On land, we find the employment of zinc to reinforce feeding structures in the fangs of spiders, and also in a variety of insect groups.

Teeth are, of course, subject to wear and various strategies have evolved to deal with this inevitability. Amongst the most interesting is the incorporation of the element zinc to harden the teeth or equivalent structure, and not surprisingly this biochemical strategy is exceedingly convergent.

Zinc in jaws of marine animals

Zinc was first identified in the jaws of the polychaete annelid Nercis, but since then it has been identified in other marine groups, notably the chaetognaths (the arrow-worms) that have a ferociously effective feeding apparatus of recurved spines and jaws. In the paper (by Bone et al. 1983) reporting this, there is also a passing reference to the possibility that the extinct conodonts (which have interesting convergences to the teeth of mammals) might have elevated levels of zinc, although the hardness of conodont teeth must be largely due to their phosphatic composition.

Zinc in jaws of terrestrial animals

On land, we find the employment of zinc to reinforce feeding structures in the fangs of spiders, and also in a variety of insect groups. Amongst the most interesting in this regard are the leaf-cutter ants (attines), but zinc is also found in such insect groups as the orthopterans, including the locust.

How Zn confers hardness to teeth

The hardness the zinc confers is evidently not achieved in a metallurgical sense, but most probably arises from cross-linking of the associated protein (except in the chaetognaths where the teeth are composed of α-chitin). What is particularly interesting is that the associated amino acids are particularly rich in histidine, and rather remarkably this has, in at least the polychaetes, the same co-ordination we see in the highly convergent enzyme, carbonic anhydrase, that is one zinc atom associated with three histidines (although in some carbonic anhydrase cysteine, which chemically is similar to histidine is substituted). There is, however, a difference because while the fourth site in carbonic anhydrase is typically occupied by water (or aspartate), in the case of zinc reinforcement a halogen such as chlorine is often present.

Other ways to reinforce teeth…

Zinc incorporation is not the only way teeth can be reinforced with recourse to mineralization. The polychaete annelid Glycera uses instead copper, and interestingly it independently has evolved a venom delivery system. So too in the jaws of cephalopods neither zinc nor copper is used, at least in the jaws of squid, but the toughness evidently depends again on protein cross-linking.