The Biology of Metamorphosis in Sea Lampreys: Endocrine, Environmental, and Physiological Cues and Events, and Their Potential Application to Lamprey Control

Abstract

The present study describes 20 years of research driven by the view that new control methods may be a product of an extensive examination of the cues and events of sea lamprey ( Petromyzon marinus) metamorphosis. A rise in water temperature (and not its magnitude) from its winter minimum is a requirement for spontaneous metamorphosis in sea lampreys. Neither density, starvation, nor photoperiod have been proved in laboratory studies to influence metamorphosis. Thyroid hormones (TH) in the serum reach a peak level just prior to metamorphosis and decline sharply at the initial stage of this phase. Precocious metamorphosis can be induced in sea lampreys and other lamprey species by most goitrogens; the induction is accompanied by a decline in serum levels of TH. Lipogenesis and lipolysis are key events in sea lamprey metamorphosis and they involve a complex interaction between TH, insulin, and somatostatin. Many larval molecules are deleted, altered, or replaced by adult forms during metamorphosis in sea lampreys and some of these do not show a similar developmental profile in a nonparasitic species ( Lampetra appendix). Albumin is one of the molecules that changes from a larval to an adult form during sea lamprey metamorphosis but is not replaced by an adult albumin in L. appendix. The functional significance of this interspecific variation needs to be examined, for it may be relevant to adult feeding behavior. Future studies that compare the development of nonparasitic and parasitic adult life histories, however, should be performed on paired (statellite) species. The redistribution of nonhaem iron at this time emphasizes the unique feature of tissue iron-loading that exists in larval lampreys. Features of sea lamprey metamorphosis are discussed in a context of their potential relevance to the control program.