval stages in two species of snails: Crepidula fornicata and Lottia gigantea. In L. gigantea, sall presents maternal expression, although later on the expression is restricted to the A and B quadrants during gastrulation and larval stage. In C. fornicata, sall has no maternal expression and it is expressed mainly in the A, C and D quadrants during blastula stages and in an asymmetric fashion during the larval stage.

Discussion

Our results suggest that the bilaterian common ancestor had a Sall protein with at least six zinc-finger domains. The evolution of Sall proteins in bilaterians might have occurred mostly as a result of the loss of protein domains and gene duplications leading to diversification. The new evidence complements previous studies in highlighting an important role of Sall proteins in bilaterian development. Our results show maternal expression of sall in the snail L. gigantea, but not C. fornicata. The asymmetric expression shown in the ectoderm of the trochophore larva of snails is probably related to shell/mantle development. The observed sall expression in cephalic tissue in snails and some other bilaterians suggests a possible ancestral role of sall in neural development in bilaterians.

Transgenerational impairments of reproduction and development of the marine invertebrate Crepidula onyx resulted from long-term dietary exposure of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47)

Polybrominated diphenyl ethers have become ubiquitous in the environment and elevated concentrations have often been found in marine organisms. Using the gastropod Crepidula onyx as a study model, this multigenerational study sets out to test the hypotheses that 1) parental dietary exposure to environmentally realistic levels of 2,2',4,4'-tetrabrominated diphenyl ether (BDE-47) would lead to transgenerational impairments on fitness traits of marine invertebrates, and 2) the organisms might develop adaptation/acclimation after exposure for one or more generations. F₀ generation of C. onyx was fed with the dinoflagellate Isochrysis galbana encapsulated with two concentrations of BDE-47 (1.78 and 16.0 ng million cells^-1, respectively), and half of the F₁ offspring from the higher concentration treatment was returned to control condition (transgenerational group), while the other half received BDE-47 treatment continuously (continuous treatment group). Bioaccumulation and maternal transfer of BDE-47 were evident in all life stages of the F₀ generation and in F₁ eggs, respectively. Exposure to BDE-47 reduced fecundity, delayed sexual maturity, and impeded embryonic development in F₀ to F₂. In particular, developmental toxicity of F₂ embryos was apparent in the transgenerational group, but not in the continuous treatment group, even when BDE-47 was not detected in the F₂ embryos nor in their mothers and they have never been exposed to the chemical. This study also suggested that the offspring might have developed adaptation/acclimation to the exposure of BDE-47 within two generations of exposure, and that the physiological alterations associated with acclimation/adaptation might have hindered the normal larval development under a stress free condition. These findings highlighted the need for long-term multigenerational studies in the ecological risk assessment of chemicals alike.

Negative effects of microplastic exposure on growth and development of Crepidula onyx

Microplastics exposure could be detrimental to marine organisms especially under high concentrations. However, few studies have considered the multiphasic nature of marine invertebrates' life history and investigated the impact of experiencing microplastics during early development on post-metamorphic stages (legacy effect). Many planktonic larvae can feed selectively and it is unclear whether such selectivity could modulate the impact of algal food-sized microplastic. In this two-stage experiment, veligers of Crepidula onyx were first exposed to additions of algae-sized micro-polystyrene (micro-PS) beads at different concentrations, including ones that were comparable their algal diet. These additions were then either halted or continued after settlement. At environmentally relevant concentration (ten 2-μm microplastic beads ml^-1), larval and juvenile C. onyx was not affected. At higher concentrations, these micro-PS fed larvae consumed a similar amount of algae compared to those in control but grew relatively slower than those in the control suggesting that ingestion and/or removal of microplastic was/were energetically costly. These larvae also settled earlier at a smaller size compared to the control, which could negatively affect post-settlement success. Juvenile C. onyx receiving continuous micro-PS addition had slower growth rates. Individuals only exposed to micro-PS during their larval stage continued to have slower growth rates than those in the control even if micro-PS had been absent in their surroundings for 65 days highlighting a legacy effect of microplastic exposure.

Morphogenesis along the animal-vegetal axis: fates of primary quartet micromere daughters in the gastropod Crepidula fornicata

Background

The Spiralia are a large, morphologically diverse group of protostomes (e.g. molluscs, annelids, nemerteans) that share a homologous mode of early development called spiral cleavage. One of the most highly-conserved features of spiralian development is the contribution of the primary quartet cells, 1a-1d, to the anterior region of the embryo (including the brain, eyes, and the anterior ciliary band, called the prototroch). Yet, very few studies have analyzed the ultimate fates of primary quartet sub-lineages, or examined the morphogenetic events that take place in the anterior region of the embryo.

Results

This study focuses on the caenogastropod slipper snail, Crepidula fornicata, a model for molluscan developmental biology. Through direct lineage tracing of primary quartet daughter cells, and examination of these cells during gastrulation and organogenesis stages, we uncovered behaviors never described before in a spiralian. For the first time, we show that the 1a²-1d² cells do not contribute to the prototroch (as they do in other species) and are ultimately lost before hatching. During gastrulation and anterior-posterior axial elongation stages, these cells cleavage-arrest and spread dramatically, contributing to a thin provisional epidermis on the dorsal side of the embryo. This spreading is coupled with the displacement of the animal pole, and other pretrochal cells, closer to the ventrally-positioned mouth, and the vegetal pole.

Conclusions

This is the first study to document the behavior and fate of primary quartet sub-lineages among molluscs. We speculate that the function of 1a²-1d² cells (in addition to two cells derived from 1d¹², and the 2b lineage) is to serve as a provisional epithelium that allows for anterior displacement of the other progeny of the primary quartet towards the anterior-ventral side of the embryo. These data support a new and novel mechanism for axial bending, distinct from canonical models in which axial bending is suggested to be driven primarily by differential proliferation of posterior dorsal cells. These data suggest also that examining sub-lineages in other spiralians will reveal greater variation than previously assumed.

Cleavage modification did not alter blastomere fates during bryozoan evolution

BACKGROUND

Stereotypic cleavage patterns play a crucial role in cell fate determination by precisely positioning early embryonic blastomeres. Although misplaced cell divisions can alter blastomere fates and cause embryonic defects, cleavage patterns have been modified several times during animal evolution. However, it remains unclear how evolutionary changes in cleavage impact the specification of blastomere fates. Here, we analyze the transition from spiral cleavage - a stereotypic pattern remarkably conserved in many protostomes - to a biradial cleavage pattern, which occurred during the evolution of bryozoans.

RESULTS

Using 3D-live imaging time-lapse microscopy (4D-microscopy), we characterize the cell lineage, MAPK signaling, and the expression of 16 developmental genes in the bryozoan Membranipora membranacea. We found that the molecular identity and the fates of early bryozoan blastomeres are similar to the putative homologous blastomeres in spiral-cleaving embryos.

CONCLUSIONS

Our work suggests that bryozoans have retained traits of spiral development, such as the early embryonic fate map, despite the evolution of a novel cleavage geometry. These findings provide additional support that stereotypic cleavage patterns can be modified during evolution without major changes to the molecular identity and fate of embryonic blastomeres.

Deviating from the Norm: Peculiarities of Aplysia cf. californica Early Cleavage Compared to Traditional Spiralian Models

Spiralia represents one of the main clades of bilaterally symmetrical metazoans (Bilateria). This group is of particular interest due to the remarkable conservation of its early developmental pattern despite of the high diversity of larval and adult body plans. Variations during embryogenesis are considered powerful tools to determine ancestral and derived characters under a phylogenetic framework. By direct observation of embryos cultured in vitro, we analyzed the early cleavage of the euopisthobranchs Aplysia cf. californica. We used tubulin immunocytochemistry to stain mitotic spindles during early cleavages, and followed each division with the aid of an autofluorescent compound inside yolk platelets, which differed from the characteristic pink-brownish pigment of the vegetal cytoplasm in zygotes and early embryos. We found that this species exhibits an unequal cleavage characterized by ooplasmic segregation, oblique inclination of mitotic spindles, and differences in size and positioning of the asters in relation to the cellular cortex. Furthermore, we detected asynchrony in cleavage timing between the two large macromeres C and D, which increases the number of cleavage rounds required to reach a particular cell stage in comparison to other spiralians. Here, we report the presence of a transient and previously undescribed U-shaped embryo in this species. The present detailed description of A. californica early development deviates considerably from stereotypical patterns described in other spiralians. Our observations demonstrate that early spiralian development can be more plastic than previously thought.

Molluscan models: Crepidula fornicata

Gastropod snails in the genus Crepidula have emerged as model systems for studying a metazoan super clade, the Spiralia. Recent work on one species in particular, Crepidula fornicata, has produced high-resolution cell lineage fate maps, details of morphogenetic events during gastrulation, key insights into the molecular underpinnings of early development, and the first demonstration of CRISPR/Cas9 genome editing in the Spiralia. Furthermore, invasive species of Crepidula are a significant ecological threat, while one of these, C. fornicata, is also being harvested for food. This review highlights progress towards developing these animals as models for evolutionary, developmental, and ecological studies. Such studies have contributed greatly to our understanding of biology in a major clade of bilaterians. This information may also help us to control and cultivate these snails.

Eye development and photoreceptor differentiation in the cephalopod Doryteuthis pealeii

Photoreception is a ubiquitous sensory ability found across the Metazoa, and photoreceptive organs are intricate and diverse in their structure. Although the morphology of the compound eye in Drosophila and the single-chambered eye in vertebrates have elaborated independently, the amount of conservation within the 'eye' gene regulatory network remains controversial, with few taxa studied. To better understand the evolution of photoreceptive organs, we established the cephalopod Doryteuthis pealeii as a lophotrochozoan model for eye development. Utilizing histological, transcriptomic and molecular assays, we characterize eye formation in Doryteuthis pealeii Through lineage tracing and gene expression analyses, we demonstrate that cells expressing Pax and Six genes incorporate into the lens, cornea and iris, and the eye placode is the sole source of retinal tissue. Functional assays demonstrate that Notch signaling is required for photoreceptor cell differentiation and retinal organization. This comparative approach places the canon of eye research in traditional models into perspective, highlighting complexity as a result of both conserved and convergent mechanisms.

Larval settlement and metamorphosis in a marine gastropod in response to multiple conspecific cues

Larvae of the marine gastropod Crepidula fornicata must complete a transition from the plankton, where they are highly dispersed, to an aggregated group of benthic adults. Previous research has shown that selective settlement of larvae on conspecific adults is mediated by a water-borne chemical cue. However, variable experimental conditions have been used to study this cue, and standardization is needed in order to investigate factors that may have weak effects on settlement. In this study, we developed a time-course bioassay based on a full-factorial design with temporal blocking and statistical analysis of larval settlement rates in the lab. We tested this bioassay by examining settlement in the presence of an abiotic cue (KCl), and biotic cues (water conditioned with adult conspecifics and conspecific pedal mucus). Results confirmed settlement in the presence of both KCl and adult-conditioned water, and discovered the induction of settlement by pedal mucus. This optimized, standardized bioassay will be used in future experiments to characterize the complex process of larval settlement in C. fornicata, particularly to measure components of potentially small effect.

Deployment of regulatory genes during gastrulation and germ layer specification in a model spiralian mollusc Crepidula

BACKGROUND

During gastrulation, endoderm and mesoderm are specified from a bipotential precursor (endomesoderm) that is argued to be homologous across bilaterians. Spiralians also generate mesoderm from ectodermal precursors (ectomesoderm), which arises near the blastopore. While a conserved gene regulatory network controls specification of endomesoderm in deuterostomes and ecdysozoans, little is known about genes controlling specification or behavior of either source of spiralian mesoderm or the digestive tract.

RESULTS

Using the mollusc Crepidula, we examined conserved regulatory factors and compared their expression to fate maps to score expression in the germ layers, blastopore lip, and digestive tract. Many genes were expressed in both ecto- and endomesoderm, but only five were expressed in ectomesoderm exclusively. The latter may contribute to epithelial-to-mesenchymal transition seen in ectomesoderm.

CONCLUSIONS

We present the first comparison of genes expressed during spiralian gastrulation in the context of high-resolution fate maps. We found variation of genes expressed in the blastopore lip, mouth, and cells that will form the anus. Shared expression of many genes in both mesodermal sources suggests that components of the conserved endomesoderm program were either co-opted for ectomesoderm formation or that ecto- and endomesoderm are derived from a common mesodermal precursor that became subdivided into distinct domains during evolution.

Moderate genetic drift is driven by extreme recruitment events in the invasive mollusk Crepidula fornicata

Effective population size (Ne) is a measure of genetic drift and is thus a central parameter in evolution, conservation genetics and invasion biology. Interestingly, in native marine species, Ne is typically several orders of magnitude lower than the census size. This pattern has often been explained by high fecundity, variation in reproductive success and pronounced early mortality, resulting in genetic drift across generations. Data documenting genetic drift and/or Ne in marine invasive species are, however, still scarce. We examined the importance of genetic drift in the invasive species Crepidula fornicata by genotyping 681 juveniles sampled during each annual recruitment peak over nine consecutive years in the Bay of Morlaix (Brittany, France). Observed variations in genetic diversity were partially explained by variation in recruitment intensity. In addition, substantial temporal genetic differentiation was documented (that is, genetic drift), and was attributed to nonrandom variance in the reproductive success of different breeding groups across years in the study species. Using a set of single-sample and temporal estimators for Ne, we estimated Ne to be three or four orders of magnitude smaller than the census size (Nc). On one hand, this reduction in Ne relative to Nc appeared congruent with, although slight higher than, values commonly observed in native marine species. Particular life-history traits of this invasive species may play an important role in buffering genetic drift. On the other hand, Ne still remained far below Nc, hence, possibly reducing the efficiency of selection effects.

A Whole Mount In Situ Hybridization Method for the Gastropod Mollusc Lymnaea stagnalis

Whole mount in situ hybridization (WMISH) is a technique that allows for the spatial resolution of nucleic acid molecules (often mRNAs) within a 'whole mount' tissue preparation, or developmental stage (such as an embryo or larva) of interest. WMISH is extremely powerful because it can significantly contribute to the functional characterization of complex metazoan genomes, a challenge that is becoming more of a bottleneck with the deluge of next generation sequence data. Despite the conceptual simplicity of the technique much time is often needed to optimize the various parameters inherent to WMISH experiments for novel model systems; subtle differences in the cellular and biochemical properties between tissue types and developmental stages mean that a single WMISH method may not be appropriate for all situations. We have developed a set of WMISH methods for the re-emerging gastropod model Lymnaea stagnalis that generate consistent and clear WMISH signals for a range of genes, and across all developmental stages. These methods include the assignment of larvae of unknown chronological age to an ontogenetic window, the efficient removal of embryos and larvae from their egg capsules, the application of an appropriate Proteinase-K treatment for each ontogenetic window, and hybridization, post-hybridization and immunodetection steps. These methods provide a foundation from which the resulting signal for a given RNA transcript can be further refined with probe specific adjustments (primarily probe concentration and hybridization temperature).

Physical and Chemical Interactions with Conspecifics Mediate Sex Change in a Protandrous Gastropod Crepidula fornicata

The protandrous marine snail Crepidula fornicata has been a theoretical and empirical model for studies of sex change for many decades. We investigated the social conditions under which sex change occurs in this species by manipulating physical and chemical contact with conspecifics. Male snails were either in physical and chemical contact with females or in chemical contact with, but physically isolated from, females. Males were tested both with living females and with empty, sterilized shells. Males that were physically touching a living female were less likely to change sex than the isolated controls, while males in chemical (but not physical) contact with females changed sex no slower than the isolated controls. These results provide experimental evidence that the factor controlling sex change in C. fornicata is due to a contact-borne inhibitor associated with female conspecifics. These findings serve as a basis for future studies of sex change in this model system.

Crepidula Slipper Limpets Alter Sex Change in Response to Physical Contact with Conspecifics

Chemical signaling, especially signaling with waterborne cues, is an important mode of communication between conspecifics of aquatic organisms. Although conspecific associations play an important role in sex allocation of sequential hermaphroditic slipper limpets, the mode of signaling is unknown. We tested the hypothesis that the effects of conspecifics on animal size and time of sex change in the tropical slipper limpet Crepidula cf. marginalis are mediated by waterborne cues. In our experiment, pairs of snails (one small and one large) were kept in cups, either together or partitioned off with fine or coarse mesh, or partitioned, but switched from side to side to allow contact with the cup mate's pedal mucus. The larger snails that were allowed contact with the smaller companions grew faster, and generally changed sex sooner, than did the larger snails in the barrier treatments, which allowed no physical contact. The smaller snails that were allowed contact with the larger cup mate delayed sex change compared to those separated from their cup mates. We were, therefore, able to reject the hypothesis that waterborne cues mediate communication between these snails. Our results suggest that the cue that affects size and time to sex change requires some kind of physical interaction that is lost when the snails are separated. Furthermore, contact with another snail's pedal mucus does not compensate for the loss of physical contact. Since males often attach to the shell of larger females, direct contact may mediate this kind of physical interaction via positional information, physical stimulation, or contact-based chemical communication. Whatever the cue, contact with conspecifics influences both partners, resulting in, surprisingly, a higher growth rate in the larger animal and delayed sex change in the smaller animal.

Role of the Substrate in Feeding and Growth of the Marine Suspension-Feeding Gastropods Crepidula fornicata and Crepipatella peruviana

Unlike lamellibranch bivalves, suspension-feeding calyptraeid gastropods lack siphons and paired shell valves to regulate water inflow. This study was designed to determine if calyptraeid gastropods use the solid surface to which they attach to facilitate food particle capture. Juveniles of both Crepidula fornicata and Crepipatella peruviana were maintained with phytoplankton for 3 to 6 wk in the laboratory, either attached to solid substrate or without solid substrate. Individuals of C. fornicata and C. peruviana that were reared on solid substrate grew about five to ten times more, or two times more, respectively, than those deprived of solid substrate. Final tissue weights were also significantly greater for individuals of both species that had been reared on solid substrate. For the two species, phytoplankton clearance rates were about two to three times higher for individuals attached to solid substrate than for those without solid substrate; rates of food cord production from the gills were also significantly higher. About 50% of C. peruviana that were deprived of solid substrate died during the first 3 wk of observation, and about 60% were dead by 6 wk. In contrast, most individuals of C. peruviana that were attached to solid substrate survived for the entire 6-wk study period, and all of the C. fornicata survived whether or not they were attached to solid substrate. The solid substrate to which calyptraeid gastropods attach clearly plays an important role in their feeding biology, although the precise role remains to be explored.

The seminal receptacle and implications for reproductive processes in the invasive gastropod Crepidula fornicata

The calyptraeid gastropod Crepidula fornicata is the object of considerable research attention, due to its invasive status in the North-Eastern Atlantic, its introduction to habitats throughout the Northern hemisphere, and its scientific interest as a model organism for the study of developmental and reproductive processes in the Metazoa. Since the knowledge concerning the structural foundations for its reproductive processes is surprisingly weak, we investigated the seminal receptacle, a key structure in the reproductive biology of other metazoans, using histology, scanning electron and transmission electron microscopy. The seminal receptacle consists of 9-11 lobes, each subdivided into small, narrow lobules. The inner epithelium of the lobules appears to be highly dynamic, characterised by the perforation and attachment of received spermatozoa, the progressive degeneration of this epithelium, and the concomitant detachment of the spermatozoa. The allocation of spermatozoa to many different lobules, in different phases, may explain the extended reproductive season of C. fornicata, and thereby contribute to its colonizing and invasive success. The same compartmentalisation, as well as the complete covering of the inner epithelium of the lobules by spermatozoa and the large amount of spermatozoan debris in the lumina, suggest that the C. fornicata seminal receptacle may be a site of sperm competition in this polyandrous species.

Trophic Ecology of Benthic Marine Invertebrates with Bi-Phasic Life Cycles: What Are We Still Missing?

The study of trophic ecology of benthic marine invertebrates with bi-phasic life cycles is critical to understand the mechanisms shaping population dynamics. Moreover, global climate change is impacting the marine environment at an unprecedented level, which promotes trophic mismatches that affect the phenology of these species and, ultimately, act as drivers of ecological and evolutionary change. Assessing the trophic ecology of marine invertebrates is critical to understanding maternal investment, larval survival to metamorphosis, post-metamorphic performance, resource partitioning and trophic cascades. Tools already available to assess the trophic ecology of marine invertebrates, including visual observation, gut content analysis, food concentration, trophic markers, stable isotopes and molecular genetics, are reviewed and their main advantages and disadvantages for qualitative and quantitative approaches are discussed. The challenges to perform the partitioning of ingestion, digestion and assimilation are discussed together with different approaches to address each of these processes for short- and long-term fingerprinting. Future directions for research on the trophic ecology of benthic marine invertebrates with bi-phasic life cycles are discussed with emphasis on five guidelines that will allow for systematic study and comparative meta-analysis to address important unresolved questions.

Spiralian gastrulation: germ layer formation, morphogenesis, and fate of the blastopore in the slipper snail Crepidula fornicata

Background

Gastrulation is a critical step in bilaterian development, directly linked to the segregation of germ layers, establishment of axes, and emergence of the through-gut. Theories about the evolution of gastrulation often concern the fate of the blastopore (site of endomesoderm internalization), which varies widely in a major branch of bilaterians, the Spiralia. In this group, the blastopore has been said to become the mouth, the anus, both, or neither. Different developmental explanations for this variation exist, yet no modern lineage tracing study has ever correlated the position of cells surrounding the blastopore with their contribution to tissues of the mouth, foregut, and anus in a spiralian. This is the first study to do so, using the gastropod Crepidula fornicata.

Results

Crepidula gastrulation occurs by epiboly: the first through third quartet micromeres form an epithelial animal cap that expands to cover vegetal endomesodermal precursors. Initially, descendants of the second and third quartet micromeres (2a–2d, 3a–3d) occupy a portion of the blastopore lip. As the blastopore narrows, the micromeres’ progeny exhibit lineage-specific behaviors that result in certain sublineages leaving the lip’s edge. Anteriorly, cells derived from 3a² and 3b² undergo a unique epithelial-to-mesenchymal transition involving proliferation and a collective movement of cells into the archenteron. These cells make a novel spiralian germ layer, the ectomesoderm. Posteriorly, cells derived from 3c² and 3d² undergo a form of convergence and extension that involves zippering of cells and their intercalation across the ventral midline. During this process, several of these cells, as well as the 2d clone, become displaced posteriorly, away from the blastopore. Progeny of 2a-2c and 3a-3d make the mouth and foregut, and the blastopore becomes the opening to the mouth. The anus forms days later, as a secondary opening within the 2d² clone, and not from the classically described “anal cells”, which we identify as the 3c²²¹ and 3d²²¹ cells.

Conclusions

Our analysis of Crepidula gastrulation constitutes the first description of blastopore lip morphogenesis and fates using lineage tracing and live imaging. These data have profound implications for hypotheses about the evolution of the bilaterian gut and help explain observed variation in blastopore morphogenesis among spiralians.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0019-1) contains supplementary material, which is available to authorized users.

CRISPR/Cas9-mediated genome modification in the mollusc, Crepidula fornicata

The discovery and application of the CRISPR/Cas9 genome editing method has greatly enhanced the ease with which transgenic manipulation can occur. We applied this technology to the mollusc, Crepidula fornicata, and have successfully created transgenic embryos expressing mCherry fused to endogenous β-catenin. Specific integration of the fluorescent reporter was achieved by homologous recombination with a β-catenin-specific donor DNA containing the mCherry coding sequence. This fluorescent gene knock-in strategy permits in vivo observations of β-catenin expression during embryonic development and represents the first demonstration of CRISPR/Cas9-mediated transgenesis in the Lophotrochozoa superphylum. The CRISPR/Cas9 method is a powerful and economical tool for genome modification and presents an option for analysis of gene expression in not only major model systems, but also in those more diverse species that may not have been amenable to the classic methods of transgenesis. This approach will allow one to generate transgenic lines of snails for future studies.

Effect of food on metamorphic competence in the model system Crepidula fornicata

Food quality and quantity, as well as temperature, are all factors that are expected to affect rates of development, and are likely to be affected by expected climatic change. We tested the effect of a mixed diet versus a single-food diet on metamorphic competence in the emerging model species Crepidula fornicata. We then compared our results with other published studies on this species that examined time to metamorphic competence across a range of food concentrations and rearing temperatures. Ours was the only study to test the effects of single food versus a mixed diet on metamorphic competence for this species. Diet composition did not affect metamorphic competence or survivorship. Comparing results across studies, we found that the shortest time to metamorphic competence was typically found when the food availability per larva was the greatest, independent of rearing temperature. Unfortunately, some published studies did not include important metadata needed for comparison with other studies; these data included larval rearing density, food density, frequency of feeding, and rearing temperature. Mortality rates were not always reported and when reported were often measured in different ways, preventing comparison. Such metadata are essential for comparisons among studies as well as among taxa, and for the determination of generalizable patterns and evolutionary trends. Increased reporting of all such metadata is essential if we are to use scientific studies performed to their fullest potential.

The effects of experimentally induced adelphophagy in gastropod embryos

Adelphophagy, development where embryos grow large by consuming morphologically distinct nutritive embryos or their own normal siblings is widespread but uncommon among animal phyla. Among invertebrates it is particularly common in some families of marine gastropods and segmented worms, but rare or unknown in other closely related families. In calyptraeid gastropods phylogenetic analysis indicates that adelphophagy has arisen at least 9 times from species with planktotrophic larval development. This pattern of frequent parallel evolution of adelphophagy suggests that the embryos of planktotrophic species might be predisposed to evolve adelphophagy. Here we used embryos of three species of planktotrophic calyptraeids, one from each of three major genera in the family (Bostrycapulus, Crucibulum, and Crepidula), to answer the following 3 questions: (1) Can embryos of species with planktotrophic development benefit, in terms of pre-hatching growth, from the ingestion of yolk and tissue from experimentally damaged siblings? (2) Does ingestion of this material from damaged siblings increase variation in pre-hatching size? and (3) Does this experimentally induced adelphophagy alter the allometry between the velum and the shell, increasing morphological similarity to embryos of normally adelphophagic species? We found an overall increase in shell length and velum diameter when embryos feed on damaged siblings within their capsules. There was no detectable increase in variation in shell length or velum diameter, or changes in allometry. The overall effect of our treatment was small compared to the embryonic growth observed in naturally adelphophagic development. However each embryo in our experiment probably consumed less than one sibling on average, whereas natural adelphophages often each consume 10-30 or more siblings. These results suggest that the ability to consume, assimilate, and benefit from yolk and tissue of their siblings is widespread across calyptraeids.

SeaBase: a multispecies transcriptomic resource and platform for gene network inference

Marine and aquatic animals are extraordinarily useful as models for identifying mechanisms of development and evolution, regeneration, resistance to cancer, longevity and symbiosis, among many other areas of research. This is due to the great diversity of these organisms and their wide-ranging capabilities. Genomics tools are essential for taking advantage of these "free lessons" of nature. However, genomics and transcriptomics are challenging in emerging model systems. Here, we present SeaBase, a tool for helping to meet these needs. Specifically, SeaBase provides a platform for sharing and searching transcriptome data. More importantly, SeaBase will support a growing number of tools for inferring gene network mechanisms. The first dataset available on SeaBase is a developmental transcriptomic profile of the sea anemone Nematostella vectensis (Anthozoa, Cnidaria). Additional datasets are currently being prepared and we are aiming to expand SeaBase to include user-supplied data for any number of marine and aquatic organisms, thereby supporting many potentially new models for gene network studies. SeaBase can be accessed online at: http://seabase.core.cli.mbl.edu.

Artificial seawater culture of the gastropod Crepidula fornicata for studies of larval settlement and metamorphosis

The slipper limpet, Crepidula fornicata, is a gastropod mollusc of growing importance as a research model in developmental biology and as an invasive organism. The large (>1 mm) veliger larvae of this species are well suited for neuroethological investigations of settlement and metamorphosis. In this chapter, methods are described for conditioning adult broodstock, growing microalgal food for larvae, and culturing larvae to metamorphic competence in artificial seawater. A protocol is also presented for obtaining electrophysiological recordings of ciliary arrest spikes from intact, behaving larvae, as putative neural correlates of larval settlement.

Swimming speed of larval snail does not correlate with size and ciliary beat frequency

Many marine invertebrates have planktonic larvae with cilia used for both propulsion and capturing of food particles. Hence, changes in ciliary activity have implications for larval nutrition and ability to navigate the water column, which in turn affect survival and dispersal. Using high-speed high-resolution microvideography, we examined the relationship between swimming speed, velar arrangements, and ciliary beat frequency of freely swimming veliger larvae of the gastropod Crepidula fornicata over the course of larval development. Average swimming speed was greatest 6 days post hatching, suggesting a reduction in swimming speed towards settlement. At a given age, veliger larvae have highly variable speeds (0.8–4 body lengths s⁻¹) that are independent of shell size. Contrary to the hypothesis that an increase in ciliary beat frequency increases work done, and therefore speed, there was no significant correlation between swimming speed and ciliary beat frequency. Instead, there are significant correlations between swimming speed and visible area of the velar lobe, and distance between centroids of velum and larval shell. These observations suggest an alternative hypothesis that, instead of modifying ciliary beat frequency, larval C. fornicata modify swimming through adjustment of velum extension or orientation. The ability to adjust velum position could influence particle capture efficiency and fluid disturbance and help promote survival in the plankton.

PCR survey of 50 introns in animals: cross-amplification of homologous EPIC loci in eight non-bilaterian, protostome and deuterostome phyla

Exon Primed Intron Crossing (EPIC) markers provide molecular tools that are susceptible to be variable within species while remaining amplifiable by PCR using potentially universal primers. In this study we tested the possibility of obtaining PCR products from 50 EPIC markers on 23 species belonging to seven different phyla (Porifera, Cnidaria, Arthropoda, Nematoda, Mollusca, Annelida, Echinodermata) using 70 new primer pairs. A previous study had identified and tested those loci in a dozen species, including another phylum, Urochordata (Chenuil et al., 2010). Results were contrasted among species. The best results were achieved with the oyster (Mollusca) where 28 loci provided amplicons susceptible to contain an intron according to their size. This was however not the case with the other mollusk Crepidula fornicata, which seems to have undergone a reduction in intron number or intron size. In the Porifera, 13 loci appeared susceptible to contain an intron, a surprisingly high number for this phylum considering its phylogenetic distance with genomic data used to design the primers. For two cnidarian species, numerous loci (24) were obtained. Ecdysozoan phyla (arthropods and nematodes) proved less successful than others as expected considering reports of their rapid rate of genome evolution and the worst results were obtained for several arthropods. Some general patterns among phyla arose, and we discuss how the results of this EPIC survey may give new insights into genome evolution of the study species. This work confirms that this set of EPIC loci provides an easy-to-use toolbox to identify genetic markers potentially useful for population genetics, phylogeography or phylogenetic studies for a large panel of metazoan species. We then argue that obtaining diploid sequence genotypes for these loci became simple and affordable owing to Next-Generation Sequencing development. Species surveyed in this study belong to several genera (Acanthaster, Alvinocaris, Aplysina, Aurelia, Crepidula, Eunicella, Hediste, Hemimysis, Litoditis, Lophelia, Mesopodopsis, Mya, Ophiocten, Ophioderma, Ostrea, Pelagia, Platynereis, Rhizostoma, Rimicaris), two of them, belonging to the family Vesicomydae and Eunicidae, could not be determined at the genus level.

Comparative ontogenetic changes in enzyme activity during embryonic development of calyptraeid gastropods

A modification of a semi-quantitative color-based enzyme assay was used to quantify the activity of 19 enzymes (5 peptidases, 3 lipases, 3 phosphotases, and 8 carbohydrases) during five stages of development in eight species of calyptraeid gastropods. Sixteen of the 19 enzymes showed a significant effect of mode of development on the concentration of the reaction product after incubation of homogenates standardized for protein content. The overall pattern was that planktotrophs showed the highest activities, followed by adelphophages, and nonfeeding embryos, which had the lowest enzyme activities. Thirteen enzymes showed significant differences across developmental stages. Of these, eight showed a clear increase during development. Only one of the enzymes showed a sudden jump in activity between the unfed, pre-hatching stage and post-hatching stages that were fed Isochrysis galbana. In three cases, ANOVA identified two exclusive, significantly different groups of species. In naphthol-AS-BI-phosphohydrolase, the measured absorbance of Crucibulum spinosum samples was significantly higher than in all of the other species. The activity of α-fucosidase in Crepipatella occulta was significantly greater than in the other seven species. Finally, the activity of β-galactosidase was significantly higher in C. occulta, Crucibulum spinosum, and Bostrycapulus calyptraeformis than in the four Crepidula species. This is the only enzyme for which there is an indication of a phylogenetic effect. Relative enzyme activities were similar to those reported for other herbivorous gastropods, with the three phosphohydrolases, four carbohydrases (β-galactosidase, β-glucuronidase, N-acetyl-β-glucosaminidase, and α-fucosidase), and leucine arylamidase showing high activities.

Cleavage pattern and fate map of the mesentoblast, 4d, in the gastropod Crepidula: a hallmark of spiralian development

Background

Animals with a spiral cleavage program, such as mollusks and annelids, make up the majority of the superphylum Lophotrochozoa. The great diversity of larval and adult body plans in this group emerges from this highly conserved developmental program. The 4d micromere is one of the most conserved aspects of spiralian development. Unlike the preceding pattern of spiral divisions, cleavages within the 4d teloblastic sublineages are bilateral, representing a critical transition towards constructing the bilaterian body plan. These cells give rise to the visceral mesoderm in virtually all spiralians examined and in many species they also contribute to the endodermal intestine. Hence, the 4d lineage is an ideal one for studying the evolution and diversification of the bipotential endomesodermal germ layer in protostomes at the level of individual cells. Little is known of how division patterns are controlled or how mesodermal and endodermal sublineages diverge in spiralians. Detailed modern fate maps for 4d exist in only a few species of clitellate annelids, specifically in glossiphoniid leeches and the sludge worm Tubifex. We investigated the 4d lineage in the gastropod Crepidula fornicata, an established model system for spiralian biology, and in a closely related direct-developing species, C. convexa.

Results

High-resolution cell lineage tracing techniques were used to study the 4d lineage of C. fornicata and C. convexa. We present a new nomenclature to name the progeny of 4d, and report the fate map for the sublineages up through the birth of the first five pairs of teloblast daughter cells (when 28 cells are present in the 4d sublineage), and describe each clone’s behavior during gastrulation and later stages as these undergo differentiation. We identify the precise origin of the intestine, two cells of the larval kidney complex, the larval retractor muscles and the presumptive germ cells, among others. Other tissues that arise later in the 4d lineage include the adult heart, internal foot tissues, and additional muscle and mesenchymal cells derived from later-born progeny of the left and right teloblasts. To test whether other cells can compensate for the loss of these tissues (that is, undergo regulation), specific cells were ablated in C. fornicata.

Conclusions

Our results present the first fate map of the 4d micromere sublineages in a mollusk. The fate map reveals that endodermal and mesodermal fates segregate much later than previously thought. We observed little evidence of regulation between sublineages, consistent with a lineage-driven cell specification process. Our results provide a framework for comparisons with other spiralians and lay the groundwork for investigation of the molecular mechanisms of endomesoderm formation, germ line segregation and bilateral differentiation in Crepidula.

Polybrominated diphenyl ethers (PBDEs) alter larval settlement of marine intertidal organisms across three phyla via reducing bacterial abundance on the biofilms

Polybrominated diphenyl ethers (PBDEs) have been widely used as flame retardants over the last three decades, and are now ubiquitous in the marine environment. While the harmful effects of PBDEs on the abnormal development and reproductive impairment in mammals and fish are well documented, the effects on marine invertebrates remain virtually unknown. Using three model intertidal species accross three phyla, including the polychaete Hydroides elegans (Phylum Annelida), the gastropod Crepidula onyx (Phylum Mollusca), and the barnacle Balanus amphitrite (Phylum Arthopoda), this study demonstrated that (a) chronic exposure to BDE-47 (at spiking concentrations up to 1000 ng L(-1)) throughout the entire larval stage did not affect settlement, development or growth of all three species per se, despite bioaccumulation was clearly evident (measured body burden ranging from approximately 7000 to 13 000 ng BDE-47 g(-1) lipid), and (b) BDE-47, at measured concentrations of 15 and 113 ng g(-1) lipid, reduced the bacterial abundance in biofilms and resulted in a concomitant change in larval settlement pattern of all the model intertidal species across three phyla.

Spiral cleavage and early embryology of a loxosomatid entoproct and the usefulness of spiralian apical cross patterns for phylogenetic inferences

Background

Among the four major bilaterian clades, Deuterostomia, Acoelomorpha, Ecdysozoa, and Lophotrochozoa, the latter shows an astonishing diversity of bodyplans. While the largest lophotrochozoan assemblage, the Spiralia, which at least comprises Annelida, Mollusca, Entoprocta, Platyhelminthes, and Nemertea, show a spiral cleavage pattern, Ectoprocta, Brachiopoda and Phoronida (the Lophophorata) cleave radially. Despite a vast amount of recent molecular phylogenetic analyses, the interrelationships of lophotrochozoan phyla remain largely unresolved. Thereby, Entoprocta play a key role, because they have frequently been assigned to the Ectoprocta, despite their differently cleaving embryos. However, developmental data on entoprocts employing modern methods are virtually non-existent and the data available rely exclusively on sketch drawings, thus calling for thorough re-investigation.

Results

By applying fluorescence staining in combination with confocal microscopy and 3D-imaging techniques, we analyzed early embryonic development of a basal loxosomatid entoproct. We found that cleavage is asynchronous, equal, and spiral. An apical rosette, typical for most spiralian embryos, is formed. We also identified two cross-like cellular arrangements that bear similarities to both, a "molluscan-like" as well as an "annelid-like" cross, respectively.

Conclusions

A broad comparison of cleavage types and apical cross patterns across Lophotrochozoa shows high plasticity of these character sets and we therefore argue that these developmental traits should be treated and interpreted carefully when used for phylogenetic inferences.