The island rule and the evolution of body size in the deep sea

Our goal is to test the generality of the island rule - a graded trend from gigantism in small-bodied species to dwarfism in large-bodied species - in the deep sea, a non-insular but potentially analogous system. Shallow-water and deep-sea benthic habitats in the western Atlantic Ocean from the North to South Poles. We conducted regression analyses of body size of deep-sea gastropods species relative to their shallow-water congeners using measurements from the Malacolog ver. 3.3.3 database. Our results indicate that, consistent with the island rule, gastropod genera with small-bodied, shallow-water species have significantly larger deep-sea representatives, while the opposite is true for genera that are large-bodied in shallow water. Bathymetric body size clines within the deep sea are also consistent with predictions based on the island rule. Like islands, the deep sea is characterized by low absolute food availability, leading us to hypothesize that the island rule is a result of selection on body size in a resource-constrained environment. The body size of deep-sea species tends to converge on an optimal size for their particular ecological strategy and habitat.