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Abiotic factors that prompt major ecological transitions: are fish on land to escape an intolerable aquatic environment?

Ord, Terry1; Surovic, Elizabeth1; Vaz, Diego2; Irisarri, Iker3

Published Sep 25, 2024 on Dryad. https://doi.org/10.5061/dryad.ht76hdrrd

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Abstract

Colonisation of novel habitats are important event in evolution, but the factors that initially prompt such ecological transitions are often unknown. The invasion of land by fish is an extreme habitat transition that offers an opportunity to empirically investigate the causes of major ecological transitions. The intertidal ecotone—and rockpools in particular—have been an important staging ground for transitions onto land. Classic hypotheses focus on the adverse abiotic conditions of rockpools at low tide as the instigator of fish voluntarily stranding themselves out of water, which can then lead to the evolution of an amphibious lifestyle. To test these hypotheses, we studied the abiotic conditions of 54 rockpools on the island of Guam where there are various species of aquatic, amphibious, and terrestrial blenny fishes. We found little support for the expected deterioration of abiotic conditions in standing pools at low tide (salinity, pH, and oxygen), and fish were not seen to be excluded from those pools that were found to exhibit poor abiotic conditions (temperature, salinity, and pH). Hypoxia was the only factor that might account for the absence of blennies from certain rockpools. Next, we experimentally measured oxygen depletion by an aquatic, mildly amphibious, and highly amphibious species of blenny in a simulated rockpool to infer the proportion of rockpools at low tide outside the tolerable range of blennies. Rockpools were found to have oxygen levels within the requirements of most blennies and other marine fishes. We conclude that the abiotic environment of rockpools alone was unlikely to have instigated the evolution of amphibious behaviour in blennies. Instead, the broad range of abiotic conditions experienced in rockpools suggests these conditions could have primed blennies to better endure the novel conditions on land. Any ecotone typified by fluctuations or gradients in abiotic conditions is likely a key transitional environment for the invasion of novel habitats and, as such, is an important location for adaptive evolution and species diversification.