title of learn-about section on goose barnacles of A SNAIL'S ODYSSEY
  Predators & defenses

photograph of ochre star Pisaster ochraceus eating goose barnacles Pollicipes polymerusMajor predators of adult goose barnacles are birds, most notably gulls Larus spp., sea stars, whelks, and perhaps fishes.  Predators of juveniles include nemerteans and various snails.  Defenses include a tough, leathery stalk, and shell plates.

NOTE  the chief predators of goose barnacles are probably humans.  Goose barnacles Pollicipes cornucopeae?, known in the tapas trade as percebes, are harvested extensively for the Spanish and Portuguese tapas markets.  Harvest is generally wholesale, that is, similar to forest clear-cutting, and regulations on collection appear to be weak and/or unenforced.  Of all the types of tapas eaten, percebes may be one of the most expensive.  This owes partly to difficulty of harvesting, and partly to increasing scarcity



An ochre star Pisaster ochraceus appears to be attacking
some goose barnacles Pollicipes polymerus but, in view of
the presence of sea mussels Mytilus californianus, it may
be one of these more preferred prey that is being eaten.
Although this is a common sight on the west coast,
no-one seems specifically to have looked at
this predator-prey relationship 0.6X

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Research study 1

photograph of nudibranch Fiona pinnata crawling on a favoured prey, the goose barnacle Lepas, courtesy Bill Rudman and the Sea Slug ForumPelagic goose barnacles Lepas anatifera are preyed upon by a pelagic nudibranch Fiona pinnata.  The action of predation on Lepas is interesting and deserves a brief mention.  Fiona attacks at the junction of peduncle and shell plates using its jaws to gain purchase.  The radula then rasps the tissues.  At this time the barnacle may dislodge the nudibranch by shaking its body.  Shortly, and after more work by the nudibranch, the barnacle gapes. and the nudibranch enters between the tergal and scutal plates, and begins to feed.  It is not known whether a toxin is involved.  Holleman 1972 Veliger 15: 142. Photograph courtesy Bill Rudman and the Sea Slug Forum.





Several nudibranchs Fiona pinnata crawl on their favourite
prey species, the goose barnacle Lepas sp. 2X

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Research study 2

photograph of gull Larus occidentalisIn the Pacific Grove area of California western gulls Larus occidentalis prey upon goose barnacles Pollicipes polymerus.  This is shown by presence of identifiable shell plates in 65% of the gulls’ regurgitation pellets (102 pellets examined). Moore 1975 Veliger 18(Suppl): 51.





Western gull Larus occidentalis (or, possibly, a glaucous-wing gull
L. glaucescens - the 2 species are similar in appearance)

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Research study 2.2

photograph of glaucous-winged gulls Larus glaucescens feeding a chick courtesy Pearson College UWC, VictoriaA comparison by a researcher at the Canadian Wildlife Service, British Columbia of diets of glaucous-winged gulls Larus glaucescens on east and west coasts of Vancouver Island shows a differential selection of prey goose barnacles Pollicipes polymerus.  Analyses of 210 and 373 regurgitated pellets from adults and chicks, respectively, reveal an 87% occurrence of Pollicipes remains in west-coast pellets versus only 2% in east-coast ones.  The difference likely relates to availability, as the prey species is much less common on east- versus west-coast shores.  Pollicipes remains are virtually absent in chick pellets on either coast (<1% occurrence), perhaps owing to their toughness for the youngsters.  Vermeer 1982 The Murrelet 63: 80. Photograph courtesy Pearson College UWC, Victoria and Race Rocks Ecological Reserve.



A hatchling Larus glaucescens
being fed by attentive adults

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Research study 3

Goose barnacles Pollicipes polymerus have patchy open-coast distributions, possibly a result of competition with sea mussels Mytilus californianus or, as some researchers believe, a product of the “patchiness” of areas of suitable hydrodynamics enabling Pollicipes to feed passively in down-swash flows off of graph showing predatory effects of birds on populations of goose barnacles Pollicipes polymerusrocks.  In a study at the Bodega Marine Laboratory, California another possibility is explored.  This new idea is that “patchiness” in bird predation, mainly involving surfbirds Aphriza virgata, western gulls Larus occidentalis, and glaucous-winged gulls Larus glaucescens, all active winter predators on both goose barnacles and sea mussels, may in turn explain the patchiness in goose-barnacle distributions and also that in sea mussels.  A 3-yr study using large-mesh cages to exclude the birds shows, indeed, that percentage cover of goose barnacles is greater in the absence of birds (see graph).  These results may not fully explain goose-barnacle distributions, but they do add to our knowledge of the role of avian predators in coastal ecologyMeese 1993 J Exp Mar Biol Ecol 166: 47.

NOTE the author includes similar data for sea mussels, but as the cover of sea mussels in the area is an order of magnitude less than that of goose barnacles, and as there is no significant effect of birds on their abundance, the data are not included here

NOTE the data are significant only on the last 3 survey dates, indicated by purple asterisks

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Research study 4

schematic showing community interactions of predation and competition in a rocky intertidal assemblage of invertebrates at Tatoosh Island, WashingtonIn a similar type of exclusion study on Tatoosh Island, Washington, birds are excluded from eating goose barnacles and other dominant invertebrates in wave-exposed rocky-shore communities.  Here, mesh cages1 are used to exclude2 glaucous-winged gulls Larus glaucescens, black oystercatchers Haematopus bachmani, and northwestern crows Corvus caurinus from selected parts of the shore.  The first 2 species are known to eat goose barnacles Pollicipes polymerus, mussels Mytilus californianus, sea stars Leptasterias hexactis, and whelks Nucella3 spp., while crows are known to eat whelks.  The 2-yr study involves a series of carefully designed experiments: 1) to show the direct4 and indirect effects that birds have on the intertidal hypotheses generated by observations on predator-prey and competition interactions in an intertidal invertebrate assemblage at Tatoosh Island, Washingtoncommunity, and 2) to test the efficacy of a statistical treatment known as path analysis in predicting the level and direction of these interactions. In preparation for the study, the author develops a "community interaction" web that models the expected interactions between the main players (see figure top Left). Start at the top with birds eating sea stars Leptasterias, which allows greater survival of acorn barnacles Semibalanus and whelks Nucella.  This provides more food in the form of Semibalanus to Nucella, and the resulting increase in whelks leads to fewer Pollicipes.  Fewer Pollicipes leads to more space for Mytilus.  Birds also eat Nucella, which permits more growth of its prey Semibalanus and Pollicipes and, in turn, leads to less space for Mytilus.  There are countless more interactions than these, but this is a start. The experiment is run over 2 seasons.

To explain the results, the author proposes 3 hypotheses (see food chains on Right).  Each hypothesis fits the facts as obtained in the results.  In Hypothesis 1 the birds eat Pollicipes which allows Semibalanus and Mytilus to expand to fill the freed-up space.  The increased cover of Semibalanus provides more food for Nucella and their numbers increase.  In Hypothesis 2 the birds eat Leptasterias leading to more Nucella.  This results in fewer Pollicipes which allows more space for histograms showing results of manipulation experiments with various bird predators and invertebrates in a rocky shore community at Tatoosh Island, WashingtonSemibalanus and Mytilus to grow.  In Hypothesis 3 the birds eat Leptasterias leading to more Semibalanus.  This provides more food for Nucella and so their numbers increase, leading in turn to fewer Pollicipes.  This frees up space for Mytilus.  Note that there are 11 predictions encompassed by these 3 hypotheses.

So, which of these hypotheses is the more likely? Path analysis, which requires that a priori hypotheses be tested, initially shows strongest support for Hypothesis 1.  The author, in meticulously correct statistical procedure, then tests each prediction of the path analysis independently in separate experiments, all involving manipulation of densities of one or other of the "players". The results5, shown in the histograms on the Left, provide strong support for Hypothesis 1. The author thus shows the important role that birds play, both directly and indirectly, in regulating the dynamics of the Tatoosh-Island invertebrate community and, additionally, the usefulness of path analysis in predicting the direction and extent of the various interactions. Wooten 1994 Ecology 75: 151.

NOTE1  plastic mesh on wire, measuring 29 x 34 x 7.5cm.  Other nearby same-sized areas on the shore, not enclosed, are designated as control plots

NOTE2  other bird species are also excluded, but the ones listed are the important ones

NOTE3  includes N. lamellosa, N. ostrina, and N. canaliculata in densities 50-370 . m-2

NOTE4  direct effects include all physical interactions of consumption, territoriality, and interference competition.  Indirect effects include non-physical interactions: for example, a sea star eats a chiton, which indirectly allows a seaweed to grow

NOTE5  the results are consistent for both years of the study and so are presented here in combined form

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Research study 5

drawings of some of the many intertidal invertebrates eaten by glaucous-wing gulls Larus glaucescens at Tatoosh Island, WashingtonObservation of birds preying on intertidal invertebrates on Tatoosh Island, Washington, shows that while glaucous-wing gulls Larus glaucescens consume more than 30 species of intertidal organisms, a single species, the goose barnacle Pollicipes polymerus, comprises almost 90% of the prey.  An average-sized gull can eat goose barnacles at a rate of 2 individuals . min-1.  Wooten 1997 Ecol Monogr 67: 45.

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