The role of hydrostatic pressure in constraining the bathymetric distribution of marine ectotherms

Abstract

A hyperbaric and thermal physiological bottleneck at bathyal depths is thought to contribute to bathymetric zonation of marine benthic invertebrates and demersal fishes on deep continental margins. The focus of this thesis was to investigate hyperbaric tolerance in the lithodid crab Lithodes maja as a case study for the effects of hydrostatic pressure on upper bathyal marine ectotherms. Experimental hyperbaric exposures revealed that hyperbaric tolerance is oxygen- and capacity-limited. Hyperbaric tolerance appears proximately oxygen-limited, but ultimately limited by cardiac capacity: adverse hyperbaric impacts on cardiac capacity appear mediated by the effects of pressure on membranes and membrane related functions. However, bathymetric range appears constrained by increased metabolic cost at elevated hydrostatic pressure. Hyperbaric limitation of bathymetric range supports a role for hydrostatic pressure in structuring bathymetric zonation in the deep sea, and lineage-specific physiological tolerances appear to contribute to global phylogenetic bottlenecks. Further, physiological effects of high hydrostatic pressure and low temperature at bathyal depths, acting on shallow-water taxa at the lower limits of their distribution, may invoke a stress–evolution mechanism. The resulting bathymetric variation in speciation rates could drive a unimodal diversity–depth pattern, typically peaking at bathyal depths, over time.

Marine ectotherms’ thermal tolerance is also oxygen- and capacity-limited, and functionally associated with hypoxia tolerance. Comparing hypoxia thresholds and hyperbaric thresholds of taxonomic groups of shallow-water fauna revealed significant correlation, supporting the proposition that hydrostatic pressure tolerance is oxygen- limited. Consequently, it appears that the combined effects of temperature, pressure, and oxygen concentration constrain the fundamental ecological niches of marine invertebrates and fishes. Including depth in a conceptual model of oxygen- and capacity-limited fundamental ecological niches’ responses to ocean warming and deoxygenation confirms that polar taxa are most vulnerable to the effects of climate change, but reveals for the first time that temperate fauna as well as tropical fauna may experience substantial fundamental ecological niche expansion with ocean warming and deoxygenation.

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