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

At least 4 Nucella spp. and several other species of whelks prey on west-coast mussels.  Their feeding activities are considered elsewhere in the ODYSSEY: FOODS, FEEDING, & GROWTH.

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  Potential predators such as snails are considered here, while FISHES & BIRDS, SEA STARS, and LOBSTERS & CRABS are considered in other sections.
Research study 1


table showing gaping responses of mussels Mytilus trossulus to the presence of different species of snails on their shellsdrawings showing possible interactions between a prey mussel and its chief predator, whelksMussels seem to be completely susceptible to attack by boring snails but, surprisingly, they have a few defensives.  Studies in Oregon show that Mytilus trossulus responds to contact of whelks Nucella spp. with valve gapings, repetitive valve closures, mantle retractions, and foot extensions (see drawings photograph showing mussels Mytillus californianus being preyed on by whelks Nucella canaliculataabove). Other scavenging or herbivorous snail species elicit little response (see table). These actions may not seem particularly aggressive, but may result in the snail being dislodged or at least kept away from the valve edges.  Occasionally, a snail’s foot or proboscis may be pinched. Valve movements may also force the predators into central, thicker regions of the shell where more time/energy is required to bore into the shell.  A more aggressive response by the mussel is to fasten byssus threads to the snail.  The snail can sometimes twist and break these but, if trussed to complete immobilisation, death may result. The author suggests that gaping and other behaviours noted are most consistent with anti-predator activity. Wayne 1987 Veliger 30: 138..

A crowded community of goose barnacles, barnacles, mussels
Mytilus californianus
and whelks Nucella canaliculata. While
no whelk is trussed with threads, note that one individual (10
o'clock position) has a thread attached to the apex of its shell 1X

Research study 2

map showing study locations in Alaska for investigation of predation by whelks Nucella lima on mussels Mytilus trossulusgraph showing effect of whelks Nucells lima on survival of mussels Mytilus trossulus in Kasitsna Bay, AlaskaAfter a severe freeze in south-central Alaska in 1989, where temperatures dropped to -31oC over a 3-wk period, abundance of mussels M. trossulus in the upper part of their vertical range was greatly reduced. For 2mo following this event, researchers monitor mussel recovery at sites with and without their chief predator, the whelk Nucella lima (see map on left). At the Nubble-Point site where the whelk is rare, mussel populations recover to occupy over 70% of the available space (see graph upper Right). Where whelks are abundant, as at the Kasitsna-Bay site, recovery is less, and up to 81% of mussel mortality is due to predation by the snails. 

Experiments in which snails are caged with Effect of density of whelks Nucella lima on survival of mussels Mytilus trossulus in cage experiments in Alaskamussels show that even “AVERAGE” or normal field densities of snails can reduce mussel cover significantly over just a few weeks, even moreso at "2X" this density (see graph lower Right).  Based on known feeding rates and measured field densities, the authors calculate that predation by N. lima will remove 60-90% of all mussels in a given season.  Contrary to the general belief that mussel recovery from a catastrophic disturbance, such as freezing temperatures, is inevitable, the authors show that the mussels are unlikely to increase their space utilisation owing to the intense predation by N. lima.  Thus, rather than reaching size refuge and establishing a long-term mussel-dominated community, the catastrophic disturbance shifts the emphasis to a long-term whelk-controlled community where recruitment is controlled by the predator.  Carroll & Highsmith 1996 Mar Ecol Progr Ser 138: 125.

NOTE densities of snails in the treatments: CONTROL = 0 snails per cage, AVERAGE = 5 snails, comparable to field densities, and 2X = 10 snails

Research study 2.1

Mussels in beds attach to one another and to the substratum using their byssus threads and in so doing form clumps.  The solid mass created is thought to be better able to withstand wave action and there is also evidence that crushing predators like crabs and lobsters find it more difficult to gain purchase on clump members, but is it possible that clumping imay also protect against predatory snails?  This idea is tested by researchers at Friday Harbor Laboratories, Washington by comparing drilling success of snails Nucella lamellosa on individual and clumped mussels Mytilus trossulus (see photographs on Right).  Results show significantly decreased drilling frequency within the treatment containing clumped mussels, suggesting that clumping may, indeed, act as an effective strategy against drilling predators.  The authors note that drill-hole placement does not differ between the 2 photographs of arenas used in mussel-clumping study with whelk predators Nucella lamellosagroups, although these data may have been biased by the particular protocol used.  They also remark on several instances where byssus threads are attached to attacking snails, sometimes resulting in the predator pulling mussels behind it as it crawls about the aquarium space.  Casey & Chattopadhyay 2008 J Exp Mar Biol Ecol 367: 174.

NOTE  the designs used for the treatments are not appropriate for statistical analysis because snails are allowed to wander freely and so an individual may contribute more than once to the data photograph showing a clump of mussels Mytilus californianuspool.  Fences in the “individual” treatment cage are used to isolate a snail and its prey during an attack, but instead of releasing the snail onto the shore afterwards, the researchers allow it to roam freely in the enclosure to attack again.  On the “clumped” side of the enclosure, multiple attacks are actually encouraged by not screening.  This, in turn, may invite bias because one feeding snail could stimulate other snails to attack in the same part of the clump as the first, or perhaps even on the same mussel

Preferred orientation of mussels Mytilus in clumps is with the anterior-posterior axis at an angle
to the substratum surface

Research study 3

graph comparing survival of two mussel species Mytilus californianus and M. galloprovincialis over a several-month periodHow do native predatory snails respond to the presence of non-native prey mussels?  Do they immediately feed on the exotic prey and have they the potential to limit its spread?  This is tested at the Bodega Marine Laboratory with native predatory snails Nucella ostrina and the Mediterranean/southern-Californian mussel Mytilus galloprovincialis.  In laboratory preference tests the snail preferentially eats thinner-shelled species M. galloprovincialis and M. trossulus (a native species) over the thicker-shelled native Mytilus californianus.  Clusters of M. galloprovincialis translocated to field locations are observed to attract snails and predation does occur by snails and possibly other predators (although most notable are the presence of drilled shells; see graph).  However, the fact that after 1yr about 15% of M. galloprovincialis are still alive suggests to the authors that predation alone may not be enough to inhibit establishment of the invader.  Shinen et al. 2009 Mar Ecol Progr Ser 378: 47.

NOTE   in contrast, 2 other predatory native species also tested in the study, the crab Cancer antennarius and the sea star Pisaster ochraceus, eat all 3 mussel species indiscriminately

NOTE   the mussels are moved in late springtime to a 5 x 10m2 area of the rocky shore inhabited by M. californianus.  Flexible plastic mesh is used to hold the mussels close to the substratum for 4wk while they firmly attach with byssus threads

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