Comparison of the internal morphology of adults of a population of lampreys that contains a nonparasitic life-history type, Lampetra richardsoni, and a potentially parasitic form, L. richardsoni var. marifuga

Abstract

Histological techniques were used to study adults of a single population of the brook lamprey, Lampetra richardsoni, in Morrison Creek, Vancouver Island, British Columbia, to establish whether there are any differences between the internal morphology of a "silver" variety (L. r. var. marifuga) and that of the nonparasitic variety (L. richardsoni). The results of the study support the view of two life-history types in this creek. When L. richardsoni is sexually mature and has an atrophied intestine and sparse haemopoietic tissue, most L. r. var. marifuga possess a maturing (not totally mature) gonad, a highly developed alimentary canal containing ingested food, and extensive haemopoietic tissue. Although the gills, thyroid gland, and the definitive kidneys are similar in both forms, incomplete regression of the larval kidney and the absence of a cranial pancreas in all L. r. var. marifuga imply that a slow sexual maturation is accompanied by a slow, and perhaps incomplete, metamorphosis. The incomplete metamorphosis is particularly exemplified in female L. r. var. marifuga and may account for the abnormal sex ratio of this variety. The retardation of sexual maturation in male L. r. var. marifuga permits the retention of a functional digestive system and, hence, provides an opportunity for feeding. The significance of these observations to present views on the evolution of lampreys is discussed.