Little Big Monsters - gigantism or miniaturization of deep-sea fauna
August 2006, CIESM News

Based on the narration of ancient mariners and their fear of the deep sea as home of giant creatures, ocean myths and legends developed. As a result, sea monsters frolicking in the ocean were often illustrated in antique cartographic atlases. There is some truth to this: deep down in the ocean there live “giants”, whereas their relatives in shallow regions are midgets in comparison. Biologists statistically compared the size of marine snails in shallow waters with species belonging to the same genus in deep-water environments. A pattern emerged: small-bodied, shallow-water species tend to have significantly larger deep-sea relatives; vice versa, snail species with a large size in shallow areas tend to have much smaller deep-water relatives.

The results of this study * , which will be published in an upcoming issue of the Journal of Biogeography, suggest that snails adapting to life in the deep sea follow similar evolutionary trends as species adapting to life on isolated islands. In other words the so-called “ island rule”, describing the evolution of body-size under specific constraints, may have more applications than previously thought, although classical factors such as reduced habitat, predation and competition may not apply in the deep sea.

There, food seems to be the limiting factor; less food is available, and at greater distances. The authors hypothesize that in the deep sea large gastropods became smaller because of a lack of food, while small ones became larger because this enabled them to expand their operating radius on the see floor and to store food surplus when available. The body size of deep-sea species would converge towards an optimal size related to their particular ecological strategy and habitat.

To test the new hypothesis on the rule’s applicability and its possible causes in marine systems, it will be necessary to extend such studies to other marine animals, including bivalves (such as clams) and cephalopods (such as squids).
  Large shallow-water snails tend to evolve into smaller species in deep water, while tiny snails often grow larger. As described in a new biogeographical study * : the "island rule" applies to deep-sea snails.
Image: (c) 2006 MBARI

This photograph shows three medium-sized shallow-water snails, along with three tiny deep-sea snails (upper left). Image: (c) 2006 Craig McClain

A crustacean, the isopod Bathynomus giganteus, is supporting the hypothesis. Found in many gardens as a tiny “pill bug”, its marine relative at the sea floor is a “monstrous” animal. And who knows what else we can find in the still poorly explored abyssal world ** …

This deep-sea isopod (Bathynomus giganteus) is an example of an animal that has evolved to a much larger size in deeper water. Image: NOAA

* C. R. McClain, A. G. Boyer, G. Rosenberg (2006): The island rule and the evolution of body size in the deep sea. Journal of Biogeography. doi: 10.1111/j.1365-2699.2006.01545.x

CIESM information on the Mediterranean deep-sea is available here:
CIESM SUB campaigns
**CIESM Monograph no. 23 - Mare Incognitum? Exploring Mediterranean deep-sea biology.