The escape of veligers from the egg capsules of Nassarius obsoletus and Nassarius trivittatus (gastropoda, prosobranchia)

Abstract

Many species of prosobranch gastropod deposit their eggs in tough capsules affixed to hard substrates. Generally, there is a small opening near the top of such capsules, occluded by a firm plug (operculum) which must be removed before the veligers can escape. Although the removal of the operculum is generally attributed to embryonic secretion of enzymes, there is little experimental support for this sugsestion. In the limited experiments which have been reported, all dealing with species that emerge as juvenile snails, no attempt was made to determine the properties of the hatching substance, or the timing of its production. My research has dealt with the escape of veligers from the egg capsules of two related species, Nassarius obsoletus and N. trivittatus. Their egg capsules are quite similar in size, number of eggs contained, general morphology, and the thickness of the material plugging the opening at the top. Both hatch as swimming veligers, after about one week of encapsulated. development. By adding fresh plug material to small volumes of sea-water containing veligers obtained prior to, or at known times after their normal hatching, I have demonstrated conclusively the essentially chemical nature of operculum removal for these two species. In addition, the hatching substance was found to be produced in a short pulse, to be functionally short-lived, and to be species-specific in its action for the two species considered. There is no evidence that the secretion of the hatching substance is stimulated by short pulses of light or increased temperature; the capsules of N. obsoletus contain many more embryos than are needed to successfully remove the plug, so that complete synchrony of hatching substance production by all individuals within a capsule is probably not necessary. Lastly, the observed rates at which N. obsoletus veligers leave their egg capsules were compared with those predicted from an equation assuming random movement of individuals. A close agreement was found, the capsules losing 98% of their residents within 45 to 55 minutes of the first escape. Thus, the location of the exit by an individual is probably by chance.