The egg strings of sea slugs are often as noticeable as the creatures themselves. Usually white, though sometimes pink or orange, they can be shaped like ribbons, rosettes or strings of pearls. A single slugs egg ribbon can contain more than a million eggs. They are attached to the seabed or to sedentary animals on which the adults have been feeding.

Sea slugs are molluscs without an obvious shell. Some, like the sea hare, have a much-reduced internal shell, but the nudibranch sea slugs lack one entirely. Nudibranch means naked gill, as the gills of these animals have no protection and simply form a frill on the animals back.

With no hard shell in which to hide, and without the pace to outrun predators, sea slugs have to protect themselves in other ways. Most manage this by producing noxious substances which make them unpalatable. One species uses sulphuric acid, and it can give a nasty burn if handled out of the water.

Sea slugs are usually fussy eaters, each species specialising in a certain type of prey such as sponges, hydroids or sea squirts. Like most molluscs, they have a cunning feeding mechanism called a radula. This strap-shaped structure bears teeth and acts both as a file for rasping at prey and a conveyor belt for transporting fragments of food.

Some sea slugs eat animals with stinging cells, such as sea anemones and hydroids. They manage to pass these cells through to the surface of their bodies without setting them off, so that they can use them in their own defence. A fish that is tempted to take a nibble will trigger the cells and get a nasty surprise.

Sea slugs can act as male and female simultaneously, because they are hermaphrodites. They cannot fertilise themselves, but a mating pair will fertilise each other when they come together. Sea hares form mating chains or circles, with each animal acting as a male to the one below and a female to the one above.