Metamorphosis of the Southern Brook Lamprey, Ichthyomyzon gagei

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Abstract
Metamorphosis of the nonparasitic southern brook lamprey, I. gagei, is described within 7 stages on the basis of external structural changes. The sequence of external changes including the development of eyes, oral disc, teeth, tongue, oral fimbriae and lateral line was remarkably consistent among individuals. The initiation of metamorphosis during late Aug.-early Sept. is considerably later than that found for many northern-hemisphere lampreys. The duration of metamorphosis in I. gagei, approximately 6 mo., is longer than that required by most species. Sex ratio of metamorphosing and adults showed a slight preponderance of females. No evidence was found to indicate differences in distribution by sex. The alimentary canal, aside from the formation of a nonfunctional adult esophagus, regressed during metamorphosis, presumably serving as an ancillary energy source. Integument thickness increased progressively throughout all but the final stage of metamorphosis. Changes in hematocrit values and Hb concentration suggest the site of hemopoiesis changes from the typhlosole and nephric fold of the kidney to the fat column after stage 2 of metamorphosis. Some of the changes realized during metamorphosis of I. gagei are important to the adult period: others represent a manifestation of their presumed ancestral origin.

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