Can crabs kill like a keystone predator? A field-test of the effects of crab predation on mussel mortality on a northeast Pacific rocky shore

Abstract

Predation can strongly influence community structure and ecosystem function, so the loss of key predators can have dramatic ecological consequences, unless other predatory species in the system are capable of playing similar ecological roles. In light of the recent outbreak of sea star wasting disease (SSWD) and subsequent depletion of west coast sea star populations, including those of the keystone predator Pisaster ochraceus, we examined whether large mobile crabs could play a role as predators on mussels (Mytilus californianus) on a rocky shore in Northern California. Using a combination of sea star removal and predator exclusion cages we found that mussel mortality was 43–294 times greater in uncaged treatments versus caged treatments. Mortality on uncaged mussels at low tidal elevations was due to predation by large mobile crabs (Cancer productus and Romaleon antennarium); confirmed by the presence of mussel shell fragments and documented attacks on wax snail replicas. Laboratory feeding assays indicated that crabs, on a per unit biomass basis, can consume almost twenty-five times as many mussels per day than sea stars, which together with the results of our field experiment, suggest that large predatory crabs could play an important role in maintaining ecosystem function through their predation on mussels on rocky shores where P. ochraceus are rare, absent, or have been depleted by SSWD.

Dynamic measurements of black oystercatcher (Haematopus bachmani) predation on mussels (Mytilus californianus)

“…For example, during high tide submerged mussels may be attacked by relatively fast‐moving predators, such as lobsters or crabs, that crush mussel shells to gain access to the mussel's tissue. The mussel's primary protection comes from closing the shell valves tightly and relying on overall size and shell thickness to resist these crushing forces (Hull & Bourdeau, ; Robles et al., ). High tide can also bring slow‐moving predators, including sea stars, which slowly pull the shell valves apart over the course of hours (Paine, ), or whelks, such as members of the Nucella genus, that drill and dissolve their way through the shell using a radula and secretions from an accessory boring organ (Dayton, ; Miller, ).…”

“…For example, during high tide submerged mussels may be attacked by relatively fast-moving predators, such as lobsters or crabs, that crush mussel shells to gain access to the mussel's tissue. The mussel's primary protection comes from closing the shell valves tightly and relying on overall size and shell thickness to resist these crushing forces (Hull & Bourdeau, 2017;Robles et al, 1990).…”

“…Among the many stresses of life in the intertidal zone, responding to the threat of predation must often be balanced against the need to carry out basic life functions such as aerobic respiration, feeding, and waste removal. The sea mussel Mytilus californianus Conrad 1837, found along the western coast of North America, must contend with a number of different predators, including sea stars (Paine, ), muricid whelks (Dayton, ), crabs (Hull & Bourdeau, ), lobsters (Robles, Sweetnam, & Eminike, ), sea otters (Estes, Riedman, Staedler, Tinker, & Lyon, ), and bird predators such as oystercatchers (Hartwick, , ). These predators can attack by prying, crushing, or drilling, and the mussel's primary defense is to hold the shell valves tightly closed, cutting itself off from water, oxygen, and food in the process.…”

Missing link: California rock crabs serve as intermediate hosts for the parasite Helicometrina nimia

“…Beyond their economic importance, rock crabs also fulfil key ecosystem roles as controllers of non-native species like the European green crab ( Carcinus maenas ) (Jensen et al ., 2007) or tunicates (Epelbaum et al ., 2009). Cancer productus and R. antennarium may also be important controllers of mussel populations, which, left unchecked, can become dominant in lower intertidal communities (Hull & Bourdeau, 2017). With the loss of keystone predators due to sea star wasting disease, researchers have suggested that rock crabs may be essential in compensating for sea star population decline (Hull & Bourdeau, 2017).…”

“…Cancer productus and R. antennarium may also be important controllers of mussel populations, which, left unchecked, can become dominant in lower intertidal communities (Hull & Bourdeau, 2017). With the loss of keystone predators due to sea star wasting disease, researchers have suggested that rock crabs may be essential in compensating for sea star population decline (Hull & Bourdeau, 2017). They are also important prey items of the southern sea otter ( Enhydra lutris nereis ) (Fujii et al ., 2017), as well as benthic fishes, octopus and bottom-foraging sharks (Carroll & Winn, 1989).…”