Maturation, Spawning and Development in Siphonaria Pectinata Linnaeus (Gastropoda: Pulmonata) at Gibraltar

A comparative study of some aspects of the reproductive biologyof Siphonaria pectinata was carried out at Gibraltar. Maturationoccurred at 5-6 mm shell length corresponding toan age of 6-12months. At the more sheltered Atlantic site spawning was evidentfrom March toJune 1992 with peak spawning occurring in May-June1992. The patterns of spawning activityfor the following twoyears were very similar. Egg ribbons were very scarce at themore exposedMediterranean site and were found in considerablyless quantities than at the Atlantic site.Individuals at bothsites contained mature eggs in their gonads at the onset, andthen throughoutthe duration of the spawning season, but exhibitedspent gonads by October. The spawn of S.pectinata consistedof small oval ribbons filled with numerous egg-shaped capsulesspacedirregularly in a jelly-like matrix. Spawn was depositedmainly away from the home scar inshallow depressions, cracksand crevices and between barnacles and mussels. S.pectinataexhibited planktonic development. The fertilised eggs took between14 and 21days to develop at ambient temperature (14-19°),and hatched as free-swimmingplanktotrophic veligers. The timingof spawning activity and the placement of egg masses inprotectivemicrohabitats on the shore are discussed in terms of adaptationsto reduce desiccationstress, predation and possible food shortages.The mode of development in S. pectinata is placed in contextwith current theories regarding siphonariid ancestry and evolution. (Received 6 January 1998; accepted 25 June 1998)