title of learn-about section on goose barnacles of A SNAIL'S ODYSSEY
  Ecological interactions
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Research study 1

schematic drawings showing successional events in a wave-exposed rocky shore communityA 3-way competition for space between goose barnacles Pollicipes polymerus and several species of acorn barnacles, sea mussels Mytilus californianus, and sea palms Postelsia palmaeformis is one of the subjects in an excellent treatise on competition and community organisation in rocky intertidal, wave-exposed shore areas of northern Washington State.

Early stages in the successional events involved in establishing such communities is settlement of opportunistic algae onto bare rock surfaces, followed by their replacement by perennial algae such as corallines (see accompanying schematic). These last are squeezed out by acorn barnacles, goose barnacles, and then by sea mussels, which are predicted to become the eventual dominant species in the community. Space competition can be direct, as just described, or indirect, such as when goose barnacles filter out mussel larvae in their cirral nets as food. Sea palms interact with all of these stages. For example, when sea palms attach to mussels and are then ripped out with their mussel hosts during severe storms, new space can be created in mussel beds leading to new colonisation . Dayton 1971 Ecol Monogr 41: 351.

Some interactive events relating to goose barnacles in such communities are shown in the these photographs:

photograph of members of a goose-barnacle community
Here, a sea mussel Mytilus californianus is seemingly trapped among a group of goose barnacles Pollicipes polymerus, and acorn barnacles Balanus glandula and Semibalanus cariosus. With no room to move, the mussel could soon be smothered
photograph of elements of goose barnacle/mussel community
An equal opportunity for both mussel and goose-barnacle protagonists? Probably not, as sea mussels on rocky west-coast shores are the eventual dominants, eventually displacing Pollicipes polymerus and other rock-inhabiting organisms
photograph of a monoculture of goose barnacles Pollicipes polymerus
If a virtual "monoculture" of goose barnacles is established, there is little opportunity for encroachment by sea mussels until some part of the population is removed, such as in storm waves
photograph of goose barnacles, barnacles, and algae on wave-exposed rocky shores
In the lower part of the shore, goose barnacles Pollicipes polymerus are out-competed for space by giant barnacles Balanus nubilis. Coralline algae find secure and more-or-less permanent spots to settle on and attach to these large barnacles
photograph of a sea palm Postelsia palmaeformis squeezing out potential competitors in the form of goose barnacles Pollicipes polymerus
Here, the holdfast of a sea palm Postelsia palmaeformis squeezes out potential competitors such as goose barnacles P. polymerus and mussels M. californianus. The sea palm is annual, however, and on its death survival of the other species may be enabled
photograph of goose barnacle/sea mussel/limpet community
Appearance of a clear patch in the community potentially allows successional events to be initiated. However, by feeding on settling spores of algae, the resident limpets Lottia digitalis (6 indivicuals are shown here) may disrupt the recolonisation pattern
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