Weapon size versus body size as a predictor of winning in fights between shore crabs, Carcinus maenas (L.)

Relative body size (carapace width) and weapon size (chela length) were used as indicators of resource holding potential (RHP) in the agonistic behaviour of male shore crabs, Carcinus maenas (L.). Weapon size was found to be a more reliable predictor of the outcome of pairwise fights than body size. Crabs with longer chelae than their opponents were more likely to win fights than crabs with relatively larger bodies. Body size had less influence on the outcome of fights. Relative body and weapon size did not influence initiation of contests but did affect the likelihood of winning; however, this was significant only for weapon size. Winning crabs had heavier claws with greater surface area than losing crabs. There was no relationship between relative size and fight duration. The frequency of cheliped display increased with chela length and win- ners performed significantly more displays than losers. Received: 5 February 1997 / Accepted after revision: 20 May 1997     

Foraging behavior of juvenile Carcinus maenas (L.) and Cancer pagurus L.

Information concerning the way juvenile crabs choose their diet from a variety of prey types can be useful for a better understanding of community dynamics, as well as for the adequate management of natural resources. Prey size and species selection by juvenile Carcinus maenas (15-35 mm carapace width, CW) and Cancer pagurus (20-40 mm CW) feeding on four bivalves of contrasting shell morphology were investigated. When offered a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas, and Cerastoderma edule presented individually, crabs generally showed evidence of size-selective predation. Cancer pagurus selected larger mussels relative to the size of their chelae (relative prey size, RPS) than did Carcinus maenas of similar and even larger carapace width. However, the RPS of selected O. edulis and Cerastoderma edule were similar for all crabs, suggesting that certain prey features constitute effective barriers even to the powerful chelae of Cancer pagurus. When offered a wide size range of mussels and oysters simultaneously, all crabs consistently selected mussels. When offered O. edulis and Crassostrea gigas, crabs consumed both these oyster species in similar numbers. Carcinus maenas consumed similar numbers of mussels and cockles; Cancer pagurus, however, showed no preference for either prey in the smaller size classes but selected more mussels than cockles as prey increased in size. Although previous studies report that adult Carcinus maenas select prey species according to their profitability (amount of food ingested per unit of handling time, milligrams per second), consumption rates of the size classes of prey selected by juvenile shore crabs did not always parallel prey value. Although variations in crab strength can account for many of the differences between the foraging strategy of juvenile and adult C. maenas, our results suggest that juvenile crabs are less species selective than adults as a result of the restrictions imposed on small individuals that have limited access to larger prey.     

Size assessment via a visual signal in snapping shrimp

Snapping shrimp are highly aggressive decapod crustaceans, with large, asymmetric chelae. Body size determines the outcome of both inter- and intrasexual interactions. Both the body and chela sizes of mated pairs are correlated, but the body size correlation is significantly stronger. In competitive interactions between individuals of the same sex, larger individuals usually win. Because the size of the major chela is a function of body size in both males and females, chela size could be used to assess body size early in interactions, before engaging in more high risk behaviors. To determine whether the major chela is used in size assessment, I presented shrimp with isolated chelae. Male snapping shrimp responded aggressively to isolated chelae when they were fixed open in a display posture, and the degree of aggressive response depended on the relative size of the chela. These data provide direct experimental evidence for the use of a visual signal in size assessment. Females, in contrast, responded aggressively to both the open and closed chela, and their responses did not depend on relative size. This sex difference in response may be due to differences in the value of certain resources, such as shelters, to males and females: females may be more willing to respond aggressively regardless of the apparent size of their opponent, in order to acquire more reliable information regarding size, motivation or fighting ability. Received: 30 December 1994/Accepted after revision: 14 August 1995     

Structure and function of the chelate pereiopods of the banana prawn Penaeus merguiensis

Penaeus merguiensis de Man systematically searches the substratum and removes small particles of food from it using the small chelae of the first three pairs of pereiopods. One or more chelae are used to transfer food particles to the mouth. The propus and dactylus of these limbs bear numerous setae arranged in discrete groups along the length of the segments. The tip of each seta is elaborately sculptured and has a large sub-terminal pore. It is probable that these are chemosensory organs responsible for discriminating edible from non-edible material. Proximal to almost every setal group is a much-branched seta (rarely two) which probably measures the depth to which the chela has been inserted into the substratum. At the articulation of the propus and carpus of the first pereiopod are three groups of differently sculptured setae, whose function is to clean the other chelate pereiopods. An elaborate system of pegs and a ridge on the apposable fringes of the chelae might be a mechano-receptive device with a particle size discriminating function. The ridges merge into opposable terminal denticulate pads whose function is to grasp food and other particles, which may be quite small (10 to 20 m upwards).     

Sperm removal and ejaculate size correlate with chelae asymmetry in a freshwater crayfish species

Asymmetry in traits of sexual relevance may impair copulation behaviour and sexual performance of males, ultimately resulting in a fitness cost. Freshwater crayfish males use chelae, a sexually selected trait, to secure and position the female prior to and during mating. Thus, a relatively large chelae asymmetry, resulting from accidental loss and regeneration of one cheliped after autotomy, could have great consequences for male sexual behaviour. We studied copulatory behaviour and sperm expenditure of males paired to a mated female in Austropotamobius italicus, a freshwater crayfish species where both male and female mate multiply and where last-mating males are able to actively remove previously deposited sperm. We aimed at assessing whether male sperm removal and expenditure varied according to sperm allocated by first-mating males, and according to copulation behaviour and phenotypic traits (carapace length, chelae length and relative chelae asymmetry) of second-mating males. Second-mating males did not adjust their ejaculate size in relation to first-mating male ejaculate, nor to the first-mating male’s sperm removed. Moreover, the amount of sperm removed by second-mating males increased with increasing first-mating males ejaculate size, and first-mating male sperm remaining after removal did not correlate with the original first-mating male ejaculate size. Interestingly, the amount of sperm removed by second-mating males decreased with increasing relative chelae asymmetry, while increasing with male body size. However, second-mating (but not first-mating) asymmetric-clawed males produced larger ejaculates than symmetric-clawed ones. Importantly, the proportion of second-mating male sperm remaining after the two matings did not vary with relative chelae asymmetry nor with body size of second-mating males. Thus, small, asymmetric-clawed crayfish males appear to adopt sperm allocation tactics that allow them to fully compensate for their inferior sperm removal ability.     

Evidence of habitat structure-generated bottleneck in the recruitment process of the SW Atlantic crab<Emphasis Type="Italic"> Cyrtograpsus angulatus</Emphasis>

Shelters generated by the introduced reef-building polychaete Ficopomatus enigmaticus (Serpulidae) significantly enhance settlement of the crab Cyrtograpsus angulatus (Grapsidae) in a SW Atlantic coastal lagoon (Mar Chiquita, 37°32S; 57°26W; Argentina). However, their ultimate role in recruitment does not appear to be very important, suggesting a habitat-generated bottleneck that occurs between settlement and recruitment. Laboratory and field experiments show that newly settled crabs actively select refuges similar in size. Inside the reefs, decreases in crab density of each newly settled cohort mirror the ratio of decreases in the number of reef refuges of similar sizes, suggesting that habitat fractal structure determines the mortality rate after settlement. Field experiments using artificial shelters show that as crabs increase in size, they move outside reefs. Field tethering of juvenile- and adult-sized crabs without access to refuges showed that juvenile crabs suffer a much higher risk of mortality than adults. Our results show that the availability of small refuges provided by polychaete reefs enhances crab settlement, but, then, due to a decrease in the number of refuges as size increases, produces a demographic bottleneck during recruitment. Thus, independent of settlement intensity, enhanced survivorship in the smallest size classes due to refuge is not transmitted to larger size classes. This is likely one reason why stock-recruitment relationships may not hold in species that use shelters during both recently settled and juvenile life stages. These results provide a good example of why habitat enhancement programs need to incorporate a comprehensive evaluation of the species ontogenic ecology to avoid class size-specific bottlenecks.Communicated by P.W. Sammarco, Chauvin     

Notes on some factors affecting aggressive behaviour in Carcinus mediterraneus

The relationships between aggressive behaviour and animal density and between aggressive behaviour and metabolic excretion have been investigated in Carcinus mediterraneus Czerniavsky. Increasing the crab density by increasing the number of crabs or reducing the available area revealed that the aggressive interactions perindividual did not increase. The frequency of interactions reached a maximum in several minutes following the introduction of crabs. Individuals isolated between 6 and 24 h were more aggressive than non-isolated ones. The water in which a group of crabs had been held had an inhibitory effect (not abolished by commercial aquarium filters) on aggressiveness of pairs of crabs grater than plain water or water in which single crabs had been kept. No home-group effect was observed. The effect of group water was enhanced by increasing the concentration of crabs in the duration of confinement, and it is probably due to the accumulation of catabolites in the medium. Crabs kept in groups had a greater excretion than isolated crabs and the nitrogen oupput decreased with an increase in the number of crabs or whith a decrease in the size of the tank (with a constant volume of water and number of crabs).     

Significance of mangrove–mudflat boundaries as nursery grounds for the mud crab, Scylla paramamosain

Fisheries managers frequently try to protect juveniles in order to preserve stocks. Juveniles can be protected by either implementing changes designed to avoid catching immature animals (e.g. increasing mesh size or altering fishing techniques) or protecting nursery grounds. To prevent the capture of immature animals, an estimate of size at maturity is required as well as a knowledge of both fishing methods and the exact location of the nursery grounds. Strong demand for juvenile mud crabs to stock aquaculture ponds has resulted in development of fisheries targeting crabs of all sizes from instar 1 to mature individuals. Using five different fishing methods, different stages in the life cycle of Scylla paramamosain were followed for a period of 16 months in an estuarine population in the Mekong Delta, Vietnam. Mangrove habitat utilisation begins when crabs settle out from the plankton at instar 1 [modal internal carapace width (ICW), 0.5 cm] amongst the pneumatophores at the mangrove fringe. Increasingly larger crabs were found deeper into the mangrove but they were still living on the surface (modal ICW size class, 1.5 cm). As their size increases, the crabs either dig burrows (modal ICW size class, 4.5 cm) or they live in the sub-tidal zone, migrating into the mangrove with each tide to feed (modal ICW size class, 4.5 cm). Larger crabs were caught offshore (modal ICW size class, 12.5 cm) where females accounted for 60% of the catch although of these, only 63% were mature. Recruitment of early instars was continuous but peaked in December to February. Subsequent peaks in the catch rates of larger size classes indicated the development of a single cohort with an estimated growth rate of 2.0 cm ICW per month. On the basis of abdominal width, females were estimated to mature at 10.2 cm ICW although at 9.7 cm ICW, 50% of females had disengaged abdomens. Abdominal disengagement occurred in males at the slightly smaller size of 9.1 cm ICW. Allometric relationships between chela height and carapace width suggested 50% of males acquire mature chelae at 10.2 cm ICW. These results demonstrate the close linkage between early life stages of S. paramamosain and certain specific niches within mangrove habitats, with the main adult population found to be living sub-tidally at some distance from the mouth of the estuary. The study also highlights the special importance of the mangrove fringe, the border between the mangrove forest and the sea, an area which is particularly vulnerable to physical and anthropogenic impacts.     

Evidence for multiple recruitment-cohorts of Puget Sound Dungeness crab,Cancer magister

While sampling intertidally in Puget Sound, Washington, USA, for juvenile Dungeness crab (Cancer magister) in 1984, we found evidence of two distinct cohorts of the same year-class based on sizes of first-instar juveniles (J1) and the spatial/temporal patterns of settlement. In 1988, three distinct cohorts were observed to settle in Puget Sound and its approaches. Settlement of one cohort occurred during May in the Strait of Juan de Fuca and in those areas of Puget Sound closest to the Strait. J1 individuals of this cohort were large (x=7.4 mm carapace width, CW) and comparable in both size and timing of settlement to populations along the Washington coast (e.g. Grays Harbor and Willapa Bay). Initial settlement density of the May cohort was as high as 215 crabs/m² in intertidal eelgrass beds along the Strait of Juan de Fuca and decreased to <2 crabs/m² within Puget Sound and the Strait of Georgia. A second cohort apparently originated in Hood Canal (a deep inland fjord), its size upon settlement in June was significantly smaller (J1 x= 5.3 mm CW) than the May cohort, and it was limited to Hood Canal and areas of Puget Sound close to the mouth of Hood Canal. A third cohort, which settled in late July and August, was the smallest of the three cohorts (J1 x= 4.8 mm CW), and was widely distributed around Puget Sound from Seattle in the south to the USA/Canadian border in the north. We hypothesize that most juvenile recruitment in Hood Canal and Puget Sound originates from parental stocks endemic to their respective basins (Hood Canal and Puget Sound cohorts), but that, on occasion, oceanographic conditions allow substantial influx of Pacific Ocean Dungeness crab larvae (oceanic cohort) through the Strait of Juan de Fuca into Puget Sound. Tracking of spatial/temporal settlement patterns and comparison of J1 sizes proved useful for estimating the probable sources and dispersion of Dungeness crab larvae. Differences in size and time of settlement between various larval cohorts of C. magister may prove useful as biomarkers for tracing circulation patterns within and between inland waters of Washington and the Pacific Ocean. Causes of smaller size and later settlement of the Puget Sound cohort relative to oceanic conspecifics of the same year-class are discussed.Contribution No. 856, School of Fisheries, University of Washington, Seattle     

Claw allometry in green crabs, Carcinus maenas: heterochely, handedness, and sex

Claw loss and reversal of handedness during regeneration are common phenomena in heterochelous decapod crustaceans, which typically have one large ‘crusher’ claw on the right side and a smaller ‘cutter’ claw on the left. Little is known about the relative importance of claw growth vs. body growth during claw regeneration. Here the relationship between claw size and body size of green crabs, Carcinus maenas, was examined to test for differences in claw allometry as a function of handedness and sex, as there are differences in how males and females use their claws. A total of 730 crabs (range = 15.7–83.6 mm CW) were collected from Maine to New Jersey, USA from May to October 1997, 2000, and 2004–2005. Claw growth, particularly crushers, was accelerated in left-handed crabs and in males compared to right-handed crabs and females respectively. These differing growth strategies highlight the role of sexual dimorphism in claw usage and the importance of achieving heterochely after claw injury. These results imply that handedness should be an important factor to consider in future studies of crab morphology, behavior, and morphometrics. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Diamondback Terrapin (Malaclemys terrapin) Mortality in Crab Pots

The entrapment of diamondback terrapins (  Malaclemys terrapin) in crab pots frequently results in drowning and death of the trapped turtles. We determined the rate of capture, size, sex, and age of terrapins captured, and the potential impact crab pot mortality has on local terrapin populations. We estimated terrapin capture rates of 0.17 terrapins/pot/day in shallow water areas of Chesapeake Bay (Maryland, USA). The sex ratio of terrapins caught in crab pots was 3:2 male biased because female terrapins become too large to enter crab pots by the time they reach 8 years of age. Males, however, remain vulnerable to entrapment throughout their life. Our estimates of capture rates and local population size suggest that 15–78% of a local population may be captured in a single year. As a consequence, crab pots may be the major reason terrapins are extirpated in coastal, shallow water areas with heavy crab pot fisheries. Additionally, the selective removal of males may also contribute to female-biased sex ratios observed in this diamondback terrapin population. We developed and tested a modified crab pot that increases terrapin survival and does not reduce the number of crabs caught. Our modified crab pot maintained permanent access to air and prevented the drowning of terrapins. Additionally, our modified crab pot caught more crabs than standard commercial crab pots, suggesting that the modified crab pot could be a viable alternative to standard traps that result in terrapin mortality.     

In situ variation in heart rate of the shore crab Carcinus maenas in relation to environmental factors and physiological condition

Heart rate of Carcinus maenas was recorded continuously for two days in situ, together with water temperature, salinity, depth and light intensity. Of each sex, 25 crabs were used and assigned to groups on the basis of size and colour (the carapace changes colour with prolongation of the intermoult phase). Wet weight:dry weight ratios of midgut gland, muscle, gonads and whole body were examined together with midgut gland lipid content and haemolymph protein concentration. Heart rates and wet wt:dry wt ratios for all the tissues examined were higher in early intermoult than in late intermoult stages of the adult C. maenas (P<0.05). Heart rate, lipid content and haemolymph protein concentrations were higher and wet wt:dry wt ratios of midgut gland, gonads and whole body were lower in juvenile crabs than in adults (P<0.05). The relationship between heart rate and wet wt:dry wt of whole body differed among C. maenas in early and late intermoutl. Heart rate was positively correlated with midgut gland lipid content (a Darwinian fitness parameter) in crabs that were in late intermoult. Physico-chemical environmental parameters, sex and colour accounted for 12% of the variation in heart rate of C. maenas recorded in situ. Temperature and prolonged intermoult were the most important factors influencing heart rate. The findings are discussed in relation to laboratory studies of cardiac activity and observed behaviour of C. maenas in the field.     

Changes in the excretory patterns of the fresh-water field crab Paratelphusa hydrodromous upon adaptation to higher salinities

Changes in the exeretory patterns of the fresh-water field crab Paratelphusa hydrodromous were studied in relation to salinity adaptation. Fifty percent sea-water medium was found to be the threshold of a change from ammonotelism to ureotelism in the crabs. Adaptation for 11 days in the 100% (=34 S) sea water brought about this change. Sea-water-adapted crabs excreted more urea, uric acid, and trimethylamine than controls, irrespective of starvation. Starvation reduced the quantitative excretion. Blood aspartic and glutamic acid levels increased on adaptation to sea water, in contrast to the levels of asparagine and glutamine. The activities of asparaginase and glutaminase in the tissues decreased on salinity adaptation. The reduction in amidase activity in 100% sea-water-adapted crabs is discussed with reference to acid-base homeostasis in the crabs.Dedicated to the memory of the late Professor K. Pampapathi Rao, who died on 23 June, 1973.     

Dispersal and use of sponges and ascidians as camouflage by Cryptodromia hilgendorfi (Brachyura: Dromiacea)

From May 1977 to February 1979, the use of sponges and ascidians by Cryptodromia hilgendorfi was studied in Moreton Bay, Queensland, Australia. The aim was to investigate patterns of seasonal use, cap making behaviour, cap turnover, the effect of intraspecific interactions on cap life and the effect of movement of crabs between hosts on background matching. C. hilgendorfi uses 12 (of 16 available) species of sponge and 3 species of ascidians to construct caps, which are carried by the crabs using their last two pairs of legs. Cap area increases non-linearly with crab size, and caps are normally two to three times as large as the crabs. Cap making behaviour is described. It occurs during intermoult periods, with females making most of their caps at night. Caps decrease in size with time, but conceal the crabs which commonly occupy exposed sites on sponges. Cap life is independent of crab size, differs between different cap species and is influenced by the presence of other crabs who can dislodge caps through aggressive behaviour. Caps are made from the sponge Suberites carnosus more often than from other available sponges. S. carnosus caps also decay less rapidly than caps made from other sponges. Use of sponge and ascidian species varies seasonally, with Halichondria sp. and S. carnosus being used in all months. C. hilgendorfi exhibits a preference for certain sponges. The majority of crabs carried caps which matched their host sponge or ascidian, but mis-matches varied seasonally with a winter peak following the breeding season. Young C. hilgendorfi settle only on S. carnosus sponges and disperse from this host to other species in the environment. Males and females differ in their rate of discovery of new hosts. Males, despite their greater mobility, find new hosts slower than females. It is hypothesized that males occupy “home ranges” which females do not. Crabs frequently move between sponges, mostly at night. Sponges and ascidian species grow in intimate association with each other, and sponge crabs act a selective asexual propagation mechanism. Depending upon the nature of the interactions between sponge and ascidian species (co-operative or competitive) and whether competitive hierarchies or networks are involved, the sponge crabs may have either stabilizing or destabilizing effects on the sponge community.     

Annual fluctuation of life-history traits in the mud crab Macrophthalmus japonicus

Interannual fluctuation of life-history traits in the mud crab Macrophthalmus japonicus (De Haan) was studied for a 7-yr period (1981 to 1987) in an intertidal mudflat in the estuary of the Tatara-Umi River, Fukuoka, Japan. Density of juveniles fluctuated greatly, and maximum annual density was low in 1981 to 1984 (22.0 to 44.0 m^-2) and very high in 1985 to 1987 (98.5 to 188.6 m^-2). In 1980, recruits may have been very few, though there is no data on or before 1980, because no crab of the 1980 cohort was found during the study period. Density of adult crabs was almost stable (11.0 to 32.6 m^-2), perhaps because of density-dependent mortality. Under the high density conditions prevailing from 1985 to 1987 crabs grew slowly and longevity was shorter. Male chela and female abdomen were dwarfish, and females produced few and small broods. The changes of life-history traits under high density conditions were thought to result from strong intraspecific competition for space.     

Shell-size selection by intertidal sympatric hermit crabs

This study evaluated selection for shell size by three species of tropical intertidal hermit crabs, Clibanarius antillensis, C. sclopetarius, and C. vittatus, from species of shells which are frequently used in nature. Crab size and weight were strongly and significantly related to all measured parameters of the selected shells. The strength of these relationships (r² values) depended neither on the crab nor on the shell variables taken into account. The relationships between crab size and the dimensions of the selected shells showed higher r² values than the corresponding relationships with the shells that the crabs had occupied when they were collected (0.482–0.903 in comparison to 0.091–0.652, respectively), indicating that the crabs were occupying sub-optimal shells in nature. Negative allometry was frequently found in the relationships between crab and shell variables, indicating that large crabs select and use proportionally lighter shells than do small crabs. This negative allometry was stronger for the shells used in nature (except for C. antillensis), i.e. larger crabs tended to select heavier shells in the laboratory than in nature. Different allometric relationships were also recorded among the dimensions of shells used in nature and those selected by the hermit crabs in free-access experiments: as shell length increased, the selected shells were heavier and had larger apertures than the shells used in nature. The relationships between crab size and the length and weight of the selected shells did not depend on the species of crab or species of shell, but only on crab size. Therefore, analyses using these variables can be performed without taking the species of crab or shell into account, i.e. data from different crab or shell species can be pooled. The influence of crab and/or shell species was recorded only in the models fitted for aperture length and width, variables which were more related to shell architecture than did shell length or weight. In contrast, if crab weight is used as an independent variable, different crab or shell species can be analyzed together independently of the particular shell parameter. This indicates that crab weight may be less susceptible than crab shield length to shell morphological constraints. Finally, the results indicate that the preferred size of a given shell type chosen by a given hermit crab will depend more on crab size or weight, than on the crab or shell species under consideration, i.e. crab shell-size relationships are not species specific.Communicated by P.W. Sammarco, Chauvin     

Spatial,annual and seasonal patterns in the condition and muscle size of snow crab

The extent of spatial (depth and locality) and temporal (season and year) variabilities in condition and relative muscle size (a direct proxy of growth) were examined in male and female adult and non-adult snow crabs Chionoecetes opilio. Condition, determined from the relative size of the digestive gland and moisture content of the muscle and digestive gland, and muscle size, determined as the ratio of merus muscle mass over merus volume, separated as different processes in a principal component analysis. Snow crabs showed a wide range of condition and muscle size values. Overall, the condition was better in non-adult than in adult crabs, with adult females being in worst condition, and muscle size was larger in males than in females. Condition variability was greater for seasonal compared to annual samples, probably reflecting annual molt cycles. In contrast, the muscle size variability was greater for annual compared to seasonal samples, possibly as a result of changing crab abundance and competition intensity during recruitment pulses. Condition and muscle size increased through summer in males and immature females, although to different extents depending on instar, but did not change in adult females. Both condition and muscle size were highly variable at the investigated spatial scales. Condition and muscle size had a significant effect on gonad size, once the effect of crab size was removed, suggesting a direct link between these two parameters and reproductive capability.     

Buoyancy in Porcellana platycheles

Intermoult Porcellana platycheles (Pennant) are slow moving, crawling crabs, incapable of swimming upwards from the substrate. Newly moulted porcelain crabs, however, are capable of effective upward swimming. An increased efficiency of tail-fan propulsion in newly moulted crabs was thought unlikely, and it was suspected that newly moulted P. platycheles were more buoyant than intermoult crabs. Results are presented to show that this is the case, and calculations were performed which indicate that fluid uptake at moult is insufficient to produce the observed increase in buoyancy. It is concluded that loss of heavy minerals and scleroprotein at moult cause the increased buoyancy. The adaptive significance of the enhanced swimming ability in newly moulted crabs is thought to lie in an improved ability to escape predation by intertidal fish and crabs.     

Endogenous swimming rhythms in estuarine crab megalopae: implications for flood-tide transport

Up-estuary migration of crab larvae to adult habitats is thought to be accomplished by selective tidal transport in which late-stage larvae enter the water column on flood tides and remain on or near the bottom on ebb tides. This study measured endogenous rhythms in swimming by the last larval stage (megalopa) of blue crabs Callinectes sapidus and fiddler crabs Uca spp. Previous field studies found that megalopae of both species were only abundant in the estuarine water column on nocturnal rising tides. Megalopae were collected from the Newport River Estuary, North Carolina (34°41N; 76°40W) during August–September 1992 and swimming activity was recorded for 4.5 to 7 d under constant conditions with a video system. Rhythms exhibited by both genera in the laboratory were not identical to those recorded in the field. Uca spp. displayed a circatidal rhythm, with maximum swimming occurring near the time of high tide in the field. Rhythm amplitude increased when crushed oyster shells were present, which suggested that megalopae bury or cling to the substrate during quiescent periods. In contrast, C. sapidus had a circadian rhythm in which maximum swimming coincided with the day phase in the field. In most trials, the activity of blue crab megalopae was unrelated to the expected tidal cycle. It was concluded that a tidal rhythm in swimming was the behavioral basis of flood-tide transport for fiddler crab larvae. The endogenous rhythm in blue crabs does not participate in transport, which probably results from behavioral responses to environmental cues associated with flood tide.     

Food and feeding habits of the king crab Paralithodes camtschatica near Kodiak Island,Alaska

Stomach contents from 809 king crabs, Paralithodes camtschatica (Tilesius), from 6 areas near Kodiak Island, Alaska, and 9 sampling periods (1978–1979) were exammed quantitatively; 713 (88%) contained food. Mollusca (mainly the bivalves Nuculana spp., Nucula tenuis, and Macoma spp.) and Crustacea (mainly barnacles) were the dominant food groups in terms of percentage wet weight and frequency of occurrence; fishes were the next most important group of prey. No significant differences in feeding between sexes occurred; however, significant differences were apparent in the quantity of food consumed from different sampling periods, areas, depths, size groups, and crab molt-classes. Consumption was greater in spring and summer and in offshore locations at depths of 126 to 150 m. In addition, king crabs <140 mm carapace length (CL) consumed more food than crabs 140 mm CL. Adult, newshell (individuals that molted during the last molting period) females greater than 95 mm CL, and newshell males greater than 100 mm CL, each contained more food than did juvenile, newshell females <120 mm CL.Contribution No. 449, Institute of Marine Science, University of Alaska, Fairbanks, Alaska 99701, USA