Demonstration of sperm chemotaxis in Echinodermata: Asteroidea, Holothuroidea, Ophiuroidea

Abstract The sperm of starfish (Asteroidea), sea cucumbers (Holothuroidea), and brittle stars (Ophiuroidea) are chemotactically attracted to the tip of a micropipette releasing very small volumes of egg water or seawater solution of an alcoholic extract of ovaries or spawned eggs. The sperm are unresponsive to injections of seawater or a weak solution of ammonium chloride in seawater. No other echinoderm tissue tested yields an extract with sperm‐attracting activity. A complex set of species‐specificities has been demonstrated between many of the genera and species used in these experiments. Plotting of the paths of chemotactic sperm reveals that they approach the tip of the pipette along a path consisting of small loops alternating with straight segments orientated directly up the gradient. This is similar to the chemotactic behavior previously reported for the sperm of hydrozoans, molluscs, and urochordates. Sperm velocity does not change in response to the sperm attractant. Attracted sperm remain motile long after they have been attracted and do not agglutinate. In at least some cases the attraction response is reversible. This is the first direct evidence of sperm chemotaxis in echinoderms.