No. 39–Barnacles

Press reports this past December carried news of a new invasive barnacle species, Megabalanus coccopoma showing up in our coastal waters. Reportedly at 50 to 100 times the size of native species they will be hard to miss should they find their way to Port Royal Sound. It is hoped that the species that hitched a ride to Charleston Harbor from home waters of the southern West Coast of the US extending into South America will not become a problem here.

According to a December 19, ‘06 article in the Beaufort Gazette, the species has caused considerable damage where it has invaded and become established. One scientist quoted in the article gives us hope that our chilly winter waters will deter these creatures from establishing permanent residence. To view this spectacular creature you may Google “Megabalanus coccopoma and click on SCDNR News Release.

With fossil records of the class Cerripedia (“curled foot”) dating back to the Silurian period (400 mya) and long classified as mollusks due to their outward appearance, it wasn’t until scientific observations of their larva in 1830 that barnacles were properly recognized as crustaceans. The early misclassification is understandable as two of the three orders live within a hard calcareous shell as do clams, oysters, etc. This masks the fact that the creature within possesses an exoskeleton of chitin as do shrimp and crabs. On occasion I am able to observe the molted feathery fan like exoskeletons of the Estuarium’s barnacles drifting in a tank.

The two barnacle species most familiar locally belong to the order Thoracica (“breast plate”). Looking like small volcanoes, members of the suborder Balanomorpha (“acorn-shaped”) go by the catchall name acorn barnacles. The most readily observable of these are the ivory and the fragile barnacles. The ivory is the larger (3cm) of the two species and is familiar to anyone who has scraped the hull of a boat or enjoyed local roasted oysters.

The fragile barnacle is much smaller (1cm). Because of its small size it requires less food and thereby less time under water which enables it to colonize higher in the intertidal zone. Look for them the next time you have an opportunity to observe a dock piling, seawall or shore riprap.

The fact that it has an exoskeleton is one of the few characteristics the barnacle shares with other crustaceans. Among the many differences the most glaring, aside from the calcareous shell, is the barnacle’s sedentary life as an adult.

Much like an oyster, only as larvae do barnacles have freedom of movement as they search for a hard surface on which to build a permanent home. The nauplius larval stage it is recognizably crustacean and in its final bivalve cypris configuration it resembles a microcrustacean from which it takes its name. In a very uncrustacean like process, however, the cypris cements its head to a suitable surface and the enveloping bivalve carapace morphs into a mantel that will secrete calcareous plates forming exterior walls. Four additional plates controlled by muscles form a central dome which opens for feeding. I’ve found children to be particularly fascinated as they watch the six pairs of feathery legs called cirri (as in Cirripedia) reach out to rhythmically sweep plankton from the surrounding water.

A second suborder of Thoracica called Lepadomorpha contains the goose barnacle (Lepas anatifera) which may be found washed up on our beaches from time to time. They are thought to be directly descended from the Silurian ancestors who lived attached by fleshy stalks to sea scorpions of the time.

L. anatifera also has a fleshy stalk called a peduncle with which it attaches to floating objects in offshore waters. The calcareous plates are located on the end of the peduncle and resemble the head of a goose leaving little doubt how the common name came into being.

Space does not permit the introduction of other barnacle species and related creatures I came across while researching this piece. I suspect we will revisit this fascinating order of creatures at some point in the future.