The California banana slug was named Arolimax californicus by J.G. Cooper in 1872. The pigmentation of a slug depends largely on genotype, but diet, exposure to light and moisture levels also influence it. All banana slugs produce extremely thick mucus, which improves mobility and bodily protection. The slime also releases pheromones in order to attract potential mates. If the climate has become too arid, the slug will produce large amounts of slime for hydration and bury itself in debris on the forest floor. It then falls into a semi-hibernation and awaits more favorable climate conditions.
The Pacific banana slug, or Ariolimax columbines, ranges in color from white to light brown, and some have blotchy, brown spots. The dark spots on a brown background can give the appearance of an overripe banana. Growing between 7 and 10 inches in length, banana slugs are the second largest slug type in the world. The Pacific slugs are especially known for their wide foot, which is what all mollusks use to move. Banana slugs often fall prey to birds, snakes, rodents and raccoons. Predators often roll the slug in soil to bind the slime, which could make the slug nearly impossible to swallow.
Arolimax dolichophallus, the slender banana slug, has a thinner foot margin than the Pacific and California varieties. All banana slugs have similar physical features: the majority of the body comprises the foot with a small, fleshy mantle that protects delicate organs near the head. Attached to the head are two pairs of antennae, which are used to sense light and smell. The antennae retract into the head for protection in the instance of danger. The slender banana slug is usually vivid yellow in color, but can also be white or brown.
The Arolimax genus is closely studied because of its peculiar post-coital behavior. Banana slugs have both male and female reproductive systems, making them hermaphroditic and able to achieve self-fertilization; however, the usual behavior is to reproduce sexually with another banana slug. This method is preferable because it diversifies the gene pool, improving the chances of survival for the species. Another benefit of sexual reproduction is sperm storage, which the slugs achieve by chewing off each other's male genitals. These organs house valuable sperm that is stored internally in preparation for the next batch of mature eggs.