A comparison of the metamorphosing and macrophthalmia stages of the lampreys Lampetra fluviatilis and L. planeri

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Abstract
Comparisons of metamorphosing and macrophthalmia stages of the closely related species, L. fluviatilis (L.) and L. Planeri (Bloch), have shown that these can be distinguished within a few weeks of the onset of metamorphosis by characteristic differences in colouration and body form. Measurements of several body intervals have disclosed differences between the macrophthalmia stages of the two species. A sharp distinction between the blunt teeth of L. planeri and the supposedly sharp teeth of L. fluviatilis has not been confirmed in these early stages, but significant differences have been found in the numbers of teeth in the anterior field of the oral disk and in the lateral and posterior marginal series. For material from several rivers, the range of length of metamorphosing and macrophthalmia stages of L. fluviatilis is83–119 mm (mean 99.3 mm). Weights varied from 0.71‐2.5 g (mean 1.51 g). Regression coefficients for weight on length are much lower in the macrophthalmia of L. fluviatilis than in comparable stages of L. planeri. Total oocyte counts on macrophthalmia of L. fluviatilis gave values from8000–20,000 which are in general agreement with egg counts for adults of this species in the river Severn. Observations on the earliest metamorphosing forms have shown that it is not possible at this stage to distinguish the males of the two species by the structure of the testes. The development of the lumen in the adult foregut of L. fluviatilis has been shown to be variable and in some instances the gut does not become patent until early Spring. The possibility has also been raised that, in exceptional cases, a temporary lumen may also be present for a short period in L. planeri. Observations on the foregut and dentition, together with field data, suggest that the climax of downstream migration and the onset of parastic feeding takes place in late March or April.