42 — Micro-molluscs, Pyramidellidae

Unidentified pyramidellids, unknown location,
 New Zealand (shells are ~5mm in length).

Turbonilla sp., unknown location, New Zealand.

These small shells (~5mm) are in the family Pyramidellidae, most probably in the genus Turbonilla. These micro-molluscs are thought to be vampiric ectoparasites: they suck the blood of their hosts. The New Zealand ones can be difficult to identify, as there are many species and their descriptions aren't very helpful. You end up trying to count the axial ribs using a microscope. There are also quite a few species, many of which are yet to be described.

The morphology of the shell is really interesting, as the early whorls (the protoconch) is sinistral (coiled left handed), but the adult shell is dextral (coiled right handed), so the shell changes direction after the first few whorls.

I get the impression that there may be some grey literature regarding these little shells. Grey literature are reports or unpublished non-peer reviewed articles.  Often the material within them is very valuable. An example would be this account in Tane: The Auckland University Field Club Journal. Before the internet this sort of material was difficult to find, so it's really great that much of it is now being made available.

As far as I can tell, very little is known about the possible hosts for the New Zealand species. But it has been speculated that the hosts could be other bivalve molluscs and/or polychaete worms. Probably a research project here for someone...


More info:

https://en.wikipedia.org/wiki/Pyramidellidae