title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
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Explanations for barnacle-gregariousness quiz:

By settling in the same area as the adults, they are assured of hospitable environmental conditions in which to live.  Yes, conditions are virtually guaranteed to favour survival.

High densities enable cooperative feeding.  No.  Barnacles feed on phytoplankton and other organic particles and each individual feeds for itself.  Having said this, researchers conjecture that barnacle masses create turbulence and localised vortices that favour retention of food particles as the seawater flows by.

High densities are necessary for reproduction.  No, this is only partly correct.  Reproduction in hermaphroditic barnacles requires only that a single neighbour be within penis length.  A greater density of individuals wouldn’t necessarily increase reproductive output in an individual, but it would ensure greater genetic diversity among the sperm being contributed.  The answer is partly correct in that barnacles must settle close enough to ensure reproductive success.

Higher density increases overall defense against predators.  No.  Predators of barnacles, such as sea stars and whelks, are not likely to be deterred by a greater density of prey.  In fact, increased density of barnacles, especially when they form hummocks, may even attract the attention of predators such as birds through greater visual stimulus, or of sea stars and whelks through greater chemical stimulus.  It is also possible that predators could use surrounding barnacles as perches on which to make their attack.

Many barnacles lead to greater attachment strength.  Yes, but only initially.  As the animals grow and space becomes limited, the centrally located individuals become longer and have thinner shell plates.  Over time, this actually leads to weaker overall attachment strength of the cluster, and it becomes susceptible to shearing wave forces and to battering by logs.

 
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