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

photographs of musses Mytilus trossulus & M. galloprovincialis courtesy Linda Schroeder, Pacific Northwest Shell ClubDefenses in other adult bivalves, such as clams, are mostly passive, relying on thick shells and burrowing for protection against predators.  Mussels, however, live on rocks in the intertidal zone and are openly exposed to predators.  Paradoxically, most species of mussel, the bay mussels Mytilus trossulus and M. galloprovincialis being good examples, seem poorly designed for defense: too thin to withstand crushing forces of crabs and fishes, too soft to prevent boring by snails, too brittle to prevent being cracked by birds, and with closing muscles too weak to prevent being pulled apart by sea stars.  Photograph of mussels courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

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photograph of oystercatchers Haematopus bachmani flyingSo, how do mussels survive in face of potential vertebrate and invertebrate predators? Vertebrate predators such as sea otters, fishes, & birds, are considered below in separate subsections, and SEA STARS, LOBSTERS & CRABS, and SNAILS, are dealt with in their own sections.

The account begins with a short section on horse mussels Modiolus rectus.

Several species of birds, including black
oystercatchers Haematopus bachmani, commonly prey
on mussels. Here are a few leaving the mussel bed

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Horse mussels, Modiolus
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Research study 1
 

photograph of horse mussel Modiolus modiolusLittle is known about predators of horse mussels Modiolus spp., but they possess 2 features of interest relating generally to protection/defense.  The first of these is a greatly expanded byssus apparatus consisting not of attachable threads that we are familiar with in intertidal mussels Mytilus spp. but, rather, thousands of long, fine filaments.  These divide into 2 bunches as they exit the mantle cavity, one bunch paralleling the shell axis and the other, heading off perpendicularly.  The filaments adhere to themselves and to larger sand grains and pebbles, forming an effective anchor to hold Modious in place in their sandy/mud-sediment habitats.  Photograph courtesy Dave Cowles, Walla Walla University, Washington.

 

Horse mussel Modiolus modiolus showing
one bunch of byssus filaments 0.9

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Research study 2
 

photographs of horse mussel Modiolus sp.The second feature of interest in horse mussels Modiolus spp. pertaining to this section on defenses is the presence of a periostracal coating on the shell valves that extends into stiff, hairs around the posterior opening of the valves (see photograph).  An investigation of the possible function of these hairs in Modiolus rectus at Friday Harbor Laboratories, Washington suggests that they may: 1) interfere with settling larvae, 2) protect against boring-type predators or boring worms and sponges, 3) provide camouflage, 4) protect against valve-chipping predatory crabs, or 5) provide tactile sensory perception at a distance from the shell (early-warning system for approaching predators).  With respect to the last, a touch of a metal probe to the posterior hairs elicits movement of the mantle edges, while a touch to the mantle itself elicits shell closure.  As for possibility #2, the authors observe that boring sponges are absent in Modiolus, despite other bivalves in the collecting area being infested.  The authors do not know whether this owes to physical or chemical properties of the periostracum.  The authors include a discussion of patterns of evolution of periostracal structures in bivalves.  Bottjer & Carter 1980 J Paleontology 54 (1): 200.

NOTE  a molluscan periostracum is formed from tough protein secreted from the outer mantle fold (the inner fold secretes the nacreous lining of the shell and the middle fold secretes the prismatic shell layer)

NOTE  as the study uses specimens collected from around the San Juan Islands, the species may actually be M. modiolus

Views of the periostracal hairs of a horse mussel Modiolua rectus.
Note what appears to be a sponge (top view), and a number of
protist foraminiferans (bottom view) growing amongst the hairs
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Vertebrate predators: sea otters

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Research study 1
 

photograph of a sea otter eating a musselmap showing sites used for mussel diversity studySea mussels Mytilus californianus are eaten by sea otters Enhydra lutris throughout the latter’s range.  The otters dive down, tear the mussels free of their byssus-thread attachments, carry them to the surface and crack them open, either with their teeth or by banging them on rocks that they pick up from the sea bottom (see photo on Left), and eat all the soft tissues.  The question arises as to what effects other than on mussel abundance the sea otters might have on mussel communities through their depredations.  This is investigated by researchers from the University of British Columbia and Simon Fraser University at 4 locations along the west coast of Vancouver Island, British Columbia and nearby Olypic Peninsula, Washington (see map on Right).  The 4 sites include 3 in which otters have been established for variable lengths of time (5, 20, and >20yr) and one in which otters have not colonised.  The authors collect and measure mussels from 10 different plots from at 2 or more intertidal locations, measure depths of beds and assess biodiversity richness.  They histogram comparing lengh-frequencies of sea mussels Mytilus californianus at sites with and without sea otters also quantitatively estimate density and size distributions of another important predatory species at the 3 northern sites, the ochre star Pisaster ochraceus

Results show as predicted that mussels are significantly smaller in size and less abundant at sites occupied by otters for ≥20yr than at sites occupied for 0-5yr (see selected data in histograms at Left).  The former shallower, preyed-upon beds are no less diverse than the others, but total biomass of organisms associated with mussel beds is 3-fold less.  Although differences exist among sites in sea-star abundance and in certain oceanographic conditions such as degree of upwelling, hinting at possible involvement of other influences on community dynamics of M. californianus the researchers convincingly argue that sea otters likely play an important role in these events.  Singh et al. 2013 PLoS ONE 8 (5): e65435.

NOTE  for example, size distribution, bed depth and biomass, and community diversity are topics of special interest to the researchers

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Fishes

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Research study 1
 

photograph of a pile perch Rhacochilus vacca courtesy James Watanable, Pile perches Rhacochilus vacca commonly eat bay mussels Mytilus trossulus and possibly also juvenile sea mussels M. californianus.  A perch grabs a mussel in its jaws, tears it from its attachment, and crushes it between large grinding plates situated at the back of the pharynx.  The importance of predation by perches or other fishes on the population ecology of mussels is not known.  Brett 1979 Can J Zool 57: 658. Photograph courtesy James Watanabe, Stanford University, California.

NOTE  the feeding behaviour of perches is considered in more detail elsewhere in the ODYSSEY: LEARN ABOUT LITTORINES: PREDATORS & DEFENSES: FISHES

 


Pile perch Rhacochilus vacca 0.6X

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Birds

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Research study 1
 

photograph of an oystercatcher Haematopus bachmani Don DesJardinAn early study on predation of sea mussels Mytilus spp. by black oystercatchers Haematopus bachmani in Sitka Sound, Alaska provides details on methods of capture and feeding.  A bird approaches a prospective mussel prey lengthwise, inspecting it carefully.  If it fits a preferred size of about 40-60mm in length, the oystercatcher delivers sharp blows to the edge of the gaping valves that eventually displaces one valve on the other and creates a gap.  The bill is then deeply inserted by pounding it in, several times if necessary, to the depth of the bill.  The bird then twists its head forcefully from various angles, until the valves are pried open or one of them cracks.  The author adds that the bird does all this from its own left side, a behaviour consistent with that of European oystercatcher species, and one that creates a measureable asymmetry in the skull of this species.  The bulk of the flesh is removed in a few large mouthfuls, with the remaining remnants being cleaned out by scissor-like snips of the beak while it is scraped lengthwise along the inner side of each of the valves.  Mussels M. trossulus and M. californianus comprise about 35% of the bulk diet of oyster catchers in this area over the course of the year.  Webster 1941 The Condor 43 (4): 175. Photograph (taken from a video) courtesy Don DesJardin, California.

NOTE  other prey in the area include limpets (3 species L. digitalis, L. scutum, and Acmaea mitra comprising about 44% of the oyster-catcher’s diet), chitons Katharina tunicata (5%),goose barnacles Pollicipes polymerus (15%) and worms Nereis sp. (1%)

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Research study 2
 

graph showing relationship of tissue content of saxitoxins in sea mussels Mytilus californianus with season and dinoflagellate bloomgraph showing diet-switching in oystercatchers Haematopus bachmani in relation to saxitoxin content of their prey mussels and limpetsSea mussels Mytilus californianus are generally preferred prey for oystercatchers Haematopus bachmani  and other shorebirds, but does this change seasonally as tissue content of paralytic-shellfish poisoning toxins (PSPTs,  specifically, saxitoxins) increases in the prey? Note in the graph on the Left how PSPT levels increase during summer in mussels correlative with a local bloom in PSPT-containing dinoflagellates Alexandrium catenella. This interesting question is investigated by researchers at California State University Monterey Bay.  Results show, indeed, that when tissue concentrations of PSPTs exceed 150u saxitoxin eq . 100g-1 the oystercatchers tend to discard the mussel tissue and switch to non-PSPT-accumulating limpet prey Lottia spp. (see graph on Right).  Based on behavioral observations of feeding birds, it seems (not surprisingly) that they taste the prey before swallowing, and then discard any tissues with higher-than-threshold levels of PSPTs.  Kvitek & Bretz 2005 Mar Ecol Progr Ser 293: 303.

NOTE  the authors mention other shorebird species whose behaviour is modified by content of PSPTs in their prey, including godwits, sanderlings, whimbrels, and willets, but the data they present focus mainly on oystercatchers.  The authors also include another prey species in the study, namely, sand crabs Emerita analoga but, as the results for this species are similar to those for mussels, they are not included here

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