BARNACLES HAVE BEEN DESCRIBED AS SHRIMP THAT STAND ON THEIR HEADS AND FLYFISH WITH THEIR FEET. Despite the hard mollusk-like shells they build, acorn and gooseneck barnacles are crustaceans, related to shrimps, crabs and lobsters.
Look closely and you’ll see fine, feathery extensions constantly being flicked into the current from within those shells, like expert fishermen casting and recasting into a stream. The creatures within use these “cirri” – adapted leg-like appendages – to capture organisms from the plankton, their daily diet.
ACORN AND GOOSENECK BARNACLE TYPES
Generally speaking, the 1,200 species of barnacles divide into two types. “Acorn” barnacles live within calcareous homes glued directly to hard substrates. “Gooseneck” barnacles float in their hard castles above the substrate, attached with long flexible stalks that, fancifully, resemble the long necks of…well, you get the idea.
They’re the only crustaceans that are sessile – they attach themselves permanently to substrates as varied as granite boulders, dock pilings, ship hulls, clam and oyster shells – and whales and turtles.
CHITIN SHELLS, LIMESTONE HOMES
Within their shells, barnacle bodies sport traditional crustacean exoskeletons of fingernail-like chitin. But, then, around their bodies, they secrete calcareous walls for protection from predators and the elements.
The elements part is most significant. Many species of acorn barnacles position themselves in intertidal zones, subject to both harsh wave action and low-tide exposure to open air.
The calcareous structures – plates of, essentially, limestone – enable them to stand up to pounding surf and to seal themselves inside in a moist cocoon during open-air exposure. And, different species of acorn barnacles occupy different zones in the intertidal, reflecting different relationships with the varying effects of the surf.
Acorn and Gooseneck barnacles do have swimming phases – as larvae moving in the great planktonic soup, maturing and searching for surfaces to glue themselves to. Once they progress into juvenile barnacles, the little guys land on home bases, guided their by chemical cues. In the right spot, glands at the base of their antennae secrete a brown glue that secures them to their spots – head first.
Barnacles spend their lives standing on their heads.
Then they begin secreting the calcium to create the plates that surround them, then moveable plates that serve as “doors” they can close at low tide. Underwater, with doors open, six feathery cirri work constantly to filter the passing current for food.
The cirri they filter with are adapted leg-like appendages. Barnacles capture their food with their feet.
For more evidence about the neatness of barnacles, see Random Barnacle Facts, by the Numbers.
PRINCIPAL SOURCES: Crustacea Guide of the World, Helmut Debelius; “Barnacles,” Woods Hole Oceanographic Institute; “Lepas anatifera,” “Semibalanus balanoides,” Animal Diversity Web, University of Michigan; “Biology of Barnacles,” Museum Victoria Australia; “Barnacle,” Wikipedia.com.