title for learn-about section of A SNAIL'S ODYSSEY
  Population & community ecology

photograph of a mussel bed Mytilus californianus in Barkley Sound, British ColumbiaIn open-coast areas sea mussels Mytilus californianus dominate all other sessile organisms in upper and middle intertidal regions.  As such, the species is involved in many different and important parts of the economy of shore communities. 





Mussel bed Mytilus californianus in
Barkley Sound, British Columbia

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  Mussel-bed diversity
  This section on population & community ecology of musses deals with mussel-bed diversity, while topics of COMMUNITY SUCCESSION, INTRASPECIFIC COMPETITION, INTERSPECIFIC COMPETITION, and EXTENT OF GENETIC DIFFERENTIATION are considered in other sections.
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Research study 1

drawing of a mussel bed Mytilus californianus
A mussel bed is comprised of a physical matrix of living and dead mussel shells, several to many cm deep, which hosts a diverse assemblage of plants and animals.  One author notes that such beds, sometimes containing a hundred or more species, are one of the most diverse temperate communities described to date.  A bed is not static in its diversity; rather, it changes in time as the bed grows.  With increasing depth of the matrix, many factors including sunlight, temperature, relative humidity, and sedimentation change in magnitude, thus creating a myriad of microhabitats. Overall, more than 300 taxa have been described from west-coast beds of mussels Mytilus californianusSuchanek 1992 Northw Envir J 8: 150.






Beds of mussels Mytilus californianus provide moist,
protected refuges for a myriad of invertebrates
and fishes, including juveniles of many species

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Research study 1.1

photograph of the inner part of a clump of sea mussels Mytilus californianusSea mussels Mytilus californianus are so competitively dominant that they may occupy virtually all of the space on the rock,  Other rock-inhabiting species, such as barnacles, are excluded.  When predators or waves remove the mussels, diversity of other rock-inhabiting species goes up.  However, because mussel beds are more spatially complex than bare rock, overall diversity in the area will actually go down on removal of the mussels.  In sea-mussel beds at Point Conception, California, for example, at least 59 species of plants and animals live on and among the mussels and most of these (95%) are found on both rock and mussel-shell substrata.  Lohse 1993 J Exp Mar Biol Ecol 166: 1.

NOTE  overall, about 300 species of plants and animals are known to live on, between, and under mussels




The spatial complexity of a mussel bed
provides a 3-dimensional habitat for species
of plants and animals that could otherwise
not survive on wave-exposed bare rock 1X

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Research study 2

diagram of oil-drilling platform with mussel community off the coast of southern CaliforniaHabitat availability may be an important limiting factor on mussel distribution and abundance.  In providing hard substrata for colonisation of mussels, offshore oil-drilling platforms also contribute to formation of benthic shell mounds that host a diversity of motile and sessile invertebrates.  On offshore oil and gas platforms in southern California the principal components of the sessile and sedentary invertebrate assemblage are mussels Mytilus californianus and M. galloprovincialis, bivalves Chama arcana and Crassoderma giganteus, and various barnacles and sea anemones.  Shell mounds may be 10m in height and 70m in diameter, with estimates of up to 1200kg wet mass of litter falling per week from the mussel photograph of mussel-bed community on an oil-drilling platform in southern California courtesy Kevin Lee, Fullerton, Californiacommunity above. The litter-fall provides food for benthic organisms and 3-dimensional habitat for colonisation by other organisms. The authors provide data on abundances of echinoderms, molluscs, and crustaceans in various parts of mounds under several platform sites in Santa Barbara Channel.  The authors comment that within 5yr after removal of a platform the enhancing effect on the benthic community largely disappears. Bomkamp et al. 2004 Mar Biol 146: 201. Photograph courtesy Kevin Lee, Fullerton, California diverKevin.


Mussel-bed community on an platform in southern California.
Visible are sea urchins, sea stars, brittle stars, sea anemones,
cup corals, hydroids and, of course, mussels (ID uncertain)

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Research study 3

photograph of students on a field tripA research group in California asks the question whether regulated marine reserves are effective in protecting the integrity of mussel-bed communities along the California coast.  Their concern is based on a perception of weak enforcement of “no-take” laws in most marine reserves (MRs), and lack of regulations to protect against tramplng, turning of rocks, and handling by visitors.  Indeed, results of surveys at 24 sites along about 1000km of coastline to address these issues suggest that at sites with high levels of human visitation, mussel populations are less dense than at low-use sites, and this is equally true whether the sites are within or without MRs.  The authors recommend that regulations be broadened in all areas of high visitor impact to include all potentially damaging activities, not just manual harvesting of intertidal organisms.  The paper provides a rich references-cited section for those interested in this type of conservation topic.  Smith et al. 2008 Environ Managem 41: 599.

NOTE  10 of these sites represent 5 pairs of adjacent sites with distinct “high” and ‘low” levels of use.  At least 7 sites are in MRs

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Research study 4

map showing study locations for survey of shell mounds below southern California oil rigsA 5yr study on shell mounds beneath 15 southern California oil and gas platforms by researchers at the Marine Science Institute, Santa Barbara employs video transects by submersible to assess  diversity and abundance of large invertebrates.  The researchers identify 37 invertebrate taxa with 8 species of sea stars making up 77%  of total organisms present ( about 11,000 organisms in total; see sample photographs below).  The importance of mussels in maintaining high diversity and biomass under the platforms is evidenced by the fact that the dominant species are carnivores and omnivorous scavengers being sustained by the food subsidy delivered from mussels and other fouling organisms growing on the upper regions of the platforms. The study provides important information on the ecological value of the platforms as artificial reefs in offshore areas of southern California, required in future decisions about their decommision and possible removal.  Goddard & Love 2010 Bull Mar Sci 86 (3): 533.

NOTE  not specifically identified by the authors but presumed from the photographs to be Mytilus californianus and M. trossulus

NOTE  average depth of the 15 platforms is about 140m

Sample photographs courtesy of the authors:
photograph of crabs Cancer antennarius in shell mound beneath southern California oil rig photograph of sea star Rathbunaster californicus in shell mound beneath southern California oil rig photograph of sea star Orthasterias koehleri in shell mound beneath southern California oil rig
Crabs Cancer antennarius Sea star Rathbunaster califoricus Sea star Orthasterias koehleri
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