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Patterns in Early Embryonic Motility: Effects of Size and Environmental Temperature on Vertical Velocities of Sinking and Swimming Echinoid Blastulae

Friday Harbor Laboratories and Department of Biology, University of Washington, 620 University Road, Friday Harbor, Washington 98250-9299

^* E-mail: kamcdon{at}u.washington.edu

Early embryonic swimming is widespread among marine invertebrates, but quantitative information about swimming behaviors is scarce. Swimming may affect encounters with predators, positioning in the water column, and nutrient absorption. Measured rates and patterns of swimming and sinking for blastulae of four eastern Pacific echinoid species show that sinking speeds equal or exceed swimming speeds. Swimming speed scaled negatively with embryo size, though sinking speed did not scale with size. Analysis of swimming paths of Strongylocentrotus franciscanus revealed a temperature dependency in swimming pattern that affected speed of upward movement. Sinking speeds were significantly greater at 10 °C than at 14 °C for blastulae of all four species examined. In Dendraster excentricus, killing the blastulae annulled this temperature effect, indicating an active density regulation by these embryos. Finally, measurements of particle velocities around sinking and swimming D. excentricus blastulae show that swimming creates a more localized disturbance than sinking. Embryonic swimming may therefore decrease rather than increase encounters with pelagic predators. Results from subsequent experiments in which embryos were reared in low-oxygen environments suggest that any oxygen-absorption advantages of swimming have little, if any, effect on the development of D. excentricus embryos.