Strange as it may seem, fossil slitnails were known long before any living specimen was ever found, and the fossils are some of the earliest known gastropods dating back hundreds of millions of years to the Paleozoic. Back in 1856, a hapless hermit crab wearing a slitsnail on it's back crawled into a fish trap and the following discovery that "fossil" organisms were alive was received by a wave of enthusiasm. Science aside, this is a fantasy come true--living fossils!
Shells of living slitsnails are virtually indistinguishable from some of the fossil forms and even more remarkable is the anatomy of these animals. Unlike most living gastropods, they have a bilaterally symmetrical body plan that has long been thought to be primitive. Slitsnails have a deep mantle cavity with paired organs such as ctenidia (gills), hypobranchial glands, and kidneys, that are bathed by inhalent currents entering at the front of the mantle cavity and exiting at the back of the slit. Most other gastropods have evolved a more asymmetrical arrangement of mantle cavity organs, in the process losing some organs (ctenidium and hypobranchial gland) while modifying others to new functions.
The striking slit in the shell is a characteristic that is unfamiliar to most beachcombers and amateur collectors since slitsnails are never found on beaches (finding one is about as likely as a ball spontaneously rolling up a flight of stairs). The unusual slit begs for some explanation. Having a deep mantle cavity containing ctenidia on both sides poses a problem--how to direct streams of water across the gills to exchange oxygen and carbon dioxide. The slit extends to the posterior edge of the gills, and the animal closes off most of it with the mantle edges, but leaves an opening at the rear, thus converting the mantle cavity into a tube so that water enters at the front and leaves through the rear opening.
Most known species, genera, and families of the superfamily of slitsnails are extinct. Of the more than 1,500 named species, less than 30 are living today. Not only is this an amazing tenure on Earth, it also speaks to patterns of evolution, expansions and collapses in the diversity of groups, and extinction as a rule.
Slitsnails have been collected from depths as shallow as 100 feet, to more than 2,500 feet. They tend to live in temperate and tropical regions, preferring hard bottom to soft, and are often on very steep slopes, including vertical rock walls. They live in bathyal habitats, mostly below the level of photosynthesis, feeding on sponges, soft corals, stalked crinoids, and perhaps other invertebrate fauna.
They are a kind of animal grazer, that is to say they feed on sessile animals, or mostly so. Of all the food items they include in their diets, sponges are the oddest. Most deep water sponges have internal skeletons made from tiny pointed shards, known as spicules, that are rich in silicon. The spicules are like fiberglass and they are oriented in ways that make ingesting them more hazardous than if they were all aligned with the same orientation. Handling one of these sponges for even a moment, as two team members did, leaves the unprotected hand stuck with painful, glassy slivers. Pleurotomarids have a highly specialized digestive system that has evolved to process spicules by aligning and packing them together before eliminating them. This is tricky business, but pleurotomarids are equipped with a digestive system to feed on one of the most challenging organisms.
Slitsnails have one of the most remarkable fossil records of any living animal, and this legacy is carried by only a few living species. These few open the past from the present.