Endogenous swimming rhythms of juvenile blue crabs, Callinectes sapidus, as related to horizontal transport

The blue crab Callinectes sapidus settles and metamorphoses in areas of aquatic vegetation in estuaries. Crabs in the first-fifth instar stages (J1-5) then emigrate from these areas by walking on the bottom or pelagic dispersal throughout estuaries. The present study was designed to characterize the timing of this migration pattern relative to the light-dark and tidal cycles. Field sampling in Pamlico Sound, NC, USA indicated that J4-5 juveniles were most abundant in the water column during the night. J4-5 juveniles were collected from Pamlico Sound in an area near Oregon Inlet that has semi-diurnal tides, a Mid-Sound area where tides are weak, and on the West side where regular tides do not occur. Crabs from all three sites had a circadian rhythm in which they swam up in the water column during the time of darkness in the field. Peak swimming consistently occurred at about 0300 h, but was not related to the timing of the tidal cycle. Similar results were obtained for juvenile crabs from an adjacent estuary having semi-diurnal tides. Dispersal at night reduces predation by visual predators, and allows early juvenile blue crabs to disperse planktonically from initial settlement sites.     

Vertical movement of mud crab megalopae (Scylla serrata) in response to light: Doing it differently down under

Selective tidal-streaming is a model frequently used to explain how planktonic larvae invade estuaries. The ability of larvae to move vertically in the water column to selectively ride favourable currents and maintain ground gained is critical to this process. The mud crab (Scylla serrata) is a widely distributed, commercially and recreationally important portunid crab but little is known about its estuarine recruitment mechanisms or the vertical migration behaviour of its megalopae. In studies of the blue crab (Callinectes sapidus), important factors identified in the recruitment mechanism include altered vertical swimming behaviours in estuarine and offshore water and an endogenous circadian rhythm. Using laboratory experiments we examined the vertical displacement response of mud crab megalopae to illumination in estuarine and offshore water during the day and the night. Mud crab megalopae released into 1 m high towers swam higher when illuminated than when in darkness. This behaviour was repeated during the day and the night and in offshore and estuarine water. Given the apparent indifference to water type and the fact that mud crab megalopae are rarely caught in estuaries, we propose the model that these crabs do not invade estuaries as megalopae, but settle and metamorphose into small crabs on the coastal shelf before moving along the sea bed into estuarine habitats.     

Sensory physiology and behavior of blue crab (Callinectes sapidus) postlarvae during horizontaltransport

Adult blue crabs (Callinectes sapidus) live in estuaries and release larvae near the entrances to estuaries. Larvae are then transported offshore to continental shelf areas where they undergo development. Postlarvae, or megalopae, remain near the surface and undergo reverse diel vertical migration. The behaviors underlying this migration pattern are responses to light and a solar day rhythm in activity, in which megalopae are active during the day and inactive at night. Onshore transport probably occurs by wind‐generated surface currents. Once in the vicinity of an estuary, megalopae move up the estuary by selective tidal stream transport, in which they swim in the water column on rising tides at night and are on or near the bottom at all other times. Light inhibits swimming during the day. The ascent into the water column on nocturnal rising tides does not result from a biological rhythm in activity, but rather is cued by the rate of increase in salinity during rising tides. Megalopae have separatebehavioural responses in coastal/shelf areas and in estuaries, which are induced by chemical cues in offshore and estuarine waters.     

Developmental shift in the selective tidal-stream transport behavior of larvae of the fiddler crab Uca minax (LeConte)

Following hatching, larvae of the fiddler crab Uca minax (La Conte) are exported from the adult habitat in estuaries to coastal and shelf waters where they undergo development prior to re-entering estuaries as postlarvae (megalopae). Studies of the spatial distribution of both newly hatched zoeae (Stage I) and megalopae indicate they undergo rhythmic vertical migrations associated with the tides for dispersal and unidirectional transport (selective tidal-stream transport) both within estuaries and between estuaries and the nearshore coastal ocean. We tested the hypothesis that U. minax zoeae possess a circatidal rhythm in vertical migration that facilitates offshore transport in ebb tidal flows, while postlarvae (megalopae) return to estuaries using a similar flood-phased endogenous rhythm. We also determined if the expression of the rhythm was influenced by the salinity conditions zoeae and megalopae experience as they transition between low-salinity regions of estuaries and high-salinity coastal waters. Stage I zoeae were collected by holding ovigerous female crabs in the lab until hatching. Megalopae were collected from the plankton and identified to species using molecular techniques (PCR-RFLP). Under constant laboratory conditions, both zoeae and megalopae exhibited endogenous circatidal rhythms in swimming that matched the principal harmonic constituent of the local tides (12.39 ± 0.07 h; X¯ ± SE). Upward swimming in Stage I zoeae occurred 2.5-4 h after high tide near the time of expected maximum ebb currents in the field. Rhythmic swimming of megalopae occurred slightly earlier in the tide (2.5 ± 0.09 h after high tide; X¯ ± SE) but was not entirely synchronized with flood currents, as expected. Salinity conditions had no apparent effect on the expression or pattern of the rhythms. Results indicate that this circatidal rhythm forms the behavioral basis of selective tidal-stream transport (STST) in early stage U. minax zoeae, but does not undergo a sufficient phase shift to account for vertical distribution patterns exhibited by megalopae in the field.     

Environmentally-controlled,density-dependent secondary dispersal in a local estuarine crab population

The mechanisms driving the pelagic secondary dispersal of aquatic organisms following initial settlement to benthic habitats are poorly characterized. We examined the physical environmental (wind, diel cycle, tidal phase) and biological (ontogenetic, density-dependent) factors that contribute to the secondary dispersal of a benthic marine invertebrate, the blue crab (Callinectes sapidus) in Pamlico Sound, NC, USA. Field studies conducted in relatively large (0.05 km²) seagrass beds determined that secondary dispersal is primarily undertaken by the earliest juvenile blue crab instar stages (J1 crabs). These crabs emigrated pelagically from seagrass settlement habitats using nighttime flood tides during average wind conditions (speed ~5 m s^–1). Moreover, the secondary dispersal of J1 crabs was density-dependent and regulated by intra-cohort (J1) crab density in seagrass. Our results suggest that dispersal occurs rapidly following settlement, and promotes blue crab metapopulation persistence by redistributing juveniles from high-density settlement habitats to areas characterized by low postlarval supply. Collectively, these data indicate that blue crab secondary dispersal is an active process under behavioral control and can alter initial distribution patterns established during settlement. This study highlights the necessity of considering secondary dispersal in ecological studies to improve our understanding of population dynamics of benthic organisms.     

Entrainment of the activity rhythm of the mole crab Emerita talpoida

The mole crab Emerita talpoida migrates with the tide in the swash zone of sand beaches. A circatidal rhythm in vertical swimming underlies movement, in which mature male crabs show peak swimming activity 1-2 h after the time of high tides at the collection site. In addition, there is a secondary rhythm in activity amplitude, in which crabs are maximally active following low amplitude high tides and minimally active following high amplitude high tides. The present study determined the phase response relationship for entrainment of the circatidal rhythm with mechanical agitation and whether the cycle in activity related to tidal amplitude could be entrained by a cycle in the duration of mechanical agitation at the times of consecutive high tides. After entrainment with mechanical agitation on an orbital shaker, activity of individual crabs was monitored in constant conditions with a video system and quantified as the number of ascents from the sand each 0.5 h. Mechanical agitation at the times of high tide, mid-ebb and low tide reset the timing of the circatidal rhythm according to the timing relationship to high tide. However, mechanical agitation during flood tide had no entrainment effect. In addition, a cycle in duration of mechanical agitation entrained the rhythm in activity amplitude associated with tidal amplitude. Both rhythms and entrainment effectiveness over the tidal cycle may function to reduce the likelihood of stranding above the swash zone.     

Endogenous rhythms and entrainment cues of larval activity in the horseshoe crab Limulus polyphemus

The American horseshoe crab, Limulus polyphemus (Linnaeus), typically inhabits estuaries and coastal areas with pronounced semi-diurnal and diurnal tides that are used to synchronize the timing of spawning, larval hatching, and emergence. Horseshoe crabs spawn in the intertidal zone of sandy beaches and larval emergence occurs when the larvae exit the sediments and enter the plankton. However, L. polyphemus populations also occur in areas that lack significant tidal changes and associated synchronization cues. Endogenous activity rhythms that match predictable environmental cycles may enable larval horseshoe crabs to time swimming activity to prevent stranding on the beach. To determine if L. polyphemus larvae possess a circatidal rhythm in vertical swimming, larvae collected from beach nests and the plankton were placed under constant conditions and their activity monitored for 72 h. Time-series analyses of the activity records revealed a circatidal rhythm with a free-running period of ≈ 12.5 h. Maximum swimming activity consistently occurred during the time of expected falling tides, which may serve to reduce the chance of larvae being stranded on the beach and aid in seaward transport by ebb currents (i.e., ebb-tide transport). To determine if agitation serves as the entrainment cue, larvae were shaken on a 12.4 h cycle to simulate conditions during high tide in areas with semi-diurnal tides. When placed under constant conditions, larval swimming increased near the expected times of agitation. Thus, endogenous rhythms of swimming activity of L. polyphemus larvae in both tidal and nontidal systems may help synchronize swimming activity with periods of high water and inundation.     

Factors influencing planktonic, post-settlement dispersal of early juvenile blue crabs (Callinectes sapidus Rathbun)

An emerging body of literature points to post-settlement, planktonic dispersal as a key determinant of distribution and abundance patterns of aquatic organisms, yet little is known about mechanisms inducing such dispersal. Recent evidence suggests that early juvenile blue crabs (Callinectes sapidus Rathbun) may use planktonic emigration as a means of post-settlement dispersal. The goal of this study was to identify mechanisms inducing post-settlement, planktonic dispersal of early juvenile blue crabs. A combination of field mark-recapture experiments in large seagrass beds within a 2x3 km region near Oregon Inlet, North Carolina, USA, and a series of laboratory flume experiments examined the effects of day vs. night, crab size (first-second juvenile benthic instars: J1-J2 vs. third-fifth juvenile benthic instars: J3-J5), crab density and current speed on planktonic dispersal of early juvenile blue crabs. Transport of dead crabs in the flume experiment identified that planktonic dispersal was an active behavioral response rather than a passive response to increasing current speed. The experimental results demonstrated that planktonic dispersal can range from 4 to 18% under medium to high flow conditions. Planktonic dispersal of juvenile crabs is (1) an active behavioral response, (2) increased significantly with current speeds above 20 cms(-1), and (3) was higher for relatively large (J3-J5) than small (J1-J2) instars. There was a non-significant trend towards greater dispersal at night than during the day in the field experiment, and no effect of crab density on dispersal in the flume experiment. The results from this study highlight the need to consider mechanisms inducing post-settlement, planktonic dispersal when attempting to understand and predict recruitment and population dynamics of aquatic organisms, as well as when linking hydrodynamics, animal behavior and planktonic dispersal.     

The effect of abrupt salinity change on oxygen consumption in crab megalopae of Chincoteague Bay,USA

Variability in salinity is an environmental stressor that crab megalopae encounter as they are carried by tides and currents throughout Chincoteague Bay. We exposed blue crab (Callinectes sapidus) and fiddler crab (Uca spp.) megalopae to abrupt salinity changes from 10 to 31 ppt and measured their oxygen usage. It was hypothesized that the megalopae would cope with the changes in a manner reflective of the documented abilities and tolerances of adult crabs. It was also hypothesized that lower salinities would have a particularly detrimental effect on the megalopae reflected by both increased oxygen usage and mortality. The megalopae of both species did exhibit an increase in oxygen use at lower salinities, although the effect was more pronounced during the initial transition and decreased during acclimation. The megalopae mirrored the adult responses, with blue crab larvae consuming more oxygen per mg of wet weight at lower salinities, whereas fiddler crab larval oxygen consumption was relatively uniform at all salinities. Mortality of some blue crab postlarvae was observed at 10 ppt while all larval fiddler crabs survived. Coupled with the introduction of additional fresh water into the global water system, these results indicate that further investigation into this subject is necessary.     

Rhythms of locomotion expressed by Limulus polyphemus, the American horseshoe crab: II. Relationship to circadian rhythms of visual sensitivity

In the laboratory, horseshoe crabs express a circadian rhythm of visual sensitivity as well as daily and circatidal rhythms of locomotion. The major goal of this investigation was to determine whether the circadian clock underlying changes in visual sensitivity also modulates locomotion. To address this question, we developed a method for simultaneously recording changes in visual sensitivity and locomotion. Although every animal (24) expressed consistent circadian rhythms of visual sensitivity, rhythms of locomotion were more variable: 44% expressed a tidal rhythm, 28% were most active at night, and the rest lacked statistically significant rhythms. When exposed to artificial tides, 8 of 16 animals expressed circatidal rhythms of locomotion that continued after tidal cycles were stopped. However, rhythms of visual sensitivity remained stable and showed no tendency to be influenced by the imposed tides or locomotor activity. These results indicate that horseshoe crabs possess at least two biological clocks: one circadian clock primarily used for modulating visual sensitivity, and one or more clocks that control patterns of locomotion. This arrangement allows horseshoe crabs to see quite well while mating during both daytime and nighttime high tides.     

Limited gene flow in Uca minax (LeConte 1855) along a tidally influenced river system

For crab larvae, swimming behaviors coupled with the movement of tides suggests that larvae can normally move upstream within estuaries by avoiding ebb tides and actively swimming during flood tides (i.e., flood-tide transport [FTT]). Recently, a 1-D transport model incorporating larval behavior predicted that opposing forces of river discharge and tidal amplitude in the Pee Dee River/Winyah Bay system of South Carolina, USA, could limit dispersal within a single estuary for downstream transport as well as become a dispersal barrier to recruitment of late stage larvae to the freshwater adult habitats of Uca minax (LeConte 1855). We sequenced 394-bp of the mitochondrial cytochrome apoenzyme b for 226 adult U. minax, from four locales along a 49-km stretch of the Pee Dee River/Winyah Bay estuary, above and below the boundary of salt intrusion. Results of an analysis of molecular variance (AMOVA) and an exact test of population differentiation showed a small, but statistically significant (α=0.05) population subdivision among adults of the 4 subpopulations, as well as all subpopulations being significantly differentiated (α=0.05). This pattern fitted with model predictions, which implies that larval transport within the tidally influenced river system is limited.     

Larval life history strategies of sub-tropical southern African estuarine brachyuran crabs and implications for tidal inlet management

Soon after hatching the larvae of many estuarine crabs migrate from estuaries to adjacent coastal waters soon, where larval development is completed before the post larval stages recruit to estuaries to settle. This study investigated the larval flux of several brachyuran crabs resident within a subtropical estuary on the east coast of South Africa. Plankton sampleswere collected over two intensive sampling periods during the spring of1997 (19 days) and the summer of 1998 (26 days). Larvae were releasedmaximally when high tide within the estuary was crepuscular, whilerecruitment occurred during nocturnal flood tides, with peak abundanceson the maximum amplitude nocturnal flood tides. The data are discussedin relation to freshwater inflow problems facing South African estuaries andthe active management of the tidal inlets of affected systems.     

Metamorphosis of mangrove crab megalopae, Ucides cordatus (Ocypodidae): Effects of interspecific versus intraspecific settlement cues

It has recently been shown that metamorphosis of Ucides cordatus megalopae is triggered by substrata from the mangrove forest habitat, and, in particular, adult conspecific odours. Here we demonstrate that the gender of the odour-emitting crabs is insignificant for the metamorphic response in this species. We further investigate whether other estuarine crabs (Goniopsis cruentata, Uca spp., and Callinectes danae) also induce settlement and metamorphosis of U. cordatus megalopae. This is of special interest for population recovery in areas hit by lethargic crab disease (LCD), a fungus that selectively kills U. cordatus but not co-occurring species. Ucides megalopae were reared in four treatments with interspecific-conditioned seawater and tested against the effects of conspecific-conditioned seawater (positive control) and pure seawater (negative control). All megalopae in the positive control metamorphosed successfully, while only one (2%) moulted in the negative control, with a delay of 10 days compared with the latest metamorphosis in the former treatment. In seawater conditioned with U. maracoani and C. danae, which occur on sediment banks and in tidal creeks respectively, all larvae died before reaching the juvenile stage. In the treatments with odours of species that share the same mangrove forest microhabitat as U. cordatus, i.e. G. cruentata and a group of five fiddler crab species (mixed-odour treatment), 20 and 10% respectively of the megalopae moulted with a delay of up to 11 days. No specimens metamorphosed after day 39, but megalopae lived up to 93 days. Since only the conspecific- and coexisting-species treatments stimulated development, we hypothesize that Ucides megalopae are able to precisely identify species-habitat-specific settlement cues. This will be investigated in more detail in future studies, which will also test the effects of the odours of the five forest fiddler crab species separately. The impact of the interspecific odour treatments was much smaller than that of the conspecific odours, nevertheless elevated moulting rates of up to 18% relative to seawater may still significantly accelerate the repopulation of U. cordatus in areas lacking conspecifics, e.g. after massive crab mortalities or at first colonization.     

Adaptive significance of hatching rhythms and dispersal patterns of estuarine crab larvae: avoidance of physiological stress by larval export?

Estuarine crabs commonly display two larval dispersal patterns in which larvae are either exported from or retained within estuaries. The semiterrestrial fiddler crab Uca minax (LeConte, 1855) hatches on nocturnal spring high tides in the upper estuary and larvae are rapidly transported downstream. The mud crab Rhithropanopeus harrisii (Gould, 1841) hatches on nocturnal high tides of any amplitude and larvae are retained behaviorally in the upper estuary throughout development. If larvae are exported from the estuary to avoid environmental stress, then exported larvae should be less tolerant of high temperatures and low salinities than retained larvae. Larvae of these two species of estuarine crabs were hatched at 20‰ and 25 °C and subjected to salinities of 0, 5, 10,20, and 30‰, temperatures of 25 and 35 °C, and exposure times of 2, 6, 12, and 48 h. Larvae of both species reared at 30 or 20‰ survived well, while those reared in fresh water all died within 2 h regardless of temperature. Mud crab larvae reared at 5 and 10‰ survived better at the lower temperature (25 °C), higher salinity, and shorter exposure times. There was no significant effect of temperature or salinity on the survival of fiddler crab larvae, although survival decreased with increasing exposure time. Thus, the hypothesis that fiddler crab larvae are exported into stable coastal waters to reduce physiological stress is not supported. However, fiddler crab larvae may have evolved to be very tolerant of extreme temperature and salinity stress because they, unlike mud crabs, often release their larvae into shallow creeks. Most fiddler crab larvae are released on nocturnal spring high tides, which facilitates dispersal from tidal creeks. However, freshwater runoff and heat transferred from the marsh surface to flooding waters may still create stressful conditions for larvae soon after they are released. Larval release on spring high tides may facilitate dispersal from tidal creeks.     

Patterns of temporal occurrence of brachyuran crab larvae at Saco mangrove creek, Inhaca Island (South Mozambique): implications for flux and recruitment

Temporal patterns of larval occurrence of brachyuran taxa weredescribed from Saco mangrove creek, Mozambique, based on planktonsamples. Brachyuran larvae were collected hourly in four 24h cycles during neap and spring tidal periods at a fixed station,in November 1997 and February 1998. Results indicate a semilunarcycle of larval release activity for most species. Newly-hatchedstages of sesarmids showed a peak occurrence in post-crepuscularebbing tides, Macrophthalmus spp. and Uca spp. showed highestdensities during ebbing tides at night. Dotilla fenestrata andPinnotheridae, in spite of showing a semilunar pattern, didnot display significant differences between day and night. Mostmangrove taxa showed thus a clear larval exportation behaviourduring ebb tides, with little return during the following floods,with varying degrees of dependence on the diel period. Megalopaewere restricted to the night period, and highest values wereobtained during spring tidal periods. However, maximum valuesof megalopae were obtained both during flood and ebb periods,contrary to most previous studies. This is interpreted as abi-directional transport of non-competent megalopae into andfrom the mangrove area.     

Patterns of larval release in the Florida stone crab, Menippe mercenaria

Larval release patterns in brachyuran crabs are often synchronized with environmental cycles. While previous studies have focused extensively on supratidal and intertidal taxa, there have been relatively few investigations of subtidal species. This study examined patterns of larval release by the Florida stone crab, Menippe mercenaria, from three different tidal regimes. Ovigerous stone crabs were collected from Sebastian Inlet on the east coast of Florida, Tampa Bay on the west coast of Florida, and the Florida Keys. Patterns of larval release were monitored in the laboratory in relation to local tidal and diel cycles. Results showed a significant diel pattern in initiation of hatching by crabs from each of three study areas. Larval release consistently occurred during the diurnal phase despite the maintenance of females in constant laboratory conditions for up to 96 h prior to hatching. This implies that release may be controlled by a circadian clock. Patterns of release by stone crabs in relation to tidal cycle were more variable. Larval release by females from populations near Tampa Bay and Sebastian Inlet were not synchronized with the tides, whereas females collected from the Florida Keys exhibited a pattern that was strongly related to tidal cycle. These results may be explained by differences in tidal amplitude at the three sampling locations.     

Adult fiddler crabs Uca pugnax (Smith) enhance sediment-associated cues for molting of conspecific megalopae

Adult-associated chemical cues can stimulate settlement and metamorphosis of invertebrate larvae into habitats with an enhanced likelihood of juvenile and adult survival. For example, sediments from adult fiddler crab habitat stimulate fiddler crab megalopae to metamorphose (molt) sooner than sediments without adult cues. A similar stimulation of molting occurs after exposure to waterborne chemical cues from adult habitats and to exudates and extracts of adult crabs. We tested whether sediments from habitats without adult Uca pugnax (Smith), which do not stimulate molting of their megalopae, could become stimulatory through brief exposure to adult crabs. Sediments were collected from tidal flats at several distances (∼ 1 m, ∼ 50 m, and ∼ 5.4 km) from adult habitats, and incubated for 24 h with or without adult crabs. Molting rates of laboratory-reared megalopae exposed for 48 h to adult-conditioned sediments were compared to those for untreated controls. Sediments collected in or within 1 m of adult habitat elicited the highest molting rates, and natural sediments from 50 m and 5.4 km had little or no effect on molting. However, incubating sediments collected away from adult habitat with adult crabs produced a higher molting response, and the magnitude of the enhancement increased with distance from adult habitat. Results suggest that the chemical cues that adult crabs release are retained by sediments and consequently stimulate molting of megalopae, regardless of the nature of the sediments themselves. Lack of chemical cues may retard colonization of newly created or heavily disturbed habitats that are otherwise suitable settlement and adult habitat.     

Comparison of low salinity tolerance in Callinectes sapidus Rathbun and Callinectes similis Williams postlarvae upon entry into an estuary

Planktonic larvae of estuarine crabs are commonly exported to the continental shelf for development and then return to coastal and estuarine areas as postlarvae (megalopae). Megalopae returning to estuaries must be adapted to survive in brackish water whereas those of coastally distributed species should not need such adaptations. We investigated 1) whether megalopae of the estuarine crab Callinectes sapidus and the coastal crab Callinectes similis undergo changes in salinity tolerance upon entry into an estuary and 2) what factors induce those changes. Megalopae were collected at a coastal site and a nearby estuarine site and exposed to a range of salinities (5, 10, 15, 20 and 30) for 6 h. Percent survival was determined after 24 h reintroduction to the collection site water. We also investigated 1) whether increased salinity tolerance was induced by reduced salinity or estuarine chemical cues, 2) the time to acclimation and 3) the salinity necessary for acclimation. C. sapidus megalopae from the estuarine site were more likely to survive exposure to low salinities than those from the coastal site. C. sapidus megalopae from the coastal site exhibited increased survival after acclimation to salinities of 27 and 23 for 12 h. Estuarine chemical cues had no effect on salinity tolerance. C. similis megalopae were less likely to survive at low salinities and did not exhibit an acclimation response upon exposure to reduced salinities. These results suggest that megalopae of C. sapidus are physiologically adapted to recruit to estuaries whereas megalopae of C. similis are unable to acclimate to low salinity conditions.     

Entrainment of the larval release rhythm of the crab Rhithropanopeus harrisii (Brachyura: Xanthidae) by cycles in hydrostatic pressure

This study investigated the entrainment of a larval release rhythm by determining whether a tidal cycle in hydrostatic pressure could entrain the circatidal rhythm in larval release by the crab Rhithropanopeus harrisii (Gould). Ovigerous females were collected from a non-tidal estuary. The time of larval release by individual crabs was monitored under constant conditions with a time-lapse video system. Crabs with mature embryos at the time of collection had a pronounced circadian rhythm in larval release with a free running period of 25.1 h. Crabs with immature embryos that were maintained under constant conditions from the time of collection until larval release retained a weak circadian rhythm. Other crabs with immature embryos were exposed to a tidal cycle in step changes in hydrostatic pressure equivalent to 1 m of water. This cycle entrained a circatidal rhythm in larval release. The free-running period was 12.1 h and larvae were released at the time of the transition from low to high pressure. Although past studies demonstrated that a tidal cycle in hydrostatic pressure could entrain activity rhythms in crustaceans, this is the first study to show that pressure can entrain a larval release rhythm.     

Timing of the water-to-land transition and metamorphosis in the land hermit crab Coenobita compressus H. Milne Edwards: evidence that settlement and metamorphosis are de-coupled

After metamorphosing from the last larval stage to the transitional megalopal stage in the marine plankton, the hermit crab Coenobita compressus moves ashore where it undergoes a second metamorphosis to the first juvenile instar on land. In two experiments using laboratory-reared crabs, I moved megalopae from water to land after different amounts of time at this stage and investigated the impact of this manipulation on the timing of and survival through the second metamorphosis. In the Involuntary Settlement experiment, megalopae were moved to land when they were 3, 6, 9, 12, or 15 days old. None of those moved between the ages of 3 and 6 days survived through metamorphosis, but the majority of 9-day-old megalopae survived, as did most 12- and 15-day-old megalopae. This suggests that developmental changes early in the megalopal stage prepare C. compressus for terrestrial life. Once on land, megalopae that had been moved to land at 9 days spent about nine additional days there before metamorphosing, while 12- and 15-day-old megalopae metamorphosed after spending about 5 and 4 days, respectively, on land. In the Voluntary Settlement experiment, megalopae were given access to land when they were 1, 5, 10, or 15 days old, but were not forced to make the transition. Those given access to land after 1 day voluntarily left their dishes for the first time after an average of 7 days in water. Those given access when they were 5 days old remained in water about 4 days longer, while those given access when they were 10 and 15 days old left after less than a day. In both experiments, the timing of metamorphosis relative to settlement (i.e., transition to land) showed that these events are dissociated to a degree and revealed the presence of a metamorphic clock. I discuss why the dissociation of settlement and metamorphosis may have been favored in the land hermit crab and in another anomuran crab.