Patterns of reproductive morphology in the genus Gobiodon (Teleostei: Gobiidae)

The reproductive system of gobiid fishes (family Gobiidae), especially among hermaphroditic goby species, is morphologically diverse. Two hermaphroditic species in the genus Gobiodon, G. okinawae and G. oculolineatus, have several modifications of the gonoduct and gonad that are associated with secretion production and storage. In this study, an examination of six additional Gobiodon species, G. citrinus, G. fulvus, G. histrio, G. micropus, G. quinquestrigatus and G. rivulatus, revealed similar reproductive modifications. Among these six Gobiodon species, a number of features were found to be shared amongst each other and with G. okinawae and G. oculolineatus. All individuals had either an ovariform gonad or an ovotestis; no individuals had a purely testicular gonad. The gonadal lobes extended caudally past, rather than terminating at, their union with the gonoduct. Accessory secretory structures associated with the reproductive complex, termed accessory gonoduct structures, or AGdS, always originated from the gonoduct. The ovariform gonad was comprised entirely of ovarian tissue, while the ovotestis was divided into three morphologically distinct regions. Only one of the ovotestis regions was strictly gametogenic, consisting of both early stage oocytes and sperm-filled seminiferous lobules. The second region of the ovotestis was made up of stromal tissue surrounding some compressed lumina and a small number of early-stage oocytes. The third region was highly lobulated and acted as a storage region for eosinophilic secretions. Anteriorly, the stromal region of each of the two ovotestis lobes disappeared and the gametogenic and secretory storage regions of the ovotestis separated into two discrete lobes. In all of the examined Gobiodon species, all individuals having an ovotestis also had AGdS. However, AGdS presence among individuals having an ovariform gonad varied in a species-specific manner, with the AGdS being fully differentiated and well-developed in a number of species, and either in a very early stage of development, or absent, in others. The distribution of these AGdS states among Gobiodon species corresponds with that of several other morphological features that have been proposed by Harold et al. (Bull Mar Sci 82:119–136, 2008) as phylogenetically informative for intra-generic clade identification. Reproductive characters may prove informative in the development of hypotheses of relationships among gobiid fishes.