A strange symbiosis

Let me tell you about an organism I know of which has a very peculiar lifestyle. Actually, it is composed of two biologically distinct organisms, known as the Slug and the Shell, each with its own DNA and reproductive cycle. However, the Slug and the Shell are so closely bound together for most of their life cycle that it makes more sense to think of them as two parts of a single organismic system.

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At first glance, it’s hard to see why a Slug needs a Shell at all. Slugs are complete, self-contained organisms in their own right and are quite capable of living and thriving alone, without being bonded to a Shell. In fact, because of their greater fecundity, Slugs greatly outnumber Shells, and at any given time most of the Slugs in the world are unbonded, living without Shells. However, unbonded Slugs do have one striking liability: an inability to reproduce. They produce gametes in great abundance but have no external genitalia and hence no way of releasing those gametes.

Shells, on the other hand, are wholly dependent on Slugs and, except during pupation, are unable to live independently. While a Shell is a complete, living organism (not a lifeless mass of calcium carbonate, as the name might suggest), it is a seriously deficient one, with an incomplete digestive system (no mouth or anus) and no means of locomotion. Unlike a Slug, it does have a fully functional reproductive system, including external genitalia, but its inability to move around nevertheless makes reproduction a practical impossibility.

A few days prior to hatching, Shell eggs produce a powerful pheromone that attracts unbonded Slugs, which will hang around waiting for the eggs to hatch so they can bond to the newborn Shells. (If no Slugs are attracted, or if the bonding is unsuccessful, the Shells usually die in a matter of hours.) When a Shell bonds with a Slug, it plugs into the Slug’s digestive system, siphoning some of the Slug’s food into its own stomach and then routing its own fecal matter back into the Slug’s digestive tract to be excreted. Because it is mostly immobile, the Shell uses relatively little of the Slug’s food, but it is nevertheless essentially a parasite at this stage in its life cycle, living off the Slug and giving it nothing in return. The Slug-Shell unit at this point in its development is known as a Protosnail.

When Protosnails reach maturity, they mate. However, while the Slug does all the work of finding a mate and fighting off other Protosnails, it is only the Shells that mate, only the Shells that lay eggs, and only the Shells’ DNA that is passed on — for what hatches from the egg is not a complete Protosnail but just a Shell, which must attract a new Slug of its own.

Shortly after mating, the Shell disconnects from the Slug, drops off, and pupates. During the Shell’s pupal stage, the Slug once more lives independently. However, it stays in the general vicinity of the pupal Shell and will not bond with any other Shell during this time. During its Protosnail phase, the Shell has imprinted on its Slug’s DNA, and after pupation the adult Shell will bond only with that same Slug. And the Slug, as we shall see, has a very good reason for wanting to bond with its Shell again; such re-bonding is its only hope of getting any return on the investment it has made in its erstwhile parasite.

When the adult Shell emerges from its chrysalis, its Slug is generally there waiting for it, and they immediately bond again to form the final stage in their collective life cycle: the Permasnail. This time the bonding is much deeper and more pervasive, and it is irreversible. If the Protosnail is a bit like a hermit crab (albeit with a living shell), the Permasnail is more like lichen: functionally a single organism. However, the Slug and Shell components still retain their own separate DNA. Most importantly, from the Slug’s point of view, in the Permasnail the Shell’s external genitalia connect to the testes or ovaries of the Slug, finally allowing the Slug to reproduce its own kind.

When Permasnails mate, the eggs they lay hatch into Slugs, which grow to maturity and then start looking around for Shells to bond to in order that they may move on to the next stage of their life cycle.

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So what am I really talking about? What is the real meaning of this zoological treatise à clef? I’ll reveal the answer later, but first feel free to guess.