[3] They are included in the infraclass Cirripedia, the barnacles, members of which are sessile suspension feeders with two active swimming larval stages, the nauplius and the cyprid.

Pedunculata is not itself a single monophyletic group but forms a transitional series of lineages moving towards the sessile acorn barnacles.

It has a muscular interior and the leathery surface is covered in bands of minute spiny scales on short stalks.

The capitulum, at the end of the peduncle, is up to 5 centimetres (2 in) long and contains the rest of the body including all the limbs and other appendages except the first pair of antennae.

[4] Pollicipes polymerus is found in the north eastern Pacific Ocean, its range extending from southern Alaska to Baja California.

They are held at a suitable angle to intercept moving water and are periodically withdrawn into the capitulum with any food items that have been trapped.

Examination of the animal's gut contents show that it feeds on copepods, amphipods, barnacle larvae, small clams, polychaete worms and hydrozoans as well as detritus and algae.

[6][8] A research study undertaken by Robert T. Paine in Makah Bay, Washington State in 1966[9] showed the importance of predators in maintaining a biodiverse community.

In an effort to understand this better, another study, undertaken by Wootton in 1994, excluded birds from an area where these two species were found on Tatoosh Island, Washington.

His results demonstrated the important part that predation by birds can play in the dynamics of gooseneck barnacle populations.

[8] Gooseneck barnacles compete with a number of other organisms in a complex struggle for survival in the limited available space in their rocky intertidal habitat.