Barnacles. These cone-shaped, shelled animals that attach to piers, boats and driftwood fool people into believing they are mollusks. Mollusks, which include clams and snails, are soft-bodied animals enclosed in a hard calcareous shell. At first glance, a barnacle looks like a mollusk, as it is entirely enclosed in a hard shell and does not appear to have any legs.
But the barnacle is actually a crustacean, more closely related to crabs and shrimp. Crustaceans are characterized by a hard exoskeleton and jointed legs. Hidden by its external shell, a barnacle has been described as a "shrimp-like animal...in a limestone house."
A barnacle's shell is actually six overlapping plates with an opening at the top covered by two hard flaps. When submerged in water, the two flaps, acting as doors, open up, and the barnacle unfolds a fan of feathery legs. Upon closer inspection, one sees the jointed legs that make up this fan. The barnacle will wave these legs through the water to sweep tiny food particles into its "shell."
Barnacles are hermaphroditic, each individual possessing both male and female organs. To reproduce, though, a barnacle's eggs must be fertilized by another individual. This is accomplished by a sperm tube that protrudes from one barnacle into a neighboring one.
Fertilized eggs are nurtured in the barnacle until they hatch into tiny larvae and are released into the water during May and June, when the water is thick with literally thousands of larvae. Barnacle larvae are a favorite food for many young fish and are consumed in such large numbers that few of the larvae settle and develop into adults.
Barnacle larvae pass through two stages. The first stage, the nauplii, is a triangular form that exists for a few days before molting into the cypris, which looks like tiny transparent seeds. The cypris larvae swim about for a few days searching for a suitable place to attach. Cypris larvae often attach to areas occupied by other barnacles of the same species.
It is thought that a chemical released by older barnacles attracts the cypris larvae. The cypris attaches itself at its head using cement secreted by its antennal glands. After its attachment, the barnacle begins to produce the calcareous plates that will encase it.
The barnacle's shell, composed of calcium carbonate, grows along with the animal. The shell is enlarged by adding calcium carbonate along the edges of each plate, increasing the size of the inside cavity. Then, like other crustaceans, the barnacle sheds its old exoskeleton.
Barnacles adhere to piers, boats, plants, rocks and shells in the intertidal zone. This is an area that is submerged by tides and then exposed to air as the tides recede. When the barnacle is exposed to the air during low tide, the two flaps shut tightly. This keeps the animal inside moist until the tide rises again.
Barnacles, though well protected, are susceptible to dryness, extreme cold and harsh winds. Many animals feed on barnacles including sponges and bryozoans, which grow over and smother them, and worms-especially the little oyster flatworm.
Of course, anyone who owns a boat, curses these tiny creatures and the tedious and difficult task of removing them. But barnacles are a necessary part of the Chesapeake Bay food chain, removing particles from water and providing food to other animals.
Barnacles...yeah we're stuck with them!