109 — The rare sand chiton, Pseudotonicia cuneata

Pseudotonicia cuneata, The base of
Mt. Maunganui, Opposite Stony Point, 2017

Pseudotonicia cuneata, The base of
Mt. Maunganui, Opposite Stony Point, 2017

Pseudotonicia cuneata, The base of
Mt. Maunganui, Opposite Stony Point, 2017

There are a few dozen species of chitons (coat-of-mail shells) found on New Zealand coasts. However, as most of them could be considered to be unremarkable-looking, they aren't really noticed by your average beach-goer.

Many chitons are light-adverse and have light-sensing organs (ocelli) on their upper surface. They use these to orient themselves away from the light. As a consequence these species are often found under rocks or ledges. Most of the shallow water chitons are thought to be vegetarian grazers, feeding on algal films on hard surfaces. 

Pseudotonicia cuneata (Suter, 1907), is quite a rare and unusual chiton. It's found mostly around North Island and the top of South, at and below low tide down to a few metres deep.

What makes this chiton species unusual is that, unlike almost all other chitons, it's found in sand. But not just any sand, P. cuneata lives in areas with clean fast-flowing waters, but not those not open to wave action, and where there are also dead shells for it to attach to. These are conditions which aren't all that common, so as a consequence not many people have seen one.

It has been speculated that this species grazes on the algal films found on dead bivalve shells, as it's often found attached to them. However, this is another one of those species for which virtually nothing is known. These ones were about 40–50mm in length and from sand around the base of Mt. Maunganui during a spring low tide.

I've only ever found this species in length of beach about 10–20m long, so their habitat is quite vulnerable.



More info:


Ashby, E. (1928). The rediscovery of Tonicia cuneata Suter and Acanthochites thileniusi Thiele (Order Polyplacophora) together with the description of a new Genus and short review of the New Zealand Acanthochitonidae. Transactions and Proceedings of the Royal Society of New Zealand 1868–1961, 58, 392–607.

Powell, A.W.B. (1979). New Zealand Mollusca. Collins