Evidence of physiological assortment and movement dynamics among social groups of a coral reef fish

The trade-offs of group living are modulated by the phenotypes of individual members of a social group, particularly in dynamic and diverse habitats like coral reefs. Little is known about the patterns of physiological traits among fishes within social groups and the mechanisms that promote these patterns, which could elucidate the drivers of group composition and their downstream ecological and evolutionary impacts. Here, in the gregarious damselfish species Chromic viridis , we examined inter-group differences in whole-animal physiological traits and the tendency for fish to move either within sites (i.e., sections of continuous reef) or among habitats (i.e., reefs separated by sandy substratum) to a new social group. Using oxygen uptake as a proxy for aerobic metabolic rate, we found significant differences in maximum metabolic rate (MMR) and aerobic scope (AS) among schools from different habitats, with these traits higher in habitats with faster water flow rates. However, we found no differences in any metabolic traits (standard metabolic rate, SMR, MMR, AS) between groups from the same site.

These trends could stem from a range of mechanisms, as mark-recapture studies of this species indicated a willingness to migrate to a new social group in over 30% of recollected fish. However, there were no effects of either body size or perceived habitat risk on the distance moved or movement type (i.e., over coral or sand). Our results indicate that, in social species, a combination of mechanisms may influence phenotypic differences among groups over different spatial scales.