ABSTRACT
Gelatinous zooplankton reproduce in an environment inimical to fertilization success because of the high potential for gamete dilution and the often wide dispersion of reproductive adults. Organisms in this group that shed sperm externally but fertilize internally face additional barriers to high fertilization success. Special reproductive adaptations probably exist to counter this problem. However, most of the inferences made about fertilization success in marine organisms are based on results using benthic species, gelatinous zooplankton being very difficult to capture and maintain in sexual condition. As a result, very little has been done to determine what sort of adaptations to enhance fertilization rates exist in gelatinous species. Furthermore, there is little direct evidence supporting mutual approach of the sexes that would bring the ripe gametes closer together. Close approach may be accomplished by a number of physical agencies, however, and possibly by gamete adaptations such as long-lived sperm or special egg investments which trap sperm. Chance contact chemoreception (which leads to mutual spawning, sperm transfer during psuedocopulation, or true copulation) coordinates reproductive behavior in some species. Free-spawned gametes may be mixed and more widely dispersed as the result of active spawning behaviors which are temporally coordinated by environmental factors such as light and temperature. Sperm chemotaxis has been described in gelatinous zooplankton, though its effect on fertilization rates is, at present, unknown. Except for sperm behavior in response to egg or tissue-associated factors, other chemosensory interactions or adaptations that might increase fertilization rates in gelatinous zooplankton have yet to be described.
