title for learn-about section of A SNAIL'S ODYSSEY
  Predators & defenses
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Lobsters & crabs


Potential predators such as lobsters are considered here, while FISHES & BIRDS, SEA STARS, and SNAILS are considered in other sections.

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Research study 1

photographs showing different degrees of bulbous-shape in mussels Mytilus californianus in relation to how easily they can be crushed by lobsters Panulirus interruptusIn Santa Catalina Island, California spiny lobsters Panulirus interruptus prey on mussels Mytilus californianus.  The lobsters leave their dens at night to forage for mussels at high water.  Attack strategy varies with size of prey, and includes cracking and chipping by the mandibles at the free edge of the shell on smaller lower-shore mussels, and prying open and direct feeding on the exposed flesh of larger upper-shore mussels.  Although an adult lobster is capable of photograph of several lobsters Panulirus sp.eating any size of mussel in the bed, they prefer low-intertidal individuals, not only for their smaller size and correspondingly weaker shell, but for their flatter, more easily cracked posterior shell edges. Both features lead to shorter handling times by the lobsters (see photos on Right).  High-intertidal mussels have a more rounded or “bovine” shape, with a curvature often too large for the gape of the lobster’s mandibles, leading to longer handling times of up to several hours.  Robles 1987 Ecology 68: 1502; Robles et al. 1990 Ecology 71: 1564.


Several spiny lobsters amongst oyster
cultch in the Newport Aquarium, Oregon
(these are likely Panulirus argus)

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Research study 2

histogram showing abundances of mussels Mytilus californianus and M. trossulus in areas around Santa Catalina Island, California in relation to degree of wave exposureLater studies by the same research group on mussels in Santa Catlina Island reveal that in removing a dominant competitor, mussels, predation by lobsters frees up space on the rocks for growth of a red-algal turf.  In their disproportionate influence on the community dynamics of the mussel/algae community, the question arises as to whether the lobsters are acting as keystone predators.  Based on the author’s results, the answer seems to be “yes” in wave-exposed areas where settlement of Mytilus californianus is high and where this mussel species dominates, and “no” in protected areas where M. californianus has poor settlement and where Mytilus galloprovincialis predominates (see accompanying histogram). In these protected sites mussel densities are so low that no predation is necessary to maintain the algal turf.

The extent of “keystone predation” in this area, then, depends upon a spatial gradient in recruitment density of the mussels.  Only on wave-exposed shores does experimental removal of lobsters lead to a mussel-dominated community and loss of algal turf in classic keystone-predator fashion.  In showing the effect of this spatial-gradient variability, the authors' studies provide new insight into the keystone-predator concept, not just for spiny lobsters, but also possibly for sea stars, sea otters, and other keystone-predator “candidates”. Robles 1997 Ecology 78: 1400; see also Robles & Robb 1993 J Exp Mar Biol Ecol 166: 65.

NOTE  the turf is comprised of coralline algae Corallina officianalis, and of red algae Chondracanthus
(Gigartina) canaliculatus and Osmundea (Laurencia) pacifica

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Research study 3

Extreme wave-beaten shores are thought to provide refuges for mussels Mytilus californianus because predators are damaged or at least hindered in their foraging by hydrodynamic stresses.  In Santa Catalina Island, California, however, this is not the case.  Here, densest aggregations of mussels occur at midshore levels of the most wave-exposed sites.  Underwater time-lapse photography reveals that dense aggregations of spiny lobsters Panulirus interruptus prey on the mussels from below during nocturnal high tides.  Although foraging time is restricted to just a brief period, the lobsters’ high motility and rapid handling of the mussels allow them to exploit the intertidal prey.  The lobsters preferred prey are juvenile mussels and the seasonal occurrence of these, in turn, regulates the lobster densities.  The lobsters are photograph of wave-beaten shore at Botanical Beach, British Columbiaable to select juveniles from among the larger mussels. So, although percentage cover may be little affected, the overall effect of predation is to decrease the densities of the “young of the year” mussels.  The study casts new light on the long-held idea that locally harsh physical conditions will create refuges from predation. Robles et al. 2001 Oecologia 128: 142.

NOTE  usually <1cm shell length; included are some Mytilus galloprovincialis along with M. californianus


Very big waves beating on sea-mussel
beds, yes, but not on Santa Catalina
Island; rather, Vancouver Island

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Research study 4

histogram comparing predation by crabs and sea stars on different morphotypes of mussels Mytilus trossulusTwo morphologically distinct forms of mussels Mytilus trossulus occur in areas of Howe Sound, British Columbia.  One form, characterised by thicker shells and more massive adductor muscles, inhabits intertidal rocky shores, while another form, with thinner shells, smaller adductor muscles, but more tightly sealing shell valves, is found in fouling communities.  The question addressed by a researcher from Simon Fraser University, Burnaby is whether crab and sea-star predators will preferentially select mussel prey based on differences in morphological features.  Results of preference tests show no significant selection by crabs for mussels of either morphotype, but a significant selection by sea stars, as expected, for mussels from the intertidal community characterised by larger gaps in shell closing (see histogram).  The findings confirm that morphological features of mussels are important in their selection as prey by crab and sea-star photograph of mussels Mytilus trosulus in a fouling communitypredators.  Addison 2009 J Shellf Res 28: 299.

NOTE  the author does not indicate the substratum inhabited by this fouling population

NOTE  predators used are red-rock crabs Cancer productus and ochre stars Pisaster ochraceus


Mussels Mytilus trossulus in a fouling community along
with tunicates Styela montereyensis and hydroids 0.5X

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