Selective predation of Pachygrapsus crassipes and Hemigrapsus oregonensis on the California horn snail, Cerithidea californica in the laboratory

Background/Question/Methods

Predation can have a great impact on populations and communities. A previous study showed that Pachygrapsus crassipes preferred to consume larger Cerithidea californica, California horn snails, as its size increased and concluded that C. californica was consumed at such a low rate that their populations were unlikely to be impacted by predation.  The early developmental stage, egg mass, of C. californica was not included in the predation study therefore the impacts of egg mass predation on the C. californica population needs further study.  We investigated size specific predation by two species of crabs P.  crassipes and Hemigrapsus oregonensis on C. californica and its egg mass.  Investigating if egg mass predation is an important source of mortality on C. californica populations is necessary to fully assess the potential impact of predation.  It was hypothesized that P. crassipes and H. oregonensis would prefer to consume egg mass over C. californica.  Specimens were collected during the summer in Carpinteria Salt Marsh Santa Barbara Co., California.  12 P. crassipes of various carapace lengths ranging from 10 mm to 50 mm were separated into small, medium, medium-large, and large class sizes.  6 H. oregonensis with carapace lengths ranging from 10 mm to 30 mm were separated into a small and a medium class size.  Each class size was in triplicates.  Each crab was offered one snail from four size classes ranging from 10 mm to 30 mm of C. californica and an egg mass of 20 mm in length.

Results/Conclusions

Pachygrapsus crassipes and H. oregonensis strongly preferred egg mass over C. californica regardless of carapace length.  Predation on C. californica might be greater than previously thought since egg mass predation may have a significant impact on its population densities.