Pea Crab Research Articles

Spatial distribution of the crab larvae (Decapoda: Anomura et Brachyura) in Possyet Bay (Peter the Great Bay of the Sea of Japan) in 2000–2001

The analysis of the data obtained during the plankton surveys in Possyet Bay (Peter the Great Bay of the Sea of Japan) in 2000–2001 is presented. The larvae of eight crab species were registered in the plankton: the red king crab Paralithodes camtschaticus (Tilesius, 1815), the snow (opilio) crab Chionoecetes opilio (Fabricius, 1780), the Arctic lyre crab Hyas coarctatus ursinus (Leach, 1815), the kelp crab Pugettia quadridens (de Haan, 1839), the helmet crab Telmessus cheiragonus (Tilesius, 1815), the Japanese swimming crab Charybdis japonica (A. Milne-Edwards, 1861), the pea crab Pinnixa rathbuni (Sakai, 1934), and the porcelain crab Pachycheles stevensii (Stimpson, 1858). These species belonged to six families: Lithodidae, Atelecyclidae, Majidae, Portunidae, Porcellanidae, and Pinnotheridae. The role of the variability of some of the hydrological factors in the meroplankton larvae development was studied. In May, the crab larvae were found mostly in the open part of the bay. In June, they were transported by the current to the northern part of the bay. In July, they spread uniformly in all the bay areas; however, patchiness was observed. The maximal population density of the crab larvae was registered for July and varied from 6.8 to 23.3 ind. m−3. The crab larvae appeared in the plankton in 2000 and 2001 earlier than for the average season.

Detrimental effects of the isopod, Edotia doellojuradoi, on gill morphology and host condition of the mussel, Mytilus edulis platensis

There are many reported associations between mussels and other invertebrates, such as pea crabs, polychaetes, turbellarians and copepods, which live in their mantle cavities. The boundary between commensalism and parasitism is often indistinguishable because of insufficient knowledge or because the interaction is variable. Preliminary evidence led to a closer examination of the relationship between the mussel, Mytilus edulis platensis, and an isopod, Edotia doellojuradoi, previously described as commensalism. Monthly intertidal samples of mussels were taken from September 2004 to August 2005 at Caleta Cordova Norte (45°43′S, 67°22′W) in southern Argentina and assessed for the prevalence and abundance levels of isopods. Mussels with and without isopods were measured, examined for evidence of gill damage and their condition (soft tissue dry weight) was determined. The overall isopod prevalence in mussels was 57.9% and infestation varied with mussel length, with maximum occurence at 30.2 mm (medium-sized mussels). Experimental evidence indicated that the position of the isopod inside the mussel depended on the feeding activity of the mussel. Female isopods were observed grasping the ventral food groove of the gill demibranchs and feeding on the mucous food strand produced by the mussel. Juveniles and males were observed clustered together on the dorsal side of the single female in each occupied mussel, suggesting extended maternal care. Gill damage was observed in 58.2% of mussels at the Argentine site and was significantly associated with isopod occurrence. Categorical regression analysis showed that the most important factor associated with the degree of gill lesions was the number of male and juvenile isopods per mussel, followed by the length of female isopods and the developmental stage of juveniles. Conversely, the degree of gill damage decreased with increasing mussel length. Overall, E. doellojuradoi had a significant effect on mussel condition throughout the year, with low flesh weight in mussels with isopods, except during the austral summer and early autumn. In contrast to previous studies, which concluded that the isopod was a commensal, the present study clearly demonstrates that E. doellojuradoi is a parasite of M. edulis platensis. Other symbiotic interactions formerly classified as commensal might not be innocuous on further investigation, especially if samples are taken at multiple sites and at different times of the year.

Life history of the pea crab, Pinnotheres sinensis, in terms of infestation in the bivalve mollusc, Septifer virgatus

The inter-relationship between the pea crab Pinnotheres sinensis and its bivalve host Septifer virgatus was studied from July 2002 to December 2004. The infested bivalve generally contained a single crab (72.4–94.7%), but multiple infestation was also observed in some cases (5.3–27.6%). Seasonal fluctuations in the prevalence rate, sex ratio and ovigerous rate were examined. The prevalence rate and sex ratio increased in autumn and decreased from spring to summer. By contrast, the rate of the ovigerous female increased in summer and decreased in autumn. The carapace width of females was larger than that of males, and seasonal fluctuation in the carapace width was observed in the female crabs. All the females had a soft carapace whereas most males had a hard carapace. No females were observed outside the host, though a small number of males were observed outside it. Mating probably took place inside the host where females remained stationary, with males visiting different bivalves as well as spending much time outside the bivalves.

Symbiosis between the pea crab Dissodactylus primitivus and its echinoid host Meoma ventricosa: potential consequences for the crab mating system

In many symbioses involving marine crustaceans the nature of the relationships and the population biology remain overlooked, obscuring the understanding of their ecology and evolution. The association between the pea crab Dissodactylus primitivus and the irregular sea urchin Meoma ventricosa was investigated in the Caribbean Sea. The work tested the predictions provided by an evolutionary model, in which host-switching behaviour and polygynandry are expected for the mat- ing system of D. primitivus. The crab prevalence (infected sea urchins) and mean burden were high and variable between sites and/or years. All post-metamorphic stages of crabs were found on the host, and no exclusion effect was observed between juvenile and adult crabs. In adults, although a general 1:1 sex ratio was stable over the years, there was no evidence for social monogamy. Adult males occurred more often on host-harbouring non-ovigerous females, suggesting that they search for females available for mating. This assumption was supported by a recolonization experiment showing that the males move between hosts. However, both juveniles and females also moved between hosts, indicating that this behaviour could occur due to causes other than mate searching. D. primitivus inflicts external lesions on M. ventricosa teguments. The most severe lesions were asso- ciated with a lower gonad volume during spawning, suggesting a reduction in fitness of the host. The demography of D. primitivus and the recolonization experiments indicate that the mating system of this crab is the 'pure-search polygynandry of mobile females', but the parasitic nature of the relationship could confound this assumption.

Morphological Development of the Commensal Pea Crab (Arcotheres Sp.) in the Laboratory Reared Specimens

Abstract Thirty-seven crabs (8 males and 29 females) of the genus Arcotheres, which inhabit the sessile bivalve Barbatia virescens (Reeve, 1844), were reared for 135 days in the laboratory and their morphological development was described. There was a strong size correlation between female crabs and their host bivalves. During laboratory culture, seven males and 18 females molted once, and one male and four females molted twice. The crabs showed little growth, perhaps because the crabs and bivalves were not fed during the study and because crab growth may be restricted by the host size. One small female molted from pre-hard stage to hard stage and progressed further to Stage II. Some adult females in Stage III or IV developed swimming setae following the molt. In past studies, only one metamorphosis was detected within a crab, and adult females were not thought to develop swimming setae. Thus, our study is the first to detect successive changes in body form within a crab. Furthermore, this study confirmed...

First record of an early pea crab stage on Tonicia chilensis (Frembly, 1827) (Mollusca, Polyplacophora)

Pea crabs, or pinnotherids (Crustacea, Decapoda), known since Aristoteles from mussels like Pinna nobilis Linnaeus, 1758, can be found in association with various marine evertebrates, such as tunicates, tube worms, sea urchins and others. Among molluscs, they are not only found in bivalves, but also in several gastropods (see Geiger & Martin 1999). However, there is only one report to date of a pea crab-polyplacophoran association, that of Opisthopus transversus Rathbun, 1893 and Cryptochiton stelleri (von Middendorff, 1847) from the Californian coast (Webster 1968).

Use of the introduced bivalve, Musculista senhousia, by generalist parasites of native New Zealand bivalves

Introduced species are often thought to do well because of an escape from natural enemies. However, once established, they can acquire a modest assemblage of enemies, including parasites, in their new range. Here we quantified prevalence and effects of infection with copepods (family Myicolidae) and pea crabs (Pinnotheres novaezelandiae), in three mussel species, the non‐native Musculista senhousia, and two native mussels, Perna canaliculus and Xenostrobus pulex, at Bucklands Beach, Auckland, New Zealand. Copepod prevalence was highest in X. pulex (17.9%), whereas pea crab prevalence was highest in P. canaliculus (33.6%). Both parasites infected M. senhousia, but at a much lower prevalence. Dry tissue weight was significantly lower in P. canaliculus infected with pea crabs. In addition, we experimentally investigated host species selection by pea crabs. In an experimental apparatus, pea crabs showed a significant attraction to P. canaliculus, but not so for X. pulex or M. senhousia. When the mussels were presented in combination, pea crabs showed a weak attraction for X. pulex. Pea crab attraction to M. senhousia was not significant. It appears that the introduced M. senhousia largely escapes the detrimental effects of infection with either parasite species compared with native mussels occurring in sympatry.

Metazoan parasites in an intermediate host population near its southern border: the common cockle (Cerastoderma edule) and its trematodes in a Moroccan coastal lagoon (Merja Zerga)

The metazoan parasite community of Cerastoderma edule was studied in the southern geographical range of the host (the coastal lagoon Merja Zerga, Morocco). A total of 11 metazoan species was found in cockles. Nine of these were trematodes using cockles as either first intermediate host (three species) or second intermediate host (six species). In addition, two other endo-metazoan species (Pinnotheres pisum and Paravortex cardii) were recorded from cockles in the studied lagoon. All the observed metazoans in cockles from Merja Zerga have previously been recorded at sites north of Africa.Up to 10% of the cockles in the studied size-groups were first intermediate hosts to castrating parasites (Gymnophallus choledochus, Labratrema minimus and Monorchis parvus). Among trematodes having metacercariae in cockles (second intermediate host) Meiogymnophallus minutus was the most widespread as it was observed in all cockles from all the examined habitats in the lagoon and it occurred in record high intensities. Different sub-communities of the trematode fauna using cockles as second intermediate host could be identified (subtidal vs intertidal associations).The richness and species composition of the macroparasite community in cockles from Morocco are discussed in relation to patterns seen in cockles from other sites along their geographical range. Migratory fish and waterbirds (final hosts) are generally responsible for the large scale spread (latitudinal spread) of trematodes. However, the distributional patterns of involved intermediate hosts in the life-cycles of the different trematode species in cockles are determining the richness and species composition patterns seen in cockles at shallow water sites along the east Atlantic shoreline.

Epibionts on Flexopecten felipponei (Dall, 1922), an uncommon scallop from Argentina

Flexopecten felipponei (Dall, 1922) is a non-commercial, seldom reported pectinid from the SW Atlantic Ocean. In this contribution we review its taxonomy, describe epifaunal species and their levels of encrustation, and discuss the composition of the macrobenthic assemblage where this scallop lives. Eighteen epibiont taxa were observed to live on the valves of these scallops. The most frequent and abundant epibionts on F. felipponei were serpulids, barnacles, and oysters. Although both valves were encrusted, the left valves had higher percentages of coverage. The benthic community contained 69 invertebrate taxa that generally characterize other mid-shelf bottoms between 37°S and 39°S. Eight pea crabs of the species Tumidotheres maculatus (Say, 1818) were found inside eight individuals of F. felipponei. Two other scallops had burrows of Polydora websteri Hartman, 1943. These were the first observations of these infestations on F. felipponei.

A field study to describe diel, tidal and semilunar rhythms of larval release in an assemblage of tropical rocky shore crabs

Available information on the larval release rhythms of brachyurans is biased to temperate estuarine species and outcomes resulting from some sort of artificial manipulation of ovigerous females. In this study we applied field methods to describe the larval release rhythms of an assemblage of tropical rocky shore crabs. Sampling the broods of ovigerous females of Pachygrapsus transversus at two different shores indicated a spatially consistent semilunar pattern, with larval release maxima around the full and new moon. Yet, synchronism between populations varied considerably, with the pattern obtained at the site exposed to a lower wave action far more apparent. Breeding cohorts at one of the sampled shores apparently belonged to actual age groups composing the ovigerous population. The data suggest that these breeding groups release their larvae in alternate syzygy periods, responding to a lunar cycle instead of the semilunar pattern observed for the whole population. For the description of shorter-term rhythms, temporal series at hour intervals were obtained by sampling the plankton and confinement boxes where ovigerous females were held. Unexpectedly, diurnal release activity prevailed over nocturnal hatching. Yet, only grapsids living higher on the shore exhibited strong preferences over the diel cycle, with P. transversus releasing their larvae during the day and Geograpsus lividus during the night. The pea crab Dissodactylus crinitichelis, the spider crab Epialtus brasiliensis and a suite of xanthoids undertook considerable releasing activity in both periods. Apart from the commensal pea crab D. crinitichelis, all other taxa revealed tide-related rhythms of larval release, with average estimates of the time of maximum hatching always around the time of high tides; usually during the flooding and slack, rather than the ebbing tide. Data obtained for P. transversus females held in confinement boxes indicated that early larval release is mostly due to nocturnal hatching, while zoeal release in diurnal groups took place at the time of high tide. Since nocturnal high tides at the study area occurred late, sometimes close to dusk, early release would allow more time for offshore transport of larvae when the action of potential predators is reduced.

Body Size of the Endosymbiotic Pea Crab Tumidotheres maculatus: Larger Hosts Hold Larger Crabs

The endosymbiotic pea crab, Tumidotheres maculatus, uses a broad range of host taxa, including several bivalve species, in the northern Gulf of Mexico. Because shelter size affects the size of other, free-living crab species, we hypothesized that pea crabs living in larger bivalve hosts should attain larger sizes. Crabs and hosts collected from 3 field sites in northern Florida show this trend. We examined crabs living in a large host, the pen shell Atrina rigida, and found them to be larger than pea crabs living in a small host, the bay scallop Argopecten irradians. Moreover, this trend was only apparent among female pea crabs, which are lifelong endosymbionts, but not among males, which are free-ranging and move among hosts. Our data support the broader conclusion that shelter size influences adult crab size in brachyuran crabs.

The prevalence of the pea crab, Pinnotheres sinensis, and its impact on the condition of the cultured mussel, Mytilus galloprovincialis, in Jiaonan waters (Shandong Province, China)

Incidence of Pinnotheres sinensis, a parasitic pea crab of the cultured mussel, Mytilus galloprovincialis, was investigated at Jiaonan, Shandong Province, China during December 2000 to December 2002. The impact of the pea crab on the host was evaluated by comparing the index of condition (percentage meat yield) of infested and uninfested mussels. The 25-month inspection revealed that the prevalence of infestation varied from 1.9% to 62.1%, with the lowest and highest prevalence occurring in July 2001 and November 2002, respectively. The number of pea crabs found in each mussel was 1 to 6, and most of the infested mussels were infested by single female (53.8%), or single male (21.4%), or a couple of a female and a male crab (18.1%). The monthly average intensity varied from 1.00 to 1.44 (total average: 1.18). Evaluated by the index of condition, female crabs showed significant impact on the condition of mussels ( P < 0.05), but male pea crabs did not ( P > 0.05). In 23 of 25 months, the mussels infested by female crabs had the index of condition significantly lower than that of uninfested individuals ( P < 0.05), and the female crabs caused more deleterious effect to the mussels in summer than that in other seasons.

A histopathological survey of some commercially exploited bivalve molluscs in northern Patagonia, Argentina

A survey of pathological conditions affecting natural beds of the most economically important bivalve species was performed for first time in the Argentinean Sea. The bivalves studied were Aequipecten tehuelchus (Pectinidae), Mytilus edulis, Aulacomya atra (Mytilidae), Protothaca (= Venus) antiqua (Veneridae), Ostrea puelchana (Ostreidae) and Pododesmus rudis (Anomiidae), all from San José gulf (42°20′S, 64°20′W), in northern Patagonia, Southwest Atlantic Ocean. Samples of about 30 adult individuals from each molluscan species were collected and processed by standard histological techniques. Rickettsia-like organisms were the commonest parasites found, followed by ciliates, Nematopsis-like spores, sporocysts and metacercariae of Trematoda, larval Cestoda, Turbellaria and two crustaceans (an isopod and a pea crab). A protozoan of uncertain taxonomic affinities, similar to that formerly reported as Perkinsus karlssoni in Argopecten irradians, was found in A. tehuelchus. DNA analysis demonstrated that it does not belong to a Perkinsus species. The commercially exploited bivalves here studied seem to be devoid of serious pathogens. None of the parasites appear to be a problem to future farming due either to low infestation levels or low pathological effect. Moreover, none of these parasites is OIE notifiable, although to confirm these initial results it is necessary to perform a more exhaustive survey in several beds from different seasons. In order to manage Argentinean stocks, a continuous, organized survey, satisfying international standards, is strongly recommended.

CONDITION INDEX OF THE EASTERN OYSTER, CRASSOSTREA VIRGINICA (GMELIN, 1791) IN SAPELO ISLAND GEORGIA-EFFECTS OF SITE, POSITION ON BED AND PEA CRAB PARASITISM

The condition index (CI) is a tool that has been used to estimate the effect that different environmental factors have on oyster meat quality. The CI of oysters in five beds in Sapelo Island and the Doboy Sound (Georgia, United States) were calculated using three different methodologies. Values from each site were used to: (1) compare two sites located in relatively large systems (river/sound) and three sites in a small creek to test for differences due to system size and related nutrient availability; (2) determine the effect of a parasite, the pea crab (Pinnotheres spp.), on oyster CI; and (3) determine the effect of differential position of oysters on a bed, with respect to CI (oysters were collected from areas of the bed that are permanently submerged and areas with the longest dry periods between tidal cycles). With respect to the three analyses mentioned earlier, it was found that oyster CIs for sites in larger systems (river and sound sites) were significantly higher than those in sites with lower flux and width (creek sites) and that the presence of pea crabs as parasites can reduce CI by ~50%. In addition, no difference in CI was found between oysters collected from different positions in the oyster bed, indicating that the CI was not altered within a site from effects of exposure. This relatively unusual finding is discussed but suggests the robustness of CI as an intersite analysis tool. Not all differences found in the comparisons were significant for all CIs used, but qualitative results among them were consistent. These variations in significance and the adequacy of each method for analysis are briefly discussed.

Abbreviated Larval Development of the Pea Crab Orthotheres Barbatus (Decapoda: Brachyura: Pinnotheridae) Described from Laboratory-Reared Material, with Notes on Larval Characters of the Pinnotherinae

Abstract The pea crab Orthotheres barbatus is one of seven species of the family Pinnotheridae reported from Venezuelan marine waters. Larval life histories are not known for any species of the genus Orthotheres. Ten ovigerous females of O. barbatus were obtained from their host gastropod, Cittarium pica, at Los Roques Archipelago, Venezuela. Larvae of O. barbatus were reared in the laboratory from hatching to the first crab stage. This species exhibits abbreviated development in both the number of larval stages (two zoeal stages preceding a megalopa) and the duration of development (minimum of four days from hatching to first crab). The number of zoeal stages for pinnotherid species described to date ranges from 3 to 5, with only Tunicotheres moseri, Pinnotheres taylori, and Nepinnotheres pinnotheres known to also have only two zoeal stages. Morphology of the larval stages of O. barbatus is described, illustrated and compared to that previously described for larvae of the Pinnotherinae. Selected larval c...

Responses of the pea crab Pinnotheres taichungae to the life history patterns of its primary bivalve host Laternula marilina

At the Kaomei tidal flat in Taiwan, the population dynamics and growth of the bivalve Laternula marilina (Reeve, 1860) and responses of the symbiont pea crab Pinnotheres taichungae Sakai, 2000 to the life history patterns of its primary host were studied. The population cycle of the bivalve appears to be initiated by larval settlement occurring as early as late March culminated by die off at the beginning of December of the same year. The maximum host life span is estimated at nine months. Pea crabs, regardless of size, preferred utilizing bivalves with a shell length exceeding 28 mm. This suggests they only spend about three months as a commensal with the primary host because the latter may take five months from the time of settlement to grow to an average size of 32 mm. In response to the brief presence of large primary hosts, the pea crab may: (1) synchronize megalopal recruitment shortly prior to the presence of large primary hosts; (2) evolve a dual infestation mode, invading the bivalve during both invasive and hard stages; and (3) have a fast growth rate while commensal with the primary host.

Intertidal distribution, symbiotic association and reproduction of Pinnotheres bidentatus (Brachyura: Pinnotheridae) from Taiwan

The intertidal distribution of the free-swimming stage of the pea crab Pinnotheres bidentatus Sakai, 1939 was examined on the Kaomei coastal flat of western central Taiwan. Most pea crabs were collected from tidal waters close to the shoreline regardless of the collecting treatments utilized. Results suggest that the pea crab population remains inshore during ebb tides. Many hard and post-hard stages of P. bidentatus were discovered, including ovigerous females. The host was found to be Laternula marilina (Valenciennes, 1860) (Mollusca: Bivalvia: Laternulidae). The infestation percentage of the bivalve was high (average 74.6%). However, this may be due to collections of the bivalve population in the present study being skewed towards large sizes. This biased size structure of the host population may be also responsible for lack of positive size correlation between number and size of male and female symbiotic crabs and the host. Multiple infestations of P. bidentatus appear to be uncommon for L. marilina. Single infestations might be a model for bivalves in tropical regions where the sea is relatively oligotrophic and food competition may be enhanced with multiple infestations. In comparison to other members of the genus Pinnotheres or related species (e.g. P. pisum (Linnaeus, 1767) and Zaops ostreum (Say, 1817)), the body size of sexually mature female P. bidentatus is relatively small, even though it produces similar size eggs (approximately 300 microns). The general rule for size correlation between fecundity and female size in brachyuran crabs holds for P. bidentatus.

POPULATION DYNAMICS OF FREE-SWIMMING STAGE PINNOTHERES BIDENTATUS (BRACHYURA: PINNOTHERIDAE) IN TIDAL WATERS OFF THE WEST COAST OF CENTRAL TAIWAN

Diel, lunar, and seasonal abundance, as well as population structure, of the free-swimming stage of the pea crab Pinnotheres bidentatus Sakai, 1939, from tidal waters off the west coast of central Taiwan were examined. A total 5,123 specimens were collected from tidal waters of the Kaomei coastal flat. These crabs were found year-round but mostly during fall to early winter (October to December 1994 and September 1995), with a peak in early December 1994. Numbers of crabs dropped sharply in mid-December 1994 and remained low during the months of January to April 1995, with slightly increasing numbers during mid-May to September of the same year. Daily abundance of the crab appeared to be related to new and full moon phases, indicating a semilunar pattern of swimming activity. Moreover, pea crabs were only collected during nighttime flooding tides, suggesting a nocturnal swimming activity which may have advantages for avoiding fish predators. Size structure of the crab indicates that P. bidentatus has life-history characteristics of which males die in the hard stage and females subsequently lose their swimming hairs and become commensal with their host. The overall sex ratio was 1:1.3 (2,206 males and 2,917 females) and varied according to month and size. The continuous occurrence of small individuals (1-mm size class) year-round indicates a long period of recruitment which may be a consequence of the long reproductive period of P. bidentatus.

Survival and molting of the pea crab larvae Tunicotheres moseri Rathbun 1918 (Brachyura, Pinnotheridae) exposed to copper.

The acute lethal toxicity of copper, as well as its sublethal effect on molting, was studied on larval and postlarval stages of the pinnotherid crab Tunicotheres moseri. The most sensitive stage was zoea I, presenting a significant mortality above 0.5 microg x L(-1). Abnormal setation of the maxillipeds was observed in zoeae II moulted from zoeae I exposed to 100 microg x L(-1) and higher copper concentrations, causing a reduced swimming activity of the larvae. This pathology could be reverted after molting of affected zoea II to megalopa, which would be the first report in crustacean larvae that states the possibility of reverting morphological abnormalities after molting. For zoeae II and megalopae, the inhibition of molting and high mortality was registered at 1,000 microg x L(-1). The effect of copper on the duration of the larval and postlarval stages was differential: It retarded the duration of the stage zoea I, did not modify that of zoea II, and accelerated the ecdysis of the megalopae to first crab. However, precocious molting to megalopae was associated with a smaller body size at the concentration of 100 microg x L(-1). This differential effect of copper on the larval stages throughout the larval development could be ecologically relevant. Because of their lecithotrophia, their short developmental period, their good survival in control conditions, and the high sensitivity showed to copper (especially zoea I), larvae of T. moseri have shown that they are an excellent model for studying the effect of pollutant on survival, molting rate, growth, and morphogenetic changes during development.

The effects of symbiotic crabs on the pumping activity and growth rates of Chaetopterus variopedatus.

This study investigates how the presence of symbiotic crabs (Pinnixa chaetopterana or Polyonyx gibbesi) in the tubes of the polychaete Chaetopterus variopedatus affects the worms' pumping activity and growth rates under laboratory and field conditions. In the field, worms whose tubes are inhabited by Pinnixa beat their fan segments significantly more frequently than do worms hosting Polyonyx, but other measures of pumping activity do not differ according to symbiont species. In the lab, worms tend to move water through their tubes at higher rates when crabs are present. In 7-month laboratory experiments, growth rates of worms hosting either species of crab did not differ from growth rates of worms without crab symbionts. Although worms hosting Polyonyx are, on average, significantly larger than worms hosting Pinnixa, this appears to be due to competition between the crab species for hosts and not due to differential effects on host growth. Unlike the crabs in this study, pea crab species inhabiting bivalves are known to have strong deleterious effects on host growth and reproduction, suggesting that the evolution of virulence in symbiotic interactions is dependent upon specific ecological context.