Physiology underpins habitat partitioning in a sympatric sister‐species pair of intertidal fishes

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Summary 1Integrative ecophysiological studies of closely related taxa are required to explore the causative factors that determine species distributions, and can offer insight into the selective forces that may have driven evolutionary divergence. 2We examined the distribution, abundance, size and microhabitat associations of two sympatric sister-species of intertidal fishes Bellapiscis medius and B. lesleyae (Family Tripterygiidae) that occupy intertidal rockpools along the coastline of New Zealand, to examine the ecological factors that determine habitat choice in these species. 3We also measured rockpool water temperatures and examined rates of oxygen consumption, ventilation frequency and hypoxia tolerance in each species at ecologically relevant temperatures to assess whether differences in respiratory physiology are associated with differences in vertical distribution. 4The results showed clear interspecific differences in vertical habitat partitioning, with B. medius occupying rockpools higher on the shore than its congener. The two species also differed significantly in adult body size and aggregation size, with B. medius being significantly larger and occurring in lower numbers per rockpool than B. lesleyae. 5Bellapiscis medius in upper-shore rockpools are exposed to higher extremes of temperature and presumably dissolved oxygen than B. lesleyae. Despite having similar rates of weight-specific oxygen consumption, B. medius showed a greater tolerance of hypoxia (lower critical oxygen tension) than B. lesleyae at both high (25 °C) and low (15 °C) temperatures, as well as a lower sensitivity to acute temperature change as indicated by differences in ventilation frequency. We thus suggest that the greater physiological tolerance exhibited by B. medius is likely to be an important factor enabling this species to exploit the higher shore environment 6Our integrative approach enabled us to demonstrate a clear link between ecological divergence and physiological performance in these intertidal fishes. Differing physiological tolerance is probably an important factor facilitating co-existence of these sister-species in sympatry, and is likely to have been a factor in their speciation.