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

Potential predators such as sea stars are considered here, while FISHES & BIRDS, LOBSTERS & CRABS, and SNAILS are considered in other sections.

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Research study 1
 

photograph of several ochre stars Pisaster ochraceus in the shallow subtidal regionAlthough Mytilus californianus lives most commonly intertidally, there seems to be no physiological barrier to constant immersion.  In fact, observations on distribution and abundance of the species in subtidal areas around Tatoosh Island, Washington at 1-18m below MLLW show not only the presence of large, competitively dominant mussel beds, but also large sizes of individuals comprising these beds.   Individuals here may be 10 times larger than ones in nearby intertidal populations.  Individuals in these deeper beds may be in spatial refuge from predation by ochre stars Pisaster ochraceus.  Paine 1976 Veliger 19: 125.

 

 

 

Several ochre stars Pisaster ochraceus feeding at high tide

 
Research study 2
 

graph showing size selection of sea-mussel prey Mytilus californianus by ochre stars Pisaster ochraceusMytilus californianus is a preferred prey of ochre stars Pisaster ochraceus, even though this species of sea star, depending upon location and time of year, may actually eat more barnacles than mussels.  Attacks are initiated as the tide rises and digestion may be completed in situ.  Alternatively, if the Pisaster is large enough, it may rip the mussel from its attachment and carry it downshore to a less exposed position to eat. Thus, while upper limits of distribution of the mussels are set by physical factors such as desiccation, temperature, and perhaps UV irradiation, lower limits tend to be set by the sea-star predators.  Pisaster’s own intolerance to physical factors creates an often abruptly defined upper boundary of their own distribution. Extensive studies on predation by ochre stars on California mussels in the Olympic Peninsula, Washington show that California mussels enter size refuge at about 10cm-shell length.  Interestingly, tphotograph of zonation pattern set by ochre stars Pisaster ochraceus preying on sea mussels Mytilus californianus on the west coast of Vancouver Islandhe graph also shows that sea stars greater than 24-cm diameter tend not eat mussels smaller than about 8cm in length.  Perhaps small mussels provide too little return in energy and nutrients for the time and effort required by large sea stars to eat them, but the consequence is that small mussels are available as prey for sea stars from an early age. Paine 1976 Ecology 57: 858.

Zonation pattern created by predation by ochre stars Pisaster ochraceus on
sea mussels Mytilus californianus on a rocky shore in Barkley Sound, British
Columbia. The lower limit of mussel distribution is immediately above the
bare-rock area, an area traversed by the sea stars when the tide comes in

 
Research study 3
 

graph showing effect of removal of ochre stars Pisaster ochraceus on vertical distribution of sea mussels Mytilus Californianus at different sites along a surge channelA later study by the same author shows that long-term removal of Pisaster ochraceus from a region dominated by Mytilus californianus, such as a surge channel at Tatoosh Island, Washington, leads to little or no change in upper limits of the mussels.  Note, however, how their lower limits of distribution markedly shift down into the intertidal zone (highlighted in blue) over 13 continuous years (1971-83) of removal of the predator.  Paine 1984 Ecology 65: 1339.

NOTE  because sea stars Pisaster ochraceus have such profound influence, not just on sea mussels, but on the overall structure and dynamics of the shore community, they are termed keystone predators, a subject considered in more detail in another section of the ODYSSEY: LEARN ABOUT SEA STARS: KEYSTONE PREDATOR

 
Research study 4
 

Mussels growing on jetties, docks, and pilings in the company of plumose anemones may gain protection from anemones attached to their shells.  Studies in New Jersey on anemones Metridium senile growing on the shell valves of mussels Mytilus edulis show that seastars investigating the mussels as possible food often get stung by the anemones’ nematocysts and quickly withdraw.  The mussel/anemone association is not thought to be symbiotic, rather, more an advantageous place for the anemone to perch.  Competition for space may be involved.  As the mussels proliferate the only space remaining for the anemones may be on the shell valves of the mussels themselves. The extent to which west-coast Mytilus species are hosts to sea anemones is not known. If there is a similar relationship to that described here for New Jersey mussels and anemones, it would make a good research study.  Kaplan 1984 J Exp Mar Biol Ecol 79: 155.

 
Research study 5
 

photograph of ochre star Pisaster ochraceus with mostly digested sea mussel Mytilus californianusgraph showing preferred sizes of mussels Mytilus californianus for ochre stars Pisaster ochraceushistogram of preferred sizes of mussels Mytilus californianus by ochre stars Pisaster ochraceus after treating the mussels to make them equally appetising and digestibleA cost-benefit analysis of predation by Pisaster ochraceus on Mytilus californianus at Santa Cruz, California helps explain the selective advantage of size-selective predation.  If an adult-sized Pisaster is presented in the laboratory with 4 clumps of mussels of mean sizes 20, 35, 55, and 85mm shell length, it chooses the intermediate size over the others (see histogram upper Right). The sea stars apparently do some exploratory crawling over the clumps and make a decision without pulling the clumps apart.  If the mussels are then treated experimentally by filing a gap between the valves and presenting as before in 4 clumps, Pisaster still selects the intermediate-sized clumps (35 and 55mm) over the others, even though the gaps should reduce attack and digestion times for all mussel sizes, and make them all seem graph showing forces to remove mussels Mytilus californianus of different sizes from intertidal substrataequally palatable. 

These results suggest that attack and eating components, such as stomach insertion, valve opening, and digestion, may not be important constituents determining the size-selective predation.  The authors then determine forces to dislodge mussels of different sizes from intertidal substrata1, the results of which show an exponential relationship with mussel size (see graph on Left). Thus, large mussels require a disproportionate amount of force to remove, a somewhat surprising result based on scaling expectations.  

graph of equivalent energy content of mussels Mytilus californianus of different sizesThe authors then determine the potential energy gain from meals of mussels of different sizes by removing the soft tissues, drying them, and converting to joules of energy using previously published conversion values for tissues of M. californianus. The authors incorrectly plot this as a linear relationship (see graph lower Right).

graph showing foraging gains vs. costs for sea stars Pisaster ochraceus feeding on mussels of different sizesFinally, the researchers integrate energy gain and energy costs, and they do this in a clever way.  They simply divide potential gain (kJ in a mussel) by "costs"2, represented by kg force to remove that same mussel from the substratum, to create a ratio that has no real meaning other than its use as an expression of "benefit/cost" (see graph lower Left).  The graph shows that intermediate-sized mussels (35-55mm shell length) provide maximum benefit for Pisaster. Selection of mussel prey by Pisaster, then, may be a compromise3 between choosing as large a size as possible for energy return, but not so large as to expend disproportionate amounts of energy in removing it from the substratum.  The study corroborates the notion of size refuge for large M. californianus from Pisaster predation.  The authors note that theirs is the first such study on size-selective predation in M. californianus. McClintock & Robnett 1986 Mar Ecol 7: 321.

NOTE1  for this the authors use a tripod arrangement with a spring scale in the intertidal zone.  Lab studies yield similar results, but of slightly less magnitude (not shown here)

NOTE2  recall that the other costs of feeding have been shown in the valve-filing experiments to be of lesser relative importance in prey selection

NOTE3 the particular shape of the benefit/cost curve (graph on lower Left) results from the exponential relationship of force to mussel length and the linear relationship of kJ to mussel length.  Division of the one into the other produces the fall-off seen at mussel lengths >50mm. But there may be problems with the data. For example, the energy/shell length relationship shown in the graph on lower Right does not agree with scaling expectation. Theoretically, kJ of energy should scale linearly with tissue volume, and volume of tissues represented by L x W x Height will scale exponentially with mussel length.  Thus, the energy/length relationship will not be linear but, rather, curvilinear, rising disproportionately as shell length increases. It may be that the shell component of the body in this population gets disproportionately heavier with size and age, thus explaining why the predicted expectation of scaling is not realised. This would need to be checked. Another unexpected scaling relationship in the results is that of tenacity vs. length shown in the upper Left graph. Expectation based on relative surface area, but with no consideration of scaling of byssus-thread attachment, would be that tenacity falls off with increasing length. In this regard, note in the graph that different results for the 2 largest individuals could markedly influence the shape of the curve. The results still make a nice story, and should stimulate further research work

 
Research study 6
 

histogram showing survival of mussels Mytilus californianus under different treatments of predator types and intensityMussels Mytilus californianus may be preyed on simultaneously by multiple carnivores, such as birds, snails, and sea stars.  The relative effect of each predator on the mussel population is likely to differ, with one of the predator species, say, the sea star Pisaster ochraceus, taking a disproportionately larger portion of the mussels as prey, and birds or snails taking a smaller portion.  What would happen if the productivity1 of the mussel population begins to increase?  Will the relative impact of the different predators differ as prey productivity increases, and can variations in prey productivity be used to predict changes in the separate impacts of the different groups of predators? 

These interesting questions are addressed in a study at Sanford Island2, Barkley Sound, British Columbia, a site where mussels M. californianus are preyed on by P. ochraceus and various birds3.  The author measures mussel settlement on plates and also growth of tagged mussels, assesses densities of sea stars using transect techniques, and measures feeding rates of sea stars and birds.  Treatments are replicated 4 times and include 3 types of predator-exclusion cages: 1) no predators of either type, 2) open/untreated control plot, and 3) 2 types of cage controls4 at both high- and low-shore levels. The study lasts for 30d. 

Results show that productivity is highest at low-shore levels (data not shown here), and this is accompanied by increased and equal predation by both predator groups, shown by "Open Plot", "Cage Control 1", and "Cage Control 2 treatments in the accompanying histogram. The increases in predation probably result from numerical increases by Pisaster and increased foraging by birds.  Note in the “open plot” data the synergistic effects of both predator types, with even high-shore mussels being affected.  Thus, despite higher productivity of Mytilus at low-shore levels, sea stars Pisaster and birds have equal ability to curb the population increases.  The author concludes that researchers need to consider carefully their choice of species/species comparisons for assessment of the relative roles of different predator groups in a community. Garza 2005 J Exp Mar Biol photograph of an ochre star Pisaster ochraceus with sea-mussel Mytilus californianus preyEcol 324: 76.

NOTE1  defined here as combined rates of settlement and body growth

NOTE2  another site at Lompoc, California is also included in the study.  The results for both sites are similar but, as they are somewhat more striking for Sanford Island, only data for this site are presented here

NOTE3  species found at the site throughout the year and known to eat mussels are glaucous-winged gulls Larus glaucesens, western gulls Larus occidentalis, black oystercatchers Haematopus bachmani, and northwestern crows Corvus caurinus

NOTE4  the 2 cage controls have different designs, one for birds (= Cage Control 1 in the histogram) and another for sea stars (= Cage Control 2)

Ochre star selects a mussel Mytilus californianus
from amongst a selection of acorn barnacles Balanus
glandula
and goose barnacles Pollicipes polymerus 0.6X

 
Research study 7
 

graph comparing levels of heat-shock proteins in mussels Mytilus californianus at high and low positions in the bedUnder the stressful conditions of intertidal life, which will be more affected: mussels Mytilus californianus, or their predators, the sea stars Pisaster ochraceus?  Based partly on what we know of Pisaster’s feeding behaviour, moving into the higher reaches of the mussel bed at high tide, then retreating to the lower parts of the bed at low tide, we would predict that the predators are more susceptible to stress than their prey.  This is tested by researchers in Oregon by translocating mid-level mussels in April to high and low edges of the mussel bed and caging half of them with Pisaster.  Three caging combinations are employed: mussels with and without sea stars, and sea stars without mussels. At four 4-weekly intervals in the summer both mussels and sea stars are assessed for growth and reproductive status, and tissues are sampled for levels of heat-shock proteins Hsp70.  Results show that mussels in the high-level cages spawn earlier and have significantly higher levels of Hsp70 than ones in the low-level cages (see graph). There is no significant difference in growth of mussels at the 2 levels.  Sea stars suffer higher mortality at the upper edge of the mussel bed, while ones at the lower edge of the mussel bed lose body mass, but mortality is less. Sea stars at either level exhibit no significant changes in Hsp70 level.  The authors conclude that intertidal stress factors such as desiccation and temperature affect the motile predator more than its sedentary prey.  The predators, through their behaviour, are able to minimise exposure to stressful conditions.  Petes et al. 2008 Oecologia 156: 671.

NOTE  regular monitoring of temperatures of “mussel mimics” shows that when the tide is out, air temperatures are 4-8oC higher at the high edge of the mussel bed than at the low edge

NOTE  more on heat-shock proteins can be found elsewhere in this mussel learn-about section: LIFE IN THE INTERTIDAL ZONE: HEAT-SHOCK PROTEINS

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