Physiological ecology of Adalaria proxima (Alder et Hancock) and Onchidoris muricata (Müller) (Gastropoda: Nudibranchia). II. Reproduction

The nudibranch molluscs Adalaria proxima and Onchidoris muricata are semelparous annuals. Although these species are ecologically, morphologically, and taxonomically similar, their reproductive characteristics differ markedly. In the present study, A. proxima individuals generally attained a larger size and commenced spawning later in the spring than O. muricata . The total energy invested in spawn production by A. proxima individuals was ≈ 1.5 times that for O. muricata , however, A. proxima produced this in fewer spawn masses and over a shorter spawning period. Correspondingly, the mean energy content of A. proxima spawn masses was greater (≈ 3 times) than that of O. muricata spawn. Nonetheless, the “average” A. proxima spawn mass contained only an estimated 675 ova in comparison to the 3300 ova in an “average” O. muricata spawn mass. This difference was due to the contrasting larval types exhibited by these species. A. proxima produces large well-provisioned ova which hatch as pelagic lecithotrophic veligers, whereas O. muricata ova are relatively small and develop into long-term planktotrophic larvae. Individuals of both species were observed to undergo degrowth throughout their spawning periods, but total production (soma plus cumulative spawn) continued to rise at a rate equal to, or slightly greater than, prespawning growth rate. Overall production patterns showed considerable intraspecific consistency, but differed between species. Degrowth rate (per day) for A. proxima was in excess ofthat for O. muricata , however, all individuals died after ≈33% of the maximum somatic weight had been lost.