Regulation of seasonal reproduction in mollusks

Neural mechanism of circadian clock-based photoperiodism in insects and snails

The photoperiodic mechanism distinguishes between long and short days, and the circadian clock system is involved in this process. Although the necessity of circadian clock genes for photoperiodic responses has been demonstrated in many species, how the clock system contributes to photoperiodic mechanisms remains unclear. A comprehensive study, including the functional analysis of relevant genes and physiology of their expressing cells, is necessary to understand the molecular and cellular mechanisms. Since Drosophila melanogaster exhibits a shallow photoperiodism, photoperiodic mechanisms have been studied in non-model species, starting with brain microsurgery and neuroanatomy, followed by genetic manipulation in some insects. Here, we review and discuss the involvement of the circadian clock in photoperiodic mechanisms in terms of neural networks in insects. We also review recent advances in the neural mechanisms underlying photoperiodic responses in insects and snails, and additionally circadian clock systems in snails, whose involvement in photoperiodism has hardly been addressed yet. Brain neurosecretory cells, insulin-like peptide/diuretic hormone44-expressing pars intercerebralis neurones in the bean bug Riptortus pedestris and caudo-dorsal cell hormone-expressing caudo-dorsal cells in the snail Lymnaea stagnalis, both promote egg laying under long days, and their electrical excitability is attenuated under short and medium days, which reduces oviposition. The photoperiodic responses of the pars intercerebralis neurones are mediated by glutamate under the control of the clock gene period. Thus, we are now able to assess the photoperiodic response by neurosecretory cell activity to investigate the upstream mechanisms, that is, the photoperiodic clock and counter.

Concomitant downregulation of neuropeptide genes in a marine snail with consecutive sexual maturation after a nuclear disaster in Japan

Consecutive sexual maturation (CSM), an abnormal reproductive phenomenon of a marine snail, Reishia clavigera, has occurred since 2017 in the vicinity of the Fukushima Daiichi Nuclear Power Plant after the nuclear disaster there. We hypothesized that alterations in animal physiology mediated through genetic/epigenetic changes could sensitively reflect environmental pollution. Understanding the mechanism of this rapid biological response should enable us to quantitatively evaluate long-lasting effects of the nuclear disaster. To determine the molecular basis for CSM, we conducted transcriptome profiling in the ganglia of normal and CSM snails. We assembled the short-read cDNA sequences obtained by Illumina sequencing, and succeeded in characterizing more than 60,000 gene models that include 88 kinds of neuropeptide precursors by BLAST search and experimental curation. GO-enrichment analysis of the differentially expressed genes demonstrated that severe downregulation of neuropeptide-related genes occurred concomitantly with CSM. In particular, significant decreases of the transcripts of 37 genes among 88 neuropeptide precursor genes, including those for myomodulin, PentaFVamide, maturation-associated peptide-5A and conopressin, were commonly observed in female and male CSM snails. By contrast, microseminoprotein precursor was the only exceptional case where the expression was increased in CSM snails. These results indicate that down-regulation of neuropeptide precursors is a remarkable feature of CSM. We also found that factors involved in epigenetic modification rather than transcription factors showed altered patterns of expression upon CSM. Comprehensive expression panels of snail neuropeptide precursors made in this study will be useful tools for environmental assessment as well as for studying marine reproductive biology.

Female reproduction-specific proteins, origins in marine species, and their evolution in the animal kingdom

The survival of a species largely depends on the ability of individuals to reproduce, thus perpetuating their life history. The advent of metazoans (i.e. pluricellular animals) brought about the evolution of specialized tissues and organs, which in turn led to the development of complex protein regulatory pathways. This study sought to elucidate the evolutionary relationships between female reproduction-associated proteins by analyzing the transcriptomes of representative species from a selection of marine invertebrate phyla. Our study identified more than 50 reproduction-related genes across a wide evolutionary spectrum, from Porifera to Vertebrata. Among these, a total of 19 sequences had not been previously reported in at least one phylum, particularly in Porifera. Moreover, most of the structural differences between these proteins did not appear to be determined by environmental pressures or reproductive strategies, but largely obeyed a distinguishable evolutionary pattern from sponges to mammals.

The effect of biologically active compounds in the mucus of slugs Limax maximus and Arion rufus on human skin cells

Molluscs are one of the sources of biologically active substances, which are now intensively studied, especially for their anti-cancer properties. Malignant melanoma originates from melanocytes, develops very quickly and is associated with poor prognosis. Therefore, the aim of the study was to assess the properties of biologically active compounds in sterile mucus isolated from slugs Limax maximus and Arion rufus. Tested mucus were isolated using the new self-developed method which is safe for the environment and the animal donors. The impact of the mucus on human keratinocytes CCD 1106 KERTr and malignant melanoma cells A-375 was examined using MTT assay and SRB assay, which allowed us to determine the cell metabolic activity and cell number after treating them with slug mucus isolated from Limax maximus and Arion rufus decreased human keratinocytes and melanoma cells metabolic activity as well as manifested properties of reducing the number of cells in both tested cell lines, and therefore can be a source of biologically active substances with anticancer potential.

Photoperiodic control of electrophysiological properties of the caudo-dorsal cells in the pond snail, Lymnaea stagnalis

Egg laying in the pond snail, Lymnaea stagnalis is regulated by the photoperiod; long-day conditions (16L8D) promote egg laying whereas medium-day conditions (12L12D) suppress it. In this snail, a caudo-dorsal cell hormone (CDCH) is produced by neurosecretory cells, CDCs in the cerebral ganglion (CG), and its release triggers ovulation and subsequent egg laying. However, the physiological basis for photoperiod-dependent egg laying remains unraveled. Here, we compared electrophysiological properties of CDCs between 16L8D and 12L12D using intracellular recording, and found that CDC excitability is higher in 16L8D than in 12L12D. Striking differences are as follows: (1) a shallower resting membrane potential in 16L8D than in 12L12D, and (2) a smaller threshold voltage (minimum depolarization from rest to elicit action potentials) in 16L8D than in 12L12D. Switching of the excitability can be a physiological basis of a photoperiod-dependent CDCH release. Simultaneous intracellular dye injection identified two morphological subtypes of CDCs, validating a previous report. Both types bear short lateral extensions in CG, some of which probably function as integration sites of photoperiodic inputs. In addition, we found two novel CDCH-immunoreactive cell groups (CDC_COM and SC_m ) in the CG besides conventional CDCs and small cells expressing CDCH. The CDC_COM with cell bodies and fibers in the neurohemal commissure may be involved in triggering ovulation. Notably, the total number of CDCs is larger than that previously reported, the right CDC cluster with more cells than the left. Our findings are instructive in following the neurophysiology of photoperiodism in L. stagnalis.

Photoperiod controls egg laying and caudodorsal cell hormone expression but not gonadal development in the pond snail Lymnaea stagnalis

Photoperiod is a reliable cue to regulate growth and reproduction for seasonal adaptation. Although photoperiodism has been well studied in Chordata and Arthropoda, less is known about Mollusca. We examined photoperiodic effects on egg laying, body size, gonad-somatic index, oocyte size and relative amounts of caudodorsal cell hormone mRNA in individual rearing conditions in the pond snail Lymnaea stagnalis. Twenty-five weeks after hatching, the percentages of egg-laying snails under a photoperiod of 12 h light and 12 h darkness (12L:12D) were significantly smaller than those under longer days. The total numbers of eggs and egg masses under 12L:12D were significantly smaller than those under longer days. Significant differences between 16L:8D and 12L:12D were not observed in the soft body and ovotestis weight, and the gonad-somatic index. Photoperiodic effects were also not observed in oocyte diameters twenty-two weeks after hatching. Twenty-seven weeks after hatching amounts of caudodorsal cell hormone mRNA were significantly lower in the cerebral ganglia with commissure under 12L:12D than 16L:8D. L. stagnalis exhibited a clear photoperiodic response in egg laying and the amount of caudodorsal cell hormone mRNA, but not in gonadal development. Under 12L:12D suppression of caudodorsal cell hormone expression might suppress egg laying.

Influence of light and temperature cycles on the expression of circadian clock genes in the mussel Mytilus edulis

Clock genes and environmental cues regulate essential biological rhythms. The blue mussel, Mytilus edulis, is an ecologically and economically important intertidal bivalve undergoing seasonal reproductive rhythms. We previously identified seasonal expression differences in M. edulis clock genes. Herein, the effects of light/dark cycles, constant darkness, and daily temperature cycles on the circadian expression patterns of such genes are characterised. Clock genes Clk, Cry1, ROR/HR3, Per and Rev-erb/NR1D1, and Timeout-like, show significant mRNA expression variation, persisting in darkness indicating endogenous control. Rhythmic expression was apparent under diurnal temperature cycles in darkness for all except Rev-erb. Temperature cycles induced a significant expression difference in the non-circadian clock-associated gene aaNAT. Furthermore, Suppression Subtractive Hybridisation (SSH) was used to identify seasonal genes with potential links to molecular clock function and revealed numerous genes meriting further investigation. Understanding the relationship between environmental cues and molecular clocks is crucial in predicting the outcomes of environmental change on fundamental rhythmic processes.

Arrested Sexual Development in Queen Conch (Lobatus gigas) Linked to Abnormalities in the Cerebral Ganglia

In the Florida Keys, queen conchs (Lobatus gigas) occur in two spatially distinct regions: nearshore in habitats immediately adjacent to the shoreline and offshore in habitats along the reef tract south of the islands. Our previous research demonstrated that adult conchs nearshore are not reproductively active, showing deficiencies in their gonadal condition compared to their offshore counterparts. Because sexual development in gastropods is controlled by hormones secreted by the cerebral ganglia, we hypothesized that the reproductive deficiencies seen in nearshore queen conchs involved the cerebral ganglia. We collected nearshore and offshore adults and made histological comparisons of their gonads and cerebral ganglia. Our results confirmed that gonadal maturity was delayed and that gamete production was reduced in nearshore conchs compared to offshore animals. These gonadal deficiencies in nearshore conchs were associated with abnormal cerebral ganglion histology (i.e., significant hypertrophy of ganglion cells and significantly lower density of ganglion cells). In addition, the shells of nearshore conchs were significantly lighter, which is particularly consequential because shell formation in gastropods is also mediated by hormones secreted by the cerebral ganglia. Given these results, it is apparent that some yet unidentified factor(s) is interfering with hormone production in the cerebral ganglia, to the detriment of gonad development and shell formation.

Size, season and origin of gastropods matter in imposex assessments

Through controlled exposure to tributyltin (TBT), the effects of season, size, and population origin were evaluated on imposex incidence in Stramonita brasiliensis. Four experiments were carried out with organisms collected from three different sites on the Brazilian coast (Torres - Rio Grande do Sul, Farol de Santa Marta - Santa Catarina, and Aracruz - Espírito Santo). S. brasiliensis were anesthetized, sexed, measured, classified by size in small (< 30 mm), medium (≥ 30 to ≤ 40 mm), or big (> 40 mm) and injected in the foot muscle with 0.5 μg g^-1 of TBT. Organisms were maintained during one month in aquariums with clear marine water under controlled laboratory conditions. One month after injection, the imposex assessments showed that the population from the southeast (tropical) region was more sensitive to TBT than the population from the south of Brazil (the temperate region). A greater sensitivity in the small and medium categories was observed. Females were also more susceptible to TBT when exposed during their reproductive period. Thus, the present study highlighted the need to ensure that intrinsic biological factors related to organisms are considered in such biomonitoring studies to avoid misinterpretation of results.

Effects of Alkalinity and pH on Survival, Growth, and Enzyme Activities in Juveniles of the Razor Clam, Sinonovacula constricta

In order to clarify the possibility of rearing razor clams (Sinonovacula constricta) in inland saline water (ISW) and to facilitate their breeding under these stressful conditions, we performed semi-static acute and chronic toxicity tests to determine the effects of carbonate alkalinity (CA) and pH on the survival and growth rate, and critical metabolic enzyme activity in juvenile of S. constricta (JSC). (1) Acute toxicity test. As the water CA increased from 1.22 to 45.00 mmol L-1, the survival rate decreased significantly, which was exacerbated by the increase in the pH. When the water CA was set at 2.5 mmol L-1, the 48 h lethal concentration 50% (LC50) for JSCs with respect to pH was 9.86. When the water pH was 9.0, 9.5, and 10.0, the 48 h LC50 values for JSCs with respect to CA were 10.38, 8.79, and 3.11 mmol L-1, respectively. (2) Chronic toxicity test. Four experimental groups comprising the control, CAS, pHS, and CA-pHS were designated according to the target ISW data. After 3 months of stress, the JSC survival rate in each group exceeded 85%, but survival was significantly lower in the CA-pHS group than the control group (p < 0.05) in the first month. For the JSCs in various groups, the shell length growth rate (SGR) and weight gain (WG) rate were significantly lower in the CA-pHS group than the other groups (p < 0.05 for SGR; p < 0.001 for WG) in the first month. However, the difference in the growth rate among groups decreased in the next 2 months. For the JSCs in the CA-pHS group, the oxygen consumption, ammonia-N excretion, Na+/K+-ATPase, aspartate aminotransferase, and superoxide dismutase levels were significantly higher than those in the other groups during the first month, but there were no significant differences between the groups subsequently. The acetylcholinesterase and lysozyme levels did not differ significantly among groups during stress for 3 months. The integrated biomarker response index showed that stressors comprising high pH and CA could be tolerated well by JSCs over long periods of stress. These results indicate that water CA and pH together affect the survival, growth, and physiological activity of JSCs. S. constricta is suitable for culture in ISW.

Chlorophyllin Bait Formulation and Exposure to Different Spectrum of Visible Light on the Reproduction of Infected/Uninfected Snail Lymnaea acuminata

Fasciolosis is a waterborne disease, caused by Fasciola species. Snail Lymnaea acuminata is an intermediate host of these flukes. Control of snail population is major tool in reducing the incidences. Variation in light intensity and wavelength caused significant changes in reproduction pattern of snails. Maximum fecundity was noted with bait containing carbohydrate (starch, 468 ± 0.10/20 snails) or amino acid (serine, 319 ± 0.29/20 snails) as attractant. Sublethal feeding of chlorophyllin bait with starch or serine attractant to infected and uninfected snails caused significant reduction in fecundity, hatchability, and survivability. These significant changes are observed in snails exposed to different spectral band of visible light and sunlight. Maximum fecundity of 536 ± 2.0 and minimum of 89.3 ± 0.4 were noted in snails not fed with bait and exposed to sunlight and red spectral band, respectively. There was complete arrest in the fecundity of infected and uninfected snails and no survivability of uninfected snails after 48 h feeding with bait containing chlorophyllin + attractant. Minimum hatchability (9.25 ± 0.5) was noted in red light exposed, chlorophyllin + starch fed infected snails and hatching period of bait fed snails was prolonged. Conclusively, chlorophyllin bait and red light reduce reproduction capacity in snails.

Serotonin Mediates Maternal Effects and Directs Developmental and Behavioral Changes in the Progeny of Snails

Many organisms survive in constantly changing environments, including cycling seasons. Developing embryos show remarkable instant adaptations to the variable environmental challenges they encounter during their adult life, despite having no direct contact with the changing environment until after birth or hatching. The mechanisms by which such non-genetic information is transferred to the developing embryos are largely unknown. Here, we address this question by using a freshwater pond snail (Lymnaea stagnalis) as a model system. This snail normally lives in a seasonal climate, and the seasons define its locomotion, feeding, and reproductive behavior. We discovered that the serotonergic system plays a crucial role in transmitting a non-genetic instructive signal from mother to progeny. This maternal serotonin-based signal functions in embryos during a short time window at exclusively early pre-neural developmental stages and modulates the dynamics of embryonic and juvenile growth, feeding behavior, and locomotion.

Efeito do fotoperíodo sobre o ciclo de vida e a morfometria da concha de Dysopeas muibum (Mollusca, Subulinidae) em condições de laboratório

Neste estudo teve-se como objetivo avaliar o efeito do fotoperíodo sobre o ciclo de vida e a morfometria da concha de Dysopeas muibum Marcus & Marcus, 1968 em condições de laboratório. Para tal, foram acompanhados o crescimento da concha, a liberação de filhotes e a mortalidade de 40 espécimes submetidos a condições naturais de temperatura, umidade relativa do ar e fotoperíodo, e 80 submetidos a condições controladas (40 com dias longos e 40 com dias curtos), desde o nascimento até 180 dias de vida, quando foram aferidas as medidas da concha. Verificou-se que o fotoperíodo não teve efeito sobre o padrão de crescimento indeterminado, a relação entre reprodução e crescimento, a iteroparidade e a baixa mortalidade antes e após a maturidade sexual. As condições controladas de temperatura, umidade relativa do ar e fotoperíodo favoreceram o crescimento, maiores valores conquiliomorfométricos e o rápido alcance da maturidade sexual, enquanto a diminuição do comprimento do dia favoreceu a fecundidade.

Localization and functional characterization of a novel adipokinetic hormone in the mollusk, Aplysia californica

Increasing evidence suggests that gonadotropin-releasing hormone (GnRH), corazonin, adipokinetic hormone (AKH), and red pigment-concentrating hormone all share common ancestry to form a GnRH superfamily. Despite the wide presence of these peptides in protostomes, their biological effects remain poorly characterized in many taxa. This study had three goals. First, we cloned the full-length sequence of a novel AKH, termed Aplysia-AKH, and examined its distribution in an opisthobranch mollusk, Aplysia californica. Second, we investigated in vivo biological effects of Aplysia-AKH. Lastly, we compared the effects of Aplysia-AKH to a related A. californica peptide, Aplysia-GnRH. Results suggest that Aplysia-AKH mRNA and peptide are localized exclusively in central tissues, with abdominal, cerebral, and pleural ganglia being the primary sites of Aplysia-AKH production. However, Aplysia-AKH-positive fibers were found in all central ganglia, suggesting diverse neuromodulatory roles. Injections of A. californica with Aplysia-AKH significantly inhibited feeding, reduced body mass, increased excretion of feces, and reduced gonadal mass and oocyte diameter. The in vivo effects of Aplysia-AKH differed substantially from Aplysia-GnRH. Overall, the distribution and biological effects of Aplysia-AKH suggest it has diverged functionally from Aplysia-GnRH over the course of evolution. Further, that both Aplysia-AKH and Aplysia-GnRH failed to activate reproduction suggest the critical role of GnRH as a reproductive activator may be a phenomenon unique to vertebrates.

Localization of a molluscan gonadotropin-releasing hormone in Aplysia californica by in situ hybridization and immunocytochemistry

Gonadotropin-releasing hormone (GnRH) plays important roles in vertebrate reproduction. Recently, molecules structurally similar to vertebrate GnRH were discovered in mollusks, including a gastropod, Aplysia californica. As an important step toward understanding the function of A. californica GnRH (ap-GnRH), the present study examined the localization of ap-GnRH peptide and transcript in the central and peripheral tissues. Reverse transcription polymerase chain reaction (RT-PCR) revealed wide expression of ap-GnRH in all ganglia (abdominal, buccal, cerebral, and pedal ganglia) of the central nervous system (CNS) and in multiple peripheral organs. However, in situ hybridization (ISH) revealed that cells positive for ap-GnRH are detectable only in the CNS, with the pedal ganglia containing the highest number of ap-GnRH-positive neurons, followed by the cerebral and abdominal ganglia. Most neurons positive for the transcript were simultaneously positive for the peptide, although some discrepancies were observed in cerebral and abdominal ganglia. Overall, our data suggest the de novo synthesis of ap-GnRH is restricted to the CNS, with the pedal ganglia being the primary source of ap-GnRH. Our results support the notion that ap-GnRH is a bona-fide neuropeptide that may assume diverse central functions, including those unrelated to reproduction.

Molecular characterization of RXR (Retinoid X Receptor) gene isoforms from the bivalve species Chlamys farreri

Background

Bivalves are among the oldest classes of invertebrates, and they exhibit diverse types of sexual patterning. However, our current understanding of the mechanisms of sex determination and differentiation in bivalves remains very limited. The retinoid X receptors (RXRs), which are members of the nuclear receptor family, are involved in sex differentiation in many organisms.

Results

In the present study, four full-length RXR-encoding cDNAs (CfRXRs) named CfRXRa, CfRXRb, CfRXRc and CfRXRd were retrieved from Zhikong scallop (Chlamys farreri). The four RXRs exhibited the conserved five-domain structure of nuclear receptor superfamily members and differed from each other only in the T-box of the C domain. The three variants, designated T (+4), T (+20) and T (+24), contained insertions of 4, 20 and 24 amino acids, respectively. The entire CfRXR gene is composed of eight exons and seven introns, and the four isoforms are generated via alternative mRNA splicing. Expression analysis showed that all four isoforms were expressed in both the testis and the ovary during the differentiation stage, whereas no expression was detected in the growth, mature or resting stages. This result suggests that CfRXRs are involved in germ cell differentiation in both sexes. The expression of the four isoforms was also detected in other tissues examined, including mantle, gill, digestive gland, and adductor muscle of sexually mature male and female Zhikong scallops, implying the multiple biological functions of CfRXRs.

Conclusion

Our study presents the first report of RXR isoforms in bivalves. Further investigation of the functional roles of different RXR isoforms may provide deep insights into the regulatory mechanism of sex differentiation in C. farreri.

Gonadotropin-releasing hormone in protostomes: insights from functional studies on Aplysia californica

Several protostomian molecules that structurally resemble chordate gonadotropin-releasing hormone (GnRH) have been identified through cloning, biochemical purification or data mining. These molecules share considerable sequence and structural similarities with chordate GnRH, leading to the current belief that protostomian and chordate forms of GnRH share a common ancestor. However, the physiological significance of these protostomian GnRH-like molecules remains poorly understood. This knowledge gap hampers our understanding of how GnRH has evolved functionally over time. This review provides a summary of our recent functional characterization of a GnRH-like molecule (ap-GnRH) in a gastropod mollusk, Aplysia californica, and presents preliminary proof for a cognate ap-GnRH receptor (ap-GnRHR). Our data reveal that ap-GnRH is a general neural regulator capable of exerting diverse central and motor effects, but plays little or no role in reproductive activation. This notion is supported by the abundance of a putative ap-GnRHR transcript in the central nervous system and the foot. Comparing these results to the available functional data from a cephalopod mollusk, Octopus vulgaris, we surmise that protostomian GnRH-like molecules are likely to assume a wide range of physiological roles, and reproductive activation is not an evolutionarily conserved role of these molecules. Future functional studies using suitable protostomian models are required to identify functional changes in protostomian GnRH-like molecules that accompany major taxa-level transitions.

Reproductive toxicity of bisphenol A and cadmium in Potamopyrgus antipodarum and modulation of bisphenol A effects by different test temperature

An OECD initiative for the development of mollusc-based toxicity tests for endocrine disrupters and other chemicals has recommended three test species with respective test designs for further standardisation. Preparing a subsequent pre-validation study we performed a reproduction test with Potamopyrgus antipodarum, determining the concentration range of the selected test substances, bisphenol A (BPA) and cadmium (Cd). At 16 °C, the recommended test temperature, the number of embryos in the brood pouch was increased by BPA and decreased by Cd (NOEC: 20 μg BPA/L and 1 μg Cd/L). Coinstantaneous BPA tests at 7 °C and 25 °C demonstrated a temperature dependency of the response, resulting in lower NOECs (5 μg/L respectively). As expected, reproduction in control groups significantly varied depending on temperature. Additional observations of the brood stock showed seasonal fluctuations in reproduction under constant laboratory conditions. The recommended temperature range and test conditions have to be further investigated.

17β-Oestradiol may prolong reproduction in seasonally breeding freshwater gastropod molluscs

Whilst the effects of oestrogenic contaminants in the aquatic environment are well documented in fish, effects in invertebrate species has been subject to debate, possibly due to differences in experimental conditions (temperature, timing and duration of exposure) between studies. It has been suggested that molluscs are only susceptible to oestrogens in periods either following the main spawning or leading up to the maturation of gametes. To investigate this possibility, two temperate, seasonally reproducing gastropods (Planorbarius corneus and Viviparus viviparus) were exposed to two concentrations of 17β-oestradiol (E2; 10ng/l and 100ng/l nominal) in an outdoor mesocosm (subject to natural seasons). In addition, P. corneus was also exposed to E2 (1, 10 and 100ng/l) in the laboratory at temperatures and photoperiods to simulate summer and autumn. In the mesocosm, both snail species produced similar numbers of eggs/embryos as reference groups in the summer, but the groups exposed to 10ngE2/l (nominal) had significantly higher productivities after the onset of autumn, when entering their quiescent phase, whilst the snails exposed to a higher concentration (100ng/l, nominal) had an increased rate of mortality, and did not experience increased reproduction. In the laboratory, the rate of egg laying in P. corneus was unaffected in simulated summer (20°C, 16h photoperiod), but snails exposed to 10 and 100ng/l (nominal) in simulated autumn (15°C, 12h photoperiod) showed a concentration-dependent inhibition of the natural decline in egg laying observed in the control snails. Overall, rather than an increase in reproductive rate, the response of this species was a perpetuation of summer reproductive rates into autumn. We conclude that exposure to E2 can affect reproduction in the freshwater gastropods studied, but in P. corneus at least, this is dependent on the seasonal conditions (temperature and photoperiod) at which exposures are made.

Toward a synthetic understanding of the role of phenology in ecology and evolution

Phenology affects nearly all aspects of ecology and evolution. Virtually all biological phenomena-from individual physiology to interspecific relationships to global nutrient fluxes-have annual cycles and are influenced by the timing of abiotic events. Recent years have seen a surge of interest in this topic, as an increasing number of studies document phenological responses to climate change. Much recent research has addressed the genetic controls on phenology, modelling techniques and ecosystem-level and evolutionary consequences of phenological change. To date, however, these efforts have tended to proceed independently. Here, we bring together some of these disparate lines of inquiry to clarify vocabulary, facilitate comparisons among habitat types and promote the integration of ideas and methodologies across different disciplines and scales. We discuss the relationship between phenology and life history, the distinction between organismal- and population-level perspectives on phenology and the influence of phenology on evolutionary processes, communities and ecosystems. Future work should focus on linking ecological and physiological aspects of phenology, understanding the demographic effects of phenological change and explicitly accounting for seasonality and phenology in forecasts of ecological and evolutionary responses to climate change.

Environmental-endocrine control of reproductive maturation in gastropods: implications for the mechanism of tributyltin-induced imposex in prosobranchs

Prosobranch snails have been afflicted globally by a condition whereby females develop male sex characteristics, most notably a penis. This condition, known as imposex, has been causally associated with the ubiquitous environmental contaminant tributyltin (TBT). Deduction of the mechanism by which TBT causes imposex has been hampered by the lack of understanding of the normal endocrine regulation of reproductive tract recrudescence in these organisms. We have reviewed the relevant literature on the environmental and endocrine factors that regulate reproductive tract recrudescence, sexual differentiation, and reproduction in gastropods. We provide a cohesive model for the environmental-endocrine regulation of reproduction in these organisms, and use this information to deduce a most likely mechanism by which TBT causes imposex. Photoperiod appears to be the predominant environmental cue that regulates reproductive tract recrudescence. Secondary cues include temperature and nutrition which control the timing of breeding and egg laying. Several hormone products of the central and peripheral nervous systems have been identified that contribute to recrudescence, reproductive behaviors, oocyte maturation and egg laying. Retinoic acid signaling via the retinoid X-receptor (RXR) has shown promise to be a major regulator of reproductive tract recrudescence. Furthermore, TBT has been shown to be a high affinity ligand for the RXR and the RXR ligand 9-cis retinoic acid causes imposex. We propose that TBT causes imposex through the inappropriate activation of this signaling pathway. However, uncertainties remain in our understanding of the environmental-endocrine regulation of reproduction in gastropods. Definitive elucidation of the mechanism of action of TBT awaits resolution of these uncertainties.

Life history cost of trematode infection in Helisoma anceps using mark-recapture in Charlie's Pond

Parasitism has the potential to affect key life history traits of an infected host. Perhaps the most studied interactions are in snail-trematode systems, where infection can result in altered growth rates, survival, and/or fecundity of the individual. Positive correlations between host size and parasite prevalence are often attributed to changes in growth rates or mortality, which have been observed in the laboratory. Extending lab-based conclusions to the natural setting is problematic, especially when environmental conditions differ between the laboratory and the field. The present study uses reproduction experiments and mark-recapture methods to directly measure key life history traits of the pulmonate snail Helisoma anceps in Charlie's Pond. Based on previous laboratory and field experiments on H. anceps, we predict a significant reduction in fecundity, but not growth rate or survival, of infected snails. Individual capture histories were analyzed with multistate models to obtain estimates of survival and infection probabilities throughout the year. Recaptured individuals were used to calculate specific growth rates. Trematode infection resulted in complete castration of the host. However, neither survival nor growth rates were found to differ between infected and uninfected individuals. The probability of infection exhibited seasonal variation, but it did not vary with size of the snail. These results suggest that the correlation between host size and trematode prevalence is not due to differential mortality or changes in growth rates. Instead, the infection accumulates in large snails via the growth of smaller, infected individuals.

The identification and distribution of gonadotropin-releasing hormone-like peptides in the central nervous system and ovary of the giant freshwater prawn, Macrobrachium rosenbergii

In the present study, we demonstrated the existence of GnRH-like peptides in the central nervous system (CNS) and ovary of the giant freshwater prawn, Macrobrachium rosenbergii using immunocytochemistry. The immunoreactivity (ir) of lamprey (l) GnRH-III was detected in the soma of medium-sized neurons located in neuronal cluster number 11 in the middle part of supraesophageal ganglion (deutocerebrum), whereas ir-octopus (oct) GnRH was observed in the soma of both medium-sized and large-sized neurons in thoracic ganglia, as well as in the fibers innervating the other medium-sized and large-sized neuronal cell bodies in the thoracic ganglia. In addition, ir-lGnRH-I was observed in the cytoplasm of late previtellogenic oocyte and early vitellogenic oocyte. These data suggest that M. rosenbergii contain at least three isoforms of GnRH: two GnRH isoforms closely related to lGnRH-III and octGnRH in the CNS, whereas another isoform, closely related to lGnRH-I, was localized in the ovary. This finding provides supporting data that ir-GnRH-like peptide(s) may exist in this decapod crustacean.

Synchronized expression of retinoid X receptor mRNA with reproductive tract recrudescence in an imposex-susceptible mollusc

The biocide tributyltin (TBT) causes the development of male sex characteristics in females of some molluscan species, a phenomenon known as imposex. Recent evidence suggests that the retinoid X receptor (RXR) participates in TBT-induced imposex. Accordingly, we hypothesized that RXR may contribute to the seasonal development of the male reproductive tract in molluscs and would be expressed in concert with this phenomenon. RXR was cloned and sequenced from an imposex-susceptble species, the eastern mud snail Ilyanassa obsoleta. The DNA-binding domain of the receptor protein was 100 and 97% identical to those of the rock shell Thais clavigera and the freshwater snail Biomphalaria glabrata. The ligand-binding domain was 93 and 92% identicalto the LBD of these two molluscan species, respectively. Phylogenetic analyses revealed that RXR is an ancient nuclear receptor whose origin predates the emergence of the Bilateria. Interestingly, though inexplicably, the molluscan RXRs were more similar to sequences of vertebrate RXRs than to the RXRs of other lophotrochozoan invertebrates. Next, the expression of RXR mRNA levels in the reproductive tract was determined through the reproductive cycle. RXR mRNA levels increased commensurate with reproductive tract recrudescence in both sexes. However, the timing of coordinate recrudescence-RXR expression differed between sexes. Results demonstrate that RXR expression is associated with reproductive tract recrudescence in both sexes; although, the timing of recrudescence may dictate sex-specific development. Retinoid signaling initiated by TBT during an inappropriate time in females may result in imposex.

The presence and distribution of gonadotropin-releasing hormone-liked factor in the central nervous system of the black tiger shrimp, Penaeus monodon

The distribution and presence of gonadotropin-releasing hormone (GnRH) in the central nervous system (CNS) of Penaeus monodon were examined by immunocytochemistry, high performance liquid chromatography (HPLC), and radioimmunoassay (RIA). We demonstrated the existence of octopus (oct)GnRH-liked immunoreactivity (ir-octGnRH) and lamprey (l)GnRH-III-liked immunoreactivity (ir-lGnRH-III) in cell bodies of medium-sized neurons of the anterior part (protocerebrum) of the supraesophageal ganglion (brain). In addition, only the ir-octGnRH was detected in the nerve fibers located in the brain and segmental ganglia (subesophageal, thoracic, and abdominal ganglia). Moreover, some branches of these fibers also innervated the neurons in the middle (deutrocerebrum), posterior (tritocerebrum) brain and segmental ganglia. There was no ir-lGnRH-I and ir-salmon (s)GnRH detected in the shrimp CNS. The results from HPLC and RIA showed ir-GnRH in the CNS using anti-lGnRH-III, but not with anti-mammalian (m)GnRH. The data from immunocytochemistry, HPLC and RIA suggest that ir-GnRH in shrimp may be more similar to octGnRH and lGnRH-III than the other forms. These findings support the hypothesis that GnRH-liked factor(s) may be an ancient peptide that also exists in this decapod crustacean.

Aplysia Bag Cells Function as a Distributed Neurosecretory Network

The anatomical organization of many neuroendocrine systems implies multiple sites of hormone release in areas mediating multiple aspects of physiology and behavior, yet this neurosecretory complexity has not often been verified. Here we probe the well-characterized hormonal model, the reproductive bag cell neuroendocrine system of the sea slug Aplysia californica. The bag cell neurons of Aplysia mediate egg-laying behavior through the coordinated secretion of a suite of peptides derived from a single gene product, the egg-laying prohormone (proELH). Although the majority of bag cell neurons are located within two major abdominal bag cell clusters, smaller groups of egg-laying hormone-expressing cells have been observed in specific pleural and cerebral ganglia regions, some of which have been reported to be electrically connected to the abdominal bag cell clusters. Releasates are sampled from discrete locations within the Aplysia CNS before and during stimulation of afterdischarge activity and subsequently analyzed with matrix assisted laser desorption/ionization time-of-flight mass spectrometry. Site-specific release profiles are observed at bag cell cluster, pleural, and genital ganglion sites after site-specific electrophysiological activation of bag cell afterdischarges. These data demonstrate that the bag cell network has multiple neurohemal release sites, exhibits descending activation that travels from the cerebral and pleural ganglia down to the abdominal bag cell clusters, and releases spatially distinct profiles of proELH-derived peptides within the Aplysia nervous system. Such distributed neurosecretory organization may be a common feature of neuroendocrine systems across higher order organisms linking multiple behavioral aspects to a single neuronal network.

Regulation of behavioral and synaptic plasticity by serotonin release within local modulatory fields in the CNS of Aplysia

In Aplysia, serotonergic neurons are widely activated during sensitization training, but the effects of exogenous serotonin (5-HT) on reflex circuits vary, inducing short- or long-term synaptic facilitation or synaptic inhibition, depending on the site of application. During learning, it is possible that specific spatial patterns of 5-HT release evoked by training may produce different phases of sensitization or behavioral inhibition. To test this hypothesis, we examined the modulation of the tail-induced siphon withdrawal reflex by repeated noxious stimuli applied to one of three sites: the (1) ipsilateral or (2) contralateral sides of the tail or (3) the head. Ipsilateral tail shock produced long-term sensitization, whereas contralateral tail shock induced only short-term sensitization, and head shock produced inhibition. In parallel cellular experiments, tail-nerve shock evoked large 5-HT release localized around the ipsilateral tail sensory neurons (SNs) and motor neurons (MNs) but only modest 5-HT release in the contralateral pleural-pedal ganglia and in the abdominal ganglion, in which the siphon MNs are located. Head-nerve shock, in contrast, produced only modest 5-HT release in the pleural, pedal, and abdominal ganglia. Thus, each training protocol evoked a specific pattern of 5-HT release within the CNS. In addition, we found that 5-HT released in the pleural ganglia was correlated with facilitation of SN-MN synapses; however, in the abdominal ganglion, it was associated with inhibition of the synapses between identified interneurons (L29s) and siphon MNs (LFSs). Because 5-HT differentially modulates synaptic efficacy at different synaptic sites, our data can explain how specific spatial patterns of 5-HT release in local modulatory fields can contribute to the induction of short- or long-term sensitization or to behavioral inhibition.

Effects of husbandry parameters on the life-history traits of the apple snail, Marisa cornuarietis: effects of temperature, photoperiod, and population density

These experiments are part of a larger study designed to investigate the influence of husbandry parameters on the life history of the apple snail, Marisa cornuarietis. The overall objective of the program is to identify suitable husbandry conditions for maintaining multi-generation populations of this species in the laboratory for use in ecotoxicological testing. In this article, we focus on the effects of photoperiod, temperature, and population density on adult fecundity and juvenile growth. Increasing photoperiod from 12 to 16 h of light per day had no effect on adult fecundity or egg hatching and relatively minor effects on juvenile growth and development. Rearing snails at temperatures between 22 degrees C and 28 degrees C did not influence the rates of egg production or egg clutch size. However, the rates of growth and development (of eggs and juveniles) increased with increasing temperature in this range, and when temperatures were reduced to 22 degrees C egg-hatching success was impaired. Juvenile growth and development were more sensitive to rearing density than adult fecundity traits. On the basis of the present results, we conclude that rearing individuals of M. cornuarietis at a temperature of 25 degrees C, a photoperiod of 12L:12D, and a density of <0.8 snails L(-1) (with lower densities for juvenile snails) should provide favorable husbandry conditions for maintaining multi-generation populations of this species.

Sexual maturation and related changes in aspartate transcarbamylase activity of gonad tissues in the soft shell clam (Mya arenaria)

The information on the relation between the gonadal aspartate transcarbamylase (ACTase) activity and the sexual maturation in molluscs is very fragmentary and is still absent in Mya arenaria. The determination of ATCase activity, energy reserves levels and maturation stages were done in gonads of clams. Results showed that the seasonal cycle of storage and utilization of energy reserves in gonads of clams are linked to the bimodal reproduction well known in this bivalve. ATCase activity was high in clams at development and ripe stages, while this activity was low for individuals found in indifferent, spawning or spent stages. This difference can be explained by the fact that during gonad development, gonadal synthetic activity increased following the mitotic events associated to the reproductive cycle. The results presented in this paper have confirmed the link between ATCase activity and gametogenesis in M. arenaria. Further work should be realized in order to assess if ATCase activity could be considered as a potential biomarker to evaluate the disruption of sexual maturation in clams collected in sites such as the St. Lawrence estuary.

Peptide and protein pheromones in molluscs

Pheromones have been implicated in the control of a number of behaviors in molluscs, but few peptide pheromones have been characterized in these animals. Peptide pheromones include: (1) a family of water-borne peptide pheromonal attractants (attractins) in the gastropod Aplysia that are released during egg laying and attract other Aplysia to form egg-laying and mating aggregations; (2) a tetrapeptide (ILME) in the cephalopod Sepia that elutes from egg masses and is thought to be involved in the transport of oocytes in the genital tract during egg laying; and (3) a Sepia sperm-attracting peptide (SepSAP; PIDPGVamide) that is released from oocytes during egg laying to facilitate external fertilization.

Chronic toxicity of tributyltin on development and reproduction of the hermaphroditic snail Physa fontinalis: Influence of population density

Tributyltin (TBT) is toxic to aquatic organisms and occurs widely in sediments and surface waters of American and European rivers and lakes. This study investigated TBT effects on development and population growth rate (r) of the common, hermaphroditic European freshwater snail Physa fontinalis. Egg ropes of similar age (1-3 days old) were exposed to a control (solvent only) and nominal concentrations of 0.01, 1.0 and 10 microg TBT l(-1) in triplicate. Hatching and mortality were recorded during 0-40 days of exposure. At day 40, 18 juveniles were randomly selected from each concentration (i.e., six from each test vessel) and individually exposed to the same concentration of TBT in 50 ml beakers. A cohort of 20 juveniles was allowed to continue developing in the original test vessels, so that individual and grouped results could be compared. Mortality and reproduction were recorded at 48-h intervals throughout the study period (110 days). Abnormal embryonic development was observed at 1 and 10 microg TBT l(-1). Although 50% of eggs hatched at 10 microg TBT l(-1), all these hatchlings failed to survive. Survivorship of hatchlings was significantly reduced by TBT at 1 microgl(-1). In general, there was a delay in egg production in isolated snails when compared with the grouped snails. Survival, fecundity and population growth rate (r) were reduced in both individual and grouped P. fontinalis at 1.0 microg TBT l(-1). Only a decline in r was observed in snails exposed individually to 0.01 microg TBT l(-1).

Sensory innervation of the ovotestis in the snail Helix aspersa

Because oviposition in the land snail Helix aspersa is a metabolically expensive process coupled to a high fixed cost, one expects oviposition to occur only when the clutch size surpasses a minimum value at which the reproductive benefit exceeds the cost. We propose that neural innervation of the gonad allows H. aspersa to monitor oocyte production and ensure an adequate supply of gametes prior to ovulation. The ovotestis is innervated by a branch of the intestinal nerve in which the majority of axon fibres measure &lt;0.2 μm in diameter. We found a strong positive correlation between the number of mature oocytes in the ovotestis and the frequency of spontaneous afferent spikes in the nerve branch. Tactile stimulation of the ovotestis resulted in a 20-fold increase in afferent spikes and an efferent reflex directed towards the ovotestis and the pericardium. Afferent activity also increased 10-fold after an experimentally induced increase in the volume of the ovotestis. These results suggest that the growing oocytes expand the walls of the acini and trigger action potentials in the mechanosensitive nerve terminals that lie within the acinar walls. We hypothesize that the resulting tonic signal is permissive for ovulation. In addition, a phasic sensory signal may occur during ovulation to trigger CNS motor output related to oviposition.