Metamorphosis in the Southern Hemisphere lamprey, Geotria australis

This paper provides the first detailed qualitative and quantitative description of metamorphosis in the lamprey Geotria australis , which is the sole representative of the exclusively Southern Hemisphere family Geotriidae. Using a description which separates metamorphosis into seven sequential stages, emphasis has been placed on the pattern of development of ihe unique adult characters found in this family. Such characters include the spatulate shape of the teeth, the markedly tridentate anterior lingual lamina, the prominent longitudinal dorso‐lateral blue‐green stripes, the pair of enlarged oral disc papillae, and the gap between the second dorsal and caudal fins. Moreover, body proportion data demonstrate that compared with other genera, Geotria has an unusually small eye and oral disc and a relatively long trunk. While during metamorphosis the length remained relatively constant, due to compensatory increases in the actual length of some parts of the body against losses in other regions, both the mean weight and the condition factor declined. The latter reductions are clearly related to the fact that the animal does not feed during the six months between the onset of metamorphosis in late January and early February and the time when the animal embarks on its movement towards the sea in late July and early August. The length and weight data suggest that G. australis enters metamorphosis at a smaller size than any other parasitic species of lamprey. The sex ratio in the 218 individuals whose gonad was examined, indicated that the populations in south‐western Australia were characterised by a slight excess of males (1♀: M8♂). The mortality encountered when downstream migrants were transferred to 100% sea water was 36% after direct transfer and 10–12% after prior acclimation to reduced salinities.