The physiological viability of bathymetric range shifts in marine ectotherms

Abstract

Organisms occupying an environment where change is occurring must move, acclimatise, or adapt in order to prevent extinction. Despite recent evidence, the potential for species to change their distribution range across depths is regularly overlooked as an alternative to well documented latitudinal shifts. The aim of this thesis was to investigate the physiological viability of bathymetric range shifts by assessing the impacts of elevated hydrostatic pressure (HP) and changing temperature in two marine ectotherms: the shallow-water caridean shrimp Palaemonetes varians; and the continental slope-depth anomuran crab Lithodes maja. Gene biomarkers were characterised in both species and their transcriptional regulation was assessed by qPCR. Acute and long-term exposures revealed that elevated HP produced significant perturbations at the transcriptional level in P. varians, and were corroborated by measurements of behaviour and respiratory response. HP-associated pathologies were exacerbated by decreasing temperature, and ameliorated by increasing temperature. Results gave a novel insight into the hierarchy and kinetics of HP-induced stress responses, and suggested that the combination of HP and temperature, in part, determines the physiological limitations to bathymetric migration in shallow-water ectotherms.

To further test the role of HP and temperature in setting depth distribution limits, gene biomarkers were characterised in a continental slope-depth crab Lithodes maja. Transcriptional regulation and respiration rate was quantified throughout larval ontogeny, and in mature adults. Early life stages showed HP tolerance beyond that of the species natural distribution limits. The observed HP tolerance window reduced through ontogeny. Reductions in HP tolerance with life stage may be analogous to established thermal tolerance reductions with ontogeny. Further, sustained HP exposures suggest that bathymetric range limits in L. maja are constrained by increased metabolic costs, nervous system sensitivities, and macromolecular damage under elevated HP.

HP and temperature, and particularly the combination of the two, play an important role in setting the depth distribution limits of marine ectotherms. For temperate shallow-water adapted marine ectotherms, water temperatures towards the upper limit of their thermal scope may facilitate bathymetric range shifts, whilst colder waters further exacerbate the negative physiological effects of elevated HP. Bathymetric range shifts may, under certain scenarios, offer an important, yet underestimated, alternative to latitudinal shifts in light of contemporary climate change. Further, the combination of HP and temperature may have played an important role in the colonisation of the deep sea from shallow waters by marine ectotherms.

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ORCID for Chris Hauton: orcid.org/0000-0002-2313-4226

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