Gonadal development and infrequent sex change in a population of the humbug damselfish, Dascyllus aruanus in continuous coral-cover habitat

The expression of protogyny often differs among populations of a single species. The humbug damselfish, Dascyllus aruanus (Linnaeus), typically lives in spatially discrete groups of several individuals around live coral colonies and has been reported to be protogynous. The size and sexual composition of groups of this species, however, vary with the size of the coral patch or the degree of coral cover. The present study examined gonadal development and sexual pattern in a population of D. aruanus living in continuous coral-cover habitat. Fish were collected in Tumon Bay, Guam (13degrees31'N; 144degrees47'E), during a 2-week period in July and August 1996. Reflecting their distributions, small fish (n = 54) were collected at depths of 1-2 in over continuous Porites spp. patches, and large fish (n = 56) were collected at depths of 2-3 in over a continuous Acropora spp. patch. Gonadal development was examined histologically. Gonads of D. aruanus first developed an ovarian lumen and primary-growth stage oocytes. From this ovarian state or from more developed ovaries with cortical-alveolus stage oocytes, some gonads developed into testes through degeneration of oocytes and development of spermatogenic tissue. Developing spermatogenic tissue only occurred in gonads having pre-cortical alveolus stage oocytes, and the size of individuals with gonads of this type overlapped mainly with the size range of individuals with gonads containing only previtellogenic oocytes (i.e. immature females). This suggested that functional female-to-male sex change was rare or absent in this population of D. aruanus. Moreover, the different habitat distributions of small and large fish suggest that sex determination in immature fish occurs in the absence of adults.