Do large predatory fish track ocean oxygenation?

Abstract

The Devonian appearance of 1-10 meter long armored fish (placoderms) coincides with geochemical evidence recording a transition into fully oxygenated oceans.¹ A comparison of extant fish shows that the large individuals are less tolerant to hypoxia than their smaller cousins. This leads us to hypothesize that Early Paleozoic O₂ saturation levels were too low to support >1 meter size marine, predatory fish. According to a simple model, both oxygen uptake and oxygen demand scale positively with size, but the demand exceeds supply for the largest fish with an active, predatory life style. Therefore, the largest individuals may lead us to a lower limit on oceanic O₂ concentrations. Our presented model suggests 2-10 meter long predators require >30-50% PAL while smaller fish would survive at <25% PAL. This is consistent with the hypothesis that low atmospheric oxygen pressure acted as an evolutionary barrier for fish to grow much above ~1 meter before the Devonian oxygenation.

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Acknowledgements

We thank John F. Steffensen, Frank B. Jensen, Per E. Ahlberg, Jan Bergström, Donald Canfield, and Andrew Knoll for thoughtful comments, additions and reviews. Villum Kann Rasmussen Foundation (T.W.D.), the Swedish Research Council (E.U.H.), and Danish National Research Foundation (NordCEE) supported this work.

Figures and Tables

Figure 1 The length of 84 fish species found in each of the Paleozoic periods from the Cambrian through the Carboniferous. In this compilation the Carboniferous stage is clearly under-sampled, while the older periods are better represented in this study.

Figure 2 Model fish size maxima are shown as a function of ambient oxygen pressure for tuna shaped fish (See appendix in Sup. Material for details). The active metabolic rate in the model fish determines its high oxygen demand, which is so high that larger gill area is unable to accommodate the required supply. Therefore, there is a limit to how big predatory fish can grow in a low oxygen environment.

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