Calcinus

Abstract: During monthly intervals over a 1—year period, 12,000 empty snail shells were added to a small, isolated, rocky intertidal reef in the San Juan Islands of Washington. The shells added were species normally used by the high intertidal hermit crab, Pagurus hirsutiusculus, and were placed in locations accessible to that species. The shell additions resulted in an increase in density of P. hirsutiusculus at the experimental reef, whereas no density change occurred at a nearby control reef, indicating the importance of shells as a limiting resource. To establish the generality of shell limitation, the hermit crab populations of four unmolested rocky intertidal sites (three of which are typical hermit crab habitats) were quantitatively samples to obtain species compositions and size distributions of hermit crabs, their shells, and unoccupied shells. Shell preference experiments determined the preferred shell sizes and species for each hermit crab species. Except for small size classes, empty shells were rate at the three typical areas. In addition, hermit crab size distributions followed shell size distributions, and all but small hermit crabs of three species occupied shells smaller than the preferred size. These results support the conclusion that empty shells are a limiting resource for these hermit crabs. Since shells constitute a common, necessary resource in short supply, these hermit crabs are in competition for available shells. The fourth area, chosen for its unusual shell—availability characteristics, exhibited a different pattern of shell utilization not suggesting shell limitation. Shell occupancy at the three representative intertidal sites was examined to determine the strength of the relationship between hermit crab species composition and resource availability. Though resource partitioning was demonstrated, the presence and numbers of each hermit crab species and its preferred shell types were poorly correlated. Differences in hermit crab species composition are explained by differences in the physical habitat, and collections from other areas show that the same shell species can support different hermit crab species in different but adjacent habitat types. Thus, the mechanism allowing coexistence apparently involves both resource and habitat partitioning.

Abstract: Field studies have suggested that the intertidal hermit crabs of the San Juan Islands of Washington normally occupy snail shells smaller than preferred. In this study the effects of shell size on protection from predation and on hermit crab shell fighting were studied in the laboratory. A predator (Cancer) presented with two hermit crabs (Pagurus granosimanus), identical except in size of occupied shell, preyed upon the hermit in the smaller shell first in 15 out of 16 trials. This results suggests that large shell size confers a selective advantage on the occupying crab. Shell fights involving two hermit crabs (P. hirsutiusculus) of unequal size were observed in which replicates differed only in the shell size of the larger crab. The probability of the larger crab effecting a shell exchange through fighting was shown to increase as the size of its shell decreased. However, shell size was shown to have no effect on the level of aggressiveness as measured by four criteria. The mechanism underlying the former result thus appears to involve a continual high level of general aggressiveness together with an increased tendency associated with occupancy of an inadequate shell by the dominant crab for that crab to evoke a shell exchange during an aggressive interaction.

Abstract: The nearshore hermit crabs Clibanarius vittatus, Pagurus pollicaris, and Pagurus longicarpus have broadly overlapping shell utilization patterns along the Texas coast. Effects of shell stress resulting from this overlap and from an overall shortage of shells on the re- productive potentials of the crabs were examined. Regression analyses indicate that shell weight and internal volume affect the clutch sizes of C. vittatus and P. pollicaris but not P. longicarpus. Clibanarius vittatus maintained in shells smaller than preferred grew more slowly than crabs maintained in shells of preferred size. Clutch size was highly correlated with crab size in this species. Several commensal animals, including polychaetes, gastropods, a hydroid, and a xanthid crab, consumed hermit crab eggs and/or zoeae in the laboratory. These egg predators were encountered in large shells occupied by e e more frequently than in shells in the size range utilized by ovigerous Y Y. These results indicate that shell size may limit hermit crab clutch size through a variety of mechanisms.

Abstract: 1. The social behavior patterns of twelve species of hermit crabs found in the waters around Curacao, N.A. are described. All species showed marked similarity in their aggressive displays, the most common of which are movements of the appendages, called here the ambulatory raise and cheliped extension. 2. Model presentation experiments proved that these positions are effective visual stimuli. These tests also showed that the white tips of the ambulatories of Clibanarius tricolor are aggressive stimuli. 3. Pagurid crabs showed a dislodging-shaking behavior pattern when crawled upon by other individuals. Experiments were carried out to determine the relationship between stimulus weight and the size of a crab showing this pattern. 4. Measurement of laboratory and field distributions indicated that some species are truly gregarious (Clibanarius tricolor, Pagurus miamensis, Pagurus bonairensis), while other species are contagiously distributed due to orientation to certain physical factors in their environment (Calcinus tibicen). 5. Laboratory and field tests showed that individuals of Clibanarius tricolor form relatively stable groups. The groups are formed and/or maintained through orientation to a “grouping pheromone”. These groups are formed daily after the crabs have been dispersed over the nightly feeding area. At night, individuals of Clibanarius tricolor oriented chemically toward a detritus-covered rock, their normal food source. Groups of Pagurus miamensis also oriented chemically to a group of conspecific individuals established on a rock. Individuals of both species oriented toward a conspecific group only during the day. 6. Diel cycle measurements were carried out for most species. The most common pattern was a nocturnal, crepuscular-peaked cycle, although Paguristes species showed an anti-crepuscular pattern. 7. Individuals of all species fought one another for gastropod shells. With the possible exception of the genus Paguristes, the direct application of force did not play a part in these shell-fights. The signals exchanged by an interacting pair were very different in the two families; in the Paguridae, the attacker shakes the defending crab back and forth rapidly by a movement of its ambulatory legs while the diogenid aggressor strikes the defender’s shell with his own by means of abdominal muscles. Measurements of shell-fighting pairs of Clibanarius tricolor indicated that smaller individuals very rarely win over larger crabs, females have a slight advantage in shell-fights and that recently moulted crabs are both more likely to be attacked and more likely to lose when attacked. 8. The sexual behavior of most species was observed and described. The precopulatory acts of the male are similar within the Families; diogenid males mainly rotate the female around an axis through the plane of her shell aperture, while pagurids jerk the female toward the male by movements of one of their chelipeds which grasps a female ambulatory leg. The normal larval releasemoult-copulate sequence was observed in most species, although pairs of Pagurus bonairensis consistently copulated while the female still had a complement of well-developed eggs. 9. A preliminary investigation indicated the presence of social order in groups of Clibanarius tricolor, Calcinus tibicen and Pagurus miamensis, but the basis for this order was uninvestigated.