Banana Slug -
 
(Ariolimax dolichophallus)

mantle pneumostome head tailpit with mucus plug US quarter approx. 24 mm or 15/16 inch in diameter striped edge of foot left optic tentacle right optic tentacle right sensory tentacle carina or keel trunk Pause your mouse (don't click) over picture to learn the names of the body parts

You might think the banana slug is a worm-like animal. To be more precise, it is a mollusk—more closely related to oysters and octopuses than to worms. The slug is in a gray area between aquatic and terrestrial organisms. It has two adaptations that allow it to live on land. First, it has solved the problem of drying out by, in effect, carrying its aquatic environment along with it in the form of slime. The slug's slime covering keeps its skin from drying out. Secondly, our slippery friend doesn't have gills but instead uses a fleshy compartment that looks like a hump or shoulder (called the mantle) to act as a primitive lung. There is a pore on the right side of the mantle through which air circulates. This hole is called the pneumostome. This is the slug's "nose." The anus is located inside the pneumostome. The gonopore or genital opening is hidden by the mantle and is approximately halfway between the pneumostome and head. (The gonopore is not evident in any of the images on this page.) You can pause your mouse (don't click) over certain regions in the pictures to learn the names of the parts of a slug's body.

mantle mouth right oral or sensory tentacle left oral or sensory tentacle right optical tentacle left eye right eye left optical tentacle striped edge of foot striped edge of foot Pause your mouse (don't click) over picture to learn the names of the body parts

What is especially interesting about the banana slug is its sex (in all possible meanings of the word). The banana slug is a hermaphrodite, i.e., it is an organisms that has both male and female reproductive organs. When banana slugs mate, there is often a reciprocal exchange of sperm—each slug produces eggs and each one also has a penis. And what a penis it is! Often the length of an A. dolichophallus's penis can exceed the total length of its body! Indeed the species name, dolichophallus, means "long penis."

Banana slug mating can occur at any time of the year but is more likely when the slugs are out and about, i.e., when it is not too dry or too cold. Presumably the slugs find each other by following their slime trails. The slime most likey contains chemicals that signal readiness to mate. Mating for A. dolichophallus can be quite an ordeal. Very often the slugs will bite one another occasionally taking out small chunks of flesh. Sometimes they will raise the front part of their body up and strike their partner like a venomous snake. The two approach one another like reciprocal commas or like a yin-yang symbol with heads meeting the right sides—that's the side of the slug with the opening in its mantle. Stimulating each other in this position for hours the genital areas just forward of the pneumostome swell as the slugs get closer and closer. Finally penetration takes place, usually reciprocal. At length (sometimes this takes several hours) the mating pair will be stimulated enough to fertilize one another. Occasionally one of the slugs will get stuck, possibly because of the prodigious size of its member. If this happens, the slugs will thrash about trying to disengage for sometimes several hours. If the two cannot pull apart, the slugs will engage in apophallation, i.e., they will take turns gnawing off the stuck penis. It is not known for certain whether slugs have the ability to regenerate a penis. If not, the emasculated slug will continue to live but now as a female only.

Apophallation raises some interesting questions concerning the evolution of slugs. First off, males of most species generally have an evolutionary advantage since they can pass on genes with minimal commitment to offspring. Males contribute sperm and their job is done. Females must devote more time and resources to nourishing the eggs that grow in their bodies. If an increase in the size of the penis makes it vulnerable to apophallation, it would seem that natural selection should favor smaller penises as long as they were adequate to get the job done since propagation of male gametes would get more of the slug's genes into the next generation. There is something that might account for the selection for larger penises. In other species of slugs, self-fertilization is common (although parthenogenesis is not). I'm not sure how often self-fertilization occurs in banana slugs. If its incidence is lower in banana slugs than in slug species with smaller penises, it might be that the increase in penile length has helped to reduce the incidence of self-fertilization. Or, it could be that a quick, high-protein meal of a penis is just the thing for nourishing developing eggs.

Have you ever wondered how a banana slug gets rid of its waste? I knew a little bit about the banana slug's anatomy, so I knew the locations of anus or cloaca and genitals (see first paragaph above) but I was not too clear on how the slug accessed these other "ports." Lucky for me, I came upon this banana slug on the Saratoga Gap Trail in Castle Rock State Park who was only too happy to demonstate how it is done. (photo to the left)

You see, the pneumostome is first dilated in order to make it possible for the mollusk to defecate. The organism has control over the size of the pneumostome and will increase the size of the aperture during periods of increased activity so that more gases can be exchanged for repiration. The pneumostome also is dilated for mating.

As any discerning banana slug knows, it is best to keep in motion while eliminating bodily waste—this way the entire operation is made much easier; the extruded waste trails behind rather than piling up. Notice the direction that the excrement exits the pneumostome. It would seem that almost any other position for the anus would be better than that of being directly behind the pneumostome. Why would the slug have such an oddly arranged body plan? Surely this is a case for a little evolutionary detective work.

The slug is essentially a snail that has lost its shell. If the slug still had a shell, it would be attached where the mantle is. An organism no longer making a shell can live in calcium-poor environments. There are trade-offs: the shell-less slug lacks a snug compartment into which it can withdraw to escape from predators or a desicating environment. The body plan of the slug, though, remains basically the same as that of its snail forebears.

References:

Fox, Richard; University of Lander website on Limax maximus 2001