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     

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.     

Diurnal and lunar patterns of larval recruitment of Brachyura into a mangrove estuary system in Ranong Province, Thailand

The present study followed the temporal recruitment pattern of brachyuran larvae in a mangrove tidal creek on the Andaman Sea coast of Ranong Province, Thailand, based on the assumption that the processes governing recruitment are important for the overall population dynamics of mangrove brachyuran crabs. Plankton net samples were taken on five occasions: on two new moon spring tides, one waxing moon neap tide, one full moon spring tide and one waning moon neap tide during October and November 1997. In addition collectors for larval crab megalopae were employed every 3 days through one dry season and one wet season (March–October 1998). Both the plankton net samples and collector samples revealed four major brachyuran groups in three families: Ocypodidae, Grapsidae and Portunidae. The grapsid group was further separated into two morphotypes which were identified as Metaplax and sesarmid species. Identified group mean numbers per cubic metre were ocypodids 3.0, sesarmids 0.8 and Metaplax 0.5, while portunid megalopae were very scarce (≪0.1 m⁻³). Further analysis of plankton net samples showed that when considering the parameters date, depth, current direction and the diel cycle, Metaplax and ocypodids distribute differently in the tidal and lunar cycle. Metaplax recruitment dominates on flood tides and on bottom layers, followed by middle and surface layers. Conversely, ocypodid abundance varied significantly with date only. Notably recruitment was not dependent on the diel cycle for either group. The collector samples of megalopae showed that recruitment of ocypodids, Metaplax and sesarmids occurred on full and new moon spring tides, while portunid megalopae preferred to settle on full moon spring tides. Since tidal currents were related to the lunar cycle megalopa groups are also cross-correlated with tidal amplitude, except for the portunid group. It is concluded that megalopae recruit in a similar manner to what has been found in other regions of the world, except that the abundance of ocypodids and Metaplax is not influenced by the diel cycle. Received: 14 February 2000 / Accepted: 24 November 2000     

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.     

Responses of estuarine crab megalopae to pressure,salinity and light: Implications for flood-tide transport

The megalopal larval stage of many estuarine brachyuran crabs appears to return to adult habitats by undergoing rhythmic vertical migrations which result in saltatory up-estuary transport on flood tides. Larval ascent into the water column during rising tides may be cued by changing hydrologic variables. To test this hypothesis, we investigated the responses of field-caught megalopae of the blue crab Callinectes sapidus and the fiddler crab Uca spp. to constant rates of pressure and salinity change under laboratory conditions. For both genera, pressure changes resulted in increased movement (barokinesis) and upward migration in the test chamber, with C. sapidus megalopae having a lower response threshold (2.8×10^-2 mbar s^-1) than Uca spp. larvae (5×10^-2 mbar s^-1). Similarly, larvae ascended in response to increasing salinity, with C. sapidus larvae being more sensitive. Larvae were negatively phototactic and failed to respond to pressure increases at light levels above 1.0×10¹⁵ and 1.0×10¹³ photons m^-2 s^-1 for C. sapidus and Uca spp. megalopae, respectively. Such responses are thought to explain the low abundances of larvae in the water column during daytime flood tides. Nevertheless, threshold sensitivities to increasing pressure for both genera were above levels experienced during floodtide conditions in the field. Similarly, it is unlikely that increasing salinity is sufficient to induce ascent in Uca spp. postlarvae. However, rates of salinity increase during midflood tide typically reach levels necessary to induce an ascent in C. sapidus megalopae. These results are consistent with the hypothesis that fiddler crab megalopae utilize an endogenous activity rhythm for flood-tide transport, while blue crab megalopae rely upon external cues, especially salinity changes, to time their sojourns in the water column.     

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     

Settlement times of blue crab (Callinectes sapidus) megalopae during flood-tide transport

Settlement by blue crab (Callinectes sapidus Rathbun) megalopae on artificial settlement substrates was monitored relative to tidal currents throughout ten nights from July to September 1997 in which the phase relationship between tides and the light dark cycle differed. Most megalopae were in intermolt, and the total number settling to collectors sampled at hourly intervals was greater than totals on collectors immersed all night. Maximum settlement occurred at slack water before ebb tide (SBE), with a smaller peak at slack water before flood tide (SBF). These results support the hypothesis that during flood-tide transport (FTT) blue crab megalopae remain swimming during flood tide at night in response to water turbulence and settle in response to the decline in turbulence occurring near SBE. Settlement peaks near SBF can be explained by a behavioral response of megalopae to increasing salinity at the beginning of flood tide, which results in an ascent response lasting only a few minutes. Depth maintenance in the water column is not maintained at SBF because of low water turbulence. Since light inhibits swimming and upward movement into the water column, settlement, and, presumably, transport were reduced when SBE occurred near the times of sunrise and sunset. Collectively, these results suggest that the phase relationship between the tide and light: dark cycles affects FTT, the timing of settlement, and behaviors associated with habitat selection. Published online: 9 August 2002     

Diel vertical migration and timing of metamorphosis of larvae of the Dungeness crab Cancer magister

Joint USA/USSR ichthyoplankton surveys off the coasts of Washington, Oregon and northern California during the years 1981 to 1985 sampled more than 120 stations each year, from 5 to 360 km offshore and between Latitude 40° and 48° N, providing information on ontogeny and diel migration of larvae of the Dungeness crab Cancer magister on a scale not studied previously. We developed a maximum likelihood method for estimating abundance and fraction in the neuston at each station from a neuston tow and an oblique bongo tow. Latestage megalopae migrate vertically on a diel basis, with the fraction in the neuston being (on average) 62% at night (19.00 to 08.00 hrs Pacific Standard Time, PST) and 8% during the day (08.00 to 19.00 hrs PST). The hourly pattern of this migration includes a peak in the early evening, possibly another in the early morning, and an intermediate level in the late afternoon. We detected no dependence of vertical migration on cloud cover or sea state. Early-stage megalopae were present in much lower fractions in the neuston, but weakly displayed the same diel pattern of migration. Zoeae appeared to be below the neuston at all times, except for 2 or 3 h in the evening. From an abrupt change in larval stage in samples from a north-south cruise, we concluded that the majority of the larvae metamorphose from zoeae to megalopae over a fairly short time span (2 to 4 wk) at a given latitude. In later cruises, 95% of the larvae were megalopae, indicating that metamorphosis over the study area either occurs at the same time or proceeds from south to north over a time span of less than a month in early spring.     

In situ swimming and settlement behaviour of larvae of an Indo-Pacific coral-reef fish, the coral trout Plectropomus leopardus (Pisces: Serranidae)

Late larvae of the serranid coral trout Plectropomus leopardus (Lacepède), captured in light traps, were released during the day both in open water and adjacent to two reefs, and their behaviour was observed by divers at Lizard Island, northern Great Barrier Reef. Coral trout larvae (n = 110) were present in light-trap catches from 18 November to 3 December 1997, including new moon (30 November). The swimming speed of larvae in open water or when swimming away from reefs was significantly greater (mean 17.9 cm s⁻¹) than the speed of larvae swimming towards or over reefs (mean 7.2 cm s⁻¹). Near reefs, larvae swam at average depths of 2.7 to 4.2 m, avoiding 0 to 2 m. In open water, swimming depth varied with location: larvae >1 km east of Lizard Island swam steeply downward to >20 m in 2 to 4 min; larvae >1 km west oscillated between 2.6 and 13 m; larvae 100 to 200 m east of Lizard Island oscillated between 0.8 and 15 m. Nearly all larvae swam directionally in open water and near reefs. In open water, the average swimming direction of all larvae was towards the island, and 80% (4 of 5) swam directionally (p < 0.05, Rayleigh's test). Larvae swam directionally over the reef while looking for settlement sites. The frequency of behaviours by larvae differed between two reefs of different exposure and morphology. Depending on site, 26 to 32% of larvae released adjacent to reefs swam to open water: of these, some initially swam towards or over the reef before swimming offshore. In some cases, offshore-swimming seemed to be due to the presence of predators, but usually no obvious cause was observed. Depending on the reef, 49 to 64% of the larvae settled. Non-predatory reef residents aggressively approached 19% of settlers. Between 5 and 17% of the larvae were eaten while approaching the reef or attempting to settle, primarily by lizardfishes but also by wrasses, groupers and snappers. A higher percentage of larvae settled in the second week of our study than in the first. Average time to settlement was short (138 s ± 33 SE), but some larvae took up to 15 min to settle. Average settlement depth was 7.5 to 9.9 m, and differed between locations. No settlement took place on reef flats or at depths <4.2 m. Larvae did not appear to be selective about settlement substrate, but settled most frequently on live and dead hard coral. Late-stage larvae of coral trout are capable swimmers with considerable control over speed, depth and direction. Habitat selection, avoidance of predators and settlement seem to rely on vision. Received: 7 July 1998 / Accepted: 26 January 1999     

Role of protein in larval swimming and metamorphosis of Bugula neritina (Bryozoa: Cheilostomatida)

Larvae of the marine cheilostomatid bryozoan Bugula neritina (L.) were prevented from settling for 1, 4 and 8 h by mechanical agitation, following which settlement and metamorphosis success were examined. Settlement rates were significantly affected by swimming time, which decreased from 100% after 2 h to 93.7 ± 4.3% after 8 h. Similarly, metamorphosis to the feeding ancestrula was significantly impaired following a swimming time of 8 h, declining from 93.7 ± 4.3% after 1 h to 65.9 ± 7.0% after 8 h. The resultant colonies grew well for the first 3 wk, following which time, growth patterns became erratic. Growth rate was in all cases highly variable, and did not correlate with enforced swimming times. Larval protein composition was examined after 1, 4 and 8 h swimming time, and post-larval composition 1, 2, 5, 24 and 48 h after settlement using sodium-dodecyl-sulphate polyacrylamide-gel electrophoresis (SDS-PAGE). Individual protein content was measured using a densitometer. Larvae did not consume protein during swimming, however a protein measuring 170 kdaltons was consumed during metamorphosis. These results are discussed in the context of larval settlement and energetics. Received: 19 July 1998 / Accepted: 3 December 1998     

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.     

Response of mud crab (Panopeus herbstii) megalopae to cues from adult habitat

During the summer of 1995, we investigated the response of mud crab megalopae (Panopeus herbstii Milne-Edwards) to cues associated with adult habitat. Natural rock/shell substratum and the rock-associated seaweed, Fucus vesiculosus L., both induced metamorphosis of competent megalopae; natural sand substratum did not induce metamorphosis. Structural mimics of preferred substrata induced metamorphosis only when covered with natural biofilm. Clean mimics did not induce metamorphosis. Water-soluble exudates from preferred substrata showed weak induction of metamorphosis. Exudates from adult P. herbstii elicited a similar weak response. Exudates from another species of crab and from two fish predators did not induce metamorphosis. We conclude that water-soluble cues associated with major components of preferred adult habitat induce the metamorphic molt of P. herbstii megalopae. The three-dimensional structure of the substratum is not important in induction of metamorphosis, but the biofilm associated with preferred substratum plays a critical role. It is not clear whether the biofilm produces the water-soluble cue or simply provides a tactile stimulus that works in conjunction with the soluble cue.     

Mechanisms of cross-shelf transport of crab megalopae inferred from a time series of daily abundance

From 20 July 1982 to 19 July 1984, crab megalopae were trapped daily from the Scripps Institution of Oceanography pier. Pachygrapsus crassipes, Portunus xantusii, Cancer spp., Hemigrapsus spp., and Majid crab megalopae dominated the catch. Yearly, seasonal, and daily variations in the magnitude of the catch were observed. Yearly and seasonal variations were probably due to a strong El Niño event that occurred during the study and to the timing of spawning and duration of the larval phase, respectively. Daily variation was correlated with oceanographic processes that can transport larvae to shore. Catch of some taxon during some seasons correlated with wind stress suggesting that transport was wind driven. The correlations were, however, weak and the sign of the correlation varied between years. The maximum daily tidal range was significantly correlated (cross-correlations and cross Fourier analysis) to both daily seawater temperature anomalies (surface and bottom) and daily catch of crab megalopae in all taxa enumerated. Significant correlations between tidal range and temperature anomalies suggest that temperature anomalies were primarily due to the shoreward transport of warm and cold water by the internal tides. The consistent and relatively strong relationship between tidal range and catch of megalopae (the cross-Fourier analysis suggests that from 20 to >90% of the variation in catch can be attributed to variation in the tidal range) suggest that much of the shoreward transport of megalopae was via the internal tides. Shoreward transport of larvae by internal tides may be due to internal cold bores or convergences over large tidally generated internal waves (solitons). Peak catches of megalopae, however, were often not associated with cold anomalies suggesting that transport was due to moving convergences over internal waves.     

Effects of chemical cues on orientation of blue crab, <Emphasis Type="Italic">Callinectes</Emphasis><Emphasis Type="Italic">sapidus</Emphasis>, megalopae in flow: implications for location of nursery areas

Megalopae (postlarvae) of the blue crab Callinectes sapidus Rathbun use flood-tide transport (FTT) for movement into and up estuaries. Since they settle around the time of slack water at the end of flood tide during FTT, it was predicted that orientation toward primary nursery areas of aquatic vegetation occurs at this time. This study tested the hypotheses that megalopae locate nursery areas by swimming upstream in the presence of chemical odors from potential nursery areas and avoid adverse microhabitats by swimming downstream when predator or adverse environmental odors are present. Megalopae were tested in a flume where they were exposed to the sequence of cues mediating FTT (i.e. 2 psu increase in salinity followed by an increase and a decrease in current speed and turbulence). The flume contained odor water either from the developmental area (offshore water), nursery area vegetation (seagrass, Zostera marina; salt marsh cord grass, Spartina alterniflora), predators (fiddler crab, Uca pugilator; mud crab, Panopeus herbstii; grass shrimp, Palaemonetes pugio), or chemicals associated with adverse environments (ammonium). Vertical positions of premolt and intermolt megalopae were similar in water devoid of estuarine chemical cues (offshore water) and water containing seagrass odor. Upstream swimming behavior (orientation) of intermolt megalopae was also similar in these waters. However, there was an ontogenetic behavioral change, as the proportion of premolt megalopae oriented upstream generally increased as the concentration of seagrass and salt marsh cord grass odor increased and as current speed decreased. Upstream orientation of premolt megalopae in response to seagrass odor decreased significantly (i.e. downstream swimming increased) in the presence of odor from U. pugilator, P. pugio, and ammonium, but not from P. herbstii. Thus, the hypothesis was supported. These results suggest premolt megalopae orient toward nursery areas by swimming upstream in response to odors from aquatic vegetation as current speeds decrease at the end of nocturnal flood tides. Moreover, these results also indicate that megalopae may discriminate among microhabitats and avoid adverse settlement habitat, as orientation toward nursery areas is reversed by predator odors and ammonium.     

Endogenous swimming rhythms underlying the spawning migration of the blue crab, Callinectes sapidus: ontogeny and variation with ambient tidal regime

Spawning female blue crabs, Callinectes sapidus, use ebb-tide transport (ETT) to migrate seaward. In estuaries with semi-diurnal tides, ETT in ovigerous blue crabs is driven by a circatidal rhythm in vertical swimming in which crabs ascend into the water column during ebb tide. The ontogeny of this rhythm was examined by monitoring swimming behavior of females before the pubertal molt, females that had recently undergone the molt but had not yet produced a clutch of eggs, and ovigerous females from an estuary with strong semi-diurnal tides. To assess variation in swimming rhythms with ambient tidal regime, swimming rhythms of ovigerous females from semi-diurnal (Beaufort, North Carolina), diurnal (St. Andrew Bay, Florida), and non-tidal (South River, North Carolina) estuaries were compared. Experiments were conducted during the summers of 2006–2008. Female crabs prior to oviposition had variable endogenous swimming rhythms (circadian, circatidal, or circalunidian). Ovigerous females from estuaries with semi-diurnal and diurnal tides had pronounced circatidal or circalunidian rhythms with swimming during the time of ambient ebb tide. Swimming rhythms of several ovigerous crabs switched between circatidal and circalunidian during the ~5-day observation period. Ovigerous crabs from a non-tidal estuary had a circadian rhythm with vertical swimming around the time of sunset. These results suggest that, while endogenous swimming rhythms are present in some female blue crabs prior to oviposition, rapid seaward movement via ETT in tidal estuaries begins following oviposition of the first clutch of eggs.     

Abundance of estuarine crab larvae is associated with tidal hydrologic variables

Abundances of brachyuran megalopae and juveniles were measured throughout consecutive tidal cycles during six 2 to 3 d sampling periods in summer 1992, and associated with rates of change of tidal hydrologic variables in the Newport River Estuary. Current speeds and rates of pressure change fitted sinusoidal (tidal) models well; however, rates of salinity and temperature change did not. Analysis of plankton samples taken during spring and neap tides showed peak abundances during nighttime rising tides for all taxonomic groups: Callinectes sapidus, Uca spp., Xanthidae, and Pinnixa spp. megalopae, and Pinnotheres spp. juveniles. Megalopal and juvenile abundances from time-intensive sampling were related to rates of changes in the hydrologic variables using stepwise logistic regression. No hydrologic variable accounted well for the presence of Uca spp. megalopae. Megalopal presence was best predicted by current speed for Pinnixa spp. megalopae, and rates of changes in pressure for xanthid megalopae and Pinnotheres spp. juveniles, and salinity for C. sapidus megalopae. These variables might act as cues causing megalopae to ascend into the water column at a particular point in the flooding tide, and subsequently descend to or near the bottom prior to ebb flow. In this way, larvae which develop on the continental shelf or lower estuary undergo transport up the estuary by behaviorally altering their swimming activity and depth concurrent with tidal changes.     

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.     

Asymptotic growth trajectories of larval sardine (Sardinops melanostictus ) in the coastal waters off western Japan

Growth trajectories of individual larvae of Japanese sardine, Sardinops melanostictus, caught in the coastal waters off western Japan were back-calculated from the first feeding stage up to date of capture (approximate size of 20 to 35 mm total length; TL) based on individually determined allometric relationships between otolith daily ring radii and fish total lengths. The larvae in January-, February-, and March-hatched cohorts in the coastal waters grew faster and more uniformly than those in the oceanic waters offshore of the Kuroshio current. Growth trajectories of the three hatch-month cohorts were similar and could be expressed by the Gompertz model. The inflection points of the growth curves were reached at 9 to 11 d after hatching, when larvae were 10.8 to 11.8 mm TL. Maximum growth rates at these points were 0.80 to 0.85 mm d⁻¹. Growth rates gradually declined after the inflection points, and larval TLs converged into the infinite length of 29 to 32 mm, the sizes at which metamorphosis from larvae to juveniles is initiated. This asymptotic growth pattern in the larval stage resulted in the narrow ranges in TLs in spite of the wide range of ages of the larvae caught by boat seiners in the coastal waters. Slow growth and therefore long duration of the metamorphosing stage could be influential in determining the cumulative total mortality in the early life stages of the Japanese sardine. Received: 14 July 1996 / Accepted: 20 August 1996     

Growth, carbon, nitrogen and caloric content of Solea senegalensis (Pisces: Soleidae) from egg fertilization to metamorphosis

Eggs and larvae of the Senegal sole, Solea senegalensis Kaup, were reared from fertilization until the end of metamorphosis, which occurs by Day 17 after hatching at 19.5 °C. Changes in energy content and biomass quality were studied in terms of dry weight and of carbon, nitrogen and energy content. S. senegalensis spawned eggs of about 1 mm diameter which hatched 38 h after fertilization. Average dry weight of individual eggs was 46 μg, the chorion accounting for about 18% of total dry weight. Gross energy of recently fertilized sole eggs was approximately 1 J egg⁻¹. From fertilization to hatching, eggs lost 8% of their total energy (chorion not included). After hatching, larvae lost 14% of their initial energy until the start of feeding which occurred about 48 h afterwards. The principal components catabolized during embryogenesis were carbon-rich compounds that decreased by 26%, while nitrogen-rich compounds decreased by only 10% and were practically unaltered from hatching to the start of feeding. Feeding larvae displayed constant growth during the period studied (specific growth rate on a dry weight basis was 0.26 d⁻¹). The relative proportion of carbon and nitrogen content revealed an accumulation of high energy compounds in the days before metamorphosis. By Day 14, the energy content reached values similar to those of recently hatched embryos, but decreased again during metamorphosis. Received: 10 June 1998 / Accepted: 28 January 1999