The Annual Slug
The annual slug, Hermissenda anno, is a member of the Nudibranchia clade, a group of shelless mollusks, or sea slugs, with external gills. At first glance undistinguishable from the other members of its order, the annual slug displays the strikingly beautiful colors associated with nudibranches. Like his cousins, the annual slug is a predatory mollusk, preying on the soft-bodied organisms he finds in his territory. What sets him apart is a rather peculiar trait when it comes to said territory.
Although nudibranches normally wander the sea floor more or less randomly in search of prey, it is not so with the annual slug. This peculiar animal follows a very precise route when in motion, and what it even more intriguing, he does so at a fixed speed. Indeed, annual slugs, once they have shed their larval shell and become adults, will move in a well-defined circle, at a speed of exactly one lap per year. Thus the name.
Why the slugs behave like this, we do not yet know. It has been observed that, depending on the terrain they have to cross, the circle on which they move will be slightly bigger or smaller. This curious fact has prompted a number of experminents by marine biologists, in which they would change the terrain of the circle randomly, making it more or less easy to cross. These studies took several years to complete, and it has been determined that the circle will always be of a size requiring precisely 2345±1 Kcal to complete. Where that number comes from, and why it is so rigourously respected by all members of the species, remains a mystery.
Being forced to stay on his circle (except when major disruptions of the sea floor make it unpracticable, or significantly change the terrain), the annual slug is unable to meet a partner for sexual reproduction. This problem has been solved by the slug explusing his reproductive cell into the water, where they will drift until they meet a cell from another individual, whereupon the two cells will fuse and start to grow into an organism. To maximize chances of encountering other reproductive cells, all annual slugs release their “eggs” at the same moment once every three years. This method of reproduction is a rarity in the nudibranches order.
The people of Hawaïi, where the annual slug lives, have long known of the annual cycle of the annual slug, and used it as a calendar sicne ancient times. Once an annual slug was found, it was observed closely each day, until its circle was known and its position could serve as a reference point. In addition to being a time-marker, the annual slug also held an important place in Hawaïian mythology, for in their eyes, it was a natural manifestation of the circle of life.