Having cake and eating too: The benefits of an intermediate larval form in a brittle star Amphiodia sp. opaque (Ophiuroidea)

Most marine invertebrate larvae either obligately feed or depend on maternally provided reserves during planktonic development. A small number of species have the capacity to do both, in a mode of development known as facultative planktotrophy. We describe facultative feeding in a larva from the Oregon coast, and identify it as being an undescribed species in the genus Amphiodia, which we refer to as Amphiodia sp. opaque. We quantified the effects of food on larval and juvenile quality by culturing larvae, collected as embryos, with and without microalgal food at 15°C. The resulting juveniles were monitored under conditions of starvation. A cohort of juveniles of larvae caught as plankton was subjected to the same starvation treatment for comparison with our laboratory-reared larvae. We observed benefits to offspring that received food: larvae provided with microalgae developed more quickly and metamorphosed at higher rates. Furthermore, juveniles resulting from fed larvae were larger and were able to avoid starvation for longer after metamorphosis. Our results varied across two experimental years, suggesting that provisions provided by parents vary between populations and years. Juveniles from planktonic larvae exhibited sizes not statistically different from larvae cultured in the absence of food, but died from starvation more quickly.

New Genera, Species, and observations on the biology of Antarctic Valvatida (Asteroidea)

Assessment of Antarctic taxonomic diversity for asteroids and other taxa for poorly studied or unknown regions, such as the deep-sea, will be important for our understanding of these understudied habitats. Eleven new species and a new genus are described from three families (Asterinidae, Goniasteridae, Solasteridae) within the Valvatida, nearly all of which were collected from deep-sea settings below 1000 m by the US Antarctic Research Program in the 1960s. A new subfamily, the Kampylasterinae subfam. nov. is designated for Kampylaster and Astrotholus nov. gen. which were supported as sister taxa on a monophyletic clade within the Asterinidae. Astrotholus nov. gen. is described to accommodate "Anseropoda" antarctica and four new bathyal and abyssal species, which are a significant morphological divergence from the typological definition of Anseropoda. New species of the goniasterid Notioceramus and the solasterid Paralophaster are also described from bathyal depths (2000-3000 m). Paralophaster ferax n. sp. is among the deepest asteroids known to brood, is the first brooding species within Paralophaster and the second species in the Solasteridae known to brood. Following examination of the type and molecular data, Lophaster densus is found to be included within Paralophaster. A review of Antarctic Lophaster species shows additional specimens of Lophaster abbreviatus which support it as a distinct species from Lophaster stellans. New occurrence data for bathyal Antarctic Asteroidea as well as unusual-gut content observations of shallower-water species are also included.

Evo-Devo in Ophiuroids: The Switch from Planktotrophy to Lecithotrophy in Ophionereis

AbstractUnderstanding the evolution of development (evo-devo) in the Ophiuroidea and the pathways in the switch from a feeding to a nonfeeding larva is complicated by the variability in the phenotype of the metamorphic larva, being a reduced yolky ophiopluteus in some species (type I development) and a vitellaria larva in others (type II development). We investigated evo-devo in the family Ophionereididae, a group dominated by lecithotrophic development through a vitellaria larva. We reared the planktotrophic larvae of Ophionereis fasciata to settlement to determine the metamorphic phenotype. Counter to expectations, O. fasciata did not exhibit type II metamorphosis through a vitellaria, although it did exhibit transient vitellaria-like features. Resorption of the larval arms in the same interradial positions where the ciliary bands form in vitellariae gave them a fleeting vitellaria-like appearance. Development of O. fasciata exhibits heterochronic features in early formation of the skeletal primordium of the third pair (postoral) of larval arms and in the presettlement juvenile early appearance of the juvenile terminal arm plates on external view in parallel with larval arm resorption. Development of the fourth pair (posterodorsal) of larval arms, the last pair to be formed, is plastic, with 44% of larvae exhibiting partial arm growth. Heterochronic traits in development, as seen in O. fasciata, may have facilitated evolution of a lecithotrophic mode of development in Ophionereis. Comparison of the ophiopluteus of O. fasciata and the vestigial pluteus of O. schayeri provided insights into the simplification of larval form from the ancestral (feeding larva) state in Ophionereis. The diverse metamorphic phenotypes in ophiuroids indicate that type I and type II development may not be completely divergent lines of evo-devo and point to selective pressure in the pelagic-benthic transition in the evolution of ophiuroid development.

The arm of the starfish: The far-reaching applications of Patiria miniata as a model system in evolutionary, developmental, and regenerative biology

Many species of echinoderms have long been considered model research organisms in biology. Historically, much of this research has focused on the embryology of sea urchins and the use of their extensive gene regulatory networks as a tool to understand how the genome controls cell state specification and patterning. The establishment of Patiria miniata, the bat sea star, as a research organism has allowed us to expand on the concepts explored with sea urchins, viewing these genetic networks through a comparative lens, gaining great insight into the evolutionary mechanisms that shape developmental diversity. Extensive molecular tools have been developed in P. miniata, designed to explore gene expression dynamics and build gene regulatory networks. Echinoderms also have a robust set of bioinformatic and computational resources, centered around echinobase.org, an extensive database containing multiomic, developmental, and experimental resources for researchers. In addition to comparative evolutionary development, P. miniata is a promising system in its own right for studying whole body regeneration, metamorphosis and body plan development, as well as marine disease.

Life-History Analysis of Asterinid Starfishes

AbstractThe starfish family Asterinidae shows a diversity of reproductive modes, and a number of species have sufficient life-history data that can be used for analysis, using life-cycle graphs. These include four species that reproduce by fission (Aquilonastra yairi, Nepanthia belcheri, Aquilonastra burtonii, and Ailsastra heteractis), a viviparous species (Parvulastra vivipara), two species with benthic egg masses (Asterina gibbosa and Asterina phylactica), one with planktonic larvae that do not feed (Cryptasterina pentagona), and one with larvae that feed in the plankton (Patiria miniata). Species are compared using adult and first-year survival and, for some species, the age at first reproduction, number of offspring (eggs or newly released juveniles), and individual growth parameters of the von Bertalanffy model. The sensitivity of population growth, fitness, to changes in these traits is shown by elasticity analysis, which aids in understanding possible consequences of environmental forces as well as possible directions of selection.

Transcriptomic analysis of sea star development through metamorphosis to the highly derived pentameral body plan with a focus on neural transcription factors

The Echinodermata is characterized by a secondarily evolved pentameral body plan. While the evolutionary origin of this body plan has been the subject of debate, the molecular mechanisms underlying its development are poorly understood. We assembled a de novo developmental transcriptome from the embryo through metamorphosis in the sea star Parvulastra exigua. We use the asteroid model as it represents the basal-type echinoderm body architecture. Global variation in gene expression distinguished the gastrula profile and showed that metamorphic and juvenile stages were more similar to each other than to the pre-metamorphic stages, pointing to the marked changes that occur during metamorphosis. Differential expression and gene ontology (GO) analyses revealed dynamic changes in gene expression throughout development and the transition to pentamery. Many GO terms enriched during late metamorphosis were related to neurogenesis and signalling. Neural transcription factor genes exhibited clusters with distinct expression patterns. A suite of these genes was up-regulated during metamorphosis (e.g. Pax6, Eya, Hey, NeuroD, FoxD, Mbx, and Otp). In situ hybridization showed expression of neural genes in the CNS and sensory structures. Our results provide a foundation to understand the metamorphic transition in echinoderms and the genes involved in development and evolution of pentamery.

Transcriptomic analysis of Nodal - and BMP- associated genes during development to the juvenile seastar in Parvulastra exigua (Asterinidae)

The molecular mechanisms underlying development of the pentameral body of adult echinoderms are poorly understood but are important to solve with respect to evolution of a unique body plan that contrasts with the bilateral body plan of other deuterostomes. As Nodal and BMP2/4 signalling is involved in axis formation in larvae and development of the echinoderm body plan, we used the developmental transcriptome generated for the asterinid seastar Parvulastra exigua to investigate the temporal expression patterns of Nodal and BMP2/4 genes from the embryo and across metamorphosis to the juvenile. For echinoderms, the Asteroidea represents the basal-type body architecture with a distinct (separated) ray structure. Parvulastra exigua has lecithotrophic development forming the juvenile soon after gastrulation providing ready access to the developing adult stage. We identified 39 genes associated with the Nodal and BMP2/4 network in the P. exigua developmental transcriptome. Clustering analysis of these genes resulted in 6 clusters with similar temporal expression patterns across development. A co-expression analysis revealed genes that have similar expression profiles as Nodal and BMP2/4. These results indicated genes that may have a regulatory relationship in patterning morphogenesis of the juvenile seastar. Developmental RNA-seq analyses of Parvulastra exigua show changes in Nodal and BMP2/4 signalling genes across the metamorphic transition. We provide the foundation for detailed analyses of this cascade in the evolution of the unusual pentameral echinoderm body and its deuterostome affinities.

Evolutionary modification of gastrulation in Parvulastra exigua, an asterinid seastar with holobenthic lecithotrophic development

Gastrulation is a fundamental morphogenetic process in development. In echinoderms with ancestral-type development through feeding larvae, gastrulation involves radially symmetrical invagination of cells around the blastopore. Gastrulation in the seastar Parvulastra exigua, a species with non-feeding larvae deviates from this pattern. Microinjection of cells with fluorescent lineage tracer dye revealed that early blastomeres contribute unequally to ectoderm and endoderm. In embryos injected at the two-cell stage, asymmetry was evident in the fluorescence at the top of the archenteron and animal pole ectoderm. Archenteron elongation is driven by asymmetrical involution of cells with more cells crossing the blastopore on one side. Lineages of cells injected at the four-cell stage also differed in allocation to endoderm and ectoderm. In embryos injected at the eight-cell stage ectodermal and endodermal fates were evident reflecting the animal and vegetal fates determined by third cleavage as typical of echinoderms. Modification of gastrulation associated with evolution of development in P. exigua shows that this foundational morphogenetic process can be altered despite its importance for subsequent development. However, observations of slight asymmetry in the lineage fates of blastomeres in asterinids with planktotrophic development indicates that gastrulation by asymmetrical involution in P. exigua may be a hypertrophic elaboration of a pre-existing state in ancestral-type development. As for echinoids with lecithotrophic development, involution as a mechanism to contribute to archenteron elongation may be associated with the impact of extensive maternal nutritive reserves on the mechanics of cell movement and a novel innovation to facilitate early development of the adult rudiment.

Developing in a warming intertidal, negative carry over effects of heatwave conditions in development to the pentameral starfish in Parvulastra exigua

Ocean warming and increasing incidence of marine heat waves (MHW) are having far-reaching impacts on coastal ecosystems. The small intertidal asterinid starfish, Parvulastra exigua, in south-eastern Australia, occurs in a global warming hotspot. Development occurs in the intertidal as this species lays eggs and has benthic larvae. The impact of temperature on development to the juvenile was determined over a broad temperature range (12-28 °C) encompassing temperatures experienced during the breeding season (16-20 °C) and cool (- 4 °C) and warm (+10 °C) extremes with the higher temperatures (24-28 °C) simulating a MHW. As the larva to juvenile transition involves major body reorganisation, we determined the impact of temperature on metamorphosis and formation of the normal five-armed juvenile. Development was faster at the higher temperatures 24-28 °C, but survival decreased from 1 to 5 days post fertilisation (dpf). Mortality was evident from day 15 at 22 °C and no larvae survived to 20 dpf at 28 °C. Thermal tolerance decreased over developmental time and the thermal optimum for 95% survival to the 20 day old juvenile spanned from 12 to 20.0 °C with the lethal temperature for 50% survival being 23.5 °C (5.5 °C above ambient). Juveniles reared in 26 °C were smaller, suggesting application of the temperature size rule. Increased temperature (22-26 °C) perturbed pentamery with three, four, six and no-armed juveniles present, contrasting with the low level of non-pentamerous individuals (<3%) in the cooler cultures and in nature (five populations surveyed). Despite the high thermal tolerance in premetamorphic stages, negative carry over effects were evident in the juveniles. This shows the importance of considering the whole of development in climate warming studies. As sea surface temperatures increase and heatwaves become more prevalent, habitat warming will be detrimental to P. exigua populations.

Brooding in the Southern Ocean: The Case of the Pterasterid Sea Star Diplopteraster verrucosus (Sladen, 1882)

Diplopteraster verrucosus is a sea star that incubates its offspring in nidamental chambers. The offspring rely exclusively on maternally provided nutrition. The retention of the embryonic stages allows the allocation of nutritional supplies from the female to the brooded juveniles during the brooding period. The main objectives of this study are, first, to quantify the reproductive investment of D. verrucosus and, second, to describe the morphology, energetics, and oxidative metabolism throughout early ontogenetic stages. A skewed sex ratio of 2:1 females:males was found, and 17 of 39 females were brooding. Both brooding and non-brooding females showed higher energy density and total antioxidant capacity in their gonads than males. We identified three cohorts of offspring being retained within the female body simultaneously. Energy density and reactive oxygen species increased significantly with the offspring's volume throughout ontogeny. Moreover, we found evidence of at least two key events during ontogeny. First, the depletion of antioxidants, the increase of reactive oxygen species, and the development of a complete digestive system appear to trigger feeding on the mothers's pyloric caeca. Second, another oxidative imbalance appears to be associated with the release of the brooded juveniles to the environment. Therefore, oxidative balance and energetic variances may be associated with development of autonomous feeding and juvenile release in D. verrucosus.

Testing adaptive hypotheses on the evolution of larval life history in acorn and stalked barnacles

Despite strong selective pressure to optimize larval life history in marine environments, there is a wide diversity with regard to developmental mode, size, and time larvae spend in the plankton. In the present study, we assessed if adaptive hypotheses explain the distribution of the larval life history of thoracican barnacles within a strict phylogenetic framework. We collected environmental and larval trait data for 170 species from the literature, and utilized a complete thoracican synthesis tree to account for phylogenetic nonindependence. In accordance with Thorson's rule, the fraction of species with planktonic-feeding larvae declined with water depth and increased with water temperature, while the fraction of brooding species exhibited the reverse pattern. Species with planktonic-nonfeeding larvae were overall rare, following no apparent trend. In agreement with the "size advantage" hypothesis proposed by Strathmann in 1977, egg and larval size were closely correlated. Settlement-competent cypris larvae were larger in cold water, indicative of advantages for large juveniles when growth is slowed. Planktonic larval duration, on the other hand, was uncorrelated to environmental variables. We conclude that different selective pressures appear to shape the evolution of larval life history in barnacles.

The structural and chemical basis of temporary adhesion in the sea star Asterina gibbosa

Background: Marine biological adhesives are a promising source of inspiration for biomedical and industrial applications. Nevertheless, natural adhesives and especially temporary adhesion systems are mostly unexplored. Sea stars are able to repeatedly attach and detach their hydraulic tube feet. This ability is based on a duo-gland system and, upon detachment, the adhesive material stays behind on the substrate as a 'footprint'. In recent years, characterization of sea star temporary adhesion has been focussed on the forcipulatid species Asterias rubens. Results: We investigated the temporary adhesion system in the distantly related valvatid species Asterina gibbosa. The morphology of tube feet was described using histological sections, transmission-, and scanning electron microscopy. Ultrastructural investigations revealed two adhesive gland cell types that both form electron-dense secretory granules with a more lucid outer rim and one de-adhesive gland cell type with homogenous granules. The footprints comprised a meshwork on top of a thin layer. This topography was consistently observed using various methods like scanning electron microscopy, 3D confocal interference microscopy, atomic force microscopy, and light microscopy with crystal violet staining. Additionally, we tested 24 commercially available lectins and two antibodies for their ability to label the adhesive epidermis and footprints. Out of 15 lectins labelling structures in the area of the duo-gland adhesive system, only one also labelled footprints indicating the presence of glycoconjugates with α-linked mannose in the secreted material. Conclusion: Despite the distant relationship between the two sea star species, the morphology of tube feet and topography of footprints in A. gibbosa shared many features with the previously described findings in A. rubens. These similarities might be due to the adaptation to a benthic life on rocky intertidal areas. Lectin- and immuno-labelling indicated similarities but also some differences in adhesive composition between the two species. Further research on the temporary adhesive of A. gibbosa will allow the identification of conserved motifs in sea star adhesion and might facilitate the development of biomimetic, reversible glues.

Looking for morphological evidence of cryptic species in Asterina Nardo, 1834 (Echinodermata: Asteroidea). The redescription of Asterina pancerii (Gasco, 1870) and the description of two new species

Three species of the genus Asterina are known to inhabit the Mediterranean Sea and the north-eastern Atlantic Ocean: Asterina gibbosa (Pennant, 1777), A. pancerii (Gasco, 1870) and A. phylactica Emson &amp; Crump, 1979. Differentiation of these species has primarily been based only on subtle characters (some highly debatable), such as colour or size. Therefore, this study aimed to review the morphological data characterising members of the genus, to incorporate new characters that may clarify morphological analyses and to couple morphological data with molecular evidence of differentiation based on the analysis of partial sequences of the cytochromec oxidase subunitI (COI) and 18S rDNA (18S) genes and two anonymous nuclear loci (AgX2 and AgX5). The different lineages and cryptic species identified from the molecular analysis were then morphologically characterised, which was challenging given the limited number of diagnostic characters. Two of the five monophyletic lineages obtained molecularly (COI divergence &gt;4%), further supported by differences in morphological characters and reproductive behaviour, are proposed as new species: Asterina martinbarriosi, sp. nov. from the Canary Islands, Spain (eastern central Atlantic Ocean) and Asterina vicentae, sp. nov. from Tarragona, north-eastern Spain (western Mediterranean Sea).

Life-history predicts past and present population connectivity in two sympatric sea stars

Life‐history traits, especially the mode and duration of larval development, are expected to strongly influence the population connectivity and phylogeography of marine species. Comparative analysis of sympatric, closely related species with differing life histories provides the opportunity to specifically investigate these mechanisms of evolution but have been equivocal in this regard. Here, we sample two sympatric sea stars across the same geographic range in temperate waters of Australia. Using a combination of mitochondrial DNA sequences, nuclear DNA sequences, and microsatellite genotypes, we show that the benthic‐developing sea star, Parvulastra exigua, has lower levels of within‐ and among‐population genetic diversity, more inferred genetic clusters, and higher levels of hierarchical and pairwise population structure than Meridiastra calcar, a species with planktonic development. While both species have populations that have diverged since the middle of the second glacial period of the Pleistocene, most P. exigua populations have origins after the last glacial maxima (LGM), whereas most M. calcar populations diverged long before the LGM. Our results indicate that phylogenetic patterns of these two species are consistent with predicted dispersal abilities; the benthic‐developing P. exigua shows a pattern of extirpation during the LGM with subsequent recolonization, whereas the planktonic‐developing M. calcar shows a pattern of persistence and isolation during the LGM with subsequent post‐Pleistocene introgression.

Patterns and Drivers of Egg Pigment Intensity and Colour Diversity in the Ocean: A Meta-Analysis of Phylum Echinodermata

Egg pigmentation is proposed to serve numerous ecological, physiological, and adaptive functions in egg-laying animals. Despite the predominance and taxonomic diversity of egg layers, syntheses reviewing the putative functions and drivers of egg pigmentation have been relatively narrow in scope, centring almost exclusively on birds. Nonvertebrate and aquatic species are essentially overlooked, yet many of them produce maternally provisioned eggs in strikingly varied colours, from pale yellow to bright red or green. We explore the ways in which these colour patterns correlate with behavioural, morphological, geographic and phylogenetic variables in extant classes of Echinodermata, a phylum that has close phylogenetic ties with chordates and representatives in nearly all marine environments. Results of multivariate analyses show that intensely pigmented eggs are characteristic of pelagic or external development whereas pale eggs are commonly brooded internally. Of the five egg colours catalogued, orange and yellow are the most common. Yellow eggs are a primitive character, associated with all types of development (predominant in internal brooders), whereas green eggs are always pelagic, occur in the most derived orders of each class and are restricted to the Indo-Pacific Ocean. Orange eggs are geographically ubiquitous and may represent a 'universal' egg pigment that functions well under a diversity of environmental conditions. Finally, green occurs chiefly in the classes Holothuroidea and Ophiuroidea, orange in Asteroidea, yellow in Echinoidea, and brown in Holothuroidea. By examining an unprecedented combination of egg colours/intensities and reproductive strategies, this phylum-wide study sheds new light on the role and drivers of egg pigmentation, drawing parallels with theories developed from the study of more derived vertebrate taxa. The primary use of pigments (of any colour) to protect externally developing eggs from oxidative damage and predation is supported by the comparatively pale colour of equally large, internally brooded eggs. Secondarily, geographic location drives the evolution of egg colour diversity, presumably through the selection of better-adapted, more costly pigments in response to ecological pressure.

Matrotrophy and placentation in invertebrates: a new paradigm

Matrotrophy, the continuous extra-vitelline supply of nutrients from the parent to the progeny during gestation, is one of the masterpieces of nature, contributing to offspring fitness and often correlated with evolutionary diversification. The most elaborate form of matrotrophy-placentotrophy-is well known for its broad occurrence among vertebrates, but the comparative distribution and structural diversity of matrotrophic expression among invertebrates is wanting. In the first comprehensive analysis of matrotrophy across the animal kingdom, we report that regardless of the degree of expression, it is established or inferred in at least 21 of 34 animal phyla, significantly exceeding previous accounts and changing the old paradigm that these phenomena are infrequent among invertebrates. In 10 phyla, matrotrophy is represented by only one or a few species, whereas in 11 it is either not uncommon or widespread and even pervasive. Among invertebrate phyla, Platyhelminthes, Arthropoda and Bryozoa dominate, with 162, 83 and 53 partly or wholly matrotrophic families, respectively. In comparison, Chordata has more than 220 families that include or consist entirely of matrotrophic species. We analysed the distribution of reproductive patterns among and within invertebrate phyla using recently published molecular phylogenies: matrotrophy has seemingly evolved at least 140 times in all major superclades: Parazoa and Eumetazoa, Radiata and Bilateria, Protostomia and Deuterostomia, Lophotrochozoa and Ecdysozoa. In Cycliophora and some Digenea, it may have evolved twice in the same life cycle. The provisioning of developing young is associated with almost all known types of incubation chambers, with matrotrophic viviparity more widespread (20 phyla) than brooding (10 phyla). In nine phyla, both matrotrophic incubation types are present. Matrotrophy is expressed in five nutritive modes, of which histotrophy and placentotrophy are most prevalent. Oophagy, embryophagy and histophagy are rarer, plausibly evolving through heterochronous development of the embryonic mouthparts and digestive system. During gestation, matrotrophic modes can shift, intergrade, and be performed simultaneously. Invertebrate matrotrophic adaptations are less complex structurally than in chordates, but they are more diverse, being formed either by a parent, embryo, or both. In a broad and still preliminary sense, there are indications of trends or grades of evolutionarily increasing complexity of nutritive structures: formation of (i) local zones of enhanced nutritional transport (placental analogues), including specialized parent-offspring cell complexes and various appendages increasing the entire secreting and absorbing surfaces as well as the contact surface between embryo and parent, (ii) compartmentalization of the common incubatory space into more compact and 'isolated' chambers with presumably more effective nutritional relationships, and (iii) internal secretory ('milk') glands. Some placental analogues in onychophorans and arthropods mimic the simplest placental variants in vertebrates, comprising striking examples of convergent evolution acting at all levels-positional, structural and physiological.

Complexity of Yolk Proteins and Their Dynamics in the Sea Star Patiria miniata

Oviparous animals store yolk proteins within the developing oocyte. These proteins are used in gametogenesis and as a nutritional source for embryogenesis. Vitellogenin and the major yolk protein are two of the most important yolk proteins among diverse species of invertebrates and vertebrates. Among the echinoderms, members of the subphyla Echinozoa (sea urchins and sea cucumbers) express the major yolk protein (MYP) but not vitellogenin (Vtg), while an initial report has documented that two Asterozoa (sea stars) express a vitellogenin. Our results show that sea stars contain two vitellogenins, Vtg 1 and Vtg 2, and MYP. In Patiria miniata, these genes are differentially expressed in the somatic and germ cells of the ovary: Vtg 1 is enriched in the somatic cells of the ovary but not in the oocytes, and Vtg 2 accumulates in both oocytes and somatic cells; MYP is not robustly present in either. Remarkably, Vtg 2 and MYP mRNA reappear in larvae; Vtg 2 is detected within cells of the ectoderm, and MYP accumulates in the coelomic pouches, the intestine, and the posterior enterocoel (PE), the site of germ line formation in this animal. Additionally, the Vtg 2 protein is present in oocytes, follicle cells, and developing embryos, but becomes undetectable following gastrulation. These results help elucidate the mechanisms involved in yolk dynamics, and provide molecular information that allows for greater understanding of the evolution of these important gene products.

Development of the Sea Star Echinaster (Othilia) brasiliensis, with Inference on the Evolution of Development and Skeletal Plates in Asteroidea

We describe the development and juvenile morphology of the sea star Echinaster (Othilia) brasiliensis in order to explore evolutionary developmental modes and skeletal homologies. This species produces large, buoyant eggs (0.6 ± 0.03 mm diameter), and has a typical lecithotrophic brachiolaria larva. The planktonic brachiolaria larva is formed 2-4 days after fertilization, when cilia cover the surface. Early juveniles are completely formed by 18 days of age. Initial growth is supported by maternal nutrients while the stomach continues to develop until 60 days after fertilization, when juveniles reach about 0.5 mm of radius length. The madreporite was observed 88 days after fertilization. In the youngest juvenile skeleton of E. (O.) brasiliensis, the madreporite and odontophore are homologous to those of other recent, non-paxillosid asteroids, and follow the Late Madreporic Mode. The emergence of plates related to the ambulacral system follows the Ocular Plate Rule. The development and juvenile skeletal morphology of this species are similar to those of the few other studied species in the genus Echinaster. This study corroborates the notion that the mode of development--including a short-lived lecithotrophic brachiolaria larva--in all Echinaster species shares a similar pattern that may be conserved throughout the evolutionary history of the group.

The diversity, development and evolution of polyclad flatworm larvae

Polyclad flatworms offer an excellent system with which to explore the evolution of larval structures and the ecological and developmental mechanisms driving flatworm and marine invertebrate life history evolution. Although the most common mode of development in polyclads might be direct development (where the embryo develops directly into a form resembling the young adult), there are many species that develop indirectly, through a planktonic phase with transient larval features, before settling to the sea floor. In this review, I introduce polyclad life history strategies, larval diversity and larval anatomical features (presenting previously unpublished micrographs of a diversity of polyclad larvae). I summarize what is known about polyclad larval development during the planktonic phase and the transition to the benthic juvenile. Finally, I discuss evolutionary and developmental scenarios on the origin of polyclad larval characters.The most prominent characters that are found exclusively in the larval stages are lobes that protrude from the body and a ciliary band, or ciliary tufts, at the peripheral margins of the lobes. Larvae with 4-8 and 10 lobes have been described, with most indirect developing species hatching with 8 lobes. A ventral sucker develops in late stage larvae, and I put forward the hypothesis that this is an organ for larval settlement for species belonging to the Cotylea. Historically, the biphasic life cycle of polyclads was thought to be a shared primitive feature of marine invertebrates, with similarities in larval features among phyla resulting from evolutionary conservation. However, our current understanding of animal phylogeny suggests that indirect development in polyclads has evolved independently of similar life cycles found in parasitic flatworms and some other spiralian taxa, and that morphological similarities between the larvae of polyclads and other spiralians are likely a result of convergent evolution.

Shallow gene pools in the high intertidal: extreme loss of genetic diversity in viviparous sea stars (Parvulastra)

We document an extreme example of reproductive trait evolution that affects population genetic structure in sister species of Parvulastra cushion stars from Australia. Self-fertilization by hermaphroditic adults and brood protection of benthic larvae causes strong inbreeding and range-wide genetic poverty. Most samples were fixed for a single allele at nearly all nuclear loci; heterozygotes were extremely rare (0.18%); mitochondrial DNA sequences were more variable, but few populations shared haplotypes in common. Isolation-with-migration models suggest that these patterns are caused by population bottlenecks (relative to ancestral population size) and low gene flow. Loss of genetic diversity and low potential for dispersal between high-intertidal habitats may have dire consequences for extinction risk and potential for future adaptive evolution in response to climate and other selective agents.

DNA barcoding discriminates echinoderm species

DNA barcode sequences (a 657-bp segment of the mtDNA cytochrome oxidase I gene, COI) were collected from 191 species (503 specimens) of Echinodermata. All five classes were represented: Ophiuroidea, Asteroidea, Echinoidea, Holothuroidea and Crinoidea. About 30% of sequences were collected specifically for this study, the remainder came from GenBank. Fifty-one species were represented by multiple samples, with a mean intraspecific divergence of 0.62%. Several possible instances of cryptic speciation were noted. Thirty-two genera were represented by multiple species, with a mean congeneric divergence of 15.33%. One hundred and eighty-seven of the 191 species (97.9%) could be distinguished by their COI barcodes. Those that could not were from the echinoid genus Amblypneustes. Neighbour-joining trees of COI sequences generally showed low bootstrap support for anything other than shallow splits, although with very rare exceptions, members of the same class clustered together. Two ophiuran species, in both nucleotide and amino acid neighbour-joining trees, grouped loosely as sister taxa to Crinoidea rather than Ophiuroidea; sequences of these two species appear to have evolved very quickly. Results suggest that DNA barcoding is likely to be an effective, accurate and useful method of species diagnosis for all five classes of Echinodermata.

The Biogeography of Marine Invertebrate Life Histories

Biologists have long sought to identify and explain patterns in the diverse array of marine life histories. The most famous speculation about such patterns is Gunnar Thorson's suggestion that species producing planktonic larvae are rarer at higher latitudes (Thorson's rule). Although some elements of Thorson's rule have proven incorrect, other elements remain untested. With a wealth of new life-history data, statistical approaches, and remote-sensing technology, new insights into marine reproduction can be generated. We gathered life-history data for more than 1,000 marine invertebrates and examined patterns in the prevalence of different life histories. Systematic patterns in marine life histories exist at a range of scales, some of which support Thorson, whereas others suggest previously unrecognized relationships between the marine environment and the life histories of marine invertebrates. Overall, marine life histories covary strongly with temperature and local ocean productivity, and different regions should be managed accordingly.

Relationships among egg size, composition, and energy: a comparative study of geminate sea urchins

Egg size is one of the fundamental parameters in the life histories of marine organisms. However, few studies have examined the relationships among egg size, composition, and energetic content in a phylogenetically controlled context. We investigated the associations among egg size, composition, and energy using a comparative system, geminate species formed by the closure of the Central American Seaway. We examined western Atlantic (WA) and eastern Pacific (EP) species in three echinoid genera, Echinometra, Eucidaris, and Diadema. In the genus with the largest difference in egg size between geminates (Echinometra), the eggs of WA species were larger, lipid rich and protein poor compared to the smaller eggs of their EP geminate. In addition, the larger WA eggs had significantly greater total egg energy and summed biochemical constituents yet significantly lower egg energy density (energy-per-unit-volume). However, the genera with smaller (Eucidaris) or no (Diadema) differences in egg size were not significantly different in summed biochemical constituents, total egg energy, or energy density. Theoretical models generally assume a strong tradeoff between egg size and fecundity that limits energetic investment and constrains life history evolution. We show that even among closely-related taxa, large eggs cannot be assumed to be scaled-up small eggs either in terms of energy or composition. Although our data comes exclusively from echinoid echinoderms, this pattern may be generalizable to other marine invertebrate taxa. Because egg composition and egg size do not necessarily evolve in lockstep, selective factors such as sperm limitation could act on egg volume without necessarily affecting maternal or larval energetics.

Extraordinarily rapid life-history divergence between Cryptasterina sea star species

Life history plays a critical role in governing microevolutionary processes such as gene flow and adaptation, as well as macroevolutionary processes such speciation. Here, we use multilocus phylogeographic analyses to examine a speciation event involving spectacular life-history differences between sister species of sea stars. Cryptasterina hystera has evolved a suite of derived life-history traits (including internal self-fertilization and brood protection) that differ from its sister species Cryptasterina pentagona, a gonochoric broadcast spawner. We show that these species have only been reproductively isolated for approximately 6000 years (95% highest posterior density of 905-22 628), and that this life-history change may be responsible for dramatic genetic consequences, including low nucleotide diversity, zero heterozygosity and no gene flow. The rapid divergence of these species rules out some mechanisms of isolation such as adaptation to microhabitats in sympatry, or slow divergence by genetic drift during prolonged isolation. We hypothesize that the large phenotypic differences between species relative to the short divergence time suggests that the life-history differences observed may be direct responses to disruptive selection between populations. We speculate that local environmental or demographic differences at the southern range margin are possible mechanisms of selection driving one of the fastest known marine speciation events.

Global diversity and phylogeny of the Asteroidea (Echinodermata)

Members of the Asteroidea (phylum Echinodermata), popularly known as starfish or sea stars, are ecologically important and diverse members of marine ecosystems in all of the world's oceans. We present a comprehensive overview of diversity and phylogeny as they have figured into the evolution of the Asteroidea from Paleozoic to the living fauna. Living post-Paleozoic asteroids, the Neoasteroidea, are morphologically separate from those in the Paleozoic. Early Paleozoic asteroid faunas were diverse and displayed morphology that foreshadowed later living taxa. Preservation presents significant difficulties, but fossil occurrence and current accounts suggests a diverse Paleozoic fauna, which underwent extinction around the Permian-Triassic interval was followed by re-diversification of at least one surviving lineage. Ongoing phylogenetic classification debates include the status of the Paxillosida and the Concentricycloidea. Fossil and molecular evidence has been and continues to be part of the ongoing evolution of asteroid phylogenetic research. The modern lineages of asteroids include the Valvatacea, the Forcipulatacea, the Spinlosida, and the Velatida. We present an overview of diversity in these taxa, as well as brief notes on broader significance, ecology, and functional morphology of each. Although much asteroid taxonomy is stable, many new taxa remain to be discovered with many new species currently awaiting description. The Goniasteridae is currently one of the most diverse families within the Asteroidea. New data from molecular phylogenetics and the advent of global biodiversity databases, such as the World Asteroidea Database (http://www.marinespecies.org/Asteroidea/) present important new springboards for understanding the global biodiversity and evolution of asteroids.

Introduction to symposium: Poecilogony--a window on larval evolutionary transitions in marine invertebrates

Poecilogony is the intraspecific variation in developmental mode that has been described in some marine invertebrates. Poecilogonous species produce different larval forms (e.g., free-swimming planktotrophic larvae as well as brooded lecithotrophic or adelphophagic larvae). Poecilogony can be a controversial topic, since it is difficult to identify and characterize the phenomenon with certainty. It has been challenging to determine whether poecilogony represents developmental polymorphism with a genetic basis or developmental polyphenism reflecting plastic responses to environmental cues. Other outstanding questions include whether common mechanisms underlie the developmental variation we observe in poecilogonous species, and whether poecilogony is maintained in different taxa through similar mechanisms or selective pressures. Poecilogonous species provide a unique opportunity to elucidate the cellular, developmental, and genetic mechanisms underlying evolutionary transitions in developmental mode, as well as to help clarify the selective pressures and possible ecological circumstances that might be involved. Here, we describe an integrative approach to the study of poecilogony and its role in larval evolutionary transitions highlighted during a symposium held at the 2012 annual meeting of the Society for Integrative and Comparative Biology.

The initiation of metamorphosis as an ancient polyphenic trait and its role in metazoan life-cycle evolution

Comparative genomics of representative basal metazoans leaves little doubt that the most recent common ancestor to all modern metazoans was morphogenetically complex. Here, we support this interpretation by demonstrating that the demosponge Amphimedon queenslandica has a biphasic pelagobenthic life cycle resembling that present in a wide range of bilaterians and anthozoan cnidarians. The A. queenslandica life cycle includes a compulsory planktonic larval phase that can end only once the larva develops competence to respond to benthic signals that induce settlement and metamorphosis. The temporal onset of competence varies between individuals as revealed by idiosyncratic responses to inductive cues. Thus, the biphasic life cycle with a dispersing larval phase of variable length appears to be a metazoan synapomorphy and may be viewed as an ancestral polyphenic trait. Larvae of a particular age that are subjected to an inductive cue either maintain the larval form or metamorphose into the post-larval/juvenile form. Variance in the development of competence dictates that only a subset of a larval cohort will settle and undergo metamorphosis at a given time, which in turn leads to variation in dispersal distance and in location of settlement. Population divergence and allopatric speciation are likely outcomes of this conserved developmental polyphenic trait.

Discordant distribution of populations and genetic variation in a sea star with high dispersal potential

Patiria miniata, a broadcast-spawning sea star species with high dispersal potential, has a geographic range in the intertidal zone of the northeast Pacific Ocean from Alaska to California that is characterized by a large range gap in Washington and Oregon. We analyzed spatial genetic variation across the P. miniata range using multilocus sequence data (mtDNA, nuclear introns) and multilocus genotype data (microsatellites). We found a strong phylogeographic break at Queen Charlotte Sound in British Columbia that was not in the location predicted by the geographical distribution of the populations. However, this population genetic discontinuity does correspond to previously described phylogeographic breaks in other species. Northern populations from Alaska and Haida Gwaii were strongly differentiated from all southern populations from Vancouver Island and California. Populations from Vancouver Island and California were undifferentiated with evidence of high gene flow or very recent separation across the range disjunction between them. The surprising and discordant spatial distribution of populations and alleles suggests that historical vicariance (possibly caused by glaciations) and contemporary dispersal barriers (possibly caused by oceanographic conditions) both shape population genetic structure in this species.

Bindin from a sea star

The genetic basis for the evolution of development includes genes that encode proteins expressed on the surfaces of sperm and eggs. Previous studies of the sperm acrosomal protein bindin have helped to characterize the adaptive evolution of gamete compatibility and speciation in sea urchins. The absence of evidence for bindin expression in taxa other than the Echinoidea has limited such studies to sea urchins, and led to the suggestion that bindin might be a sea urchin-specific molecule. Here we characterize the gene that encodes bindin in a broadcast-spawning asterinid sea star (Patiria miniata). We describe the sequence and domain structure of a full-length bindin cDNA and its single intron. In comparison with sea urchins, P. miniata bindin is larger but the two molecules share several general features of their domain structure and some sequence features of two domains. Our results extend the known evolutionary history of bindin from the Mesozoic (among the crown group sea urchins) into the early Paleozoic (and the common ancestor of eleutherozoans), and present new opportunities for understanding the role of bindin molecular evolution in sexual selection, life history evolution, and speciation among sea stars.

Nervous system development in feeding and nonfeeding asteroid larvae and the early juvenile

Larval and juvenile nervous systems (NS) of three asterinid sea stars with contrasting feeding and nonfeeding modes of development were characterized using the echinoderm-specific synaptotagmin antibody. In the feeding bipinnaria and brachiolaria larvae of Patiriella regularis, the species with ancestral-type development, an extensive NS was associated with the ciliary bands (CBs) and attachment complex. Lecithotrophic planktonic (Meridastra calcar) and benthic (Parvulastra exigua) brachiolariae lacked CBs and the associated NS, but had an extensive NS in the attachment complex. The similarity in the distribution and morphology of synaptotagmin immunoreactive neurons and the anatomy of the NS in the attachment complex of these closely related sea stars suggests conservation of neurogenesis in settlement-stage larvae regardless of larval feeding mode. Nerve cells were prominent on the brachia of all three species. In advanced brachiolariae the larval nervous system was localized to the adhesive disc as the larval body resorbed during metamorphosis. The structures and tissues that contained larval neurons degenerated during metamorphosis. There was no evidence that the larval NS persists through metamorphosis. In juvenile development, synaptotagmin IR was first evident in the NS of the tube feet. As the central nervous system developed, synaptotagmin IR reflected the histological organization of the adult NS. The juvenile NS formed de novo with a temporal lapse between histogenesis and synaptotagmin IR. We evaluated the ontogeny of NS organization in the change in body plan from the bilateral larva to the radial juvenile.

A new brooding species of the biscuit star Tosia (Echinodermata:Asteroidea:Goniasteridae), distinguished by molecular, morphological and larval characters

The biscuit star Tosia australis Gray, 1840 is a well known component of the shallow rocky reef fauna of south-eastern Australia. The putative T. australis species complex was subjected to reproductive, morphometric and molecular analyses. Molecular analyses of the data from three markers (mitochondrial COI and 16S rRNA and the nuclear non-coding region ITS2) confirmed the presence of a cryptic species, the morphology of which does not agree with any of the existing nominal species. Two separate reproductive modes were observed within the complex and documented via scanning electron microscopy. T. neossia, sp. nov., described herein from south-eastern Australia, is shown to release gametes from gonopores on the actinal surface. Embryos develop first into non-feeding, non-swimming brachiolaria, and then into tripod brachiolaria before metamorphosis. No surface cilia are present at any point throughout development of T. neossia. T. australis sensu stricto is shown to release gametes from the abactinal surface. Embryos develop into non-feeding, swimming brachiolaria before metamorphosis. Whereas T. australis var. astrologorum is confirmed as synonymous with T. australis, the status of the putative Western Australian taxon T. nobilis remains unresolved.

Discovery and cross-amplification of microsatellite polymorphisms in asterinid sea stars

Variation in tandem repeats of two- to six-base nucleotide motifs (microsatellites) can be used to obtain inexpensive and highly informative multi-locus data on population genetics.We developed and tested a large set of cross-amplifiable sea star (Asterinidae) microsatellite markers from a mixed pool of genomic DNA from eight species. We describe cloned sequences, primers, and PCR conditions, and characterize population-level variation for some species and markers. A few clones containing microsatellites showed considerable similarity to sequences (including genes of known function) in other sea stars and in sea urchins (from the Strongylocentrotus purpuratus complete genome). The pooled genomic DNA method was an efficient way to sample microsatellites from many species: we cloned 2-10 microsatellites from each of eight species, and most could be cross-amplified in 1-7 other species. At 12 loci in two species, we found 1-10 alleles per microsatellite, with a broad range of inbreeding coefficients. Measures of polymorphism were negatively correlated with the extent of cross-amplification.

The evolutionary ecology of offspring size in marine invertebrates

Intraspecific variation in offspring size is of fundamental ecological and evolutionary importance. The level of provisioning an organism receives from its mother can have far reaching consequences for subsequent survival and performance. In marine systems, the traditional focus was on the remarkable variation in offspring size among species but there is increasing focus on variation in offspring size within species. Here we review the incidence and consequences of intraspecific offspring-size variation for marine invertebrates. Offspring size is remarkably variable within and among marine invertebrate populations. We examined patterns of variation in offspring size within populations using a meta-analysis of the available data for 102 species across 7 phyla. The average coefficient of variation in offspring size within populations is 9%, while some groups (e.g., direct developers) showed much more variation (15%), reflecting a fourfold difference between the largest and smallest offspring in any population. Offspring-size variation can have for reaching consequences. Offspring size affects every stage of a marine invertebrate's life history, even in species in which maternal provisioning accounts for only a small proportion of larval nutrition (i.e., planktotrophs). In species with external fertilization, larger eggs are larger targets for sperm and as such, the sperm environment may select for different egg sizes although debate continues over the evolutionary importance of such effects. Offspring size affects the planktonic period in many species with planktotrophic and lecithotrophic development, but we found that this effect is not universal. Indeed, much of the evidence for the effects of offspring size on the planktonic period is limited to the echinoids and in this group and other taxa there is variable evidence, suggesting further work is necessary. Post-metamorphic effects of offspring size were strong in species with non-feeding larvae and direct development: bigger offspring generally have higher post-metamorphic survival, higher growth rates and sometimes greater fecundity. Although there is limited evidence for the mechanisms underlying these effects, the size of post-metamorphic feeding structures and resistance to low-food availability appear to be good candidates. There was limited evidence to assess the effects of offspring size on post-metamorphic performance in planktotrophs but surprisingly, initial indications suggest that such effects do exist and in the same direction as for species with other developmental modes. Overall, we suggest that for direct developers and species with non-feeding larvae, the post-metamorphic effects of offspring size will be greatest source of selection. Offspring-size variation can arise through a variety of sources, both within and among populations. Stress, maternal size and nutrition, and habitat quality all appear to be major factors affecting the size of offspring, but more work on sources of variation is necessary. While theoretical considerations of offspring size can now account for variation in offspring size among mothers, they struggle to account for within-brood variation. We suggest alternative approaches such as game theoretic models that may be useful for reconciling within-clutch variation. While some of the first theoretical considerations of offspring size were based on marine invertebrates, many of the assumptions of these models have not been tested, and we highlight some of the important gaps in understanding offspring-size effects. We also discuss the advantages of using offspring size as a proxy for maternal investment and review the evidence used to justify this step. Overall, offspring size is likely to be an important source of variation in the recruitment of marine invertebrates. The quality of offspring entering a population could be as important as the quantity and further work on the ecological role of offspring size is necessary. From an evolutionary standpoint, theoretical models that consider every life-history stage, together with the collection of more data on the relationship between offspring size and performance at each stage, should bring us closer to understanding the evolution of such a wide array of offspring sizes and developmental modes among species.

Adaptations to benthic development: functional morphology of the attachment complex of the brachiolaria larva in the sea star Asterina gibbosa

The asteroid Asterina gibbosa lives all its life in close relation to the sea bottom. Indeed, this sea star possesses an entirely benthic, lecithotrophic development. The embryos adhere to the substratum due to particular properties of their jelly coat, and hatching occurs directly at the brachiolaria stage. Brachiolariae have a hypertrophied, bilobed attachment complex comprising two asymmetrical brachiolar arms and a central adhesive disc. This study aims at describing the ultrastructure of the attachment complex and possible adaptations, at the cellular level, to benthic development. Immediately after hatching, early brachiolariae attach by the arms. All along the anterior side of each arm, the epidermis encloses several cell types, such as secretory cells of two types (A and B), support cells, and sensory cells. Like their equivalents in planktotrophic larvae, type A and B secretory cells are presumably involved in a duo-glandular system in which the former are adhesive and the latter de-adhesive in function. Unlike what is observed in planktotrophic larvae, the sensory cells are unspecialized and presumably not involved in substratum testing. During the larval period, the brachiolar arms progressively increase in size and the adhesive disc becomes more prominent. At the onset of metamorphosis, brachiolariae cement themselves strongly to the substratum with the adhesive disc. The disc contains two main cell types, support cells and secretory cells, the latter being responsible for the cement release. During this metamorphosis, the brachiolar arms regress while post-metamorphic structures grow considerably, especially the tube feet, which take over the role of attachment to the substratum. The end of this period corresponds to the complete regression of the external larval structures, which also coincides with the opening of the mouth. This sequence of stages, each possessing its own adhesive strategy, is common to all asteroid species having a benthic development. In A. gibbosa, morphological adaptations to this mode of development include the hypertrophic growth of the attachment complex, its bilobed shape forming an almost completely adhesive sole, and the regression of the sensory equipment.

The active evolutionary lives of echinoderm larvae

Echinoderms represent a researchable subset of a dynamic larval evolutionary cosmos. Evolution of echinoderm larvae has taken place over widely varying time scales from the origins of larvae of living classes in the early Palaeozoic, approximately 500 million years ago, to recent, rapid and large-scale changes that have occurred within living genera within a span of less than a million years to a few million years. It is these recent evolutionary events that offer a window into processes of larval evolution operating at a micro-evolutionary level of evolution of discrete developmental mechanisms. We review the evolution of the diverse larval forms of living echinoderms to outline the origins of echinoderm larval forms, their diversity among living echinoderms, molecular clocks and rates of larval evolution, and finally current studies on the roles of developmental regulatory mechanisms in the rapid and radical evolutionary changes observed between closely related congeneric species.