Monday, August 16, 2010

Sailing with Barnacles

One of the most common sights on the intertidal zone of a rocky shore are barnacles encrusting the rocks. Water washing over the animals brings plankton which the crustaceans filter out using their legs.

Acorn barnacles attached to rock surface

However some species (a whole order in fact, Pedunculata) operate quite differently. While still attaching themselves to a substrate, they prefer the more dangerous life out on the ocean waves. Both the Goose Barnacle Lepas anatifera and the Buoy Barnacle Dosima fascicularis attach themselves to substrates floating in the pelagic zone and remain adrift at the mercy of winds, waves and the ocean currents, which often wash them ashore.

Lepas anatifera

L. anatifera is covered with five translucent plates, bluish white in appearance, and attaches to its substrate of choice by a long retractable stalk which is dark brown in colour (Sterry, 2004 Collins Complete Guide to Irish Wildlife p. 170). In the past, this substrate was often driftwood (but now is more likely to be plastic rubbish). This, along with its appearance, lent it its name “Goose Barnacle” as it was thought that L. anatifera was an immature form of the Barnacle Goose (Branta leucopsis). This idea supposed that there were trees that grow the barnacles as fruits on their branches. When ripe they would metamorphose into geese, but sometimes branches with unripe specimens could break off and were washed up on shore (Scholtz, 2008 Journal of Biology 7 pp. 16.1-16.4). Even though this had been rubbished as early as the 13th century by the great philospher-theologian Albertus Magnus, the idea persisted even to the point where the great plantsman John Gerard included a "Barnacle Tree" in his Herball of 1597 (Pavord, 2005 The Naming of Names pp. 336-337). Persistence with this myth may have had a practical purpose as it meant that B. leucopsis could be classed as a plant, not meat and thus be consumed at lent.

Dosima fascicularis

In appearance, D. fascicularis is similar to L. anatifera, except its plates are almost see through, being a light brown colur, with the stalk similarly pigmented. It differs from L. anatifera in one important aspect however: it produces its own buoyancy aid. Upon attachment to a small piece of sustrate, D. fascicularis secretes a cement that acts as a float (Barnes and Blackstock, 1974 Journal of Experimental Marine Biology and Ecolgy 16 pp. 87-91).

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