Variation of group composition and spatial structure with group size in a sex-changing fish

Abstract

Current knowledge of stable social groups of different size and structure suffers from a paucity of data on processes of group formation and development. If, for stable groups of one species, fission, growth, or other features of development reflect a uniform process operating on all groups, one might expect any group at one point in time to represent a single stage in the developmental process. Thus, unless group development is synchronous, a population should contain groups at all stages. A simple way to provide information about group development is to describe a large number of groups in a uniform habitat and to arrange the data along a variable axis, such as group size, that is thought to vary directly with development. A study was conducted on the tropical marine fish Anthias squamipinnis. Groups ranged in size from 1 to 370 fish. Small groups were spatially unitary. Intermediate-sized groups were divided into two spatially separate subgroups, i.e. were bipartite. Large groups were divided into three subgroups, i.e. were tripartite. Subgroups within each group were classified by relative size as small, medium, or large. Unitary groups were statistically similar to the larger subgroups in bi- and tripartite groups, but differed from smaller subgroups in sex ratio, juvenile membership, and relation to the substrate. While subdivision of groups could, in theory, be explained by proximity among randomly dispersed aggregations or by discontinuity of underlying substrate or available food, existing evidence does not favour these hypotheses. Group subdivision is probably the consequence of successive fissions of a previously unitary group.