Sand Crabs & George MacGinitie

compiled by
Robert Roy J. van de Hoek
June 1, 2001
Malibu, California

In a classic marine biology article, published 63 years ago in 1938, George MacGinitie gives us an eloquent and educational description of the Sand Crab, found on the sandy beaches of California. For me it is particularly interesting to revisit this crab through MacGinitie. It was in an Invertebrate Zoology course at California State University Northridge, 24 years ago, in 1977 that my Marine Biology professor, Dr. Earl Segal, introduced me to this crab on Santa Monica Beach. Now 20 year later I have watched these sand crabs, surf scoters, and shorebirds for the last three years in Malibu, I have been watching this crab, particularly at Point Dume Beach and Zuma Beach. I have watched a unique marine duck, the Surf Scoter, for hours, at Las Flores and La Costa Beach, in Malibu, preying upon the Sand Crab.

As an example of breaking through, MacGintie also transitions us from the Sand Crab to discussing the linkage to birds as predators of the Sand Crab, and in a sense therefore to vertebrates, such as ourselves that prey upon these Crabs, both in prehistory with Native Americans, and today with fishermen who use them as bait. But he is also clear to show us that other marine invertebrates and fish may have influenced the burying behavior of the Sand Crab as an evolutionary survival feature.

At this time, I would like to point out that sandy beaches, as well as tidepools, are considered wetlands, according to today's scientists, and in fact, the federal government, through the Environmental Protection Agency (EPA) considers beaches as wetlands. It makes good sense and is quite logical, as it is part of the land that is covered and uncovered with water on a periodic basis, and forms a unique soil substrate type, i.e. sand or cobble beach.

The web page was created and compiled to guide the curious individual into the realm of "knowledge is power" and "breaking through." There is something to be said for just plain education, pure knowledge, and curiosity to know about natural landscapes from the perspective of WILD NATURE. That kind of knowledge is in all of the writings of George MacGinitie. It is also hoped that this web page will help educate us all about the inter-relationships of the land and the sea, through the eyes of a gifted marine naturalist of marine biology.

The American Midland Naturalist
Volume 19 ................. 1938 .................... Number 4

Movements and Mating Habits
of the
Sand Crab,
Emerita analoga

by
G.E. MacGinitie

Feeding and General Activities

Because of the unstable conditions on sandy beaches, due to shifting sand, heavy surf, and tidal exposure, the number of species of animals indigenous to such localities is few compared with that of the more secure rocky regions. However, animals that have become adapted to the open sandy beach usually occur in great numbers of individuals, no doubt because of the lack of competition for space. As far as food is concerned, the sandy beach is capable of supporting almost as many animals as are the rocky regions, the most unfavorable factor of the open beach being the lack of protection. Practically all animals that live on the beach seek refuge beneath the sand, and are equipped to bury themselves very quickly.

The sand crab or mole crab, Emerita analoga (Simpson) (Fig. 1), which is found from Oregon to Panama, is very abundant on the beaches of Southern California. The sand crab is exceedingly well adapted to living on the open beach. It can bury itself rapidly, can swim fairly efficiently by means of the uropods, which are modified into swimming or sculling organs, and its carapace is streamlined from the flexure of the abdomen toward the head. Emeritas burrow backward, the burrowing being facilitated by the movements of the anterior pair of legs as well as by the sculling organs, and they settle into the sand facing oceanward.

. . . if the crabs are left too high on the beach they fall prey to birds, and if they are too far down the beach they fall prey to shore feeding fishes.

The concealment of Emerita in the sand serves no doubt as a refuge for this animals and its enemies. What led Mead to the conclusion that only birds were at fault is not clear. In the course of evolution there are many other animals which could have been as effective as birds in causing these crabs to conceal themselves in the sand. These sand crabs may have had the burrowing habit long before sandpipers and curlews became shore feeding birds. I have seen sandpipers, sanderlings, godwits, black-bellied plovers, willets, curlews, scoters, and even Brewer's Blackbirds feeding upon these crabs, but without disturbance to the activities of the remaining animals. The crabs are preyed upon by animals lower in the water, and these animals perhaps had more effect upon the evolutionary choice of the burying activities of Emerita than had birds, for they no doubt were preying upon the crabs during the latter's evolution toward becoming sand-dwelling forms. As the waves wash down and leave some of the Emeritas "dry" they always withdraw their antennae and remain concealed under the sand whether or not there are any birds or other animals, including human beings, about.

. . .

Scoters feed upon the crabs under water, and therefore, upon larger individuals. Cancer crabs, swimming crabs (Portunus xantusii), croakers, corbina and the imported striped bass feed on the oceanward side of the beds of Emerita. Two other sand crabs, Blepharipoda occidentalis and Lepidopa myops (for descriptions and figures, see Schmitt, 1921), which are rather common in colonies of Emerita, apparently do not feed upon the living crabs, but upon the dead Emeritas and other dead or decaying flesh. Of one thing we can be sure, the Emeritas must remain buried to escape dessication and to hold their position in the surf.

REFERENCES
MacGinitie, G. E. 1935. Ecological aspects of a California marine estuary. American Midland Naturalist 16(5):629-765.

MacGinitie, G. E. 1937. Notes on the natural history of several marine crustacea. American Midland Naturalist 18(6):1031-1037.

Mead, Harold Tupper. 1917. Notes on the natural history and behavior of Emerita anaoga (Stimpson). Univ. Calif. Pub. Zool. 16(23):431-438.

KERCKHOFF MARINE LABORATORY
CORONA DEL MAR, CALIFORNIA


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