Slide 5 of 50: The sex life of these gooey, yellow mollusks is even more bizarre than their coloring or the fact that they inch down from high tree branches on thin strands of slime in much the same way as spiders utilize their webbing. For starters, according to a 1916 paper by Stanford University zoology professor Harold Heath, they are hermaphrodites. Two, a slug penis, which emerges from its head, can be as long as its entire body. That makes for some of the largest male genitalia of any species in proportion to its overall size. Three, reproductive sessions last hours and sometimes end in apophallation, which is when a banana slug gnaws off and eats its partner's privates. They do not grow back.

California: Banana slug

The sex life of these gooey, yellow mollusks is even more bizarre than their coloring or the fact that they inch down from high tree branches on thin strands of slime in much the same way as spiders utilize their webbing. For starters, according to a 1916 paper by Stanford University zoology professor Harold Heath, they are hermaphrodites. Two, a slug penis, which emerges from its head, can be as long as its entire body. That makes for some of the largest male genitalia of any species in proportion to its overall size. Three, reproductive sessions last hours and sometimes end in apophallation, which is when a banana slug gnaws off and eats its partner’s privates. They do not grow back.

Banana slugs are often bright yellow (giving rise to the banana nomenclature) although they may also be greenish, brown, tan, or white. The species Ariolimax columbianus sometimes has black spots that are so extensive that the animal looks almost entirely black. Individual slugs will change colors with alterations in food consumption, light exposure, and moisture levels. Color may also indicate whether a slug is healthy or injured or what age it is.[4]

The Pacific banana slug is the second-largest species of terrestrial slug in the world, growing up to 25 centimetres (9.8 in) long,[5] and weights of 115 grams (4.1 ounces).[6] (The largest slug species is Limax cinereoniger of Europe, which can reach 30 centimetres (12 in) in length.) Banana slugs have an average lifespan of 1–7 years.[4]

Banana slugs (like other gastropods and many other mollusks) have a radula, a ribbon-like anatomical structure covered in rows of microscopic teeth; the radula is used for feeding.[citation needed] Individuals can move at 6 12 inches (17 cm) per minute.[7]

Slugs use two pairs of tentacles to sense their environment. The larger, upper pair, termed “eyestalks,” are used to detect light or movement. The second, lower pair are used to detect chemicals. The tentacles can retract and extend themselves to avoid damage. If a predator bites off a tentacle, the slug can grow a new one.[8]

Banana slugs have a single lung (on the right side) which opens externally via a pneumostome. The pneumostome lung cavity is heavily vascularized to allow gas exchange. Dehydration is a major problem for the mollusk; to combat this, banana slugs excrete a thick coating of mucus around their bodies and can also aestivate. To do so, they secrete a protective layer of mucus and insulate themselves with a layer of soil and leaves. They remain inactive in this state until the environment becomes moist again. Due to their susceptibility to desiccation, they are more commonly active at night, but also appear during cool, moist days.[9]

This individual Ariolimax columbianus has numerous black spots. The patterning may be so extensive as to make the animal look almost solid black. Banana slugs have a single lung which opens externally via a pneumostome. The placement of the pneumostome on the mantle helps to distinguish the species of Ariolimax.

The slime also contains pheromones to attract other slugs for mating. Slugs are simultaneous hermaphrodites, and reproduce by exchanging sperm with their mate. They produce up to 75 translucent eggs, which are laid in a log or on leaves. Slugs mate and lay eggs throughout the year. The adults provide no further care for their eggs beyond finding a suitable hiding spot, and the eggs are abandoned as soon as the clutch is laid.[citation needed]

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