Barnacles are only found in marine environments, usually living in shallow and tidal waters although some are found at depths up to 600 metres. Over 1,200 barnacle species have been name so far. As adults they live permananetly fixed to hard surfaces including rocks, boat hulls, jetty piles, other marine animals like crustaceans and molluscs and even turtles and whales.

Barnacles are suspension feeders feeding on plankton whilst some species are parasites. Their predators include worms, marine snails (e.g. whelks), sea stars, some fish and some shorebirds. They also need to compete for scarce living space with limpets, mussels and other barnacles.

They spend their adult lives in their shell (usually made up of of six plate) ] and rhythmically beat their six pairs modified legs (cirri) located on the thorax in the water. these feathery appendages pull plankton and decayed organic materials into the shell to the mouth. Their calcium carbonate shells are impermeable (waterproof) and two of its plates can slide can slide shut when they are not feeding. These plates protect against predators and prevent the barnacles from drying out (dessication) as many live in harsh, intertidal zones.

They attach themselves to hard surfaces with "cement" produced by glandsin the first pair of antennae, they live attached by their foreheads upside down. In some species, the cement glands are fixed to a long muscular stalk, but in most they are part of a plate.

Inside the shell, the animal lies on its back, with its appendages pointing upwards. There is little segmentation with the body evenly divided between the head and thorax. The adults only have a single, poorly developed, pair of antennae attached to the cement gland.

Their main sense is touch, with the hairs on the limbs being especially sensitive. The adult also has a single eye to detect light and dark.

Parasitic barnacles have a much simpleranatomy than the free-living speies with no shell or appendages and an unsegmented sac-like body. They feed by extending thread-like stalks of living cells into the host's body from where they are attached.

Most barnacles are hermaphrodes (possessing male and female reproductive organs). As they cannot leave their shells to mate, barnacles have the largest penis to body size ratio of the animal kingdom. A fertilised egg hatches into a nauplius: a one eyed larva with a head and a tail, without a thorax or abdomen. This grow for 6 months of before transforming into the final cyprid stage. Nauplii are tusually brooded by the parent, and released as free-swimming larvae after the first moult.

The cyprid stage lasts from days to weeks during which time the barnacle searches for a place to settle using its modified antennules.