Lepadidae, Balanidae, Chthamalidae - Barnacles
Barnacles are sedentary crustaceans (related to shrimp and crabs) that are usually found attached to hard substrates. They will attach to just about any solid object, including other barnacles and mammals such as whales. Most are covered with a set of overlapping hard calcareous plates that protect the body. Some, like the goose barnacle, have a stalk (peduncle) that attaches to hard surfaces and can extend the body out. Others, like the acorn barnacles, have no stalk and the body, covered by its plates, attaches directly to the hard surface. A set of feather-like appendages extend from the plates to filter the water for food. Adult barnacles can secrete substances that attract barnacle larvae to the area. That way they can form dense populations, making it easier to reproduce. Since many are found around jetties and other intertidal areas, they can adapt to dry conditions by closing their plates tightly to conserve moisture.
In Texas, a parasitic barnacle, Loxothylacus texanus, invades the bodies of blue crabs. A female larva injects a blob of cells into the crab through the crab's carapace. The larva dies and the blob of cells become the adult parasite. It grows in the crab, wrapping itself around the crab's internal organs. The barnacle stunts the growth of the crab and adjusts the crab's hormones so that both juvenile male and female crabs take on the appearance of a mature female crab by widening the abdominal flap. The crab becomes infertile and is unable to molt or regenerate its appendages. The barnacle finally protrudes to the outside the abdomen of the crab as a bulging sac, resembling an egg-carrying female crab. The sac carries the eggs of the barnacle. When the eggs are fertilized by a male barnacle, the crab carries and nurtures the barnacle's eggs until they hatch. The barnacle remains with the crab until the crab dies and may reproduce several times while the crab remains small and unreproductive for the rest of its life.