Gonads of larval Geotria australis destined to become ovaries can be readily identified well before the start of metamorphosis, whereas the majority of presumptive testes of large larvae and some metamorphosing individuals remain indistinguishable from the undifferentiated gonads of smaller larvae. Gonial proliferation in the female germ line, which occurs earlier and is more intense than in the male line, produces cystic gonads containing large numbers of meiotic germ cells. The growth rate of those oocytes surviving the larval cytoplasmic growth phase (25%) increases by about three times at metamorphosis. Although mean cell counts for the undifferentiated and future male testes rose slightly with increasing body length, many larvae of metamorphosing length, as well as transforming animals, possess a minute gonad with cell numbers no greater than those of small ammocoetes.

Differences in the timing of gonial proliferation and the onset of oogenesis during the larval life of holarctic lampreys have been correlated with differences in the fecundity and body size of the mature adult. Geotria conforms to this trend and parallels most closely landlocked Petromyzon marinus, a form whose fecundity and body size lie between those of the largest anadromous parasitic lampreys and the small nonparasitic species. However, the pattern of gonadogenesis and sex differentiation in Geotria differs markedly from holarctic lampreys. Both male and female gonads are relatively ‘immature’ and more variable at the onset of metamorphosis. At this stage, the ovaries contain small oocytes (43·1 μm diameter), frequently with undeveloped cytoplasmic basophilia, and in many cases still possess premeiotic cysts. During metamorphosis, the future testes show no acceleration of mitotic activity and remain undifferentiated, small and variable. Geotria also exhibits a more direct type of sex differentiation by lacking the characteristic juvenile hermaphrodite phase of holarctic lampreys. Thus no evidence was found for the origin of testes through the extensive regression and atresia of gonads of the ovarian type. Indeed, meiotic gonia and oocytes were seen in only 17% of future male testes.