Swimming activities and costs in free-living coral reef wrasses

Abstract

Estimates of the energetic costs associated with locomotion in free-living animals are difficult to find for aquatic animals. This is largely due to the technical difficulties of applying methods commonly used for air-breathing terrestrial taxa (e.g. doubly-labelled water) to aquatic environments. Consequently, there is a paucity of information on the activity costs of free-living fishes. Preliminary indications are that swimming can be a significant energetic expense for fishes. Swimming may therefore affect the allocation of metabolic energy to other fundamental functions such as growth, reproduction and cellular maintenance and thus is expected to be significant target for natural selection. However, without quantitative information on either the time or energy budgets associated with field swimming behaviours, we are unable to explore this further. In this study I observed both similarities and differences in the swimming activity during foraging of three species of coral reef wrasses. Regardless if measured in terms of time or frequency of events, all three species allocated most of their time to steady swimming (75-98% of total time). Searching and feeding were also prominent behaviours, with the extent varying across species with different feeding ecologies (benthic macrocarnivore, benthic microcarnivore and planktivore). Antagonism also varied across and within species, possibly as a consequence of the varying territoriality among species. When combined with the observed differences in field swimming speeds among species, it appears that wrasses vary their activity budgets according to their particularly trophic ecology and social structure. Combining the field observations of swimming behaviour with lab-based measures of the cost of swimming yielded estimates of the aerobic costs associated with locomotor activity by these wrasses. Such estimates of activity cost varied approximately seven-fold across species. Comparing our results with the limited current data available for field activity costs in other fishes, Stethojulis bandanensis appears to have the highest mass-specific activity cost reported for a fish to date. More broadly, it appears that my estimates for these ectothermic species are well below those for locomotor activity costs calculated for a range of endothermic taxa. Estimating and validating field activity costs in free-living fishes is still a challenge. Moreover, placing these estimates of activity cost within a broader context of field energetics is hindered by a lack of consistency in how values are reported in the literature. A convention on a common currency of costs would greatly facilitate future comparative analyses of field energetics, both within and across taxa from aquatic and terrestrial realms.