BARNACLES: Class Cirripedia
The Cirripedia belong to the class Maxillopoda, an ancient form of relatively small, primarily marine crustaceans which today, are presently known by 11 subclasses, five of which are completely parasitic. Out of the six nonparasitic subclasses, the Ostenocarida and Skaracarida are extinct, dating back to the Cambrian period, and the Mystacocarida are now generally restricted to a narrow band of the marine interstitial environment. On the other hand, of the primarily nonparasitic groups, the Ostracoda and Cirripedia (from Cambrian period) and Copepoda are very diverse and occupy a wide variety of aquatic habitats. Of the nonparasitic groups, only the mystacocarids and cirripedes are exclusively marine. (Britannica, 1999)
ClassificationThe subclass Cirripedia is divided into two superorders, Acrothoracica and Thoracica. Members of the Acrothoracica are commonly known as the "burrowing barnacles" because they burrow into calcareous substrates such as limestone, corals, and mollusk shells. The principle superorder, however, is the Thoracica. Included under this division are the orders Pedunculata, which includes the common goose barnacle (pedunculate cirripedes) and Sessilia, which include the acorn (sessile) barnacles.
Natural HistoryAs a group, the cirrepedes are the most highly modified of the Crustacea, being either sessile or dwellers in other organisms in a parasitic manner ( Barnes, 1993). These marine animals differ from other crustaceans and most other arthropods in that they are attached to a substrate. Due to their sessile habit, the body is enclosed within a bivalved carapace that is covered and protected by calcareous plates. Their attached condition and calcareous covering makes them so unlike other members of the class that it was not until 1830, when their larvae were discovered, that the barnacles were recognized as crustaceans (Barnes, 1973).
If one is able to imagine an ostracod, an animal that is also attached to a substrate (by antennae) and enclosed within a bivalved carapace, with the valves covered by calcareous plates, one has some idea of the structure of barnacles. The late larval stage of barnacles is also enclosed within a bivalved carapace and looks very similar to an ostracod. They are headless, most lack an abdomen, and there is little or no segmentation.
All barnacles are attached, but the group can be divided into two forms: sessile and stalked. Sessile barnacles, which are members of the first Thoracian order, Sessila, are commonly referred to as acorn barnacles. They can be described as a tapering, sessile animal whose soft body is contained within a cavity that is protected by an outer wall. This wall, which resembles a bag-like carapace, is comprised of an even number of calcareous plates, ranging in quantity from few to many, which are cemented to the substratum. Like the other cirripedes, initial attachment to, in this case the substratum (rock, shell, or seaweeds) is effected by the antennules. After a successful attachment, this pre-oral region may enlarge to form an elongate column, as in the goose or stalked barnacles, or it may form only a thin attachment disc, as exemplified by the acorn barnacles (Barnes, 1993). An opening at the top can be closed by two pairs of plates, an operculum, through which feathery, jointed legs (cirri) can be extended into the water to capture small drifting plants and plankton (Britannica, 1999).
Members of the second Thoracian order, Pedunculata, are similar to the sessile barnacles in having the main part of the body contained within a protective covering, or wall. They differ from acorn barnacles in that the plates do not form a separate wall and operculum. Instead, the wall and the cirri are elevated above the substratum by a peduncle. This peduncle contains the ovaries and some musculature. Gooseneck barnacles are probably the most commonly observed of the pedunculate cirripedes.
It has been said that a barnacle is a shrimp-like crustacean that attaches by the top of its head and then "kicks" food into its mouth with its feet (Britannica, 1999). This kind of descriptive definition actually distinguishes barnacles from their allies and provides for a fair idea of how the animal operates. Both stalked and sessile barnacles are filter feeders and their trunks contain six pairs of long, coiled, biramous appendages called cirri, from which the name of the class is derived (Barnes, 1973). In feeding, the cirri unroll and project through the gape of the carapace and perform a rhythmic, scooping motion. The many fine setae of the appendages are responsible for the filtering of plankton out of the water.
Correlated with their attached existence, barnacles, unlike most other arthropods, are hermaphroditic, having both male and female reproductive systems in the same individual, with either or both being able to activate at one time. Although some species are known to self-fertilize if no partners are present, most shallow-water species cross-fertilize. In species in which populations are sparsely distributed, a hermaphrodite may be accompanied by one or more small "complemental" males, or the larger individual may develop into a female, and in that case, a smaller individual attaching to it becomes a "dwarf" male. In both groups of barnacles, the penis may be very long in order to reach into nearby attached (and therefore sessile) individuals. (Klepal, 1985)
The acorn barnacles are now the dominant shallow-water barnacles. Species are found in almost all habitats, from equatorial to polar regions, from estuarine waters and the highest intertidal zones to depths of 2,000 meters. Although some stalked barnacles are found on rocks, many species live attached to floating objects such as wood, or on the floating bodies of larger swimming animals. Whales, porpoises, sea turtles, crabs, and even jellyfish are used as substrata by different species. Most sessile barnacles have become adapted for life on rocks or other hard substrata, and their heavy, somewhat fused, wall of plates most likely represents an adaptation for protection against currents, pounding waves, and predators. However, like stalked barnacles, some sessile barnacles have become adapted for living on other animals such as crabs and whales. (Barnes, 1973).
Barnacle larvae prefer to attach next to another member of their own species, or on a place where one of their species has just been removed. They avoid settling on top of another member of their species, even though they readily settle on a member of another species. To some extent, they react to chemicals that are specific to their species. As a result then, barnacles aggregate in colonies, but within a colony space themselves out. (Klepal, 1985).
The evolution of parasitism in barnacles is most likely logical consequence of living attached to other animals (Kane, 1992). Approximately one-third of the members of the class are parasitic and are so highly modified that they are recognizable as barnacles only by their larval stages (Barnes, 1973). An example is the extreme form of the order Rhizocephala, which resemble nothing so much as a bracket fungus. The rhizocephalans have an unusual life cycle. A cyprid larva destined to become a female seeks out a host, such as a crab, and attaches where the cuticle is thin, usually on a gill or at the base of a seta. The cyprid metamorphoses, and most body parts (excluding the head) are discarded. When this process is completed, a hollow, ventral stylet is forced either directly into the host or into the host after passing through one of the cyprids first antennae. Once in the hosts body, the cells of the larva migrate into a central position under the gut, where they send out rootlike absorbtion processes to all parts of the crabs body. Once the parasite has established itself internally, a hollow reproductive body develops in between the hosts thorax and abdomen, and there it enlarges to fill the space where the crab normally broods its eggs. The crab then cares for the parasite as if it were its own eggs. (Britannica, 1999).
Importance to humans
Barnacles are major fouling organisms on pier pilings, sea walls, bouys, and the submerged portions of marine vessels. Much research has been expended to develop anti-fouling measures such as anti-fouling paint, for a badly fouled ship may have its weight increased and its speed reduced by as much as 35 percent (Barnes, 1973). The cement by which barnacles attach themselves to the substratum sets under water and even sticks to plastics with low surface tensions. Because of its unusual properties, it has recently been the subject of a number of investigations in the hope that there might be practical applications for dentistry.
Barnes, R., C.A. Villee, and W.F. Walker. 1973. General Zoology. W.B. Saunders Company, Philadelphia, PA, pp.608-611.
Barnes, R.S.K., P. Calow, and P.J.W. Olive. 1993. The Invertebrates: a new synthesis. Blackwell Science, Ltd., Cambridge, MA, pp.206-207.
Kane, K. 1992. Zoology. Wm. C. Brown Publishers, Dubuque, IA, pp. 357-358.
Klepal, W. 1985. Ibla cumingi (Crustacea Cirripedia): A Gonochoristic Species (Anatomy, Dwarfing, and Systematic Implications). Marine Ecology 6 (1): 47-119.
1994-1999. Encyclopedia Britannica Online. Cirrepede. Available: http://www.eb.com:180/bol/topic?eu=118971&sctn=1&pm=1. September 22, 1999.