Morphology and Physiology of Lamprey Metamorphosis

Metamorphosis is highly synchronized in both landlocked and anadromous sea lampreys, Petromyzon marinus, and can be divided into seven clearly defined stages. Although there may be some subtle internal changes several months before the beginning of external metamorphosis, most internal and external changes are initiated simultaneously in early July in P. marinus. The time and degree to which various external anatomical features take place does not differ to any large degree in landlocked and anadromous sea lampreys. Observations on P. marinus and other lamprey species shows that variations in the time of completion of transformation or development of organs reflect the immediate needs of the animal. For example, transformation of the liver and development of the kidneys occur very early (stage 4) whereas organs and tissues involved in osmoregulation, feeding, and digestion do not reach their definitive adult condition until later stages. All organ systems seem to undergo some morphological transformation during metamorphosis. After the beginning of metamorphosis lipid is utilized as the primary source of energy during the nontrophic period which may last up to 10 mo. Physiological modifications accompanying the initiation of metamorphosis include changes in rate of oxygen consumption, in serum osmolarity and thyroxine levels, in the biochemistry of the blood, and in aspects of intermediary metabolism. There is no definitive information as to the factor(s) which control the metamorphic process in lampreys.Key words: lampreys, metamorphosis, morphology, physiology