Barnacles are a type of arthropod that belong to the subclass Cirripedia in the subphylum Crustacea, and are related to crabs and lobsters. They are exclusively marine and tend to live in shallow and tidal waters, typically in erosive settings. There are around 1,000 known species of barnacles. Some key facts about barnacles include:

• Appearance: Barnacles have an external shell-like covering that encases their body. They have modified legs called cirri that they use to sweep tiny food particles from the water column and pass them to their mouth parts inside their protective plates.

• Habitat: Barnacles attach themselves to a hard substrate or a symbiont such as a whale, a sea snake, or another crustacean. They like places with lots of activity, like underwater volcanoes and intertidal zones, where they reside on sturdy objects like rocks, pilings, and buoys.

• Reproduction: Most barnacles are hermaphrodites, meaning they have both male and female sex organs. To create baby barnacles, they must be fertilized by a neighbor. A retractable tube containing sperm can reach outside the shell as far as several inches to fertilize a nearby barnacle.

• Predators: Barnacles feed on plankton they sweep from the water with their fan-like feet. Their main predators are whelks, which envelop the barnacles cone and force the valves open. The blue mussel is a competitor for space, possibly outgrowing and smothering barnacles.

• Economic impact: Barnacles can cause significant economic and environmental costs. Large barnacle colonies cause ships to drag and burn more fuel, leading to increased costs.

In summary, barnacles are a type of arthropod that live in marine environments and attach themselves to hard substrates or other organisms. They have an external shell-like covering and modified legs that they use to feed. Barnacles are hermaphrodites and have both male and female sex organs. They have predators such as whelks and compete with other organisms for space.