Reproduction and larval development of Polydora robi (Polychaeta : Spionidae), an obligate commensal of hermit crabs from the Philippines

The reproduction of a recently described spionid polychaete, Polydora robi, is examined from the Philippines. Adults inhabit a burrow in the apex of g astropod shells occupied by hermit crabs. Females were found to deposit bro ods of 18-94 egg capsules in the summer (June-August) and winter months (Ja nuary-March) sampled. Paired or single egg capsules are attached by stalks to the inside wall of the burrow and contain 40-106 eggs which average 97 m um in diameter. The total number of eggs per brood ranges from 941-8761 egg s and is positively correlated with the total number of segments and length of female worms. Adults of P. robi are polytelic, producing less than or e qual to9 successive broods over a 3-month period; a mean of 6.7 d was exhib ited between broods in the laboratory. Females utilize sperm stored in the seminal receptacles during successive spawnings. Development occurs within egg capsules until the 3-segment stage, at which time the planktotrophic la rvae are released. Juveniles of similar to 20 segments are competent to set tle on gastropod shells inhabited by hermit crabs. Members of P. robi are r elatively fecund, semicontinuous breeders; the life-cycle in this species i s similar to the only other known obligate polydorid commensal of hermit cr abs.