METAMORPHOSIS IN THE PAIRED SPECIES OF LAMPREYS, LAMPETRA-FLUVIATILIS (L) AND LAMPETRA-PLANERI (BLOCH) .1. DESCRIPTION OF THE TIMING AND STAGES

A large number of larval, metamorphosing and adult nonarasitic lampreys, lampetra planeri, were collected from three different rivers and placed in a morphological series. The characteristic changes were then described and used to propose a sequence of Stages (1–9). The first signs of metamorphosis (Stages 1–2), which could occasionaly be found as early as late June, are characterized by the eruption and enlargement of the eyes. This is followed by rapid transformation of the the oral hood into an oral disc, and changes in the shape of the pharyngeal region (Stages 2–5). Pronounced alterations in body pigmentation, enlargement of the fins and the development of teeth occur during Stages 4 to 6. The vast modifications involved in Stages 2 to 6 take place relatively repidly in the period between approximately mid‐July and mid‐September. The subsequent changes which occur more slowly, eventually lead to the production of immature (Stage 7) and sexually mature adults (Stage 8). Mature males are characterized by the presence of a urinogenital papilla, while the females posses a post‐cloacal fin‐like fold and greatly distended trunk. Spawning took place between late March and late April with the spent animals (Stage 9) dying soon afterwards. The above sequence of Stages was then compared with those found in a smaller sample of the parasitic and ancestral species L. fluviatilis. The early metamorphosing stages in both species are apparently indistinguishable but clear differences atart to appear at Stage 5 when the body surface of L. fluviatilis is assuming a distinct silvery sheen. This trend becomes more marked in Stages 6 and 7, at wich time the river lamprey also has a more pronounced eye and disc and a slimmer trunk. Stage 7 in L. fluviatilis represents animals migrating to the sea in either the autumn or spring to commence their trophic phase. On the other hand this Stage in L. planeri may be regarded as representing a massive contraction of adult Stages and is in many ways similar to the early upstream migrants of parasitic species. Copyright © 1979, Wiley Blackwell. All rights reserved