To determine how physical processes and biological behaviors influence larval dispersal on the inner shelf, time series of larval concentrations were quantified during August 1994, on the Outer Banks of North Carolina, U.S.A. Zooplankton pumps, moored in 21 m of water at 3.2, 8.7, and 12.2 m above bottom, collected larvae every 3 h for 3 weeks. Physical variables and larvae were sampled at similar time and space scales. Larval concentrations were typically 10 2 -10 4 m Ϫ3 for polychaetes, bivalves, and gastropods and 10-10 2 m Ϫ3 for brachyuran crab zoeae. There were two dominant scales of variability, 3-6 h and 2-10 d. The high-frequency signal is partially explained by diel vertical migrations-nocturnal ascent and daytime descent. This pattern would allow larvae to feed in subthermocline waters while avoiding visual predators. Low-frequency variations tracked with water temperature. Highest concentrations of worm larvae occurred in cool (upwelled) water and of crab zoeae in warm (downwelled) water. At least two larval groups comprised the clam and snail time series, one with fairly high abundances in cool water and the other with peak concentrations in warm water. Wind-driven cross-shelf transport is the most plausible explanation for these low-frequency fluctuations. For example, dense patches of worm larvae overlying parental habitat (offshore muds) would be carried shoreward in cool, upwelling flows. In contrast, brachyuran zoeae in nearsurface waters would descend at the coast during downwelling and, together with larvae aloft nearshore sediments, be transported seaward below the thermocline. Thus advected by strong along-shore and weaker cross-shelf currents, larvae zigzag up and down the coast. Vertically traversing the water column while they feed and grow, larvae ultimately seek a suitable habitat in which to settle.The inner portion of the continental shelf (roughly 5-30 m depth) is a dispersal corridor between intertidal and offshore habitats. Replete with planktonic larvae, it also provides soft sediments for benthic adults. Physical processes, such as wind-driven upwelling and downwelling, thermal fronts, eddies, tides, internal waves, tidal bores, surface waves, and storms, operate over spatial scales of 1 m to 10 3 km and temporal scales of hours to weeks (Mann and Lazier 1991). In this high-energy environment, physics should play 1 Previously published as Cheryl Ann Butman. To whom correspondence should be addressed. Present address: Department of Biology, University of California, 621 Charles E. Young Drive South, Los Angeles, California, 90095-1606. AcknowledgmentsWe thank D. Simoneau, W. Ostrom, J. Bouthillette, L. Costello, C. Marquette, J. D. Sisson, V. Starczak, and the captains and crews of the R/V Endeavor and R/V Cape Hatteras for deploying and recovering the moorings. K. Doherty, K. Fairhurst, S. Longworth, and J.D. Sisson engineered and constructed the plankton pumps and moorings, for which we are grateful. We are indebted to numerous divers who battled the wicked Outer-Banks ...

Trematode parasites in intertidal estuaries experience constantly varying conditions, with the presence or absence of water potentially limiting larval transport between hosts. Given the short life spans (< or =24 h) of cercariae, emergence timing should be optimized to enhance the probability of successful transmission. In the present study, field measurements and laboratory experiments identified processes that regulate the emergence of cercariae from their first intermediate snail hosts in an intertidal marsh. Larvae emerged over species-specific temperature ranges, exclusively during daylight hours, and only when snails were submerged. The three factors operate over different temporal scales: temperature monthly, light diurnally (24-h period), and water depth tidally (12-h period). Each stimulus creates a necessary condition for the next, forming a hierarchy of environmental cues. Emergence as the tide floods would favor transport within the estuary, and light may trigger direct (downward or upward) swimming toward host habitats. Abbreviated dispersal would retain asexually reproduced cercariae within the marsh, and local mixing would diversify the gene pool of larvae encysting on subsequent hosts. In contrast to the timing of cercarial release, emergence duration was under endogenous control. Duration of emergence decreased from sunrise to sunset, perhaps in response to the diminishing lighted interval as the day progresses. Circadian rhythms that control cercarial emergence of freshwater species (including schistosomes) are often set by the activity patterns of subsequent hosts. In this estuary, however, the synchronizing agent is the tides. Together, exogenous and endogenous factors control emergence of trematode cercariae, mitigating the vagaries of an intertidal environment.

Foundation species provide critical resources to ecological community members and are major determinants of biodiversity. The barnacle Balanus glandula is one such species and dominates space among the higher reaches on wave-swept shores. Here, we show that B. glandula produces a 199.6-kDa glycoprotein (named "MULTIFUNCin"), and following secretion, a 390-kDa homodimer in its native state. MULTIFUNCin expression is localized in the epidermis, cuticle, and new shell material. Consequently, this molecule can specify upon contact the immediate presence of a live barnacle. Shared, conserved domains place MULTIFUNCin in the α-macroglobulin (A2M) subgroup of the thioester-containing protein family. Although previously undescribed, MULTIFUNCin shares 78% nucleotide sequence homology with a settlement-inducing pheromone (SIP) of the barnacle, Amphibalanus amphitrite Based on this and further evidence, we propose that the two proteins are orthologues and evolved ancestrally in structural and immunological roles. More recently, they became exploited as chemical cues for con- and heterospecific organisms, alike. MULTIFUNCin and SIP both induce habitat selection (settlement) by conspecific barnacle larvae. In addition, MULTIFUNCin acts as a potent feeding stimulant to major barnacle predators (sea stars and several whelk species). Promoting immigration via settlement on the one hand, and death via predation on the other, MULTIFUNCin simultaneously mediates opposing demographic processes toward structuring both predator and prey populations. As a multifunctional protein cue, MULTIFUNCin provides valuable sensory information, conveys different messages to different species, and drives complex biotic interactions.

Marine benthic invertebrates living in dense, intraspecific aggregations are important community members because they provide structural habitat for other species. Here, we determined the mechanisms that facilitate gregarious larval settlement and promote group living. Using suspension‐feeding oysters (Crassostrea gigas) residing in large assemblages (“reefs”), experiments were conducted under laboratory conditions that simulated critical aspects of natural estuarine habitats. Oyster larvae were attracted to the scent of their conspecific elders. In still‐water trials, they moved downward and settled after contacting a waterborne, adult chemical cue. Yet, mortality of larvae placed in the adult pallial cavity was very high (mean of 91.3%). This seeming paradox of larval attraction to adult cannibals was resolved via laboratory flume (2 cm/s and 6 cm/s flows) experiments. Suspension‐feeding activity did not significantly affect flow speeds or directions. Moreover, weak (mean of 1.65 mm/s) adult ciliary currents effectively entrained phytoplankton but rarely captured larvae. In fact, only a small percentage (≤4.6%) of settlers was cannibalized in flume trials, even when they passed within 1 mm of the inhalant opening, or “gape” (a narrow slit between two valves). Larvae cued by conspecifics potentially attach to any portion of the shell surface, but there is a low probability that they will land in or near the inhalant opening. On juvenile and adult oysters, for example, the mean ratio of gape to shell surface area was only 0.025. Furthermore, in surveys of juvenile/adult oysters at nine field sites (Hood Canal and eastern Olympic Peninsula, Washington, USA), the gape was ≤5.2% of the total plane surface area of the reef. Thus, an oyster larva settling onto a reef of suspension‐feeding adults is unlikely to be cannibalized. Given this low mortality risk at settlement, future fitness payoffs (e.g., improved fertilization success) may drive the evolution of a gregarious settlement cue that promotes group living.