UMBILICAL BROOD PROTECTION AND SEXUAL DIMORPHISM IN THE BOREAL PACIFIC TROCHID GASTROPOD,<i>MARGARITES VORTICIFERUS</i>DALL

Abstract

Specimens of Margarites vorticiferus Dall collected at Attu Island, Alaska, during July 1979, were found brooding eggs and young in the umbilicus. The number of developing young in each brood ranged between 471 and 3264. Each brood contained several developmental stages including uncleaved eggs, unhatched embryos, hatched pro-toconch-bearing juveniles and two distinct juvenile size classes. The umbilicus of M. vorticiferus is significantly (t-test; P < 0.001) larger in female specimens than in male specimens. Margarites vorticiferus is the largest member of the genus found at Attu and the only member known to brood its young.