Does Limited Brood Capacity Link Adult Size, Brooding, and Simultaneous Hermaphroditism? A Test with the Starfish Asterina phylactica

Abstract

In the sea star Asterina phylactica the exponent in the allometric equation that relates number of offspring in a brood to adult size decreases from early to late stages of brooding. This suggests proportionately greater losses from broods of larger adults and supports the allometry hypothesis for the association of small adult size and brooding. Allocation to reproductive effort as a male is low in A. phylactica. Production of eggs is the same as in nonbrooding asterinids. The low reproductive effort as males is possibly a result of self-fertilization and also some breeding in pairs. These observations suggest that gain from increased production of male gametes is more limited than gain from increased production of female gametes, and the hypothesis that simultaneous hermaphroditism is associated with brooding because of limited capacity to brood offspring is rejected. For this and similar cases, an alternative hypothesis for the association of brooding and hermaphroditism is prompted by the observation that when simultaneous hermaphroditism occurs in echinoderm species with separate sexes, there is no block to self-fertilization. Self-fertilizing hermaphrodites are favored when allocation to male function and decreased viability from inbreeding are both low. Inbreeding depression from selfing should be lowest in populations with no dispersal phase, as is the case for many brooders, because these animals are already inbred even when there are separate sexes. A few brooding A. phylactica contained developing offspring in the ovary. This observation supports the hypothesis that self-fertilization can lead to internal brooding.