Eight Stages were assigned to post-larval Lampetra planeri on the basis of their morphology and time of capture in three different rivers: River Honddu, South Wales; Highland Water, New Forest and a tributary of the River Wylye, Wiltshire. During transformation (Stages I - V), the ammocoete changed to resemble an immature adult while further gradual modifications culminated in maturity the following spring. Transformation was initiated at different times at the three sites, in an inverse sequence to the time of spawning. The post-larval period was therefore longest in the River Honddu and shortest at Longbridge Deverill. Measurement of total length and body intervals revealed differences in the development of the sexes. Thus, while females were longer at the onset of transformation, the position was generally reversed by spawning due to greater extension of the male prebranchial region and less shrinkage posterior to the cloaca. By this Stage, the males also had a relatively larger disc and a higher posterior dorsal fin. They were slightly more numerous than females at transformation, but comprised up to 80% of spawning catches. The water content of transforming animals from all three sites was approximately 72% but increased greatly at sexual maturity, with females having the highest values of up to 87% The weight of lipid declines to approximately half its initial value by the prespawning Stage (VI) while spent females retain less than 20%, a feature possibly correlated with their considerable loss of post-cloacal volume. In males, these trends are not so marked. Cinemicrography shows that the branchial basket contracts in synchrony with the backward stroke of the velum. When the velum beats in the absence of detectable branchial movements it probably acts as a pump, but its main function, especially under reduced oxygen tension, is to act as a valve increasing the efficiency of pharyngeal pumping.
|Date of Award||1973|