The distribution of fat in larval, metamorphosing, and young adult anadromous sea lampreys, Petromyzon marinus L.

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Abstract
A histological study using Sudan Black B has been made of the distribution and relative density of fat in the various regions of the body of larval (ammocoete), metamorphosing, and young adult stages of the anadromous sea lamprey, Petromyzon marinus L., caught in New Brunswick. The principal sites of fat storage are located in and around the fat column, lateral to the notochord, between the notochord and either the cardinal or caudal veins, surrounding the body cavity, beneath the skin, and in the myosepta and the nephric fold. Dense but more restricted sites are present lateral to the nerve cord, between the bundles of muscle fibres, and surrounding the pharyngeal cavity. Fat is most abundant in large ammocoetes captured in May and June and in those displaying the very earliest stages of metamorphosis in July. The amount of fat declines during metamorphosis, but is still abundant in November just prior to the time when the sea lamprey can commence parasitic feeding. Fat was undetectable, however, in nearly every site in those animals in which feeding was delayed until the following May. This study shows that sea lampreys accumulate large amounts of fat at the end of larval life by employing a wide variety of different storage sites. It also demonstrates that the fat in virtually all of these sites may be exhausted when the nontrophic period following the initiation of metamorphosis is of a very long duration.
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