ScienceDaily (Sep. 29, 2006) — When he started compiling an online database of seashells 15 years ago, Dr. Gary Rosenberg did not envision that his meticulous record-keeping would eventually shed light on a 40-year-old evolutionary debate.

The debate involves the mechanism underlying the island rule: that small animals isolated on islands evolve to be larger than their mainland relatives, and large animals evolve to be smaller. In a paper to be published in September in the Journal of Biogeography, "The Island Rule and the Evolution of Body Size in the Deep Sea," Rosenberg and his co-authors apply the island rule to deep-sea animals using Rosenberg's detailed database of marine snails. They find a similar pattern: when species colonize the deep sea, large-bodied species become smaller and small-bodied species become larger.

"I've been building the Malacolog database for many years as a tool for research, summarizing information on the names and distributions of species of mollusks, but I had not anticipated asking this particular evolutionary question," said Rosenberg, Vice President for the Center for Systematic Biology and Evolution at The Academy of Natural Sciences, Philadelphia's natural history museum. "That means that the data entered in the system could not have been subconsciously biased toward this result. I hope there will be many more surprising results in the years to come."

The database Malacolog (www.data.acnatsci.org/wasp) documents species of mollusks in the Western Atlantic, from Greenland to Antarctica.

Scientists have suggested several explanations for the evolution of body size in animals isolated on islands: reduced area, fewer predators, less competition, and resource limitation.

"Only resource limitation clearly applies to deep-sea animals," said Rosenberg. "We know there is less food available in the deep sea than in shallow water, but the area of the deep sea is much larger. Also, the competitors and predators of a species often don't reach an island, but competition and predation in the deep sea can be intense. A lot more study needs to be done on the relative importance of these factors, but clearly resource limitation is a key factor in the evolution of size."

This giant deep-sea isopod is an example of an animal that has evolved to a much larger size in deeper water. These isopods are distant relatives of the tiny "pill bugs" found in many gardens. They are also related to small shallow-water isopods that live in tide pools. (Image: National Oceanic and Atmospheric Administration)

Related Stories

Midgets And Giants In The Deep Sea (Jul. 10, 2006) — Biologists have long observed that when animals colonize and evolve on isolated islands, small animals tend to become larger while large animals tend to become smaller. Recent research lead by MBARI ...  > read more

When Animals Evolve On Islands, Size Doesn't Matter (Nov. 8, 2007) — A theory explaining the evolution of giant rodents, miniature elephants, and even miniature humans on islands has been called into question by new research. The new study refutes the 'island rule' ...  > read more

Mammoths Stranded On Bering Sea Island Delayed Extinction (Jun. 18, 2004) — St. Paul, one of the five islands in the Bering Sea Pribilofs, was home to mammoths that survived the extinctions that wiped out mainland and other Bering Sea island mammoth ...  > read more

Invasion Of The Island Bats (May 9, 2007) — Ever since the relationship between land area and number of species crystallized into a mathematical power function, islands and island archipelagoes have been thought of as biological destinations ...  > read more

Search ScienceDaily

Screens of the Future