Swimming and buoyancy in ontogenetic stages of the cushion star Pteraster tesselatus (Echinodermata: asteroidea) and their implications for distribution and movement

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The eggs of some marine fish (1) and benthic invertebrates such as many corals (2, 3) and lecithotrophic echinoderms (4, 5) are positively buoyant at time of release from the parent, and density increases later in ontogeny. How these eggs and larvae are distributed in the water column and eventually reach suitable habitat for settlement will depend, in part, on their vertical velocity and on the turbulence in the water (i.e., the eddy diffusivity). For eggs and unhatched stages, vertical velocity is passive and depends on egg or embryonic volume and density relative to the seawater (6, 7). For motile stages, vertical velocity depends on relative density, swimming ability, and behavior of the larvae (8, 9). We have measured the vertical velocity of eggs and larvae of the sea star Pteraster tesselatus Ives, which spawns floating eggs (1.1 to 1.5 mm diameter) that develop into nonfeeding larvae and spend several weeks in the plankton before settling to the benthos (10). Because of the simple shapes of eggs a...