Search

Activity and clustering in two intertidal species of hermit crabs, Clibanarius virescens and Calcinus laevimanus, were studied during a semilunar tide cycle in Somalia. Three groups were marked and examined throughout 16 low waters, and 12 samples of clustered and scattered crabs were collected from similar habitats. During high water, the animals took refuge in mass in holes of the cliff; they began to reappear (especially at night) and feed during ebb-tide (feeding phase). When exposed to the air, the crabs entered a resting phase with some aggregating in clusters, then when the tide rose became active again before going back to their refuges. Clusters composed of up to 95 quiescent individuals (which were even more crowded during nocturnal and spring low tides) occurred at every low tide and at approximately the same place. Size and sex composition of clumps differed from scattered samples, suggesting that the clustering habit is exclusive to crabs which either need to frequently change their shells (due to their faster growth rate) or are less efficient in the competition to acquire such resources.

Sesarma leptosomaan East African mangrove‐dwelling crab, migrates twice a day from a system of known dens among the roots to well‐defined feeding areas in the branches of trees, reaching 15 m high. Field experiments were performed to test whether chemical or visual cues are involved in the orientation and homing of this species to reach their feeding areas. Manipulation of the substratum at branch junctions, in order to alter possible chemical cues, did not affect homing ability in S. leptosoma.Moreover, crabs trained to cross an asymmetrical artificial wooden fork could still follow their preferred directions after (1) the fork branches had been switched, (2) the whole fork had been rotated around the trunk, resulting in a right‐left inversion, and (3) the inversion of two wide black and white screens hiding most of the canopy from view of the climbing crabs. These results suggest that S. leptosomamay not rely on reference systems such as chemical trail‐following and chemical or visual cues from the substratum, but probably depend on complex visual information from the surroundings trunks and/or from the sun's position integrated with junction sequence memory.

Some dynamic patterns of clustering in the mangrove-dwelling hermit crab Clibanarius laevimanus were studied during a semi-lunar tide cycle in Kenya. Clusters composed of hundreds of quiescent individuals occurred at every low tide, developing either on the mangrove prop roots or in the open (but always within a 4-m belt from the mangrove fringe). They disbanded at ebb tide, when most animals migrated in lines towards feeding grounds. There they grazed on vegetable debris during high water. When the tide receded, they came back home in crowded groups. The position of aggregating sites remained constant throughout the tides, as well as their number and size, and the shape of clusters was stable with time. Individual hermits often changed clusters (only rarely they were found scattered), but transfers only occurred between aggregations where shell size range matched the shell worn by the moving crab. Dispersal was slower than predicted by random walk models. Questions were aimed on the proximate and ultimate factors underlying this spatial strategy which contrasts the nomadic behaviour shown by the sympatric C. longitarsus.

The spatial strategy of the hermit crab Clibanarius longitarsus was studied during a semilunar tidal cycle in a mangrove swamp in Kenya. This species is isospatial, being active at HW and taking refuge during its wrong phase (air) within the mangrove roots at LW. However, as revealed by radio-telemetry, these animals (as well as other decapods) may miss one or more activity cycles. During their displacements, they followed a straight course parallel to the coast-line but limited within a narrow and constant channel from the mangrove edge to the sea. Displacement was significantly higher at spring tides (with a maximum of 450 m), but there was no difference in extent between nocturnal and diurnal tides. Animals maintained the same locomotory sense during single HWs, and over subsequent HWs. These movement patterns were fast dispersal along the x-axis, as revealed by both a low recovery rate of tagged animals and the trajectories followed by 12 radio-tracked hermits. This nomadic behaviour may be explained as a means of intercepting spatially unforeseeable resources. Empty shells are a limiting factor in this habitat, and to periodically obtain new gastropod shells is fundamental for hermits (due to the animals' growth or deterioration of old shells). Acquiring them from dead snails or conspecifics may have placed a fitness premium on this expensive spatial strategy.

The activity rhythms of the swimming Portunid Thalamita crenata have been studied on the basis of crab sightings in surveys along three transects of a mangrove swamp in Kenya. Activity is well matched to the environmental regime and is prevalent during the high tides. However, activity does not occur over the whole of the high tide, but is confined to “temporal windows”; corresponding to a water level between 10 and 30 cm. Our findings suggest that activity in Thalamita crenata is shaped by the tidal cycles and we propose that variation in hydrostatic pressure as the tide changes is the cue which elicits activity.

Larvae of Euscorpius carpathicus settled on conspecific adults can distinguish between males and females on a chemical basis, with females being preferred to males. The preference for different females depends on their reproductive phase, with mothers being the most preferred. E. carpathicus females offer a better chance of survival against dehydration when mothers than before parturition, and in both cases a better one than males. This proves that the water passage is an effective specialized mechanism which appears periodically and contributes (to what extent is not known) to improving larval survival, especially in dry periods and/or environments.

Abstract: Locomotor activity in the freshwater crab, Potamon fluviatile, was studied both in the field, by direct counts of animals along a stream section, and by means of actographs. General metabolism was evaluated through the analysis of oxygen consumption. Sex, size, and environmental factors proved to influence both the intensity and the timing of locomotion. In the laboratory, the two sexes behaved in the same way and consumed oxygen to the same extent, while the higher females' activity recorded in the field was due to their reproductive state. Younger age classes were more cryptic in the field and also less active in the laboratory, even though they had a higher weight-relative oxygen consumption. A rise in temperature, at least under the range examined in the present study, increased the crabs' activity rate, and also caused a change in their rhythmicity pattern, with a passage from diurnalism to nocturnalism. A lowering in relative humidity increased locomotion intensity, as a hygrokinetic response.