Dr. Jorge Gutiérrez was an IES postdoctoral researcher. The following article, which appeared in the July-August 2001 (18:4) Newsletter, summarizes the thesis research he conducted in Argentina. Currently, Dr. Gutiérrez is focusing on how burrowing crabs control the transfer of nutrients from salt marshes to adjacent waters.

What kind of impact can a small crab have on a large seashore? According to Jorge Gutiérrez: a considerable one.

Gutiérrez, a doctoral student at the Universidad Nacional de Mar del Plata, Argentina, is studying the interactions between burrowing crabs and stout razor clams on mudflats along the southeastern coast of Argentina. He is finding that these creatures are "ecosystem engineers"; they physically modify their intertidal environment and thereby affect many other species.

Originally investigating the effects of stout razor clam shells on sediments and the benthic communities, after reading a paper on ecosystem engineering authored by Drs. Clive Jones, John Lawton, and Moshe Shachak, Gutiérrez recognized connections to his own research.

Chasmagnathus granulata finds shelter by burrowing into mud. In so doing, this crab — whose shell, is a mere 4 centimeters (1.5 inches) across — changes the character of the sediment by stirring it up so that the finer grains are suspended, then deposited on the surface, and finally dispersed by currents. For animals that feed on sediment, fine grains have a higher nutrient content and are more easily digested than larger particles.

The environment created by the burrowing crab is attractive to sediment feeders. Clams fall into this category, and Tagelus plebeius, the stout razor clam (6.5 - 8 cm, or 2.5 - 3 in.), thrives where there are high densities of burrowing crabs. Clams feed by drawing water in through a tube called a siphon. Before exiting through a second siphon, the water passes by gills that trap sediment particles. Clams generally burrow into the sediment with only the siphon openings showing, usually in a shallow depression in the mud.

Preliminary evidence suggests that when currents are strong, burrowing crabs find adequate food in the sediments swept across and into their burrows, but when currents are weak, they explore the surroundings searching for depressions where organic particles become trapped. These depressions are frequently the ones created by stout razor clams. Gutiérrez has observed that crabs may make even more efficient traps by actually enlarging the depressions where clams reside. And, he notes, a larger pit makes life easier for the clam as well, by increasing the amount of sediment that can collect there.

Gutiérrez and colleagues are studying the role of Chasmognathus as a key species in regulating the functioning of southwestern Atlantic estuaries. The burrowing behavior of crabs has an impact on the lives of a number of animals, including sediment-dwellers — polychaetes (bristleworms), nematodes (roundworms) and ostracods (tiny crustaceans) — and shore birds, many of which are migratory species from North America that spend Northern Hemisphere winters in Southern Hemisphere summers. His current experiments include introducing burrowing crabs into areas where there are none, then measuring the impact on clams by measuring their body weight.