The "island rule" and deep-sea gastropods: re-examining the evidence

Abstract

Background: One of the most intriguing patterns in mammalian biogeography is the "island rule", which states that colonising species have a tendency to converge in body size, with larger species evolving decreased sizes and smaller species increased sizes. It has recently been suggested that an analogous pattern holds for the colonisation of the deep-sea benthos by marine Gastropoda. In particular, a pioneering study showed that gastropods from the Western Atlantic showed the same graded trend from dwarfism to gigantism that is evident in island endemic mammals. However, subsequent to the publication of the gastropod study, the standard tests of the island rule have been shown to yield false positives at a very high rate, leaving the result open to doubt.

Methodology/principal findings: The evolution of gastropod body size in the deep sea is reexamined. Using an extended and updated data set, and improved statistical methods, it is shown that some results of the previous study may have been artifactual, but that its central conclusion is robust. It is further shown that the effect is not restricted to a single gastropod clade, that its strength increases markedly with depth, but that it applies even in the mesopelagic zone.

Conclusions/significance: The replication of the island rule in a distant taxonomic group and a partially analogous ecological situation could help to uncover the causes of the patterns observed--which are currently much disputed. The gastropod pattern is evident at intermediate depths, and so cannot be attributed to the unique features of abyssal ecology.

Figure 1. Body sizes of deep-sea gastropods and their shallow-water congeners.

Part A shows how different tests of the ‘island rule’ can give qualitatively different results. “Deep-sea” species were defined as those with a depth range midpoint >200m, and all other species defined as “shallow-water”. The ordinary-least-squares regression (dashed line) differs significantly from the 1∶1 line of the null (dotted line), but the standardized-major-axis regression (solid line) shows no significant departure. Part B shows a less ambiguous case: “deep-sea” species are those never observed above 400m, and “shallow-water” species those never observed below 200m; body sizes are within-genus means, taking equal numbers of deep- and shallow-water species in each genus.

Figure 2. Comparison of effect size across depths and taxonomic groups.

The body sizes of deep-sea gastropods are plotted against those of their shallow-water congeners. “Shallow-water” species were never observed below 200m, and “deep-sea” species never observed above depths of A: 200m, B: 400m and C: 600m. Separate standardized-major-axis regression lines are shown for the Neogastropoda (black points) and all other groups (grey points). The dotted line is the 1∶1 expected under the null. Genera with fewer than two deep and two shallow species were excluded.