A Source‐Sink Hypothesis for Abyssal Biodiversity

Abstract

Bathymetric gradients of biodiversity in the deep-sea benthos constitute a major class of large-scale biogeographic phenomena. They are typically portrayed and interpreted as variation in alpha diversity (the number of species recovered in individual samples) along depth transects. Here, we examine the depth ranges of deep-sea gastropods and bivalves in the eastern and western North Atlantic. This approach shows that the abyssal molluscan fauna largely represents deeper range extensions for a subset of bathyal species. Most abyssal species have larval dispersal, and adults live at densities that appear to be too low for successful reproduction. These patterns suggest a new explanation for abyssal biodiversity. For many species, bathyal and abyssal populations may form a source-sink system in which abyssal populations are regulated by a balance between chronic extinction arising from vulnerabilities to Allee effects and immigration from bathyal sources. An increased significance of source-sink dynamics with depth may be driven by the exponential decrease in organic carbon flux to the benthos with increasing depth and distance from productive coastal systems. The abyss, which is the largest marine benthic environment, may afford more limited ecological and evolutionary opportunity than the bathyal zone.

Abyssal Solenogastres (Mollusca, Aplacophora) from the Northwest Pacific: Scratching the Surface of Deep-Sea Diversity Using Integrative Taxonomy

“…The observed rarity of the discovered abyssal Solenogastres might be an indication for source-sink mechanics, in which the low density of individuals prevents the sustainability of populations of sexually reproducing organisms and the impoverished abyssal fauna is dependent on larval influx of bathyal populations (Rex et al, 2005). Preliminary diversity assessments indicate differences in the taxonomic composition of the shallower Kuril basin in the semi-isolated Sea of Okhotsk and the abyssal fauna investigated herein with only few conspecifics (Ostermair et al, accepted).…”

“…Many paradigms in deep-sea research (e.g., on diversity gradients, bathymetric ranges, dispersal barriers, or the connectivity of populations) have been proposed based on a limited number of organisms linked to their specific biological traits and their general applicability still needs to be tested when broader data sets are available (see, e.g., Gage and Tyler, 1991;Rex et al, 2005Rex et al, , 2006McClain and Hardy, 2010;Rex and Etter, 2010). In fact, recent studies have revealed taxon-specific patterns of bathymetric and geographic distribution, therein highlighting the potential pitfalls of generalizing single-taxon studies (McClain and Hardy, 2010).…”

Does energy availability predict gastropod reproductive strategies?

“…oligotrophic) regions may reflect source/sink dynamics. Many mollusc populations in the abyssal plains, which are some of the most oligotrophic areas of the oceans, are probably maintained through the continued recruitment of larvae from source populations in more productive ocean regions [15].…”

“…These two contrasting theories alternatively predict that planktotrophic larvae should be more prevalent either at high [15] or low food availability [10]. The results in support of these two different patterns are mixed.…”

“…Indeed, planktotrophic development may be more frequent at extreme low food availability, as populations in oligotrophic regions may represent sinks maintained only through continued recruitment requiring dispersal from distant population sources [14,15]. In contrast to these predictions of larval type with production, others have concluded that 'the presence or absence of a feeding larval stage is only weakly and indirectly related to allocation of energy or materials to production' [16, p. 339].…”