ADDITIONAL INFORMATION, IF ANY. CONCERNING AUTHORS, ADDRESS. TITLE, OR CITATION DATA PLEASE TYPE ABSTRACT DOUBLE SPACED BELOW — CHAPIN, DEXTER (Hopkins Marine Station, Pacific Grove, Calif.,) Some Observations of Predation on Acmaea Species by the Crab, Pachygrapsus crassipes. (Mollüsca:Gastropoda: Prosobranchia) (Arthropoda:Crustacea:Decapoda). The Veliger Predation om Acmaea populations by various predators including the crab, Pachygrapsus crassipes was studied. Laboratory observations and experiments indicate that the crab can remove the peak of an Acmaeg shell by applying pressure with the cheliped on a fracture aone enciroling the shelll above the muscle scar. The results of a survey of Acmaea shells cast up on the beach at Pacific Grove, Galifornia, and the number of limpets dispatched by the crabs im the laboratory indicate that Pachygrapsus crassipes is responsible for sigmificant mortality in limpet popullations. — PLEASE DO NOT TYPE BELOW THIS LINE 96 SOME OBSERVATIONS OF PREDATION ON ACMAEA SPECIES BY THE CRAB, PACHYGRAPSUS CRASSIPES (Mollusca: Gastropoda: Prosobranchia) (Arthropoda: Crustacea: Decapoda) By Dexter Chapin Hopkins Marine Station of Stanford University Pacific Grove, California Species of the genus Acmaea are distributed throughout the rocky intertidal zone of the California coast in associ¬ ation with a variety of animals potentially predaceous on these limpets. Predation on Acmaea by the fish, Gibonsia elegans Hubbs 1929 was observed by Mitchell (1953). A continuation of this work by Johnston (1953) indicated that Oligocottus snyderi, Greeley; Gibonsia metzi, Hubbs 1929; and Gobiesox maeandricus (Girard, 1858) were also occasionally found with limpets in their gut. Frank (1965) has presented evidence that Leptoplana may be predaceous on some species of Acmaea on the Oregon coast. The other major predator reported in the literature is the starfish (Bullock, 1953; Feder, 1959). Recorded here are observations of predation by the crab, Pachygrapsus crassipes, Randall, 1839, a previously un- recognized predator of these molluscs. - 2 - Dexter Chapin MATERIALS: The following limpet species were included in the study: Acmaea scutum, Eschscholtz, 1833; Acmaea digitalis, Eschscholtz, 1833; Acmaea limatula, Carpenter, 1864; Acmaea pelta, Eschscholtz, 1833; and Acmaea scabra (Gould, 1864). These animals and P. crassipes were collected between Mussel Point and Cypress Point on the coast of Monterey County, California, and studied both in the field and in the laboratory. RESULTS AND DISCUSSION Pachygrapsus crassipes was never observed attacking Acmaea in the field. In the laboratory, when left undis- turbed, the crab sometimes made as many as five attacks in three hours. While all limpets were preyed upon, crabs preferentially attacked Acmaea limatula. Pachygrapsus crassipes had two methods of attacking its prey. The first was simply to pry the limpet off the rock with its cheliped. This method was successfully used if the limpet did not have its shell clamped to the rock surface and the crab could get underneath the edge. Animals attacked in this manner showed a characteristic chipping of the edge of the shell. This method was used most often on Acmaea scutum and Acmaea digitalis. The second method of attack was never directly observed. 92 -3 - Dexter Chapin However, the result of this method could be assessed by the examination of shells. Shells of limpets subjected to this form of attack had lost the peak of the shell above the muscle scar (see Plate 1). A total of seventeen shells with the tops removed were taken from the aquaria containing the crabs. Such shells were never found in any other tanks. Several shells were found that had deep scratch marks on them that were possibly made by the crab's chelipeds exert¬ ing pressure on the shell. The scars were randomly oriented on the shell and were found distributed over the entire sur- face of the shell. Attempts were made to mechanically duplicate the possible squeezing action of the chelipeds by means of a pair of needle-nose pliers suspended from a stand above the limpet, so that only a lateral pressure was exerted between the points. These experiments indicate that there is a fracture zone or weak area that encircles the shell just above the point of attachment of the shell muscles. This zone seems to be present in all Acmaea species studied, but it is most pronounced in the shells of Acmaea limatula where, if a pressure of just two pounds was applied with the pliers. the shell might break. The maximum pressure needed to break an Acmaea limatula shell along the shear zone was seventeen pounds at the tips of the pliers. - 4 - Dexter Chapin Studies on factors influencing the shell breakage showed it is important to exert the pressure exactly on the very narrow shear zone. The shells were most easily broken if the tips of the pliers were placed on the longitudinal axis rather than the lateral axis. When the pressure was exerted below the shear zone, one Acmaea limatula shell withstood a pressure of forty-five pounds. The same shell broke when a pressure of twelve pounds was exerted at the shear zone. If the shells were artificially broken in this manner, they did not always produce a clean circular break. The remaining shell fragments, however, could very easily be broken back to the shear zone but no farther so that a very even break could be achieved. A second set of experiments were run in which a cheliped that had recently been removed from a crab was used to exert pressure on the shells in the same manner that the pliers were used. The results of these experiments indicated that the cheliped could puncture the shell at the fracture zone as easily or more easily than the pliers although no exact pressures were recorded. The cheliped did not suffer any damage when the pressure was applied. By having the crab squeeze a piece of balsa wood. and then duplicating the damage with pliers with about the same squeeze area, it was possible to make a very crude determination of the pressure that could be exerted by the 5 - Dexter Chapin crab. With this method, it was found that the pressures exerted by the crab exceeded twenty-three pounds, which is more than any pressure needed to artificially break a limpet shell at the shear zone. Surveys of the intertidal on Mussel Point indicate that shells from which the peaks have been removed make up about fourteen percent of the total Acmaea shells cast up on the beach. Such shells were often not highly eroded and the loss of the peak did not appear to be due to erosion after the death of the limpet. SUMMARY The crab, Pachygrapsus crassipes, has not previously been recognized as an important predator of limpets. Labor¬ atory observations and experiments suggest that the crab çan remove the tops of the shells of some limpets by squeezing with the cheliped, thus making the viscera available for food. The results of a survey of the shells cast up on the beach, and the numbers of limpets apparently attacked by this method in the laboratory, suggest that these animals may be responsible for a significant mortality in limpet populations. - 6 - Dexter Chapin ACKNOWLEDGEMENTS The author gratefully thanks Dr. John H. Phillips and Ray Markel for their patience and assistance in carrying out this study and preparing the manuscript. This work was made possible by grant GY806 from the Undergraduate Research Participation Program of The National Science Foundation. 2 - 7 - Dexter Chapin LITERATURE CITED BULLOCK, THEODORE 1953. Predator recognition and escape responses of some intertidal gastropods in the presence of starfish. Behavior, 2: 130-140. HOWARD,M. 1959. The food of the starfish Pisaster FEDER, ochraceus along the California coast. Ecology, 40: 721-724. PETER W. 1965. The biodemography of an intertidal FRANK snail population. Ecology, 46: 831-843. JOHNSTON, RICHARD F. 1954. The summer food of some inter¬ tidal fishes of Monterey County, California. California Fish and Game Bulletin, 40, (1): 65-68. MITCHELL, D. F. 1953. An analysis of stomach contents of California tide pool fishes. American Midland Naturalist, 49 (3): 862-871. Footnote 1 Permanent address: - 8 - in 9 - Dexter Chapin Caption for Plate 1 "A". Acmaea shells broken by various means, From left to right: (1) Acmaea limatula shell found in an aquaria containing only Pachygrapsus crassipes as a predator. (2) Acmaea digitalis shell artificially broken with needle-nose pliers. (3) Acmaea limatula shell broken artificially with needle-nose pliers, and (4) Acmaea limatula found in aquaria with Pachygrapsus crassipes. "B". An artificially broken Acmaea limatula shell. The very smooth surface of a break in the cleavage zone is seen. The notch on the left was made by the tips of the pliers when the pressure was applied slightly below the cleavage zone. "C". A shell of Acmaea spp. found cast up on the beach. Illustrates the effects of the slight erosion often found in such shells.