Vertical faunal exchange in the ocean and the deep source-sink hypotheses: A review from the perspective of pressure tolerance studies

Abstract

Faunai exchanges between shallow-water and deep-sea environments, throughout geological times, have resulted in the broad colonization of oceanic depths we see today. Attempts have been made to explain the current distribution of benthic animals by the submergence (shallow to deep) hypothesis or the high-latitude emergence (deep to shallow) hypothesis. More recently, discussions of bi-directional exchanges have emerged. Here we aim to summarize and explore empirical support for the various hypotheses by reviewing over 130 studies on the tolerance (survival) of adults and larvae of -260 species following compression or decompression. Secondarily, we attempt to tease out the importance of ontogeny (comparing tolerance of adult and larval life stages), phytogeny (comparing tolerance among major phyla) and geographic location of collection (looking for latitudinal trends). Overall, empirical evidence supports the formalization of a bi-directional exchange hypothesis as the most parsimonious explanation for current bathymetric patterns of biodiversity and highlights the pressure tolerance of benthic animals occupying bathyal depths. No clear ontogenetic patterns emerged, but comparison of pressure tolerance between adult and larval stages remains tentative due to the limited number of species for which this was assessed across life stages (nearly all under decompression scenarios). Pressure tolerance varies across and within phyla, and more basal taxa do not show consistent patterns of lower mortality (or longer survival) upon pressure shifts, although organismal simplicity and protective features (e.g. calcified exoskeleton) seem to increase tolerance. Adults of deep-sea species collected at lower latitudes appear to survive to pressure in equal or higher proportions, but for much shorter durations than those collected from higher latitudes, partly supporting suggestions that bathymetric transitions may be favoured in isothermal water columns (closer to the poles). Globally, this synthesis highlights technical and conceptual gaps offering a framework for further investigation of vertical movements of marine species across depths, which will be particularly useful in predicting ecosystem shifts in response to climate change.