Barnacle (Balanus glandula), also known as ‘cirripede’, is an arthropod of the subclass Cirripedia in the subphylum Crustacea and is thus related to crabs and lobsters. There are about 850 free-living species (all marine) and about 260 species which are crabs and other crustacean’s internal parasites. They are sessile (nonmobile) and most are suspension feeders but they are parasitic in the superorder rhizocephala. They have four nektonic (active swimming) larval stages. Of the quite 1,400 species of barnacles found within the world’s waterways, the foremost common ones are called acorn barnacles.
Though barnacles were once thought to be snails related, it turns out that barnacles are actually crabs related. They are encrusters which temporarily attach themselves to a hard substrate. Typical barnacles as adults are coated with calcareous plates and are cemented; head down, from clams to whales, to rocks, pilings, hulls of ships, driftwood, or seaweed, or to the bodies of larger sea creatures. Because barnacles are sessile animals, they need no need for the walking legs that a lot of their crustacean relatives possess. Their legs have adapted over time to unique use. They use their adapted legs, called cirri, to sweep tiny particles of food from the water column and move them within their protective plates to their mouthpieces.
Balanus glandula (top left), Megabalanus californicus (Top right), Volcano Barnacle: Tetraclita rubescens(Bottom Left), Gooseneck Barnacle: Pollicipes polymerus (bottom right)
Free-living barnacles are attached to the substratum by cement glands that form the bottom of the primary pair of antennae; in effect, the animal is fixed the wrong way up by means of its forehead. With a tensile strength of 5,000 pounds per square inch and adhesive strength of 22-60 pounds per square inch, they secrete fast-curing cement which is among the most strong natural glues known. Via cirri feathery retractile organs formed by the metamorphosis of some of their swimming legs they capture tiny particles of food.
The form of barnacle most ordinarily encountered by land-lubbing humans is that the gray, volcano-shaped, stony type which will be found attached to piers, buoys, and boat hulls around the world, but this is often just one form that barnacles can take. Barnacles have no true heart, but a sinus close to the esophagus serves a similar function, with a series of muscles pumping the blood through it. The vascular blood supply is limited. Similarly, they need no gills, absorbing oxygen from the water through their limbs and also the inner membrane of their carapaces. The excretory organs of barnacles are maxillary glands.
They have a wide variety of body designs, but one of the most unusual is the rhizocephalan barnacle, which in other crustaceans is an internal parasite. They penetrate and propagate inside their host’s body, and even alter their actions and appearance. The outer reproductive structure of the rhizocephalan which grows where the crab’s own eggs would be can detect infected crabs. The crab cleans and cares for this growth as if it were it’s own. The eggs develop inside the mantle cavity, and the larvae, as in many other species of crustaceans, emerge as free-swimming forms called nauplii.
Whale barnacles attached to the throat of a humpback whale
Barnacles feed through feather-like appendages called cirri. Because the cirri rapidly extend and retract through the opening at the highest of the barnacle, they comb the water for microscopic organisms. When they sense a possible threat, they quickly return to their defensive shells. By introducing new material to their heavily calcified plates, barnacles continue to develop until metamorphosis is over and they have reached their adult stage. These plates are not molted; but the barnacle itself, like all ecdysozoans, can also molt its cuticle.
There is a particular form of barnacle that attaches itself to manatees. When manatees enter the relative warmth of the springs during the winter months, the barnacles can’t survive within the freshwater and die. They eventually fall off, leaving a round-ish scar behind on the back of the manatee. Typical barnacles (about 800 species of order Thoracica) have six pairs of cirri and more or less complete shells. Barnacles may reproduce through a technique called sperm casting, during which the male barnacle releases his sperm into the water and females pick it up and fertilize their eggs.
The order of Rhizocephala parasitary cirripedes (about 230 species), such as Sacculina, lack appendages, body, and intestines, and resemble fungi. Females parasitize decapod crustaceans (crabs and allies) through the host’s body by transmitting root-like absorptive processes; this interference prevents the host’s reproductive growth (parasitic castrations). Barnacles secrete hard calcium plates that completely encase them. A white cone consisting of six plates of calcium forms a circle around the crustacean. While present at water depths up to 600 m (2,000 ft), most barnacles live in shallow waters, with 75% of species living in water depths below 100 m (300 ft), and 25% inhabiting the intertidal region.
Different types of Barnacle
Parasites of the order Ascothoracica, the foremost primitive of cirripedes, are cyprislike as adults. An example is Laura, found imbedded in cnidarians and echinoderms. In general, the morphology of parasite barnacles is simpler than that of their free-living relatives. They have no carapace or arms, with only unsegmented bodies that are like sacs. Such barnacles feed from their attachment points, by extending thread-like rhizomes of living cells into the bodies of their hosts. Opting for internal fertilization, the exceptionally long penis of some barnacle species increases the chances that there’ll be another barnacle available to receive the sperm. The larvae are then released into the water column to settle as they’ll.
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