To examine the problem of how far coral larvae disperse from their natal reef, coral recruitment densities were experimentally determined at distances up to 5 kilometers from a small, relatively isolated platform reef, Helix Reef, on the central Great Barrier Reef for 7 months. High concentrations of recruits, accounting for up to 40 percent of all recruitment, were found downstream of the reef in areas of high water residence time, suggesting that near-field(proximal) circulation has a profound influence on dispersal and recruitment of coral larvae. Coral recruitment declined logarithmically with distance from the reef, decreasing by an order of magnitude at radial distances of only 600 to 1200 meters. On an ecological time scale, advective dispersal of semipassive marine larvae with relatively short planktonic lives(minimally days) may be extensive, but success of recruitment is highly limited. Through evolutionary time, sufficient dispersal occurs to ensure gene flow to reef tracts hundreds or possibly thousands of kilometers apart. In the short term, however, coral reefs appear to be primarily self-seeded with respect to coral larvae.

In a large-scale oceanographic study, we attempted to determine whether reefs of the central Great Barrier Reef are primarily self-seeded with respect to corals. Twenty-four moorings were deployed for 7 months around the small, relatively isolated Helix Reef (diam = 800 m). Settling plates were placed on moorings 15-l 8 m below the surface. Moorings were placed 0,0.3, 0.6, 1.2, 2.5, and 5.0 km from the reefs perimeter. Water movements were recorded concurrently via current meters and tide gauges. Most larvae settled on or within 300 m of the reec settlement was generally lower in pocilloporids than acroporids. Most spat in the far field (acroporids) were derived from corals utilizing externally fertilized eggs with an obligate planktonic larval development period of 24-72 h, implying higher effective dispersal capabilities. Densities of settling pocilloporids (obligate brooders), however, were highest directly on the reef. Postsettlement mortality of recruits decreased significantly with distance from the reef. Coral spat were concentrated in two areas where water residence times were high and flushing rates were low. Highly localized circulation patterns appear to retain larvae near the reef. Effective larval dispersal in corals seems to be a small-scale phenomenon, although measurable far-field dispersal indicates sufficient gene flow between reefal populations to prevent allopatric speciation.The distance over which reproductive propagules disperse and the probability of their successful colonization of a given habAcknowledgments

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The active space is the zone around an alga where chemical exudates are sufficiently concentrated to promote a behavioral response in a grazing copepod. Chemosensory perception of algae by copepods is investigated with a numerical model of the effects of advection in copepod feeding currents on the chemical diffusing from algae. Recent observations of encounters between algae and calanoid copepods define length and speed scales for a doubly sheared, low Reynolds number flow. These scales and shears are incorporated in an analytical description of the feeding current. Advection of the chemical in the laterally sheared flow changes the distribution of the exudates to create a deformed active space. An initially spherical active space is elongated and the edge of the active space is projected an order of magnitude further in the direction of the flow than the undisturbed radius. The advance warning received by a copepod of an approaching alga is a function of the radius of the active space, the detection threshold, the position of the alga in the stream field, and the molecular diffusivity of the exudate. An example of the relation between radius, detection threshold, and diffusivity for a detection distance of 1000 μm demonstrates that the model is well behaved for widely varying parameter values and loosely defines some bounds for the controlling parameters.

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