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The Biological Bulletin, Vol 197, Issue 3 309-314, Copyright © 1999 by Marine Biological Laboratory

RESEARCH NOTE

Swimming and Buoyancy in Ontogenetic Stages of the Cushion Star Pteraster tesselatus (Echinodermata: Asteroidea) and Their Implications for Distribution and Movement

D. Kelman and R. B. Emlet
Department of Zoology, George S. Wise Faculty of Life Sciences, Tel-Aviv University, Ramat Aviv, Tel Aviv 69978, ISRAEL

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 and larvae, we used force balance equations for drag and buoyant forces to determine the density of eggs and two larval stages. Initially the eggs were positively buoyant and floated upwards at about 1 mm/s. Even formalin-fixed eggs floated in seawater, so concentrations of light ions were not responsible for the buoyancy. The density of the larvae increased in the jrst 10 to 11 days, but it varied considerably between the three larval cohorts examined. Ten-day-old larvae that were negatively buoyant swam downward at mean speeds as high as 1.7 mm/s, while positively buoyant larvae of the same age swam upward at about 1 mm/s. These patterns of buoyancy and swimming velocity should initially facilitate dispersion and later promote settlement into subtidal habitats.


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