EXPERIMENTAL STUDY OF MERISTIC VARIATION IN A POPULATION OF THREESPINE STICKLEBACKS, GASTEROSTEUS ACULEATUS

Abstract

Eggs of 21 pairs of wild sticklebacks from Hobson's Brook near Cambridge, England, were reared under various controlled temperatures and salinities. Two hundred and fifty-three young were examined for counts of abdominal and caudal vertebrae, dorsal and anal spines and soft fin rays and basal supports, segmented and simple caudal rays, and pectoral rays. Lateral plates were counted on 95 young. Rearing in conditions of crowding or high temperature produced high proportions of females. Curves of vertebral count against rearing temperature in freshwater were V-shaped, but position of the apex differed markedly between different genotypes. Low temperature optimum, low temperature for producing minimal vertebral count, and high plate count were associated, and were largely inherited from the mother. Phenotypic vertebral variation occurred mostly in the anterior half of the body. All other meristic characters studied were also subject to both phenotypic and genotypic modification. Variation in each series was largely independent of other series. Maintenance of high variability despite selection is postulated to characterize temperate freshwater fish populations. Relation of these findings to sampling problems, and to clinal variation, is discussed.