Associations between shell morphology and land crab predation in the land snail Cerion

1. The land crab Gecarcinus lateralis is a significant predator of the abundant Bahamian land snails of the genus Cerion . The crabs typically 'scissor' the cylindrical shells in half or break the lip and peel back the shell to reach the animal which withdraws two or three whorls into the shell. Scars on shells of live adults at 73 sites in the Bahamas and Florida Keys show that about 8% (range: 0–44%) of the snails have survived attacks of this type.
2. An artificial crab claw was used to investigate the compressive force required to break Cerion shells of different morphotypes. Defining shell strength as the ability to withstand compressive forces, 10 morphotypes were found that exhibited mean relative strengths of between 30 and 300 newtons. Feeding trials with one adult crab showed that snails whose shells could withstand compressive forces of > 95 N were safe from this individual predator.
3. Multiple linear regression analyses showed that both shell size (length and width) and shell wall thickness were the ultimate determinants of shell strength. Ribs strengthen the shell by contributing to wall thickness and also by increasing overall shell width. The thickened adult shell lip and collabral ribs provide effective protection from attack by peeling.     

Experimental evidence for latent developmental plasticity: intertidal whelks respond to a native but not an introduced predator

Animals with highly inducible traits may show no inducible response when exposed to a related but wholly novel cue. This appears to be true for the intertidal whelk Nucella lamellosa faced with a voracious introduced predator. In the laboratory, we exposed whelks to effluent from two species of predatory crab, the native red rock crab Cancer productus and the invasive European green crab Carcinus maenas. Nucella and Cancer have a long shared history in the northeast Pacific, whereas potential interaction with Carcinus began here less than 10 years ago. Although Nucella responded adaptively to Cancer effluent by increasing shell thickness and decreasing somatic growth, there was no such response to Carcinus. Furthermore, thicker shelled Nucella were less likely to be eaten by Carcinus. Because Nucella produces thicker shells when exposed to Cancer cues, its ability to respond similarly to Carcinus depends only on the coupling of the Carcinus cue to the existing developmental pathways for adaptive changes in shell form. Such coupling of latent plasticity to a novel cue -- via genetic changes or associative learning -- could explain many cases of rapid phenotypic change following a sudden shift in the environment.     

Patterns of gill ventilation in some decapod Crustacea

The frequency of reversal of the respiratory current is recorded for Cancer pagurus, Macropipus puber, Corystes cassivelaunus and Homarus gammarus by cannulating the gill chamber and recording pressure changes therein. The predominant respiratory current flows backwards in Corystes and forwards in the other three, but all show rhythmic reversals of that current which appear to form an integral part of the mechanism of gill ventilation. Reversals appear to have significant gill cleaning functions only in Homarus andperhaps also Corystes.
Measurements of gill ventilation volumes support these findings. Macropipus puber takes in almost all respiratory water at the bases of the chelae, anterior to the gills, and it shows a high reversal frequency apparently to irrigate the posterodorsal aspect of the gills. Cancer pagurus takes in a considerable volume of respiratory water at the bases of the last pair of peraeopods in addition to that entering at the bases of the chelae. This species shows fewer reversals, since the posterior gills are irrigated during normal forward flow, but artificial closure of the posterior openings results in an increased reversal rate.
Comparisons are made of the gill ventilation patterns of all these species, together with Carcinus maenas described previously, and their functional inter-relationships are discussed.     

Immunocytochemical demonstration of the neurosecretory systems containing putative moult-inhibiting hormone and hyperglycemic hormone in the eyestalk of brachyuran crustaceans

Summary By use of antisera raised against purified moultinhibiting (MIH) and crustacean hyperglycemic hormone (CHH) from Carcinus maenas, complete and distinct neurosecretory pathways for both hormones were demonstrated with the PAP and immunofluorescence technique. By double staining, employing a combination of silver-enhanced immunogold labelling and PAP, both antigens could be visualized in the same section. Immunoreactive structures were studied in Carcinus maenas, Liocarcinus puber, Cancer pagurus, Uca pugilator and Maja squinado. They were only observed in the X-organ sinus gland (SG) system of the eyestalks and consisted of MIH-positive perikarya, which were dispersed among the more numerous CHH-positive perikarya of the medulla terminalis X-organ (XO). The MIH-positive neurons form branching collateral plexuses adjacent to the XO and axons that are arranged around the CHH-positive central axon bundle of the principal XO-SG tract. In the SG, MIH-positive axon profiles and terminals, clustered around hemolymph lacunae, are distributed between the more abundant CHH-positive axon profiles and terminals. Colocalisation of MIH and CHH was never observed. The gross morphology of both neurosecretory systems was similar in all species examined, however, in U. pugilator and M. squinado immunostaining for MIH was relatively faint unless higher concentrations of antiserum were used. Possible reasons for this phenomenon as well as observed moult cycle-related differences in immunostaining are discussed.     

The effects of environment upon shell construction and strength in the Rough periwinkle Littorina rudis Maton (Mollusca: Gastropoda)

The interrelationships between environment, shell construction and shell strength of the Rough periwinkle Littorina rudis are investigated in samples of shells taken from a range of shores differing in exposure and salinity level. The effects of different aspects of shell construction upon shell strength are evaluated. Shell strength is compared to chelar bite force of the predatory crab Carcinus maenas. The results are discussed in terms of a theory of factors of safety, and the observed patterns of variation in shell strength and construction with respect to environment are found to be consistent with the predictions of the theory.     

The distribution and orientation of epizoic barnacles on crabs

The distribution and orientation of Balanus crenatus on Carcinus maenas is described. Elminius modestus was also found on C. maenas but it was not common. Both species of barnacle were also found on Cancer pagurus, although neither species of barnacle appeared to be as common on Cancer as on Carcinus. Possible reasons for this are discussed.
Treating the carapaces of similar sized C. maenas as sampling units it is clear that the distribution of B. crenatus over these sampling units is not random but is aggregated. This probably arises because of the gregarious nature of the cyprids when they settle out but other possibilities are considered. B. crenatus on the carapace of C. maenas is found exclusively in the grooves and depressions on the carapace and this is because the cyprids preferentially settle in concavities.
The orientation of B. crenatus also shows a consistent pattern, with the cirral nets facing predominantly backwards. This pattern could arise because the barnacle cyprids orientate to water currents, generated by the exhalant respiratory currents, flowing forwards over the carapace.     

Poor phenotypic integration of blue mussel inducible defenses in environments with multiple predators

Aquatic prey encounter an array of threat cues from multiple predators and killed conspecifics, yet the vast majority of induced defenses are investigated using cues from single predator species. In most cases, it is unclear if odors from multiple predators will disrupt defenses observed in single-predator induction experiments. We experimentally compared the inducible defenses of the common marine mussel Mytilus edulis to waterborne odor from pairwise combinations of three predators representing two attack strategies. Predators included the sea star, Asterias vulgaris (= Asterias rubens ), and the crabs Carcinus maenas and Cancer irroratus . The mussels increased adductor muscle mass in response to cues from unfed Asterias (a predatory seastar that pulls mussel shells open) and increased shell thickness in response to unfed Carcinus, a predatory crab that crushes or peels shells. However, the mussels did not express either predator specific response when exposed to the combined cues of Asterias and Carcinus , and mussels did not increase shell thickness when exposed to cues from Cancer alone or any pairwise combination of the three predators. Shell closure or 'clamming up' did not occur in response to any predator combination. These results suggest that predator-specific responses to the Asterias and Carcinus are poorly integrated and cannot be expressed simultaneously. Simultaneous cues from multiple predators affect the integration of predator specific defenses and predator odors from functionally similar predators do not necessarily initiate similar defenses. Ultimately, the degree that prey can integrate potentially disparate defenses in a multiple predator environment may have ecological ramifications and represent a seldom explored facet of the evolution of inducible defenses.     

Ecological relations between hermit crabs and their shell-supplying gastropods: Constrained consumers

Hermit crabs are critically dependent upon gastropod shells for their survival and reproductive fitness. While anecdotal reports have suggested that hermit crabs may be capable of removing live gastropods from their shells to access the essential shell resource, no systematic experiments have been conducted to investigate this possibility. This paper reports experiments on both marine (Pagurus bernhardus) and terrestrial (Coenobita compressus) hermit crabs in which crabs were paired in the laboratory with the gastropods whose shells they inhabit in the field. Pairings included both shelled and naked crabs and spanned the full range of the gastropod life cycle. Neither marine nor terrestrial hermit crabs were successful at removing live gastropods from their shells. Furthermore, only a small fraction of the crabs (5.7%) were capable of accessing shells in which the gastropod had been killed in advance, with its body left intact inside the shell. Finally, although hermit crabs readily entered empty shells positioned on the surface, few crabs (14.3%) were able to access empty shells that were buried just centimeters beneath them. These results suggest that hermit crabs are constrained consumers, with the shells they seek only being accessible during a narrow time window, which begins following natural gastropod death and bodily decomposition and which typically ends when the gastropod's remnant shell has been buried by tidal forces. Further experiments are needed on more species of hermit crabs as well as fine-grained measurements of (i) the mechanical force required to pull a gastropod body from its shell and (ii) the maximum corresponding force that can be generated by different hermit crab species' chelipeds.     

Variation in shell morphology of Calliostoma zizyphinum (Gastropoda: Trochidae)

Differences in shell morphology in the intertidal prosobranchmollusc Calliostoma zizyphinum were studied from a number ofsites within four geographical regions of the British Isleswith varying exposures to wave action and crab predation. Meanvalues of damage scarring were highest in shells sampled fromStrangford Lough, Northern Ireland, and lowest in individualsfrom the Atlantic Coast. Shells collected from the Isle of Manwere smaller than those from either the Atlantic coast of Irelandor Strangford Lough. Shells from Strangford Lough had tallershells (higher aspect ratio) than shells from the County Downand Atlantic Coast and shells from the County Down coast weremore squat (lower aspect ratio) than those from all other areas.Shells from Strangford Lough were significantly thinner thanshells from all other geographical areas. The relationshipsbetween shell damage scarring and shell size and tallness werenot consistent among areas. Shells from Strangford Lough arenotable in showing a steep, positive relationship between shellsize and scarring and a steep, negative relationship betweenscarring and tallness. The unusual shell phenotypes observedin shells from Strangford Lough may be explained by rapid shellgrowth, which would not only allow Calliostoma to attain a sizerefuge from crab predators but also to recover successfullyfrom repeated crab attacks on the shell lip. Such a strategywould result in larger, thinner shells with a high number ofdamage scars. (Received 6 June 2006; accepted 20 December 2006)     

SEM observations on the cuticles of some decapod crustaceans

The cuticles of the crayfish Austropotamobius pallipes (Lereboullet), and the crabs Cancer pagurus L. and Carcinus maenas (L.), have been examined with the scanning electron microscope. Laminae are composed of sheets of horizontal fibres which subsequently are into the inter-laminae. The inter-laminae contain sheets of fibres which are at a slight angle from the horizontal between sucessive laminae. The disposition of the pore canals is clearly related to fibre pattern. The sinuous macrofibres described by Dennell (1973) are discrete from the pore canals.     

Differential snail predation by an exotic crab and the geography of shell-claw covariance in the Northwest Atlantic

Here we investigate if predation by the European green crab (Carcinus maenas) differs between two congeneric snails in the northwest Atlantic (Littorina littorea and L. obtusata), and ask if differential predation can help explain the geography of claw and shell forms among geographically separated populations. First, correlations between crusher-claw size and shell mass -- tested across a wide size range of animals -- were highly significant among populations of C. maenas and L. obtusata, whereas only a small number of significant correlations were found between C. maenas and L. littorea, and these were limited to the smaller size classes of snails and crabs. Moreover, among populations, L. obtusata shells were more frequently scarred than those of L. littorea, and L. obtusata were attacked and killed more frequently than L. littorea during field- and laboratory-predation experiments. Combined, results suggest L. obtusata is currently under greater selection by C. maenas than L. littorea for more crab-resistant shell forms. One possible explanation for these patterns is that L. littorea may have interacted with green crabs for centuries (in Europe) prior to their reintroduction to green crabs in America, thus predator-resistance may had already evolved.     

The competitive and predatory impacts of the nonindigenous crab Carcinus maenas (L.) on early benthic phase Dungeness crab Cancer magister Dana

We evaluate the potential competitive and predatory impacts of nonindigenous European green crab Carcinus maenas on native Dungeness crab Cancer magister in the northeast Pacific. The coastal estuaries of Washington State, USA, provide appropriate habitat for recently introduced green crab, yet these areas are important nursery grounds for Dungeness crab and contribute greatly to the coastal crab fishery. Juvenile Dungeness crabs are dependent on limited intertidal epibenthic shell for refuge habitat during early benthic life and experience increased mortality on open sand and mud as a result of predation by fish and birds. Early juveniles throughout the subtidal are similarly at risk due to predation by fish and especially adult conspecifics. Laboratory experiments and infrared video observations revealed that juvenile green crab displace Dungeness crab of equal size from shelters during one-on-one competition. Green crab also consistently win nocturnal foraging trials in which the species compete for fresh, damaged clams. Field and laboratory enclosure experiments show that juvenile Dungeness crab emigrate from oyster shell habitat as a result of competition and predation by adult green crab. Depending on the extent to which the two species overlap, interactions with the dominant nonindigenous species could have a negative influence on juvenile Dungeness crab survival and could conceivably impact recruitment to the fishery. However, current evidence indicates that the distribution of green crab in Washington State is far removed from nursery areas of Dungeness crab.     

MORPHOLOGY AND BEHAVIOR OF CRABS AND GASTROPODS FROM LAKE TANGANYIKA,AFRICA: IMPLICATIONS FOR LACUSTRINE PREDATOR-PREY COEVOLUTION

The shells of most lacustrine gastropods are typically small, weakly calcified, and modestly ornamented to unornamented. Similarly, most lacustrine crabs are usually small detritivores with weak chelae. A number of invertebrate taxa in Lake Tanganyika, however, deviate from these generalities. This study explores a predator-prey coevolution model as an explanation for the large, heavily calcified, and ornate gastropods and the robust, durophagous crabs of Lake Tanganyika. The endemic thiarid and viviparid gastropods from Lake Tanganyika have significantly thicker shells and higher frequencies of terminal apertural lip thickening than closely related cosmopolitan taxa from outside the lake. Tanganyikan gastropods also display considerably higher incidence of shell repair, following nonlethal shell damage, than cosmopolitan taxa of the same families. There is a strong positive correlation between gastropod apertural lip thickness and shell repair frequency among all the gastropod species analyzed. The endemic Tanganyikan potamonautid crab Platytelphusa armata (a molluscivore) possesses larger, more robust crushing chelae than other African potamonautid or potamonid crabs. In contrast with the cosmopolitan African crabs, the Tanganyikan crabs display molariform, rather than serrate dentition on their crushing chelipeds. In shell-crushing experiments, the Tanganyikan gastropod shells were an order of magnitude stronger than typical lacustrine gastropod shells, many well within the range of tropical marine gastropod shell strengths. Predation experiments with the endemic gastropods Spekia, Neothauma, Lavigeria spp., Paramelania spp. and the crab Platytelphusa armata showed that increased size, apertural lip thickness or shell sculpture reduced the successful predation rate of P. armata. Crabs with large chelae have a greater ratio of successful: unsuccessful attacks than crabs with small chelae. Among cases of successful predation, crabs with large chelae employed predation methods that required less time and energy (such as crushing the shell in the cheliped) than the methods employed by crabs with small chelae (such as peeling the shell from the aperture or the spire). The morphological, shell-crushing, and aquarium experiment data, considered in concert, provide strong support for the idea that the endemic gastropods and crabs of Lake Tanganyika have coevolved over the past 7 million years.     

“Exolysosomes,” Enzyme-Containing Vesicles in the Ecdysial Space of Molting Crabs

Free vesicle-like bodies (VLBs) present in the ecdysial space of cuticle regions undergoing degradation during preecdysis of the Atlantic shore crabCarcinus maenashave been interpreted either as infectious organisms or as secretion structures associated with degradation of the old cuticle. Ultrastructural, cytochemical, and immunocytological investigations were performed to test these hypotheses and to see whether VLBs are peculiar to this crab species. Similar VLBs were systematically found in two other preecdysial crabs,Cancer pagurusandMacropipus puber.InCar. maenas,they originate during early premolt inside Golgi buddings and are often gathered into large vacuoles in epidermal cells. The histochemical azo-dye technique and a cerium-based cytochemical method revealed acid phosphatase activity in both the ecdysial space and the VLBs, while Feulgen's method and immunocytological labeling always failed to reveal any DNA or RNA in either the ecdysial space or the VLBs. We conclude that VLBs are not infectious organisms but “extracellular” cuticle-degrading organelles of lysosomal origin and propose to coin them “exolysosomes.”     

Molecular cloning of crustacean pigment dispersing hormone precursor.

The cDNA encoding the precursor of the pigment dispersing hormone (PDH) of the shore crab, Carcinus maenas, was isolated and sequenced. The precursor consists of a putative 22 amino acid signal peptide, a putative 33 residue peptide of unknown function, and the 18 amino acid mature PDH, followed by a Gly residue which serves as a possible amide donor. The deduced mature PDH amino acid sequence is identical to those of Uca pugilator and Cancer magister, previously determined by Edman degradation.     

Intraspecific trait cospecialization of constitutive and inducible morphological defences in a marine snail from habitats with different predation risk

1.?Studies examining the integration of constitutive and inducible aspects of multivariate defensive phenotypes are rare. 2.?I asked whether marine snails (Nucella lamellosa) from habitats with and without abundant predatory crabs differed in constitutive and inducible aspects of defensive shell morphology. 3.?I examined multivariate shell shape development of snails from each habitat in the presence and absence of waterborne cues from feeding crabs (Cancer productus). I also examined the influence of constitutive and inducible shell morphology on resistance to crushing. 4.?Regardless of the presence of crabs, snails from high-risk (HR) habitats developed rotund, short-spired shells, while snails from low-risk habitats developed elongate shells, tall-spired shells, indicating among-habitat divergence in constitutive shell shape. Moreover, allometry analyses indicated that constitutive developmental patterns underlying this variation also differed between habitats. However, snails from HR habitats showed greater plasticity for apertural lip thickness and apertural area in the presence of crab cues, indicating among-habitat variation in defence inducibility. 5.?Both shell shape and apertural lip thickness contributed to shell strength suggesting that constitutive shell shape development and inducible lip thickening have evolved jointly to form an effective defence in habitats where predation risk is high.     

Response of hermit crabs to sinistral shells

Shell rotating behavior of the hermit crabPagurus geminus was investigated. In preliminary observations, hermit crabs motivated to change shells rotated presented shells, filled with sand, in a way that dislodged the inside material. In order to determine if this behavior is stereotyped, or flexible and dependent on shell type, hermit crabs were tested with ordinary dextral shells ofLatirulus nagasakiensis and sinistral shells ofAntiplanes contraria. Sinistral shells are not normally encountered by hermit crabs. Their rotation of the dextral shell to the left was adequate for sand discharge. Sinistral shells were rotated in both directions. Analysis of recorded videotapes showed that variation in rotation direction could be attributed to variation in the position of the crab relative to the shell. When the crab faced the shell aperture from the inner lip, it rotated the sinistral shell to the right, and to opposite direction when it faced from the outer lip side. The crab always pushed the upper side of the horizontally laid shell, regardless of shell type or its own position.