Species Crepidula Research Articles

Retinoids promote penis development in sequentially hermaphroditic snails

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

Late Quaternary Faunal Changes in Northeastern Patagonia (Argentina) according to a Dynamic Mosaic of Benthic Habitats: Taphonomic and Paleoecological Analyses of Mollusk Assemblages

The aim of this study was to describe and interpret the paleoenvironmental history of the San Matias Gulf (SMG) in northern Patagonia, Argentina, associated to possible biotic and abiotic changes occurred during the late Quaternary. In this regard a taphonomic (disarticulation, right/left valve ratio; fragmentation, abrasion, teeth preservation) and paleoecologic (alpha diversity, species abundance, life habit, substrate preference, feeding mode) analysis of A. purputata (the target species) and its accompanying faunal remains (the non-target species) in modern, Holocene and Pleistocene shell assemblages was performed. The general trend of the SMG throughout the late Quaternary is as a low energy environment, with varying intensity depending on the study area. Additionally, each study site contains different substrates showing patches of sand and rock, and this would also have determined the presence of certain species; in turn, the proportion of sandy and rocky patches may have changed over time which led to the development of different local paleocommunities. This gulf therefore presents a dynamic mosaic of environments over time, as evidenced by taphonomic and paleoecological analyses. Its waters would have been sufficiently rich in nutrients to allow the development of the filter fauna which have dominated the late Quaternary, especially A. purpurata. Dual feeder gastropods from hard substrates (Crepidula species) have increased in proportion through the Holocene. The variation in the composition of the fauna accompanying A. purpurata is due to certain physical, biological and even anthropic environmental conditions, which have changed through time. These changes coincided with the Last Glacial Maximum, the final configuration of the SMG and the Climatic Optimum of the Holocene.

Comparative Ontogenetic Changes in Enzyme Activity During Embryonic Development of Calyptraeid Gastropods

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

Impacts of Adelphophagic Development on Variation in Offspring Size, Duration of Development, and Temperature-Mediated Plasticity

Adelphophagic development, where embryos consume sibling embryos or nurse eggs, is particularly common in marine caenogastropods and some families of polychaetes. When exogenous nutrition is provided before hatching, egg size and hatching size can be uncoupled, but advantages and constraints of adelphophagic development compared to development from large eggs are unknown. Here we examine temperature-mediated plasticity in offspring size, brooding duration, and fecundity in the adelphophagic marine gastropod Crepidula cf. onyx. We use these data combined with previously published data on two planktotrophic Crepidula and two Crepidula species that develop from large eggs to test hypotheses about the consequences of adelphophagic development and patterns of variation in offspring size. In Crepidula cf. onyx, egg size shows no significant effect of temperature. Hatching size is significantly larger at 28 °C than at 23 °C but proceeds from fewer eggs per capsule at 28 °C. Hatching size is therefore decoupled from both egg size and the number of eggs per capsule. Although development is faster at the higher temperature, broods are produced roughly every 26-27 days at both temperatures. Increased rate of development has been cited as a potential advantage of adelphophagic development in muricids, but the adelphophagic C. cf. onyx did not develop more quickly than C. atrasolea or C. ustulatulina, species that produce similarly sized hatchlings from large eggs. Comparisons across calyptraeid species support the role of adelphophagy in increasing variance in offspring size. This increased variability is primarily expressed within broods or among broods from the same female, not among females.

Temperature-mediated trade-offs and changes in life-history integration in two slipper limpets (Gastropoda: Calyptraeidae) with planktotrophic development

Intraspecific variation in egg size and hatching size, and the genetic and environmental trade-offs that contribute to variation, are the basis of the evolution of life histories. The present study examined both univariate and multivariate temperature-mediated plasticity of life-history traits, as well as temperature-mediated trade-offs in egg size and clutch size, in two planktotrophic species of marine slipper limpets, Crepidula. Previous work with two species of Crepidula with large eggs and lecithotrophic development has shown a significant effect of temperature on egg size and hatching size. To further examine the effect of temperature on egg size in Crepidula, the effects of temperature on egg size and hatching size, as well as the possible trade-offs with other the life-history features, were examined for two planktotrophic species: Crepidula incurva and Crepidula cf. marginalis. Fieldcollected juveniles were raised at 23 or 28 °C and egg size, hatching size, capsules/brood, eggs/capsule, time to hatch, interbrood interval, and final body weight were recorded. Consistent with results for the lecithotrophic Crepidula, egg size and hatching size decreased with temperature in the planktotrophic species. The affects of maternal identity and individual brood account for more than half of the intraspecific variation in egg size and hatching size. Temperature also showed a significant effect on reproductive rate, with time to hatch and interbrood interval both decreasing with increasing temperature. However, temperature had contrasting effects on the number of offspring. Crepidula cf. marginalis has significantly more eggs/capsule and therefore more eggs per brood at 28 °C compared to 23 °C, although capsules/brood did not vary with temperature. Crepidula incurva, on the other hand, produced significantly more capsules/brood and more eggs per brood at the lower temperature, whereas the number of eggs/capsule did not vary with temperature. The phenotypic variance‐covariance matrix of life-history variables showed a greater response to temperature in C. incurva than in C. cf. marginalis, and temperature induced trade-offs between offspring size and number differ between the species. These differences suggest that temperature changes as a result of seasonal upwelling along the coast of Panama will effect the reproduction and evolution of life histories of these two co-occurring species differently. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, , ••‐••. ADDITIONAL KEYWORDS: adaptive phenotypic plasticity ‐ phenotypic covariance ‐ P-matrix ‐ temperature-size rule.

Effects of food availability on sex change in two species of Crepidula (Gastropoda: Calyptraeidae)

Sex allocation theory predicts that growth rates should influence the optimal size at sex change, with sex change occurring at larger sizes with increasing growth rates. We examined how variation in food availability affects growth rates and therefore influences size at sex change in 2 protandrous calyptraeid gastropods, Crepidula cf. marginalis and C. incurva. The lowest food concentration tested resulted in starvation and was discarded. The medium and high food concen- trations resulted in differences in growth rates, and in both species the penis was lost and sex change was initiated at larger sizes with more food. However, food concentration had little effect on the size at which the female stage was attained. Food-induced alterations in the timing of sex change were not in the expected direction. C. cf. marginalis raised with less food initiated sex change earlier, but completed sex change later in time than those raised with more food. C. incurva both initiated and completed sex change later when raised with less food compared to those individuals raised with more food. In both species the transitional phase was completed more rapidly with more food. These results are consistent with the idea that, as more resources become available, time dedicated to active sexual phases is maximized and the duration of the non-reproductive transitional phase is minimized. Under 'good' conditions this is manifest by a delay in the loss of the male phase, but no change in the attainment of the female phase.

New record of calyptraeid hosts for the pea crab Calyptraeotheres garthi (Fenucci, 1975) (Decapoda, Pinnotheridae) in Argentina

Calyptraeotheres garthi (Fenucci, 1975) is one of nine species of Pinnotheridae occurring in Argentina and the only species of that genus located in this region. This species can be found from the San Matias Gulf, Argentina to Rio Grande do Sul, Brazil where it lives in symbiotic association with limpets of the family Calyptraeidae. Currently, two species of limpets have been reported as hosts of this species: Crepidula protea (d’Orbigny, 1841) and Crepidula argentina Simone, Pastorino & Penchaszadeh, 2000. We report here four more species of limpets serving as hosts of C. garthi, namely Bostrycapulus odites Collin, 2005, Crepidula cachimilla Cledon, Simone & Penchaszadeh, 2004, Crepidula plana Say, 1822 and Trochita pileus (Lamarck, 1822). Thus, this pinnotherid appears to be, as others members of the family, a generalist symbiotic species since it dwells on almost all the potential limpet species reported for Argentina. However, there are notable differences in prevalence of C. garthi on each host, which suggest that the symbiont might prefer species of Crepidula over the others genera (Trochita and Bostrycapulus). Additionally, there exists an apparent relationship between host size and prevalence (e.g., Crepidula cachimilla, the limpet with the highest prevalence, is clearly larger than the others hosts). Perhaps it is premature to conclude that the size of the host might be of benefit for the symbiont, but it is a hypothesis worthy of being explored.

In silico mining and characterization of 12 EST-SSRs for the invasive slipper limpet Crepidula fornicata

In silico mining of an expressed sequence tags (ESTs) library was found to be efficient at isolating simple sequence repeats (SSRs) loci in the non-indigenous marine mollusc Crepidula fornicata. Twelve SSR loci were developed for routine genotyping. Cross-species amplification to 8 other Crepidula species showed that the 12 loci are highly specific for C. fornicata. Mendelian inheritance was shown for 11 of them (1 being monomorphic in the analyzed offspring array). The genetic diversity for 88 adults was found to be variable across the 12 loci (2–40 alleles, expected heterozygosity between 0.023 and 0.898) with a high overall exclusion probability of 0.99. The degree of genetic polymorphism found here is similar to that shown for 7 anonymous SSRs previously developed and here used on the same samples. This set of 12 specific loci is relevant to perform reliable population and relatedness analyses in Crepidula fornicata.

Larval development of Calyptraeotheres garthi (Fenucci, 1975) (Brachyura, Pinnotheridae) described from laboratory-reared material, with notes of larval character use on Pinnotheridae systematic

Calyptraeotheres garthi (Brachyura, Pinnotheridae) is a pinnotherid crab that lives in association with some species of Crepidula sp. (Calyptraeidae). The larvae of C. garthi were cultured under laboratory conditions (20°C and 35 PSU). Larval development from hatchling to megalopa took 30 days. Five zoeal stages and the megalopa are described and fully illustrated. The genus Calyptraeotheres consists only of four species. Larval characters were compared with the previous description of larvae from the southeastern Pacific species C. politus and with Tumidotheres maculatus a closely related pinnotherid crab. In addition, in light of recent molecular phylogeny of the family Pinnotheridae, we present a discussion about the Calyptraeotheres and its close related genera.

Predation on juveniles of Crepidula fornicata by two crustaceans and two gastropods

Crepidula fornicata, the slippershell snail, while native to New England, has now become a successful invasive species along coastlines in many other parts of the world. This study considers the possible control of native populations by several common predators: the invasive shore crab Hemigrapsus sanguineus, one species of pagurid hermit crab ( Pagurus longicarpus), and juveniles of the drilling gastropods Nucella lapillus and Urosalpinx cinerea. In the laboratory, juveniles of C. fornicata were especially vulnerable to the two crustacean predators tested, and to a lesser extent vulnerable as well to predation by the oyster drill U. cinerea; in choice tests, young oyster drills ate barnacles and mussels rather than slippershell snails, but ate C. fornicata when offered no other choice. Juveniles of two other Crepidula species ( C. plana and C. convexa) were less susceptible to predation by hermit crabs, probably due to differences in juvenile shell morphology and growth trajectories. Remarkably, juvenile dogwhelks ( N. lapillus) ate no Crepidula prey over several months in the laboratory, even in the absence of alternative food and although they readily consumed blue mussels. Additional work is needed to determine the role of crustacean and other predators in regulating the growth of native populations of C. fornicata in the field and to determine the extent to which the explosive growth of at least some invasive Crepidula populations reflects escape of juveniles from native predators.

Effect of inbreeding on reproduction and juvenile performance in two marine gastropods with contrasting reproductive patterns

Many species avoid inbreeding through dispersal of individuals away from their natal groups or sites. Benthic marine invertebrates with long free-living larval stages generally possess great dispersal potential and are therefore expected to have a small chance of inbreeding in the field. Species that lack such dispersive larval stages should have a higher likelihood of inbreeding. If such direct-developing species do often mate with close relatives, we expect them to exhibit less inbreed- ing depression than those with dispersive larvae, since frequent inbreeding can lead to the purging of deleterious alleles through natural selection. To test this, we compared the effects of inbreeding in 2 Crepidula species with contrasting reproductive patterns: C. fornicata, which has a long dispersive larval stage; and C. convexa, a direct developer without a free-living larval stage. In laboratory stud- ies, we compared reproductive output and several fitness traits of juveniles from snails that were forced to mate with full siblings with those from snails forced to mate with unrelated individuals. In contrast to our expectations, C. convexa showed much stronger inbreeding depression than C. forni- cata. For example, inbreeding decreased mean juvenile growth rate for C. convexa by 20 to 44%, but did not affect mean growth rate for C. fornicata. It appears that larval dispersal potential alone can be a poor predictor of the amount of inbreeding that occurs in natural populations and of the effect of inbreeding on offspring fitness.

Morphological and phylogenetic study of the Western Atlantic Crepidula plana complex (Caenogastropoda, Calyptraeidae), with description of three new species from Brazil

A detailed morphological study was performed in the following species of Crepidula: 1) C. margarita new species from Venezuela; 2) C. plana Say, 1822; 3) C. atrasolea Collin, 2000; 4) C. depressa Say, 1822, from Florida, USA; 5) C. pyguaia, new species from Santa Catarina coast, Brazil; 6) C. carioca, new species from Rio de Janeiro coast, Brazil. Additionally, five other species anatomically studied in Simone (2002) were also included: 7) C. argentina Simone, Pastorino & Penchaszadeh, 2000, from Argentina; 8) C. glauca Say, 1822, from Venezuela; 9) C. fornicata (Linné, 1758), from Mediterranean and Florida; 10) C. protea Orbigny, from S.E. Brazil; 11) C. aff. plana, here described as C. intratesta new species, from S.E. Brazil; 12) C. cachimilla Cledón Simone & Penchaszadeh, 2004, from Patagonia. The 46 characters (106 states) for these species were phylogenetically analyzed and a single cladogram was obtained (length: 93, CI: 65; RI: 64) as follows: (Crepidula cachimilla (C. argentina ((C. carioca C. pyguaia) ((C. fornicata (C. intratesta C. protea)) ((C. glauca C. margarita) (C. plana (C. atrasolea C. depressa))))))). Two outgroups were used: Bostrycapulus aculeatus (Gmelin, 1791), which most authors consider a Crepidula (operationally analyzed as part of the ingroup), and the remaining calyptraeoideans studied by the author. The monophyly of the ingroup is confirmed, supported by 25 morphological synapomorphies. Although the ingroup is fully resolved, no clades are formally named, because the phylogeny is still considered provisional. Most studied species belong to an informal group called “Crepidula plana-complex”, but it is not monophyletic, since C. fornicata, which does not belong to this complex, is part of the ingroup. This study demonstrates that detailed morphological data are useful in phylogenetic studies even at the level of closely related/sibling species, resulting in cladogram with good resolution and a good number of shared, analyzable characters. A biogeographic analysis is also performed considering the distribution of each species under the light of the obtained cladogram, a clear ascension from south to north is the main pattern of the evolutionary history of these species. Further comments on the systematics of Crepidula aplysioides Reeve and C. convexa Say is also performed.

Developmental and genetic evidence for the existence of three morphologically cryptic species of Crepidula in northern Chile

The taxonomy of Crepidula has been based principally on shell morphology, but shell characteristics in this genus may be strongly influenced by the substrates they inhabit and are thus of limited use in classification. Four species in this genus are currently recognized from the Chilean coast, of which two, Crepidula dilatata and C. fecunda, are sympatric. These species are morphologically cryptic and differ only in their larval development: C. dilatata has indirect development with a planktonic phase, and C. fecunda direct development within egg capsules. The other two species, C. coquimbensis and C. philippiana, are species that occur within gastropod shells inhabited by hermit crabs and differ from the other species in internal morphology of the shell, developmental features, and geographic distribution. Analyses of nine morphological characters, eight developmental characters, and 26 enzymatic loci were carried out on four Crepidula populations in the Coquimbo region of northern Chile. Four populations of Crepidula were identified based on morphological, developmental, and protein electrophoresis features. Our results suggest the presence of C. coquimbensis plus three morphologically cryptic species including C. dilatata, C. fecunda, and a new previously unrecognized cryptic species. All four populations showed differences in larval development pattern. C. fecunda have indirect development, with free-swimming larvae. The remaining three species have intracapsular metamorphosis, but different modes of larval nutrition. C. dilatata have nurse eggs that are consumed by mechanical destruction; C. coquimbensis engulf nutritional embryos; and the new species eat nutritive embryos by rotation. Protein electrophoresis data support the specific condition of each group with different larval development and endorse the use of developmental differences in taxonomy of Crepidula. Genetic distance shows that C. dilatata and C. fecunda are closely related, and that C. coquimbensis is closer to them than to the new cryptic species.

Embryonic Velar Structure and Function of Two Sibling Species of Crepidula With Different Modes of Development

The structure and function of the embryonic velum of two closely related species of Crepidula with different modes of development are examined. The velum of C. dilatata, a direct developer whose embryos feed on nurse eggs, does not differ substantially from the velum of C. fecunda, a species with planktotrophic larvae. Although velar ciliation develops earlier in embryos of C. dilatata, embryos of both species were able to feed on small particles, using the opposed-band ciliary mechanism. However, the embryos of C. dilatata lose this ability as they grow. The embryos of C. dilatata were not able to swim, whereas those of C. fecunda swam consistently in vials of seawater. This difference in swimming ability is probably due to differences in velum-body size allometry between the two species.

Phylogeny of theCrepidulaplana(Gastropoda: Calyptraeidae) cryptic species complex in North America

The taxonomy of Crepidula species with flat white shells is particularly difficult. These animals from the east coast of North America have generally been classified as a single species, Crepidula plana Say, 1822. Based on allozyme and developmental data, however, Hoagland (K.E. Hoagland. 1984. Malacologia, 25: 607-628; K.E. Hoagland. 1986. Am. Malacol. Bull. 4: 173-183) concluded that two species of flat white-shelled Crepidula live along the east coast of the United States, but she did not apply any name to the second species. Herein I use molecular techniques to characterize populations of flat white-shelled Crepidula species from Texas, Florida, Georgia, North Carolina, and |Massachusetts, and describe their morphology and development. DNA-sequence data support the existence of three species. One species is readily distinguished on the basis of morphology and development, but the other two are very similar. To clarify the nomenclature of these species, I designate neotypes for C. plana Say, 1822 and Crepidula depressa Say, 1822, and describe Crepidula atrasolea sp.nov.

The role of movement by Crepidula norrisiarum Williamson in altering reproductive potential

Slipper limpets in the genus Crepidula have long been used as a model system in the study of protandrous hermaphroditism. The timing of the sex change in different social situations has been a major focus of previous work. Most efforts, however, have ignored the possibility that Crepidula species may also change their social situation through movement. In this study we evaluated the situations in which movements of the Norrisia norrisi-dwelling Crepidula norrisiarum Williamson occur, and the change in reproductive potential that may result from these movements. While both sexes and a range of sizes were able to reattach following separation from the host, in a series of movement experiments only male C. norrisiarum moved on and between host N. norrisi. A female C. norrisiarum alone on a N. norrisi was the most likely social situation to attract a male, while males were least likely to move when stacked on a female C. norrisiarum. A 74% portion of male C. norrisiarum movements between host N. norrisi resulted in an increase in reproductive potential, 10% saw no change, while the remaining 16% of movements resulted in a decrease. Movement was found to be a viable alternative to sex change as a means of increasing reproductive potential. The frequency of movements associated with particular Crepidula social situations should be an important consideration when evaluating sex change tradeoff decisions in these systems.

A new species of Crepidula (Mollusca: Mesogastropoda: Calyptraeidae) from Chile: additional characters for the identification of eastern Pacific planar Crepidula group

A new species of Crepidula is described. It was collected together with Pagurus edwardsii Dana and Pagurus perlatus H. Milne Edwards, inhabiting the inside of empty shells of Tegula euryomphala Jones and other gastropods. The shell is low convex-oval in juveniles, low convex-oblong in males and concave-oblong elongated in females. In the adult phases the apex projects slightly at the posterior border. The sinusoidal septum is ‘V’ shaped at its centre, with a smooth sulcus in the left quarter. The osphradium has 2–13 digitiform leaflets according to phases of the life cycle. The characteristically thin, external-lateral teeth of the taenioglossate radula possess a row of denticles. In addition, evidence is given that: (1) males have a lanceolate penis with a spermatic tube; (2) the intracapsular-embryonic development starting with many embryos, occurs with a decrease at the larvae-protoconch stage, each one with an adhered-vitelline mass, and culminates with a few prehatching juveniles of 1 mm size. In the...

Reproduction in Crepidula philippiana (Gastropoda, Calyptraeidae) from Southern Chile

Crepidula philippiana differs from most congeneric species in having only one embryo per capsule which attains a great hatching size, a feature shared only with C. monoxyla from New Zealand. In C. philippiana, this single embryo results from the ingestion of numerous nurse eggs. The clutch size (fecundity per spawn) is accordingly reduced. A significantly smaller but more uniform hatching size is observed among the remaining species of Crepidula, and clutch size is greater. Why do Crepidula species adopt different embryo nourishing mechanisms to attain direct development? Hypotheses considering ecological aspects and reproductive strategies are discussed.

Symbiotic organisms increase the vulnerability of a hermit crab to predation

The association between the hermit crab Pagurus longicarpus Say and an ectosymbiotic colonial hydroid of the genus Hydractinia was studied to determine whether the presence of Hydractinia affects the resistance of Pagurus longicarpus to predation. Observations of tethered hermit crabs confirmed the importance of predation by the blue crab, Callinectes sapidus Rathbun, on the Pagurus longicarpus population in Nantucket Harbor, Massachusetts. In a predation experiment, caged hermit crabs in Littorina littorea (L.) shells with and without Hydractinia were exposed to single blue crabs. Shell damage indicated that blue crabs attacked hermit crabs in both shell types in roughly equal proportions: 65% of hermit crabs in shells with Hydractinia were attacked compared to 69% of hermit crabs in shells without Hydractinia. Only 20% of hermit crabs in shells with the hydroid survived attack, however, compared to 54% in shells without Hydractinia. Continuous observation of predation in aquaria showed that hermit crabs in shells with Hydractinia had significantly shorter survival times when attacked by a blue crab than hermit crabs in shells without the colonial hydroid. The crushing resistance of shells was measured with an Instron™ testing machine. A comparison of the regression equations of shell mass (g) vs. crushing resistance (N) between shell types showed that shells with Hydractinia had a lower slope than shells without Hydractinia, although because of a wide range in crushing resistance, this difference was not significant. Shells with Polydoran worm burrows, however, were significantly weaker than shells without burrows, and shells with Hydractinia were more likely to hold worms than shells without Hydractinia. The positive correlation between Hydractinia and shell-degrading worms explains the vulnerability of hermit crabs in shells with the colonial hydroid to predation by blue crabs. Other studies have shown that hermit crabs may benefit from having Hydractinia on their shells because the hydroid prevents large and potentially harmful ectosymbionts, such as Crepidula species, from colonizing. Thus, for Pagurus longicarpus the costs and benefits of the association with Hydractinia may change depending upon the presence of other organisms and this may shift the nature of the association with Hydractinia from mutualism to commensalism to parasitism

Colonization of a shell resource by calyptraeid Gastropods: Tests of habitat selection and preemption models

The influence of patch size, habitat selection, and preemption on the colonization of a patchy resource by three sympatric gastropod species ( Crepidula L.) was evaluated. These species are common epifaunal associates of hermit crabs. Although adult populations exhibit strongly divergent resource utilization patterns, these patterns are not generally the result of divergent colonization processes. All three Crepidula species selectively colonize small shells rather than large shells, are strongly inhibited from colonizing shells dominated by the colonial hydroid Hydractinia echinata Fleming (but not by the colonial bryozoan Alcyonidium polyoum Hassall), and co-occur on individual shells in the absence of other epifauna. The field distribution patterns of Crepidula species are not wholly the result of divergent colonization patterns. These results suggest that significant post-colonization biological interactions also affect Crepidula distribution patterns.