Sex allocation in a simultaneous hermaphrodite, the zebra goby Lythrypnus zebra: insights gained through a comparison with its sympatric congener, Lythrypnus dalli

Synopsis The sexual pattern of the zebra goby Lythrypnus zebra, is an apparent exception to sex allocation theory. Most L. zebra are simultaneous hermaphrodites (i.e., have active female and male gonadal tissue), yet it appears they do not reproduce as males and females simultaneously. Understanding the maintenance of simultaneous hermaphroditism in L. zebra could expand sex allocation theory. In this study, I used a comparison with the blue-banded goby, Lythrypnus dalli, a sympatric congener with a qualitatively similar sexual pattern, to investigate the role of male spawning rate, body size and sexual flexibility in determining the sexual pattern of L. zebra and to isolate differences between the species that might explain their differing sexual patterns. Using field measurements of male nesting success, I found no differences between the species in the body size of nesting males suggesting that large size is associated with successful male reproduction in both species. In addition, nesting males spawned at approximately three times the rate of females in both species; thus, reproduction via male function can be equally advantageous relative to adopting the female role. However, the nest longevity of L. zebra males was shorter than that of L. dalli males, suggesting reproduction via male function may be less reliable in L. zebra. Finally, under laboratory conditions, L. zebra females tended to prefer large mates, and L. zebra were able to re-allocate in both directions, exhibiting a greater capacity to switch than L. dalli. Given these results, I suggest that switching between the sexes plays a greater role in maintaining simultaneous hermaphroditism in L. zebra than L. dalli, perhaps because male reproduction is not as consistent in L. zebra. Sexual flexibility may be an important factor affecting patterns of sex allocation, generally.