Richard A. Tankersley

Migratory Behavior of Ovigerous Blue Crabs Callinectes sapidus: Evidence for Selective Tidal-Stream Transport

In the late summer and early fall, newly inseminated female blue crabs (Callinectes sapidus) leave low-salinity areas of estuaries and migrate seaward to spawn near the entrance. We tested the hypothesis that migration of female C. sapidus to spawning grounds is facilitated by selective tidal-stream transport (STST). We monitored the swimming direction of adult crabs from a stationary platform located about 1 km inside the entrance to the Newport River Estuary (Beaufort, North Carolina). Swimming activity near the surface occurred primarily at night and most crabs avoided swimming against tidal currents. Eighty-one percent of the crabs observed moving down-estuary toward the inlet during ebb tide were ovigerous females. Of the 36 gravid females captured traveling in ebb currents, 97% possessed dark egg masses containing late-stage embryos. Conversely, nearly all (98%) adult crabs observed traveling in flood currents lacked egg masses, and all the females captured while migrating up-estuary exhibited signs of recent spawning. These observations indicate that ovigerous blue crabs use ebb-tide transport to migrate seaward to spawn and flood-tide transport to reenter the estuary shortly after larval release.

Survival and development of horseshoe crab (Limulus polyphemus) embryos and larvae in hypersaline conditions.

The horseshoe crab Limulus polyphemus spawns in the mid- to upper intertidal zone where females deposit eggs in nests below the sediment surface. Although adult crabs generally inhabit subtidal regions of estuaries with salinities from 5 to 34 ppt, developing embryos and larvae within nests are often exposed to more extreme conditions of salinity and temperature during summer spawning periods. To test whether these conditions have a negative impact on early development and survival, we determined development time, survival, and molt cycle duration for L. polyphemus embryos and larvae raised at 20 combinations of salinity (range: 30–60 ppt) and temperature (range: 25–40 °C). Additionally, the effect of hyperosmotic and hypoosmotic shock on the osmolarity of the perivitelline fluid of embryos was determined at salinities between 5 and 90 ppt. The embryos completed their development and molted at salinities below 60 ppt, yet failed to develop at temperatures of 35 °C or higher. Larval survival was high at salinities of 10–70 ppt but declined significantly at more extreme salinities (i.e., 5, 80, and 90 ppt). Perivitelline fluid remained nearly isoosmotic over the range of salinities tested. Results indicate that temperature and salinity influence the rate of crab development, but only the extremes of these conditions have an effect on survival.

Spatial and temporal patterns of spawning and larval hatching by the horseshoe crab,Limulus polyphemus, in a microtidal coastal lagoon

Estuarine species with wide geographic distributions often experience tidal regimes that vary significantly throughout their range. Plasticity in behaviors associated with the tide is expected to enable synchronization with local tides. The American horseshoe crabLimulus polyphemus typically inhabits estuaries and coastal areas with pronounced semi-diurnal tides that play a role in synchronizing the timing of spawning and larval hatching, but also lives in areas that lack significant tides and associated synchronization cues. We investigated the spatial and temporal pattern of adult spawning and larval hatching ofL. polyphemus in a microtidal coastal lagoon (Indian River Lagoon, Florida, USA). Spawning activity and larval abundance were monitored weekly February 1998–August 2000 at sites spanning 100 km of the lagoon. To identify possible synchronization cues for spawning and hatching success, the presence of adult and larvalL. polyphemus were related to environmental and hydrologic variables using logistic regression. The presence of spawning adults varied significantly among the sub-basins of the lagoon, with the highest densities occurring in the Banana River. Large spawning aggregations were not observed and densities never exceeded 6 m−2. Spawning occurred year-round but varied seasonally with episodes of increased mating activity in the early spring. The occurrence of mating pairs was episodic and was not synchronized among sites. Larval densities were low (4 m−3) and larvae were present at only 12 of the 21 sites. Hatching success was decoupled temporally from spawning activity, with peaks in larval abundance occurring approximately 8 wk after peaks in spawning. Larval abundance was associated with periods of high water. Reproductive activity of horseshoe crabs in the lagoon differs significantly from populations inhabiting areas with semi-diurnal and diurnal tides. These differences are likely due to the lack of periodic tidally-related synchronization cues and regular beach inundation.

Selective tidal stream transport of spot (Leistomus xanthurus Lacepede) and pinfish [Lagodon rhomboides (Linnaeus)] larvae:: Contribution of circatidal rhythms in activity

Spot (Leistomus xanthusus Lacepede) and pinfish (Lagodon rhomboides Linnaeus) spawn offshore and their larvae are then transported shoreward where they migrate into estuaries for continued development. The present study determined (1) whether larvae of these species use selective tidal stream transport for up-estuary movement, and, (2) whether an endogenous rhythm in activity contributes to this movement. Field studies determined the abundance of spot and pinfish larvae over time at two locations in a tidal estuary as related to tidal phase and the light–dark cycle. At one location, both species were statistically most abundant in the water column during flood tide at night, which indicates the presence of selective tidal stream transport. Since results were inconsistent at the other location, the effectiveness of selective tidal stream transport for up-estuary movement varies with location in an estuary. Endogenous rhythms in activity of estuarine caught larvae were measured under constant conditions in the laboratory. Both species had circatidal rhythms with period lengths close to a tidal cycle and two activity peaks over a lunar day. In most cases maximum activity corresponded to the times of ebb tide in the field. During selective tidal stream transport, increased activity could represent the behavior needed to remain low in the water column or move laterally in order to reduce horizontal movements during ebb tides. During flood tides larvae could move passively with up-estuary directed currents.

Larval release behaviors in the blue crab Callinectes sapidus: role of chemical cues

Egg hatching by brachyuran crabs is often precisely timed relative to environmental cycles and may be controlled by the female, the developing embryos, or both. The current conceptual model for larval release in subtidal brachyuran crabs is that the exact time of release is controlled by the developing embryos. At the time of hatching, the eggs release pheromones that induce stereotypic larval release behaviors in ovigerous females consisting of rapid abdominal pumping. This behavior breaks open the eggs and results in synchronized hatching. To test this model, we examined the role of pheromone substances released by developing and hatching eggs in initiating this pumping behavior in ovigerous blue crabs Callinectes sapidus. Pumping behavior was used as a bioassay to determine if pumping activity changes with the developmental state of the eggs and to test the response of ovigerous crabs to (1) substances released by hatching eggs (hatch water), (2) substances present in homogenized eggs containing early- and late-stage embryos (homogenized egg water), and (3) substances released by developing eggs containing early- and late-stage embryos (egg conditioned water). Pumping activity associated with egg maintenance increased with embryo development. Pumping activity increased with increasing concentration of hatch water and the threshold concentrations for females possessing early- and late-stage eggs were similar. Water containing homogenized eggs also evoked larval release behaviors and response thresholds were the same for females exposed to early- and late-stage egg treatments. Egg conditioned water prepared from eggs containing late-stage embryos was more potent than water prepared from eggs with early-stage embryos. Collectively, these results support the model that larval release in C. sapidus is controlled by pheromones released from hatching eggs and indicate that (1) the responsiveness of ovigerous C. sapidus to the pheromones is relatively independent of the stage of embryo development, (2) homogenates of both early- and late-stage eggs contain similar pheromone concentrations, (3) pheromones are released during development and at the time of hatching, and (4) the concentration of pheromones released from developing eggs increases as the embryos mature.

Larval hatching in the horseshoe crab, Limulus polyphemus: facilitation by environmental cues

Female horseshoe crabs, Limulus polyphemus (Linnaeus), lay their eggs in nests on sandy beaches near the high water line. Embryos develop within the sand, hatch into trilobite larvae, and enter the water column when the nest is inundated. Given the diversity of tidal and shoreline inundation patterns that populations of L. polyphemus experience throughout their range (semidiurnal and diurnal tides, microtidal, and nontidal), hatching may also be facilitated by environmental triggers that serve to synchronize hatching and larval emergence with periods of high water. The objective of this study was to determine if larval hatching in L. polyphemus is triggered or facilitated by environmental cues. Stage 21 embryos were subjected to one of seven different treatments that simulated conditions experienced during inundation: (1) hydration, (2) agitation, (3) hydration and agitation, (4) hydration and agitation with sand, (5) osmotic shock, (6) terrestrial hypoxia, and (7) aquatic hypoxia. Hatching rates increased significantly under all simulated tidal conditions compared to controls and were highest (96%) for eggs simultaneously exposed to both hydration and agitation with sand. Measurements of the osmolarity of the perivitelline fluid of developing eggs collected from the field indicated that it is hyperosmotic to the ambient seawater and porewater. Thus, when inundated, eggs also experience a hypoosmotic shock, which would likely facilitate hatching by causing the eggs to swell, rupturing the egg membrane and thereby increasing the likelihood that larvae would hatch and enter the water column during periods of high water.

Endogenous swimming rhythms of larval Atlantic menhaden, Brevoortia tyrannus Latrobe: Implications for vertical migration

Atlantic menhaden, Brevoortia tyrannus Latrobe, spawn on the continental shelf. Larvae are transported shoreward where they enter estuaries and metamorphose. Field studies suggest that while offshore, larvae may undergo nocturnal diel vertical migration (DVM), in which they are near the surface during the night and at depth during the day. The DVM pattern is more pronounced in estuaries but the timing of the nocturnal ascent may be related to tides. Larvae appear to swim in the water column during nocturnal rising tide and are less abundant at all other times. This migration pattern would result in up-estuary movement by selective tidal stream transport. The present study measured endogenous swimming rhythms of Atlantic menhaden larvae and related them to possible vertical migration patterns in offshore and estuarine areas. Larvae reared in the laboratory on a diel light:dark cycle were measured at three sizes (7–9 mm, 14–17 mm and 23–27 mm total length [TL]) as representative of larvae in offshore areas, while estuarine larvae (25–30 mm TL) were collected in the Newport River Estuary (N.C.). Swimming was monitored in a column under constant conditions in the laboratory with a time-lapse video system. In all cases, larvae had a similar circadian rhythm. The smallest laboratory reared larvae ascended into the upper portion of the column during the time of night and descended during the time of day. The other laboratory reared and field caught larvae had increased swimming activity during the time of night and reduced activity during the time of day. This circadian activity rhythm would contribute to the nocturnal DVM in offshore and estuarine areas. However, there was no evidence that larvae developed a tidal rhythm in activity that could contribute to selective tidal stream transport in estuaries.