The main protection of molluscs is the shell. But their own kind can drill through this armor - and eat the host alive!
Among mollusks, the most numerous is the class of gastropods (they are also gastropods, they are also snails and slugs). There are at least 65 thousand species of them.
Most representatives of this class can be recognized by their shells (which, however, differ greatly in shape and size; in slugs, they are greatly reduced). Gastropods and food preferences differ. Some of them are herbivores, while others are ruthless predators, able to defeat not only worms, other molluscs and invertebrates, but also fish.
Some gastropods have developed a special tool for hunting mollusks covered by a shell, a "drill". Its role is played by a radula, or grater, which is located at the tip of a long proboscis and has transverse rows of chitinous teeth. In some mollusks, the radula acts on the principle of a dredger equipped with buckets: the mollusk scrapes off food particles with it, which it then swallows.
And those gastropods that feed on their own kind use the radula as a drill to open the victim's shell. These are, for example, needlefish (lat. Muricidae) and sea snails Nucella lamellosa: first they secrete a secret that softens the shell of a mollusk, and then they start drilling - it takes several hours.
If successful, the predator sucks flesh out of the hole with the help of its proboscis. Sometimes the victim still manages to survive: shells with traces of overgrown holes testify to this. Perhaps the attacker stopped drilling, underestimating the thickness of the shell or unsuccessfully choosing an especially thick section of it - or someone scared him away.
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Other mollusks - octopuses from the class of cephalopods - are also able to pierce the protective cover of the victim: with a sharp beak, they make a hole in the chitinous shell, and then inject poison and digestive enzymes there.
Among modern gastropods, representatives of the families Naticidae and Muricidae are boring predators. Naticidae, judging by various studies, drill shells exclusively by mechanical means, but Muricidae add a chemical effect to this: they use various weak acids and enzymes that corrode the victim's shell in the drilling area. After the shell is drilled, all predatory gastropods act in the same way: they push their proboscis inside and scrape out the prey with the same radula. In this case, mollusks can inject enzymes into the opened shell that soften the tissues of the victim and contribute to the detachment of their muscles from the shell walls.
Predatory gastropods, as a rule, choose a place for drilling competently: they drill other gastropods at a distance of one turn from the shell mouth (where the body of the mollusk is attached to the shell from the inside), bivalves - not far from the shell apex (where the fleshy part is located and adductor muscles, see Adductor muscles).
There are, of course, mistakes: there are healed holes (that is, the victim survived after the attack and repaired the hole) and unfinished drills that do not go through the shell wall (when the predator is tired of drilling a shell that is too thick or someone scared him away).
Cannibalism is developed in drilling gastropods. Thus, gastropods of the family Naticidae calmly drill and kill smaller representatives of their own species (and other closely related species of the same genus).
Although all modern drilling gastropods appeared only in the middle of the Mesozoic, the earliest traces of drilling are known from the Late Precambrian. Already starting with the Ordovician, drills that are very similar to modern ones become numerous (paleontologists distinguish them in the genus Oichnus). As a rule, they are found on the shells of brachiopods (the benthic animals that dominated at that time), but they are also found on gastropods, and even on the shells of nautiloids from the order Oncocerida.
Since drilling on Paleozoic shells is very similar to modern ones made by gastropods, researchers believe that in those distant times, gastropods were the drilling predators, but they belonged to other genera and families that have now become extinct. It may seem strange that sedentary gastropods crawling along the bottom could attack even cephalopods (animals actively swimming in the water column), but some Early Paleozoic Nautiloid groups, including oncocerida, lived at the very bottom and, apparently, swam very slowly, and gastropods could climb on their shells while oncocerids rested or fed.
Of course, with the fossil traces of drilling molluscs, much remains unclear. It is not known which gastropods drilled shells prior to the emergence of modern taxa. In addition, several holes are extremely rare on modern shells at once, while such cases are quite common on fossil material. Some researchers suggest that these are not traces of predation, but parasitism, others - that this is the result of outbreaks of the number of predators forced to literally repel each other from a small number of prey, or traces of erroneous attacks on already empty shells.
In modern seas, the drilling method for hunting is used not only by gastropods, but also by octopuses of the genus Octopus. They drill through the hard shells of various animals, including the shells of their distant relatives, the nautilus. Having drilled (also with the help of a radula) a small hole in the shell of a potential victim, they inject saliva inside, which contains a poison that can destroy the areas of muscle connection with the shell. However, they do not have a proboscis, and the radula is located between the jaws. Therefore, the hole turns out to be small and this method of hunting is not always successful: researchers often come across nautilus who survived such attacks and healed the holes in their shells.
The holes drilled by octopuses are markedly different from those made by gastropods in their characteristic oval shape (they were distinguished as a separate ichnospecies Oichnus ovalis) and are easily recognizable from fossil material. Although the oldest octopuses are found already in the deposits of the Late Cretaceous (more than 70 million years ago), the oldest hole drilled by an octopus is only about five million years old. That is, octopuses most likely learned to use their radula for drilling shells and shells relatively recently (on a geological time scale).