Matthew Kerruish, of East Taieri, asks :-

I found some odd shells on Ocean View Beach. They were shaped a bit like a butterfly. What are they?

Jean McKinnon, a marine scientist at the University of Otago, responded.

These are the shell plates or valves from primitive molluscs called chitons. There are about 56 species in New Zealand and perhaps around 1000 worldwide. They belong to a group called Polyplacophora, which means “many plates”. This refers to the shell plates on the upper surface of the animal; there are eight plates in all (some unusual species have seven). The plates are held in place by a leathery band called the girdle, which can also be covered in hairs, spines or scales depending on species.

This combination of shell plates and girdle means that the animal is very flexible and can fit onto an uneven rock very firmly which is very useful for water conservation in intertidal species. If they are knocked off their rock they can curl into a tight ball to protect their soft undersides. Many species have light sensors in the shell plates called aesthetes and some even have more functional “shell eyes” complete with lenses.

Most chitons are herbivores, grazing upon microscopic algae on the rock surface. They use their toothed tongue (called a radula) to scrape the algae from the rock, this has led to a rather unusual feature, to prolong the life of the teeth, they are made of metal! It’s a form of iron called magnetite, which the animal absorbs from seawater.

The most common species of chitons in this region are; the snakeskin chiton (Sypharochiton pelliserpentis); the green chiton (Chiton glaucus); the brown chiton (Ischnochiton maorianus); the hairy chiton (Acanthochitona zelandica) and the butterfly chiton (Cryptoconchus porosus). The large valves in the photo are likely to be from our biggest species the noble chiton (Eudoxochiton nobilis), which grows to 11 cm long and is not commonly seen.