The Goose Barnacles on the North West Pacific Coast of America are different from the ones we see usually see in the UK. They are a sessile species, Pollicipes polymerus, and they are attached to rocks low on the shore. They are related to a similar species that grows in warmer European waters. This compares with pelagic species like, for example, Lepas anatifera, which settles on floating objects that are washed around in the sea at the mercy of tides and currents. There were huge numbers of Pollicipes on the beach at Yachats in Oregon when I visited a few years ago.
There is something rather prehistoric about the way these barnacles look. They have a tough black leathery stalk or peduncle about 2 cm long that contains the gonads and an adhesive gland for sticking them securely to the rock. They do need to hold very fast because the waves are enormous and relentless in the pounding they give the shore. The ‘head’ end, also with black flesh, contains all the other organs and the appendages that it uses to filter food particles from the water. This capitulum is protected by a series of separated white calcareous plates which are exceedingly robust and thick – often showing microscopic damage cause by an endo-lithic lichen.
The barnacles mostly live close together in large mounds or dense carpets on the rocks. They are often associated with colonies of the big California Mussel (Mytilus californianus) with the beds of which they either alternate or intermix. They occur most frequently on the lower shore, especially where the impact of the waves is greatest. They are found on vertical surfaces as well as horizontal; framing tide pools; under overhangs; and in steep-sided narrow surge gullies.
Pollicipes feeds by spreading its cirri (appendages) rather like a net so that the water passes through them. They catch small crustaceans and plankton. When sufficient particles have become trapped on the cirri, they withdraw them into the capitulum and the food is transferred to the mouth parts. The cirri do not face the oncoming waves but are arranged so that they can take advantage of the water running off the rock rather than the water hitting the rock. All of the animals in a particular group or colony will characteristically face in the same direction to maximise use of the run-off water – and this may differ from the next cluster a short distance away.
You can compare and contrast this American species of goose barnacles with ones that I have seen in the UK by clicking here for:
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