Aspects of the reproductive biology of <i>Gonostoma bathyphilum</i> (Gonostomatidae)

Protandrous hermaphroditism among deep‐sea fishes is known only within one family, Gonostomatidae, and therein restricted to a few species of two genera, Cyclothone (C. atraria, C. microdon) and Gonostoma (G. elongatum, G. gracile). Gonostoma bathyphilum, the deepest dwelling (700–2700 m depth) of Gonostoma spp., is also protandrous and shares with Cyclothone spp. an expression of sexual dimorphism manifested by the excessive development of olfactory structures, including the olfactory lobes, in ripe males. Sex‐reversal in ripe males, therefore, involves a regression of these structures. The relationships apparent between sex, maturity and olfactory development, and those between animal size, maturity and depth of occurrence, indicate that whilst protandry is an integral component of the species' reproductive strategy some individuals develop directly as females and that among males (max. size 150 mm S.L.) sex‐reversal is not obligatory. Primary females, together with those derived from prematurely reversing males, may form an early maturing female group, spawning for the first time at about 110–120 mm S.L. Most hermaphrodites were &lt; 100 mm S.L., and within the 70–99 mm S.L. siz