Feeding and functional responses of invasive European green crabs (Carcinus maenas) in the presence of an introduced sea slug (Philine orientalis)

Background/Question/Methods

Introduced predators change native biological communities in ways that are complex and that propagate through food webs.  Predators in their introduced ranges can both compete with and facilitate other introduced species.  I investigated the potential for foraging interference between two abundant introduced predators of infauna in the heavily-invaded mudflat habitats of northern California.  Laboratory observations suggested that contact with the introduced, mucous-secreting cephalaspidean Philine orientalis could suppress the feeding of the invasive European green crab (Carcinus maenas) on otherwise palatable food.  This suppression was likely due to the secretion of acidic defensive mucous by Philine.  In laboratory and field enclosures, I investigated the potential for Philine to suppress the feeding rate of Carcinus on infaunal prey.  I allowed gem clams (Gemma gemma) to bury in 2 cm of sieved sediment, and applied the following 4 treatments:  1) one Carcinus + two non-feeding Philine (N = 7), 2) one Carcinus (N = 7), 3) non-feeding Philine control (N = 6), 4) No Predator control (N = 7).  `Non-feeding Philine’ were enclosed in small packets of window screen that prevented slugs from feeding, but allowed mucous to exude into experimental chambers. 

Results/Conclusions

Philine deterred predation by Carcinus on small bivalves in the laboratory, but not in field enclosures.  This study represents the first evidence that the defensive secretions of a predatory Opisthobranch (Philine) can suppress feeding for another predator.  I employed foraging models to bridge the gap between laboratory and field results, and to predict how changes in species abundances at different trophic levels might alter the ecological importance of crab suppression by Philine.  In these models, low prey density and high foraging efficiency of Carcinus resulted in greater proportional prey loss to crabs due to the presence of Philine.  In the field, areas of lower Gemma density occur between high density patches and crab densities are also spatially variable.  Using the model results, I predict that high Philine density, low Carcinus foraging efficiency (possibly as a result of high crab density), and low Gemma density would maximize interference of Philine with crab feeding.