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Predatory semi-slug from Crimea

Sergey Leonov

(thanks a lot to Gene Fedorov (New-York) for inestimable help with translation in English)



Bilania (syn. Daudebardia) boettgeri (Clessin, 1883)
Photo: Sergey Leonov

Certain terrestrial mollusks are usually referred to as "semi-slugs" in scientific world, because, unlike true slugs, they do have shells -- very rudimentary, however, shells which offer no protection from the environment. What is then the purpose of such a shell, one might ask? With snails, the function of their shell is clear: it can be used as a hiding place, in which the owner is protected from enemies as well as from the unfavorable weather conditions. With slugs, on the other hand, the absence of a shell allows to penetrate into small openings as narrow as earthworms' holes, and thrive in biotopes where not many snails would be able to survive with their large portable homes. The shell of "intermediate" state absolutely does not allow mollusk to hide in it, and, at the same time, can hamper their movement in narrow places, so what purpose does it serve?




Photo: Sergey Leonov
Let us, by an example of psychoanalysts, take a look into the early childhood of these creatures. No, certainly, we shall not try to speculate on what childhood fears the maturing semi-slugs had to overcome, or how well they got along with their parents (although, on the other hand, there's no wonder if the peculiarities of their relationships with mother-father and father-mother might eventually lead to certain psychic abnormalities). No, we shall simply return to the time when so-called "mother" of these babies laid eggs under some stone or in a small chamber in the ground.


Embryos of semi-slugs develop, using water contained in eggs, but after hatching they very often end up in very dry conditions - a situation that is fatal to terrestrial mollusks. Here we will discover the purpose of their seemingly useless, rudimentary shell. The hatchling is still very small and perfectly fits into its embryonic shell. While hiding in it, a young semi-slug develops a thin, transparent epiphragm (cover of the shell mouth) which protects it from drying. When rains come, the mollusk leaves its life-saving prison and starts to eat and grow actively. The body grows much more rapidly than its shell, and soon the mollusk can no longer hide in it. But, the new means and ways of survival are already open before it by then, and the shell, having performed its task in due time, remains on its tail much like a small ladies' hat.


The surface of a semi-slugs body is dry to the touch and not sticky at all. This small and muscular creature is very different from its relatives, and not only externally. But we shall not dive deep yet into the details of semi-slugs anatomy, let's just mention an interesting fact -- if most common slugs are phytivorous animals, the overwhelming majority of semi-slugs are furious and gluttonous predators (admittedly, small and hardly perceptible).


There is one predatory species in Crimea - Bilania (syn. Daudebardia) boettgeri (Clessin, 1883). Very little is known about its life. This average-sized semi-slug inhabits the Mountain Crimea, where it lives in the forests and forest-steppe zones, preferring the sites with rich forest litter. It likes to hide under large stones -- where, probably, hunts its prey. Outside Crimea, it can be found in environs of Novorossisk. B. boettgeri has a body which is almost round in section, adapted for penetrating the earthworms holes; its mantle is strongly reduced and almost hidden under its miniature shell, which is up to 5,3 mm wide.


Their skin is dense and covered by furrows -- 2 pairs among them are large: one pair starts at the edge of the mantle and continues along the back up to the eye tentacles; another pair also starts at the edge of the mantle, then goes obliquely down and forward on each side and finally merges with the edge of the sole. The sole has 3 longitudinal strips.


The sole of B. boettgeri
Photo: Sergey Leonov


When frightened, daudebardia retracts its tentacles, compressed its body and turns into what resembles a small elastic cask.
Photo: Sergey Leonov

It is not known precisely what B. boettgeri eats, or how many years lives, or which time of the year breeds, or how many eggs lays. Our attempt to answer these questions can be based on indirect data only, as we know the peculiarities of biology and ecology of related species.


By the way, this information (about absence of the information) is rather useful to you, dear students! Can you see a major opportunity for research here?


Perhaps, its most investigated relative is the typical species of genus Daudebardia - D. rufa (Draparnaud, 1805) - a species which inhabits the Northern Africa, Asia Minor and the Mediterranean countries. The nearest place where this semi-slug dwells is in the mountains of southern Bulgaria. Generally, the family Daudebardiidae is widespread in western Palaearctic. The list of peculiarities of these semi-slugs includes their way of life and their food specialization.


Daudebardia rufa (Draparnaud,1805)
Source: http://www.biolib.cz

We are used to the fact that carnivores differ from the phytovorous mammals by their powerful jaws and teeth (they even have special "predatory" teeth). With predatory mollusks, the situation is a sort of reversed: if phytovorous species have a well developed maxillary plate, the predatory ones have reduced maxillary plate. The plate can even disappear at all, as in our species - B. boettgeri. But, despite that fact, they still do have teeth - so everything is all right. Their teeth differ in structure from the teeth of phytovorous species no less than, say, teeth of the wolf from teeth of a goat. At phytovorous slugs the teeth are wide and flat, and in predators they are long, thin, and sharp, like daggers. And now, dear malacologists, I would like to take a break from this fluent narration in order to address those readers who, in their curiosity, ask themselves: "Strange indeed, jaws are not present, yet the teeth are wolf-like. How it is ultimately possible?" It is possible, - we shall answer, - if the teeth grow not on the jaws (a situation to which we all have got used to), but, instead, on the tongue. By the way, among vertebrates there are also animals without jaws and with their tongue covered by teeth, but we'll discuss them another time.




The radula of D. rufa
Source: http://www.weichtiere.at
This apparatus is called "radula". When a predatory mollusk attacks its prey, the tongue protracts out, the teeth unwrap, and when touching the prey they close and stuck into it. After that, powerful muscles pull back the tongue along with the prey, everything gets into the stomach and digestion starts. From large prey (large earthworm, for example), the predatory mollusk can tear off pieces and swallow them one by one. Besides earthworms, the predatory slugs could feed on insects' larvae and other mollusks; cases of cannibalism have been observed, too.


The hibernation period of Crimean daudebardia begins quite late in the fall (as in many other slugs), from November till December, depending on weather. On the Southern coast of Crimea, apparently, they can remain active all winter long. During this time of the year, they can be found under stones, or under overthrown trees. But during summer they, evidently, get into deep chinks, or go deep into the ground in order to escape from drying.


It is possible that their life cycle could last longer than one year, as the adult individuals can be found all year round. In general, this interesting group of predatory slugs and semi-slugs remains very poorly studied, so the researchers still have a lot to do and are welcome.


Thus daudebardia hides under the stone, going on hibernation
(as you can possibly guess, the author has turned over the stone to make this shot)
Photo: Sergey Leonov

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