Relationships between external sexually dimorphic characteristics and internal gonadal morphology in a sex‐changing fish

The mangrove rivulus fish (Kryptolebias marmoratus) is an emerging model for research in the biological sciences. The species is androdioecious, with populations consisting predominantly of self‐fertilizing hermaphrodites and a low abundance of males, but no females. Males arise either through environmental sex determination at the embryonic stage or environmentally driven sex change from hermaphrodite at the adult stage. Marked sexual dimorphisms in external morphological characters accompany gonadal transitions, including the loss of a caudal ocellus (‘eyespot’) and the development of orange color in males. How well external morphological features predict internal gonadal morphology, however, has not been systematically examined across the geographical range of this species. Our goal was to identify reliable external morphological predictors of animals possessing an ovotestis (hermaphrodite) or a testis (male) and to see if the strength of prediction is consistent throughout rivulus’ geographical range. We demonstrate that the state of the ocellus and, to a greater extent, orange coloration are key predictors of internal gonadal state. However, the degree to which these characteristics are predictive of internal gonadal state was region‐specific, with the ocellus and orange status of animals from Bahamas and Belize, respectively, having a lower probability of forecasting internal gonadal state than those from other regions. These results will help to streamline laboratory studies by allowing unambiguous assignment of sex, and will assist field studies by providing greater opportunity to determine sex ratios in wild populations. We investigated the extent to which external characteristics can predict gonadal sex in a self‐fertilizing hermaphroditic fish, the mangrove rivulus. Both coloration and the presence of an ‘eyespot’ on the back of the caudal fin were good predictors of gonadal sex but their predictive power varied across rivulus’ geographical range. This study provides insights into the correspondence between external appearance and gonadal state in an animal with extreme sexual lability.