title for learn-about section of A SNAIL'S ODYSSEY
  Population & community ecology
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  Interspecific competition
  This part of population & community ecology deals with interspecific competition, while topics of MUSSEL-BED DIVERSITY, COMMUNITY SUCCESSION, INTRASPECIFIC COMPETITION, and EXTENT OF GENETIC DIFFERENTIATION are considered in other sections.
  As much of the research on this topic deals with competition of sea mussels Mytilus californianus with bay mussels M. trossulus and M. galloprovincialis, let’s take a moment to review the major characteristics of each species (M. galloprovincialis has features similar to M. trossulus).   Both kinds of mussels potentially live in the same habitat.  Data on features partly from Suchanek 1981 Oecologia 50: 143. Photograph of M. californianus courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

photograph of bay mussels Mytilus trossulus
weak byssus threads
good crawler
small size
thin shell
fast growing
matures early (1-2mo)
single spawning period
lives +10yr

photograph of sea mussels Mytilus californianus courtesy Linda Schroeder and Pacific Northwest Shell Club
strong byssus threads
poor crawler
large size
thick shell
slow growing
matures late (4-8mo)
spawns throughout year
lives +50yr

NOTE  small size, fast growth, early sexual maturity, and related features are characteristic of opportunistic or fugitive species.  Thus, Mytilus trossulus has the potential quickly to exploit new habitats or patches created in previously existing mussel-beds by log-battering or wave impact

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Research study 1

On wave-exposed shores sea mussels Mytilus californianus usually dominate even though conditions may otherwise favour survival of the smaller bay-mussel species, M. trossulus and M. galloprovincialis.  On wave-exposed shores newly recruiting bay mussels may sometimes be overgrown and crushed by the superior competitor.  Features of strong attachment and thick shell, however, are not particularly advantageous for M. californianus in quiet-water areas, and in these areas the 2 species often coexist.  Studies of survival of Mytilus californianus and M. galloprovincialis growing in mixed clumps in pier habitats in Santa Barbara show that, by virtue of a better ability to crawl, the smaller species avoids lethal effects of siltation by moving to photograph of experimental cages for study of mussel competition at Ellwood Pier, Californiathe outsides of the clusters, thereby gaining access to cleaner, food-bearing water.  Harger 1970 Veliger 13: 44; Harger 1972 Veliger 14: 275.

NOTE  the bay-mussel species used in these several studies at Ellwood Pier and other locations around Santa Barbara, California could have been either, or both, galloprovincialis or trossulus, but not edulis as the author reports.  For more on this, see IDENTIFICATION OF MUSSELS

Mixed clumps of Mytilus californianus and M. galloprovincialis
up to 15.m in diameter grow on Ellwood Pier, California

Research study 2

graph comparing growth of mussels Mytilus californianus and M. galloprovincialis alone and together in clumpsThe destructive effect of wave action is the biggest threat to survival of mussels Mytilus californianus and M. galloprovincialis growing in mixed clumps on Ellwood Pier and nearby rocky shores in Santa Barbara, California.  Survival of mussels in these locations depends in large part of the ratio of occurrence of the 2 species within the clumps.  Thus, while survival is good for M. californianus growing alone, that for M. galloprovincialis is poor, with all members of the clump being torn away within 2wk by winter storms (see graph).  Note that survival is better when M. californianus predominates in a mixed clump than when M. galloprovincialis predominates.  In general, then, clumps of M. californianus containing intermixed M. galloprovincialis are weaker than pure clumps of M. californianus, owing to the weaker attachment threads of M. galloprovincialis.  Conversely, in mixed clumps, M. galloprovincialis benefits from the stronger threads of its competitor.  Harger 1970 Veliger 13: 147; see also Harger 1972 Veliger 14: 387 for a summary of  the factors permitting coexistence of the 2 species of mussels.  

NOTE  each clump, whether mixed or pure, starts with 100 individuals of 8-10cm shell length.  The experiments are repeated with 2 other smaller size classes of mussels, but only data for the largest size classes are included here

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What reasons other than competition between the 2 species might explain the dominance of Mytilus californianus in wave-exposed habitats, and M. trossulus in quiet-water habitats?  Consider the following possibilities, then CLICK HERE for explanations.

Predators specific to one species or the other are found in the different habitats. 

Sea mussels have stronger anchoring threads and are better able to withstand wave forces than bay mussels. 

Sea mussels have thicker shells than bay mussels and can better withstand wave forces. 

The species differ in their tolerance to low-salinity conditions. 

Mytilus californianus simply does not favour life in quiet water. 

The ribbed shell of Mytilus californianus is more hydrodynamically “stable” in extreme wave conditions. 

Research study 3

photograph of sea palm Postelsia palmaeformis attached to a sea mussel Mytilus californianusSea palms Postelsia palmaeformis inhabit areas of extreme wave-swept rocky shores where they interact with sea mussels Mytilus californianus.  The alga is an annual and requires that photograph of a sea palm Postelsia palmaeformis interfering with valve opening in a sea mussel Mytilus californianusspace be available on nearby rocks for colonisation by its spores.  The sea palms grow quickly and may overgrow sea mussels, eventually causing them to be torn free in the waves.  This creates the space necessary for settlement of spores and recolonisation by Postelsia.  Dayton 1973 Ecology 54: 433; see also Paine 1979 Science 205: 685.


A sea palm Postelsia palmaeformis attached to a mussel Mytilus californianus. In such a
circumstance, the mussel
is at great risk of being torn
free from its attachment
in waves 0.4X

Close view of a sea palm's holdfast attached to the shell valves of a
mussel and clearly impeding the
opening and closing of the valves. The shell valves rely not on muscles to
open, but on the springiness of the valve hinge. They can easily be held closed by an overgrowing holdfast 1X

Research study 4

A study on interspecific competition of Mytilus trossulus and acorn barnacles Balanus glandula at Point Atkinson in southern British Columbia shows that the type and intensity of the interaction varies with intertidal height.  At the highest intertidal level of mussel distribution, competition with barnacles plays an additional role to desiccation in limiting the mussels' distribution.  At mid-intertidal levels, where conditions are more favourable for growth and survival of Mytilus, their growth in thick mats may sometimes smother the barnacles.  At lower intertidal levels, predation by sea stars Pisaster ochraceus and, in other areas, whelks Nucella spp., acts to limit the lower distributional levels of Mytilus.  Log-battering also kills some M. trossulus.  The authors conclude that loss of mussels through competition with Balanus and predation by Pisaster, and from log-battering, provides new sites each year for settllement of both M. trossulus and B. glandula, thus permitting their continued coexistence.  Ross & Goodman 1974 Veliger 16: 388.

Research study 5

schematic drawing of wave-exposed shore with distributions of mussels Mytilus trossullus indicatedOn wave-exposed coasts, such as at Tatoosh Island, Washington Mytilus trossulus is an important component of the upper intertidal community. It thrives in a zone at about the 2.9-3.2m tidal level, but is out-competed by M. californianus lower down in the mid-intertidal zone (1.7-2.9m), but may extend through the low intertidal to subtidal regions if refuge substrata are available (see diagram on Left). Recruitment of M. trossulus occurs in winter and preferentially occurs on patches created during severe winter storms. Settlement at this time if further favoured by seasonal quiescence of whelk and sea star predators. Settlement is more successful on patches greater than about 40cm diameter. This is because smaller patches are more likely to be grazed by limpets and chitons, thus removing the larvae’s preferred secondary substratum of filamentous algae.  Settlement onto photograph of a mussel Mytilus trossulus attached to "secondary" substratum of the brown alga Fucus gardnerifilamentous algae provides the juvenile mussel with temporary spatial refuge from predation by whelks and other predators.  The patches later fill in by Mytilus californianus either “rolling” in or being pushed in by “glacier” movement of the population, or by larval settlement onto filamentous algae.  The larvae of M. trossulus apparently also preferentially settle onto the byssus threads of adults already established in the patch. The author suggests that upper limits of distribution of M. trossulus are determined by desiccation stress and lower limits probably by competition and predation. Suchanek 1978 J Exp Mar Biol Ecol 31: 105.

NOTE consisting of hydroids, bryozoans, filamentous algae, and stipes of kelp that provide protection from predation

Juvenile Mytilus trossulus firmly attached to a
frond of intertidal brown alga Fucus gardneri 2X

Research study 6

On Tatoosh Island, Washington, where the 2 types of mussels live on the same shore, Mytilus trossulus has a different strategy for survival than its competitor M. californianus. The latter is a larger, sturdier, slower-growing species with effective predator-deterring mechanisms, and is an overall superior competitor for space. Its reproductive strategy is based on continual spawning at a low level throughout a yearly cycle. In comparison, M. photograph of close view of whelk Nucella lamellosa drilling a mussel Mytilus sp.trossullus is a classic fugitive species that rarely attains large size, but that matures early and has a single spawning season. Its most important attribute for survival in the Tatoosh Island area is an ability to be spatially separated from its competitor.  There are actually 2 spatial refuges.  In the first, Mytilus trossulus lives in a narrow vertical band above the upper distributional level of M. californianus.  Individuals there are smaller, reproduce less, and contribute little to the gene pool.  The second refuge consists of patches within the lower distributional level of M. californianus.  Here, M. trossulus survives by virtue of being temporarily isolated from contact with its superior competitor. Individuals are larger than their high-level counterparts and contribute, on a per individual basis, about 40 times more reproductive output.  Their habitat is high-risk, though, because of greater exposure to predators.  Eventually these predators, mainly whelks Nucella spp., eliminate the opportunistic patch-occupiers.   Suchanek 1981 Oecologia 50: 143.

NOTE  the two populations are separated by about 1m vertical height on the shore


Close view of whelk Nucella lamellosa
drilling a mussel Mytilus sp. 3X

Research study 7

Sea mussels Mytilus californianus are often overgrown by seaweeds.  Does the presence of the seaweeds help or hinder the mussels? Possible help might include protection from insolation, and camouflage from and interference with the attempted feeding activities of predators.  Hindrance could include interference with the mussel’s own feeding activities, and increased hydrodynamic drag affecting not just valvular movements, but also photograph of mussels Mytilus californianus bearing a growth of alga Spongomorphaphotograph of mussels Mytilus californianus bearing a growth of red algae, possibly Endocladia muricataincreasing the risk of being dislodged in waves.  Other minor effects that could be present in small tidepools are benefits to the mussel from oxygen produced by photosynthesis and removal of ammonia wastes by the plant, and detriments to the mussel from competition for oxygen at night.

Mussels Mytilus californianus bearing green algae Spongomorpha sp. (Left) and an unidentified red alga (Right)


Two of these major effects, namely, interference with feeding and camouflage protection from predators, can be tested by cleaning mussels in situ of seaweeds, and then protecting half the mussels with cages and leaving the other half unprotected.  Matching groups of uncleaned mussels are similarly treated.  The study is done at Santa Catalina Island, California where sea mussels are commonly overgrown with the red algae Corallina officinalis, Gigartina canaliculata, and Gelidium coulteri.  Major predators of sea mussels in this area are night-hunting spiny lobsters Panulirus interruptus. Thus, histogram showing effects of algal growths on survival of mussels Mytilus californianus both within protective cages and withoutvisual camouflage is probably not an issue, but chemical camouflage might be.  Moreover, if the algae interfere with feeding by the mussels, then uncleaned mussels will grow less and survive less. 

Results show that survival of mussels after 16wk treatment is 100% when cleaned and caged, but only 82% when cleaned and uncaged, suggesting that predation is involved. Survival of uncleaned and caged mussels is even less, at 76%, indicating some other type of deleterious effect of being overgrown.  Finally, survivorship is lowest of all, at 47%, in sea mussels uncleaned and uncaged.  Clearly, survival costs to mussels in being overgrown by algae are not compensated by increased camouflage-protection from predators. Are the deleterious effects of being overgrown related to less food being available to the mussels?  Yes, this is possible.  Measurements over the treatment period show values 15 and 65% higher for somatic and gonadal growth,respectively, in cleaned mussels than in uncleaned mussels.  Moreover, shell lengths increase 75% more in cleaned as compared with uncleaned mussels over the same period.  Although direct measurements of feeding interference would be useful, the author suggests that the seaweed may create a stagnant boundary layer around the siphon openings from which phytoplankton food is continually depleted.  Effects of food deprivation may be twofold: 1) stultification of growth and physiological weakening of the mussel, and 2) loss of mussels from wave-action through associated weakening of byssus-thread attachment. Dittman & Robles 1991 Ecology 72: 286.

NOTE  cages are often used in ecological studies, most notably to exclude predators and herbivores.  In fact, use of cages is often the only method possible, but the question always arises as to what caging effects may be present.  In the present study, for example, do the cages themselves improve survival of the mussels, apart from excluding predators, by reducing temperature, drying, and wave-induced stresses, or perhaps by other effects?  Yes, quite possibly, but it doesn’t really matter here because uncleaned mussels die significantly more than cleaned ones whether caged or not. Scientists who use cages are usually quick to point out how their: 1) use of extra-large mesh sizes, 2) employment of custom designs such as umbrella- or inverted dog-bowl-shapes, the latter allowing water or animal movement in and out from below, or open tops that allow water or animal movement in and out from above, and 3) frequent and careful cleaning to maximise water flow and minimise shading from plant growth, all help to minimise or eliminate cage-effects.  Use of cages per se is not being criticised here, but students and other “first-timers” should be cautioned not to take what is being presented completely at face value, and should understand that the use of cages of any design cannot be completely controlled for

Research study 8

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

Research study 9

photograph of sea palms Postelsia palmaeformis in the surfschematic drawings showing annual cycle of recolonisation of sea palms Postelsia palmaeformisMussels and seaweeds interact in more direct ways.  For example, Mytilus californianus directly competes with sea palms Postelsia palmaeformis for space.  The alga is a perennial and opportunistically grows in patches in mussel beds created by waves or other impact forces, such as logs.  By their quick appearance in springtime in patches created during winter storms, the belief is that the sea palms grow from developmental stages already present within the mussel bed, but have been restricted from growing by shading from the mussels or from other causes.  If patches do not form, then the sea palms are usually out-competed by the mussels.  The exclusion processes are direct, and include spores being consumed by the mussels, or settled spores and growing gametophytes being ground between the mussels’ shell valves.  Some scientists hypothesise that the mussels may produce a chemical “antibiotic” that prevents or interferes with settlement of the sea palm’s photograph of sea palms Postelsia palmaeformis torn off in storm surge with trapped mussels Mytilus californianusspores, both on the sea mussels’ shells and on the substratum beneath. If sea palm spores do settle and survive through the gametophytic growing phase, then the later sporophytic stages may grow to such a size that the host mussel, sometimes along with other attached mussels, is torn from the rocks through wave-generated drag force, thus creating new space for colonization by sea palms. That this does not happen more commonly may owe to the presence on the sea mussel’s shells of limpets that routinely graze off the settled spores and gametophytes. Blanchette 1996 J Exp Mar Biol Ecol 197: 1. Photo above courtesy Chris Lobban, University of Guam.

Several sea palms Postelsia palmaeformis torn
off in storm surge, along with 2 sea mussels
Mytillus californianus
trapped in the holdfasts

Research study 10

It is axiomatic that to be successful an invading species such as Mytilus galloprovincialis must be competitively dominant over resident native histogram comparing growth rates of mussels under different competitive circumstancesmussel species M. trossulus and M. californianus in at least one important way.  Indeed, one-species monoculture experiments and  2- and 3-species polyculture experiments in field and lab settings at the Bodega Marine Laboratory, California consistently demonstrate that M. galloprovincialis is the superior interference competitor of the 3 potentially competing species.  In all mixed-species experiments M. galloprovincialis restricts movements, smothers, and reduces feeding of the 2 native species, leading ultimately to its own faster growth and greater survival. The results show overall that the invasive M. galloprovincialis grows the most, followed by M. trossulus and then M. californianus (see histogram on Left for growth data relating to M. galloprovincialis). Survival over the 5wk study period is +90% for the 2 bay mussel species, and 85% for M. californianus, a level significantly less than the other two.   All mortality is caused by smothering by individuals that crawl atop the other mussel clumps (see photograph on Right for M. galloprovincialis). The authors summarise their results by suggesting that from its initial introduction in the 1930s M. galloprovincialis may have been the single most important cause of displacement of native bay mussels M. trossulus from southern California embayment and harbour habitats where presently it is the only species now found.  Shinen & Morgan 2009 Mar Ecol Progr Ser 383: 187.

photographs of mussels Mytilus galloprovincialis in field and laboratory NOTE  the researchers also include tests of exploitative competition for food in their study, but the results are opposite to expectation from other studies and are not included here. Individuals of about 3cm shell length are used in all experiments.  The arrays are caged to restrict mussels crawling away and to exclude predatory whelks

NOTE  the data show growth rates of M. galloprovincialis in 1-species monoculture, and 2- and 3-species polyculture experiments. The data shown are only for growth of M. galloprovincialis in various combinations. The asterisk indicates significant difference for galloprovincialis only for the californianus/galloprovincialis combination (over the growth rate of M. galloprovincialis in monoculture = CONTROL). Thus, although galloprovincialis is certainly outgrowing trossulus in the 2-species polyculture, as stated in the text above, note that the data do not actually differ significantly from the control data (first bar on Left in the histogram)

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