Environmental and endogenous control of selective tidal-stream transport behavior during blue crab <Emphasis Type="Italic">Callinectes sapidus</Emphasis> spawning migrations

Selective tidal-stream transport (STST) is used by many estuarine organisms. Spawning blue crabs use a form of STST, ebb-tide transport (ETT), to migrate to high-salinity areas of the lower estuary and coastal ocean for larval release. In tidal estuaries, ETT is driven by a circatidal rhythm in vertical swimming with episodic ascents into the water column during ebb tide. This study examined vertical swimming behavior of migrating female blue crabs tethered in habitats they could encounter during migration. A combined bio-physical field study in the summer of 2009 simultaneously measured physical parameters of the water column and vertical swimming behavior of tethered ovigerous crabs using pressure-recording dataloggers. Tethering sites were in the tidal Beaufort Inlet drainage and the non-tidal Albemarle-Pamlico Estuarine System, North Carolina, USA. Crabs tethered in tidal areas swam primarily during ebb tides, both day and night. Swimming frequency increased as embryonic development progressed and ebb-tide swimming continued after larval release. Swimming frequency varied among habitats with the highest swimming frequency in the known migratory corridor. Swimming did not occur in the non-tidal habitat. Differences in swimming frequency among sites are hypothesized to be responses to environmental cues, including flow regime. Some habitats serve as migratory corridors while others serve as foraging stopovers. These areas are likely defined by a combination of environmental cues including flow regime.     

Circatidal activity rhythms in ovigerous blue crabs, <Emphasis Type="Italic">Callinectes</Emphasis><Emphasis Type="Italic">sapidus</Emphasis>: implications for ebb-tide transport during the spawning migration

Female blue crabs (Callinectes sapidus Rathbun) with mature embryos have a spawning migration in which they: (1) undergo ebb-tide transport for movement seaward from estuaries, (2) release their larvae, and (3) reverse direction by undergoing flood-tide transport for up-estuary movement. The study determined whether ebb-tide transport during the spawning migration is based upon an endogenous rhythm in vertical migration. Under constant conditions in a rectangular container, which limited horizontal and vertical movements, females with young and mature embryos had circatidal rhythms (periods=12.11-12.95 h) in migratory restlessness (swimming activity) and egg maintenance behavior (abdominal pumping). However, the rhythms were out of phase, as migratory restlessness occurred during the expected time of ebb tide in the field, and egg maintenance behavior, during the time of flood tide. Under constant conditions in vertical columns (1.32 m high), crabs with mature embryos had a circatidal rhythm (periods=12.2-13.7 h) in which they had frequent bouts of swimming to the surface of the column during the expected time of ebb tide in the field and remained on the bottom during the time of flood tide. This rhythm was not present in crabs with young embryos and disappeared after larval release. Thus, an endogenous rhythm in vertical migration does underlie the ebb-tide transport behavior of ovigerous blue crabs with mature embryos during their spawning migration.     

Circatidal swimming behavior of brachyuran crab zoea larvae: implications for ebb-tide transport

Larvae of the blue crab Callinectes sapidus and fiddler crab Uca pugilator are exported from estuaries and develop on the continental shelf. Previous studies have shown that the zoea-1 larvae of some crab species use selective tidal-stream transport (STST) to migrate from estuaries to coastal areas. The STST behavior of newly hatched larvae is characterized by upward vertical migration during ebb tide followed by a descent toward the bottom during flood. The objectives of the study were (1) to determine if newly hatched zoeae of U. pugilator and C. sapidus possess endogenous tidal rhythms in vertical migration that could underlie STST, (2) to determine if the rhythms persist in the absence of estuarine chemical cues, and (3) to characterize the photoresponses of zoeae to assess the impact of light on swimming behavior and vertical distribution. Ovigerous crabs with late-stage embryos were collected from June to August 2002 and maintained under constant laboratory conditions. Following hatching, swimming activity of zoeae was monitored in darkness for 72 h. U. pugilator zoeae displayed a circatidal rhythm in swimming with peaks in activity occurring near the expected times of ebb currents in the field. Conversely, C. sapidus zoeae exhibited no clear rhythmic migration patterns. When placed in a light field that simulated the underwater angular light distribution, C. sapidus larvae displayed a weak positive phototaxis at the highest light levels tested, while U. pugilator zoeae were unresponsive. Swimming behaviors and photoresponses of both species were not significantly influenced by the presence of chemical cues associated with offshore or estuarine water. These results are consistent with predictions based on species-specific differences in spawning and the proximity of hatching areas to the mouths of estuaries. U. pugilator larvae are released within estuaries near the adult habitat. Thus, ebb-phased STST behavior by zoeae is adaptive since it enhances export. Selective pressures for a tidal migration in C. sapidus larvae are likely weaker than for U. pugilator since ovigerous females migrate seaward prior to spawning and hatching occurs near inlets and in coastal waters.     

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.     

Biological and physical aspects of migration in the estuarine amphipod Gammarus zaddachi

The amphipod Gammarus zaddachi (Sexton) conducts extensive migrations along estuaries from near the limit of tidal influence in winter to more downstream reaches (where reproduction occurs) in spring. A return migration then takes place, primarily by juveniles, until the seaward areas are depopulated in winter. The present study was conducted between 1988 and 1990 in the Conwy Estuary, North Wales. This represents the first investigation on this species in a strongly tidal estuary, where the amphipods appear to migrate vertically into the water column on flood or ebb tides to control horizontal transport and to maintain preferred distributions. The timing of vertical migration seems to be largely controlled by an endogenous circatidal swimming rhythm. Phasing of peak activity relative to the time of expected high tide varies with season; upstream migrants in the autumn showed peak activity at the time of expected high tide, while in the spring at the time of downstream migration the rhythm was phase-delayed, with peak activity during the expected ebb tide. Together with the season, position along the estuary also affected the timing of peak endogenous activity; downstream migrants, originally active on the ebb tide and experimentally displaced seawards, showed a phase-advance of the rhythm relative to the time of high tide. Salinity-preference behaviour also varied between different developmental stages, with ovigerous females (downstream migrants) showing no preference between fresh and saline water, and juveniles (upstream migrants) showing a significant preference for freshwater. The interactions of endogenous rhythmicity and salinity-preference behaviour are discussed as controlling factors of migration in this species.     

Flood tide transport of blue crab, Callinectes sapidus, postlarvae: behavioral responses to salinity and turbulence

Blue crab (Callinectes sapidus) postlarvae (megalopae) use flood tide transport to move upstream in estuaries during nocturnal flood tides. The megalopae have a endogenous diel rhythm in activity that is inconsistent with this tidally timed behavior. Thus, it is hypothesized that this behavior is regulated by behavioral changes in response to exogenous cues associated with tidal currents. In a laboratory flow tank, blue crab megalopae were exposed to simultaneous changes in salinity and turbulence to simulate tides in an estuary. On simulated flood tides, megalopae ascended upon exposure to a salinity increase, remained swimming during times of high turbulence, and descended at times of low turbulence. Turbulence stimulated swimming for several hours, approximating the duration of tidal currents in estuaries. Swimming was inhibited by decreasing salinity on simulated ebb tides. These results support a model for regulation of flood tide transport by blue crab megalopae as follows: (1) blue crab megalopae are stimulated to swim into the water column by increasing salinity associated with flood tide; (2) megalopae remain swimming during flood tide in response to high levels of turbulence; (3) megalopae descend at the end of flood tide, when current speed and turbulence decline to low levels; and (4) megalopae are inhibited from swimming on ebb tides by the associated salinity decrease. This is the first model for regulation of flood tide transport in a species lacking a tidal rhythm in activity.     

Endogenous swimming rhythms of blue crab, Callinectes sapidus , megalopae: effects of offshore and estuarine cues

Larvae of the blue crab Callinectes sapidus Rathbun develop on the continental shelf. The postlarval stage (megalopa) occurs near the surface and is transported shoreward by wind-driven surface currents. It then uses selective tidal stream transport for migration up an estuary. Endogenous swimming rhythms were measured under constant dark conditions in the laboratory in megalopae collected from the Newport River Estuary (North Carolina), the Delaware Bay, and offshore from the Newport River Estuary. Megalopae from all areas had a similar circadian activity rhythm, in which they swam during the time of the day phase in the field and were inactive at night. This rhythm predicts the presence of a reverse, diel, vertical-migration pattern offshore which would contribute to the location of megalopae near the surface during the day. The rhythm lacks obvious ecological significance in estuaries because it does not contribute to selective tidal stream transport and would increase vulnerability to visual predators during the day. Attempts to entrain a circatidal rhythm in swimming by cyclic and step changes in salinity were unsuccessful, as the circadian rhythm persisted. The rhythm also continued in the presence of the eelgrass Zostera marina, which is a site of settlement and metamorphosis in the field. Thus, megalopae enter estuaries with a solar day rhythm in activity. This rhythm, however, is not expressed, because light inhibits swimming during the day upon exposure to estuarine water. Since this light inhibition is removed in offshore waters, the rhythm would be expressed if, after entering an estuary, megalopae were transported back to offshore areas. Received: 19 December 1995 / Accepted: 2 August 1996     

Rhythms in larval release of the sublittoral crab Neopanope sayi and the supralittoral crab Sesarma cinereum (Decapoda: Brachyura)

Ovigerous females of the subtidal xanthid crab Neopanope sayi (Smith) and the high intertidal grapsid crab Sesarma cinereum (Bosc) were collected during the summers of 1986 and 1987 in the Beaufort, North Carolina (USA), area and brought into the laboratory, where rhythms in larval release were monitored. When crabs with late-stage embryos were put under a 14 h light:10 h dark cycle in an otherwise constant-environment room, an apparent tidal rhythm in release of larvae was observed for both species, with N. sayi releasing near the time of day and night high tides, and S. cinereum releasing around the time of night high tides. The time of sunset relative to high tide was a complicating factor, since larval release for both species was often concentrated around sunset when evening high tides fell several hours before sunset. When a group of N. sayi and S. cinereum were brought into the laboratory and placed under constant lowlevel light for 5 d, the release rhythm of the population persisted, thus implying that the rhythm is endogenous. Larval release near the time of high tide and often at night is common among brachyurans living in tidal areas, regardless of specific adult habitat, suggesting a common functional advantage. Possibilities include transport of larvae from areas where predation and the likelihood of stranding and exposure to low-salinity waters are high, as well as a reduced probability of predation on adult females. Results of the present study suggest that the importance of release after darkness may increase with increasing tidal height of the adult.     

Relationship of daily and circatidal activity rhythms of the fiddler crab,<Emphasis Type="Italic"> Uca princeps</Emphasis>, to the harmonic structure of semidiurnal and mixed tides

Intertidal organisms may employ circatidal rhythms to track the tidal cycle, but tidal patterns may vary within a species range and necessitate adaptation to the local tides. Circatidal rhythms were examined in populations of the eastern Pacific fiddler crab Uca princeps (Smith) from four sites with differing tidal characteristics, La Paz (24°10N; 110°21W), San Blas (21°33N; 105°18W) and Manzanillo (19°6N; 104°24W), Mexico (lower amplitude, mixed semidiurnal tides) and Mata de Limon, Costa Rica (9°55N; 84°43W) (high-amplitude, semidiurnal tides). Local tides were characterized by harmonic constants of M₂, S₂, K₁, and O₁, partial tides that largely determine their semidiurnal and diurnal features. Rhythmic structure in continuously recorded locomotor activity of individual crabs held under laboratory conditions was described by cosinor and periodogram methods of time-series analysis. Both daily and circatidal rhythms were found in crabs studied in light–dark cycles set to local conditions at the time of collection. Crabs at all four sites shared a tendency toward bimodality, with a mid-morning activity peak and varying degrees of nocturnal activity. Circatidal rhythms closely matching the period of the 12.42-h M₂ partial tide were consistently present at all sites except Manzanillo. At Mata de Limon, the circatidal rhythm clearly dominated locomotor activity, but was strongly modulated by a daily rhythm in a repeating pattern at a semilunar interval. In contrast, the amplitude of the daily rhythm was higher than that of the circatidal rhythm in crabs from the three mixed tide sites on the Mexican coast, where the tidal pattern is dominated by a diurnal inequality arising from the diurnal K₁ and O₁ partial tides. These results suggest that populations of U. princeps use both daily and circatidal timing systems to track local forms of the tide generated by their M₂, S₂, K₁, and O₁ geophysical counterparts.Communicated by J.P. Grassle, New Brunswick     

Evolutionary significance of prenuptial molting in female <Emphasis Type="Italic">Pagurus</Emphasis> hermit crabs

Molting and breeding entail major energetic costs for female crustaceans. However, females of some hermit crabs perform a molt immediately prior to copulation (prenuptial molt). The evolutionary significance of the prenuptial molt was examined in Pagurus hermit crabs, and two hypotheses were tested: (1) prenuptial molt might enhance the success of the present clutch by cleaning the pleopods of females and thereby preventing eggs from being dislodged from the pleopods, and (2) prenuptial molt might function for growth and increase future fecundity at the cost of energetic expenditure on the present brood. Although these hypotheses are not mutually exclusive, our results rejected the former hypothesis and supported the latter hypothesis. All four Pagurus species examined showed significant negative relationships between prenuptial molting and continuity of breeding; i.e., they showed high molting frequency after they had a long rest period from breeding. Females of P. minutus increased their size through the prenuptial molt, and showed a decreased clutch size due to the molt. The number of dislodged eggs increased if females molted in P. minutus. These results suggest that hermit crabs undergoing a prenuptial molt might not gain any clear immediate advantage of enhanced survival of eggs in the present clutch, and that the prenuptial molt would mainly contribute to growth.     

Biochemical composition of the deep-sea red crab Chaceon quinquedens (Geryonidae): organic reserves of developing embryos and adults

Deep-sea red crabs Chaceon quinquedens (Smith) were collected in traps at depths of 860 and 1043 m in the northern Gulf of Mexico. Ovigerous crabs were maintained in the laboratory and the developing embryos were sampled every 2 wk until hatching. Proximate analysis (lipid, protein, carbohydrate, and ash) of embryos was performed to determine patterns and rates of organic reserve utilization during embryogenesis. Midgut gland, gonads, and clutch (as appropriate) of adult crabs (males, non-ovigerous females and ovigerous females) were analyzed for the same components as the embryos. Red crab embryos exhibited different patterns of yolk deposition and subsequent depletion of yolk components during embryogenesis. There was a range of lipid to protein (L:P) ratios among the different clutches examined, indicating plasticity in the relative proportions of lipid and protein yolk. The energy used for embryogenesis was estimated by converting the amounts of lipid, protein and carbohydrate in the embryos to their caloric equivalents; final values, taken from 9 mo-old embryos whose siblings were hatching as zoeae, were subtracted from the initial values of sibling embryos sampled at the time of collection (2 to 3 mo old). The amount of energy consumed during embryogenesis in the laboratory was relatively constant (0.12 to 0.13 cal egg^-1). There was considerable variability among the concentrations of organic reserves in the midgut gland of adult crabs and in the ovaries of females. Variations in midgut gland L:P ratios and ovaries were related to the reproductive status of the females, but there were no trends related to depth of capture.     

Circatidal rhythm of an intertidal crab, Hemigrapsus sanguineus : synchrony with unequal tide height and involvement of a light-response mechanism

The circatidal rhythm of intertidal animals may reflect the inequality of the tides. In addition, a light-sensitive mechanism may be involved in their internal timing systems. To test these hypotheses, the larval release activity of the intertidal crab Hemigrapsus sanguineus was monitored under different light conditions in the laboratory. Under a 24-h light–dark (LD) cycle with the phase similar to the field, the activity coincided with the times of high tide in the field and showed a tidal rhythm. This rhythm free-ran in constant, dim-light conditions, suggesting that the timing is controlled by an endogenous clock. When the population was exposed to a 24-h LD cycle with the phase changed from that in the field, the tidal rhythm was phase-shifted; while the light cycle advanced in phase from the field caused a phase-advance of the rhythm, that delayed in phase induced a phase-delay of the rhythm. Thus, a light-response mechanism is definitely involved in circatidal timing systems. But the population rhythm showed a large variability among individuals, associated with the phase-shift, and the magnitude of the phase-shift did not accurately correspond to that of the light cycle. These results suggest that the light-response system can control the phase of the rhythm less stronger than that in estuarine crabs. Most releases occurred at higher high tides, but the release of some females obviously occurred at lower high tides. The larval release pattern thus could not be accounted for by a simple synchrony with higher high tides. Hatching of H. sanguineus occurred after a “hatching program” of 49.5 to 52.5 h. This program is initiated by some factor (hatching-program inducing stumuli: HPIS) transmitted from the female to the embryos. We speculated that this factor is effectively transmitted to the embryos when the habitat is exposed to air, i.e., at lower low-tide periods, and that once each embryo is stimulated, hatching occurs synchronously 2 d later during high tide. The release of HPIS is probably controlled by the circatidal clock of the female, and the 24-h LD cycle may participate in shifting this timing to the opposite low tide. Received: 14 January 1997 / Accepted: 18 February 1997     

Migration of the sandworm Nereis virens during winter nights

There have been many previous reports of the sandworm Nereis virens Sars swimming in the water column. This behavior usually has been attributed to reproductive processes. Sandworms were found swimming in surface waters at night on ebb tides during many nights of January, February and March in a Maine (USA) estuary. None of the specimens examined contained gametes or possessed other characteristic spawning or pre-spawning modifications. Several age classes were found, with worms measuring 9 to 38 cm in length, weighing 0.5 to 19.8 g, and having 82 to 187 segments. The greatest numbers of worms were observed during near-average tides on evenings in which low tides occurred a few hours after sunset but prior to moonrise. Up to 83 worms per minute were observed swimming seaward through a 20 m transect, while none were observed swimming landward at any stage of the tide. It is concluded that sandworms swimming during winter nights is unrelated to reproduction and that it is an inherent behavior pattern.Contribution No. 102 of the Ira C. Darling Center, Walpole, Maine 04573, USA.     

Studies on portunid crabs from the Eastern Pacific. II. Significance of the unusual distribution of Euphylax dovii

Euphylax dovii Stimpson (Brachyura: Portunidae: Podophthalminae), a tropical Eastern Pacific swimming crab, has distinctive morphological adaptations for pelagic existence. Crabs in collections from the open ocean had a sex ratio approximating 1:1, with no crabs bearing eggs. Samples from the continental shelf of Colombia contained thousands of females, mostly ovigerous, but no males. Egg attachment has posed a major problem in the evolution of decapod crustaceans, and the two genera of portunid crabs thus far observed cannot attach eggs unless females can bury partly in soft sediments. This suggests that mated E. dovii females must migrate into shallow shelf waters to encounter sediments necessary for spawning. The high energetic cost of swimming while carrying eggs and the presence of abundant food for larvae are factors favoring residence of females in shelf waters until hatching is complete.     

Relationships between intertidal zonation and circatidal rhythmicity in littoral gastropods

The presence and phase of circatidal rhythmicity was correlated with vertical zonation and other ecological factors. Ten species were studied in the field and in aktographs under controlled conditions in the laboratory. Retina plicata, Melanerita atramentosa, Bembicium nanum, Austrocochlea obtusa and Morula marginalba occupied mid- or upper-littoral zonations, and were subjected to regular tidal influence. They were active at high water and for a period after the ebb, possibly as these were the times of least desiccation. Each possessed a circatidal activity rhythm under non-tidal conditions. Midlittoral Amphinerita polita and lower-littoral Theliostyla albicilla were active at low water, possibly because their prefered habitats reduced desiccation and other selective forces, for example wave action and predation, determined the phase of the rhythmicity. Supra- and upper-littoral Nodilittorina pyramidalis and Melarapha unifasciata and lower- and infralittoral specimens of Bembicium auratum displayed no overt circatidal rhythmicity, possibly because they were not subjected to regular tidal action.     

Circatidal swimming activity rhythm in a subtidal cumacean Dimorphostylis asiatica (Crustacea)

Dimorphostylis asiatica, a cumacean crustacea inhabiting the sublittoral line of the Seto Inland Sea, shows a rhythmic pattern of swimming activity coinciding with daily and tidal components in the field. This activity pattern was bimodal at first, i.e., circatidal activity coinciding with high tides at their habitat. The mean free-running period (i.e., bitidal interval) of this endogenous rhythm was 23.1 h at 10°C, which was significantly shorter than the environmental tidal cycle. In most cases (90% of the records) this bimodal activity became unimodal within 10 d. The unimodal period was 24 to 27.5 h, which is markedly longer than the preceding bimodal period. The bimodal pattern observed in the present study was not observed in the field. The difference between field and laboratory activity patterns can be explained in terms of direct response of the bimodal circatidal rhythm to the day-night cycle in the field. Field observations were made and laboratory experiments conducted between 1988 and 1992.     

Control of larval release in the Caribbean spiny lobster, <Emphasis Type="Italic">Panulirus argus</Emphasis>: role of chemical cues

The current model for larval release in subtidal crustaceans suggests that hatching time is controlled by the embryos, which release a pheromone that stimulates the parent female to undergo behaviors that synchronize larval release. Alternatively, hatching could be controlled by the females. Ovigerous spiny lobsters Panulirus argus (Latreille) exhibit stereotypic behaviors during larval release, including rapid abdominal extensions and pleopod-pumping activity. Ovigerous P. argus were collected from coral reefs in the Florida Keys, USA during the summers of 2005 and 2006. Pleopod-pumping activity was quantified to determine if a female’s pumping activity correlates with the developmental state of the embryos. The role of pheromones released by developing and hatching embryos in controlling pumping behaviors was tested by measuring the pumping response of ovigerous lobsters to (1) hatch water, (2) homogenized embryo water, (3) embryo-conditioned water (unhatched late-stage embryos soaked for 20 h), and (4) water containing homogenized post-hatch embryo cases. Bioassays were conducted under constant conditions (dim-red light) in the laboratory at random times during the day to control for any possible rhythm in pumping activity. Spontaneous pleopod-pumping activity increased significantly with increasing embryo development. Upon exposure to hatch water, ovigerous lobsters with late-stage embryos displayed increased pleopod pumping with increased treatment concentration. Water individually conditioned with homogenized late-stage embryos, intact late-stage embryos, and homogenized post-hatch embryo cases all induced larval release behaviors in females with late-stage embryos. Ovigerous females with early-stage embryos did not respond to water conditioned with homogenized early- or late-stage embryos. These results suggest that active substances are released by embryos at the time of hatching and induce the stereotypical pumping behaviors of the female that synchronizes larval release. The results support the model that larval release in subtidal crustaceans is controlled by pheromones released from hatching embryos.     

Looking for the clock mechanism responsible for circatidal behavior in the oyster Crassostrea gigas

Valve activity rhythm of the oyster Crassostrea gigas is mainly driven by tides in the field, but in the laboratory, only a circadian clock mechanism has been demonstrated. In an attempt to reconcile these results, the mechanisms underlying the circatidal rhythm were studied in the laboratory under different entrainment or free-running regimes and in the field at Arcachon (44°39′N/1°09′W) in February–April 2011). Results confirm the existence of a circadian clock in C. gigas. Under entrainment regimes (12-h dark/12-h light photoperiod and tidal cycles simulated by a reversing current flow), oysters exhibited both circadian and circatidal cycles. Under free-running conditions (e.g., continuous darkness), the endogenous rhythm appeared to be circadian. There was no experimental evidence for an endogenous circatidal rhythm, even in oysters just transferred from the field, where a clear tidal cycle was expressed. There are two possible mechanisms to explain tidal behavior in C. gigas: an exogenous tidal cue that drives tidal activity and masks the circadian rhythm and an endogenous circatidal clock that is sensitive to tidal zeitgebers and runs at tidal frequency.     

Factors affecting the interval between clutches in the hermit crab <Emphasis Type="Italic">Pagurus nigrivittatus</Emphasis>

Interval between reproductive events is an important factor for iteroparous animals because it determines the number of clutches throughout life. This study examined whether female size, clutch size, shell size and prenuptial molting affected the clutch interval in the hermit crab Pagurus nigrivittatus. Precopulatory guarding pairs of P. nigrivittatus were sampled in the field and kept in the laboratory until the female extruded eggs. The clutch interval of each female was assessed as one of two types of relatively “short” and “long” intervals by checking whether the guarded female had eggs and/or egg cases from the preceding brood or not when the guarding pair was collected. The clutch interval was longer in females with prenuptial molting than those without molting and these females usually grew larger at the prenuptial molt. This suggests that female P. nigrivittatus with a long interval might allocate energy into growth at the expense of the number of clutches during the current reproductive season. The allocation to growth is theoretically predicted to decrease with female size. Gastropod shell size is also known to affect the reproductive activity in hermit crabs. However, female size did not significantly affect the clutch interval in P. nigrivittatus, and the effect of gastropod shell size on clutch interval was not consistent with previous empirical studies. These results may be caused by differences in the gastropod species of shell occupied by the females of P. nigrivittatus.