One of the main distinguishing characteristics of sharks is their teeth. Shark teeth are not anchored to the jaws (or rather to the mandibular cartilage), but are fragilely implanted in a layer of tissue known as the basal epithelium. This, which could seem a contradiction since if the teeth are not firmly attached they could easily be lost, is just another adaptive genius of these species. This feature allows them to replenish their teeth without rest throughout their lives, always having their “tools” in perfect condition of use. This varies between species, but generically, sharks could have around 20,000 teeth in their lifetime.

Shark teeth are mainly composed of enamel-coated dentin. These teeth are generated in the inner part of the jaw and grow in rows moving forward in a similar way to that of an escalator, breaking the thin epithelium that protects them and establishing themselves as a new row of fully functional teeth.

In this way, these animals present a continuous replacement of their dentition throughout their lives. Sharks can have between 5-15 rows of teeth although only between 1 and 3 rows of teeth are functional, depending on the species. The other that can be seen are in the process of elimination (the most anterior) or formation (the later ones). In a living animal that preserves its tissues, we cannot see all those rows of teeth, but if we look at shark jaws prepared and clean of skin we can see all those rows of teeth that are in the process of being created below the epithelium.

In addition, evolutionarily, sharks and rays have developed great morphological diversity in the shapes of their teeth. So much so, that they serve as an identification tool for the different species. This could be due to an adaptive strategy in which the difference in prey (and the consequent adaptation of the teeth) would have allowed sharks to take advantage of a wide variety of food niches, minimizing competition between similar species. This, throughout evolution, has provided, as we said, a very great variety of dentitions adapted to the different eating habits of each species.

Making a general classification in large groups we can find:

• Tiny vestigial teeth of conical shape. They are non-functional teeth in the process of disappearing in those species that feed on plankton such as the whale shark (Rhincodon typus) or the basking shark (Cetorhinus maximus).
• Blunt teeth in mosaic formation. Used to crush and crush the shells or shells of invertebrates such as mollusks or crustaceans. They are presented by some species of rays and sharks, such as Smooth-hound (Mustelus spp).
• Teeth with many small pointed cusps. To tear and chop at the same time, such as Blackmouth catshark (Galeus melastomus) or Smallspotted catshark (Scyliorhinus canicula).
• Pointed, long, smooth and narrow teeth with a hook shape, to hold large prey and to be able to swallow them progressively. Once attached, the prey has no escape since the hook shape prevents it from escaping. A clear example would be the Shortfin mako (Isurus oxyrhinchus) or the Smalltooth sand tiger (Odontaspis ferox).
• Flat, triangular and serrated teeth to cut large prey or tear large pieces of meat in one bite by moving the head laterally. For example, the Great white shark (Carcharodon carcharias).
• Very different teeth (dimorphism) between the two jaws. Normally the lower teeth are flat and with many sawblade-shaped cusps and the upper teeth are fine and pointed (sometimes also with several cusps). Those above hold and those below cut. For example, the Sixgill shark (Hexanchus griseus).

One last curious detail. Some of the teeth that are lost fall out, but others are reabsorbed and in some species even swallowed to reuse the mineral substances that allow them to create new teeth.