SPERM CHEMOTAXIS IN OIKOPLEURA DIOICA FOL, 1872 (UROCHORDATA: LARVACEA)

An alcohol extract of unfertilized eggs of the larvacean, Oikopleura dioica, can attract sperm over a distance of at least 80 µm from an artificial source. The sperm, which normally swim in wide circles or straight lines, alter their path to form small loops between straight or slightly curved segments directed up the gradient. During the first loop, the velocity of sperm increases 50%. The new velocity is maintained as long as the cells are influenced by the attractant. Once sperm reach the center of the gradient, the path alters to the form of enlarging concentric circles which eventually attain the diameter of the circles made in sea water. O. dioica sperm and sperm attractant are species-specific in tests against attractants and sperm of sessile tunicates. It has not yet been possible to test the species-specificity against other larvaceans. We estimate that sperm chemotaxis in O. dioica increases the chance of sperm-egg collisions from 4 to 15 times. This is mainly due to an increase in apparent diameter of the egg and also to an increase in the velocity of attracted sperm. Rapid population increase is characteristic of O. dioica under appropriate conditions. An increase in the probability of fertilization produced by sperm chemotaxis may be an additional factor leading to decreased generation time for the population as a whole.