Associations between gastropod shell characteristics and egg production in the hermit crab Pagurus longicarpus

Summary Two populations (salt marsh and seagrass bed) of the hermit crab Pagurus longicarpus were sampled to examine associations between shell characteristics and egg production. Multivariate statistical analyses controlled for crab size and time of year, variables that otherwise could be confounded with shell effects. Although correlations between shell characteristics and reproduction existed in both populations, generalizations could not be made because associations varied within and between populations. Shell species was not associated with a female's reproductive state (i.e., whether or not she was barren when sampled) in either population. In the seagrass population, medium-large and large females occupying severely damaged or fouled shells were half as likely to be reproductive as females occupying other shell conditions. However, there was no association between shell condition and reproductive state for small and small-medium females in the seagrass population or among all females in the salt marsh population. In the seagrass population, small through medium-large reproductive females occupied shells more similar to their predicted shell size, regardless of whether the occupied shell was relatively small or large, than nonreproductive females. In contrast, relative shell sizes of reproductive and nonreproductive females were similar for large females in the seagrass population and all females in the salt marsh population. Clutch sizes were enhanced for females occupying Polinices duplicatus shells or shells larger than their predicted shell size in the seagrass population. Relative shell size also was associated with clutch size in the salt marsh population, but crabs occupying shells similar to their predicted shell size had the largest clutches.     

Avoidance of drilled gastropod shells by the hermit crab Pagurus longicarpus at Nahant, Massachusetts

Most hermit crabs depend on empty gastropod shells for shelter; competition for appropriate shells is often severe. This study determined whether shells that have been drilled by naticid gastropods are suitable for occupancy by the hermit crab Pagurus longicarpus. Differences in the characteristics of empty shells and those occupied by hermit crabs were assessed at two adjacent field sites in Nahant, Massachusetts. Drilling damage was far more frequent in empty gastropod shells than in shells occupied by hermit crabs, suggesting that individuals of P. longicarpus avoid drilled shells. They did not appear to avoid shells with other forms of damage. Laboratory experiments confirmed that these hermit crabs preferentially chose intact shells over drilled shells, even when the intact shells offered were most suitable for crabs half the weight of those tested. Final shell choices were generally made within 1 h. The hermit crabs apparently discriminated between intact and drilled shells based on tactile cues, since crabs kept in the dark showed the same preference for intact shells. The hermit crabs strongly avoided, to nearly the same extent, artificially drilled shells, naturally drilled shells, and shells with holes artificially drilled on the opposite side of the shell from where they would normally be located. Possible selective forces causing P. longicarpus to show such strong behavioral avoidance of drilled shells include increased vulnerability of crabs in drilled shells to osmotic stress, predation, and eviction by conspecifics.     

Dominance and effects of strange conspecifics on aggressive interactions in the hermit crab Pagurus longicarpus (say)

Dominance orders in the hermit crab Pagurus longicarpus were observed in the laboratory. Groups of four crabs formed loose dominance hierarchies as determined by repeated display and retreat behaviour. Stronger dominance orders were inversely correlated with the frequency of aggressive interactions. Recognition of individuals, as measured by frequency and intensity of aggressive encounters between familiar crabs and introduced strangers, was not important in maintaining dominance orders. Rather, P. longicarpus recognized the aggressive state of conspecifics, as shown by frequencies of aggressive encounters after individuals of different dominance rankings were exchanged between established hierarchies.     

New shell acquisition in the hermit crab,Pagurus geminus

The process of how the hermit crab,Pagurus geminus, acquires a new shell was investigated in the natural habitat at Ezura in Shirahama, Wakayama Prefecture, during the non-breeding season, and the following results were obtained. Hermit crabs acquired new shells most frequently by shell exchange between 2 individuals and occasionally by attacking snails. Acquisition through location of empty shells was not found. At the snail attacking site or the site of shell exchange attempts, sometimes many other individuals appeared and, frequently, confusing or complex shell changes were observed. The importance of introduction of fresh shells to a hermit crab population and the possibility for a certain individual to acquire a shell introduced by others through shell exchange attempts are discussed.     

Kinematics of walking in the hermit crab,Pagurus pollicarus

Hermit crabs are decapod crustaceans that have adapted to life in gastropod shells. Among their adaptations are modifications to their thoracic appendages or pereopods. The 4th and 5th pairs are adapted for shell support; walking is performed with the 2nd and 3rd pereopods, with an alternation of diagonal pairs. During stance, the walking legs are rotated backwards in the pitch plane. Two patterns of walking were studied to compare them with walking patterns described for other decapods, a lateral gait, similar to that in many brachyurans, and a forward gait resembling macruran walking.Video sequences of free walking and restrained animals were used to obtain leg segment positions from which joint angles were calculated. Leading legs in a lateral walk generated a power stroke by flexion of MC and PD joints; CB angles often did not change during slow walks. Trailing legs exhibited extension of MC and PD with a slight levation of CB. The two joints, B/IM and CP, are aligned at 90° angles to CB, MC and PD, moving dorso-anteriorly during swing and ventro-posteriorly during stance. A forward step was more complex; during swing the leg was rotated forward (yaw) and vertically (pitch), due to the action of TC. At the beginning of stance, TC started to rotate posteriorly and laterally, CB was depressed, and MC flexed. As stance progressed and the leg was directed laterally, PD and MC extended, so that at the end of stance the dactyl tip was quite posterior. During walks of the animal out of its shell, the legs were extended more anterior-laterally and the animal often toppled over, indicating that during walking in a shell its weight stabilized the animal.An open chain kinematic model in which each segment was approximated as a rectangular solid, the dimensions of which were derived from measurements on animals, was developed to estimate the CM of the animal under different load conditions. CM was normally quite anterior; removal of the chelipeds shifted it caudally. Application of forces simulating the weight of the shell on the 5th pereopods moved CM just anterior to the thoracic-abdominal junction. However, lateral and vertical coordinates were not altered under these different load conditions. The interaction of the shell aperture with proximal leg joints and with the CM indicates that the oblique angles of the legs, due primarily to the rotation of the TC joints, is an adaptation that confers stability during walking.     

Oogenesls in the terrestrial hermit crab,coenobita clypeatus (decapoda,anomura): II. Vitellogenesis

Oocytes from the land hermit crab, Coenobita clypeatus, in various stages of vitellogenesis were examined by light and electron microscopy. Early vitellogenic oocytes are characterized by accumulations of discrete vesicles of endoplasmic reticulum in the perinuclear cytoplasm. As oocytes develop, the endoplasmic reticulum becomes abundant, and numerous Golgi complexes are seen. There is a well developed Golgi-endoplasmic reticulum interaction. Within the confines of the reticulum are discrete intracisternal granules, which can be seen coalescing into electron-dense yolk bodies. Lipid accumulation is seen throughout the cytoplasm. Coincident with the burst of intra-oocytic metabolism are oolemma modifications and micropinocytosis, which provide ultrastructural evidence for extra-oocytic yolk production. The mature oocyte contains numerous yolk and lipid vesicles of varying electron density that comprise both intra- and extra-oocytic substrates.     

Influence of prior experience on shell selection by the white spotwrist hermit crab Pagurus criniticornis (Crustacea: Paguridae)

Individuals of Pagurus criniticornis in a free-choice situation were experimentally tested under different laboratory conditions. In order to assess the effect of recently occupied shells on the size- and type-preference by hermit crabs, individuals were held for 30 days under one of the following two conditions: (1) excess of shells and (2) absence of shells. The crabs were then allowed to select shells from a wide array of empty gastropod shells of the two most-occupied species, as observed previously in the field: Cerithium atratum and Morula nodulosa. Preferred shell type (species) and size (shell aperture width and length) were correlated with hermit-crab size. The crabs showed a strong (100%) preference for C. atratum shells, demonstrating that recent and past experience did not influence either shell-type or shell-size preferences in this pagurid. Handling editor: K. Martens     

Spermatophore transfer in the hermit crab Clibanarius vittatus (Crustacea,Anomura, Diogenidae)

Although mating has been described in several hermit crab species, the mechanics of spermatophore transfer have not previously been demonstrated. Evidence from pleopod and gonopore morphology, video observations, and inseminated females indicates that in Clibanarius vittatus the male applies a spermatophoric mass directly onto the female via the gonopores rather than with modified pleopods 1-2 (gonopods) and/or genital papillae as in many other decapods. The single second pleopod of males of C. vittatus has a simple endopod with no apparent modifications for sperm transfer. There are no genital papillae extending from the male gonopores. The globular spermatophores are aligned in rows surrounded by a seminal secretion in the male ducts (vasa deferentia that terminate in ejaculatory ducts opening to the exterior via the gonopores). During copulation, described from time-lapse video recordings, the ventral surface of the last thoracic segment of the male, bearing the gonopores, was apposed to the ventral cephalothorax of the female. A massive amount of seminal secretion containing spermatophore ribbons, termed here the spermatophoric mass and described for the first time in a hermit crab species, was observed covering the sternites and coxae of pereopods 1-5 of a recently copulated female. It is suggested that during copulation the male emits the contents of the ejaculatory ducts directly onto the female without the aid of gonopods or genital papillae. Although spermatophore transfer is simple in C. vittatus, the presence of modified anterior pleopods or elongate genital papillae (sexual tubes) in other paguroidean species suggests the possibility of a more complex insemination process in these other hermit crabs.     

Oogenesis in the terrestrial hermit crab Coenobita clypeatus (decapoda,anomura). I. Previtellogenic oocytes

Ultrastructural study of previtellogenic oocytes found in cystlike clusters scattered throughout the length of the bilobed ovary of the hermit crab Coenobita clypeatus shows a high nuclear:cytoplasm ratio. Large, round nuclei containing synaptinemal complexes serve as good temporal markers for identification of previtellogenic oocytes. The cytoplasm contains many smooth-membraned vesicles filled with granules and probably of nuclear origin. In addition to its complement of Golgi complexes, endoplasmic reticulum, mitochondria, and free ribosomes, the cytoplasm also contains stacks of annulate lamellae, a feature not previously described for decapod oocytes. Typically, the previtellogenic oocyte with its accumulation of ribosomes has the appearance of a nonsynthetic cell preparing to go through a metabolic transition.     

Morphological and physiological properties of the giant interneuron of the hermit crab (Pagurus pollicaris)

The physiological and morphological properties of the giant interneurons in the hermit crab Pagurus pollicaris are described. The cell bodies are located anteriorly in the supraesophageal ganglion, close to the mid-line. Each cell sends a neurite posteriorly and then laterally, so that they cross over in the center of the ganglion. Each axon then branches: one branch runs laterally while the other travels posteriorly and leaves the ganglion in the circumesophageal connective on the side contralateral to the cell body. The giant axons travel in the circumesophageal connectives and through the thoracic and abdominal ganglia without branching. Each giant axon makes synaptic contact with its ipsilateral giant abdominal flexor motor neuron and with a second flexor motor neuron that has its axon in the contralateral third root. In the supraesophageal ganglion there is a bidirectional synapse between the two giant interneurons. Intracellular recordings from the giant axons show that there is a delay of 0.5 to 0.75 ms that cannot be accounted for by spike propagation along the axons, and may be accounted for by a chemical synapse between the giant interneurons.     

Peripheral and central asymmetry in the swimmeret system of the hermit crab,Pagurus pollicarus

Summary The consequences of the degeneration of all pleopods on the right side in the adult hermit crab were examined by comparing the motor and sensory components of the first root on both sides of the abdomen. In the third segment, the populations of sensory receptors remain relatively unaffected, with the number of units recorded extracellularly, their position and mode of activation, and their conduction velocities similar on the two sides. In contrast, only 2 or 3 somata in the ganglion are back-filled with cobalt via the right first root, compared to approximately 20 filled through the left root. While no remnant of the pleopod musculature remains on the right side, extracellular action potentials propagating centrifugally could be elicited in the right first root. The central cytoarchitecture of the first root neurons, as revealed by cobalt staining, was also studied in 3 other macruran species. The apparent phylogenetic variability revealed in comparing the swimmeret systems of the 4 related decapods is discussed in terms of different probabilistic strategies for crustacean neuromuscular ontogeny.     

Indirect female choice mediated by sex pheromones in the hermit crab Pagurus filholi

Males of the hermit crab Pagurus filholi perform precopulatory guarding behavior, and solitary males often show aggressive behavior to take away guarded females. Males behave coercively while guarding females, so direct mate choice by females seems difficult in such a situation. By performing several experiments we examined possible indirect female choice of hermit crab. Males were attached to a shell by their left cheliped to look like guarding pairs (fake guarding pairs). The shells were filled with cotton containing either seawater or pheromone water. The fake guarding pair with only seawater caused male–male combat in 60% of trials whereas with pheromone water combats occurred in 88% of trials. Mean duration of male–male combat was significantly longer in trials with drops of seawater containing pheromones than in those without pheromones. These results suggest guarding pairs themselves cause male–male combat by visual stimulation, that female sex pheromones have further significant function in the recognition of guarding pairs and intensification of male–male combat, and that females release sex pheromones while they are guarded. As a result of the combat, the larger male ended up guarding a female. This strongly suggests that females choose males indirectly by exploiting male–male competition induced by sex pheromones under male coercive behavior.     

Antipredator defence of the hermit crab Pagurus filholi induced by predatory crabs

Hermit crabs have two antipredator tactics: taking refuge in its shell and fleeing. We examined the following two hypotheses using the hermit crab Pagurus filholi : (1) hermit crabs change their preference for shell types that they take refuge in and/or change the timing of fleeing (i.e. the duration of refuge in the shell) when they perceive a predator threat; (2) the type of shell that a hermit crab occupies affects the fleeing tactic of the individual. Under the stimulus of a crushed conspecific, hermit crabs changed neither their preference for shell species nor their refuge duration. On the other hand, under the stimulus of the predatory crab Gaetice depressus , hermit crabs increased their preference for Batillaria cumingi shells, which provide superior protection against predators, and shortened their refuge duration in the shells even when they occupied those effective against predation. Refuge duration was longer in B. cumingi shells than in the more vulnerable shells of Homalopoma sangarense . These results suggest that both antipredator defences (changing shell and timing of fleeing) are induced by the stimulus of a predator, and the timing of fleeing is affected by the shell type occupied.     

Patterns of activity in the antennular motoneurones of the hermit crab Pagurus alaskensis (Benedict).

1. Using electromyogram recordings from the antennular muscles of intact animals and recordings from the antennular nerves of partially dissected preparations, the patterns of activity in specific antennular motoneurones have been described during antennular flicking and antennular withdrawal. 2. The slow extensor motoneurone A30S is active during flicking in addition to the phasic component of the antennular motor system (A30F, A31F and A32F). 3. The flexion phase of a flick is the result of a burst of variable duration and number of spikes within flexor motoneurones A31F and A32F. 4. The extension phase of a flick is the result of a burst of variable duration and number of spikes in extensor motoneurones A30F and A30S. 5. Extension-withdrawal and slow flexion-withdrawal reflexes, tonic flexion withdrawal and maintained flexion at the MS-DS joint usually result from activity in part of the tonic component of the antennular motor system:moto-neurones A30S, A31S and A32S. 6. Fast flexion-withdrawal reflexes result from a burst of spikes in motoneurone A31F-S which constitutes the phaso-tonic component of the antennular motor system. 7. During high-frequency activity (15-60/sec), reciprocity exists between the slow flexor motoneurones A31S and A32S and slow extensor motoneurone A30S.     

Analysis of repeated signals during shell fights in the hermit crab Pagurus bernhardus

Shell exchanges between hermit crabs may occur after a period of shell rapping, when the initiating or attacking crab brings its shell rapidly and repeatedly into contact with the shell of the non-initiator or defender, in a series of bouts. There are two opposing models of hermit crab shell exchange and the function of shell rapping. The negotiation model views shell exchange as a mutualistic activity, in which the initiator supplies information about the quality of its shell via the fundamental frequency of the rapping sound. The aggression model views shell rapping as either detrimental to the defending crab, or as providing it with information about the initiator''s ability or motivation to continue, or both. The negotiation model makes no predictions about the temporal pattern of rapping, but under the aggression model it would be expected that crabs that rapped more vigorously would be more likely to effect an exchange. Repeating the signal could be expected under either model. Crabs that achieve an exchange rap more vigorously, rapping is more persistent when a clear gain in shell quality may be achieved, and the vigour is greater when the relative resource-holding potential (or ''fighting ability'') is high. These findings support the aggression model rather than the negotiation model. Contrary to the predictions of game theory, crabs that do not effect an exchange appear to signal that they are about to give up. The data suggest that rapping is performed repeatedly because the accumulation of all of the performances acts as a signal of stamina.     

Intraspecific competition and aggression for shells in the hermit crab Pagurus samuel1s

Many studies have investigated shell‐related behaviour in hermit crabs. Few studies, however, have focused specifically on the intraspecies aggression associated with shell competition. We examined intraspecies aggression in hermit crab (Pagurus samuelis) pairs as it relates to competition for a limiting resource, gastropod shells. Pairs of hermit crabs were observed in the laboratory in four different treatments that varied the presence or absence of shells for one or both of the crabs. Measurements of the latency to respond, the number of bouts, and the fight durations were recorded. There was a significant difference among treatments for all three measurements, and naked hermit crabs were much more aggressive than housed hermit crabs. There was no significant difference in aggression between males and females in any of the three treatments. The heightened aggression observed in naked P. samuelis is likely in service of acquiring a protective shell.