35 — Black nerita, Nerita (Lisanerita) melanotragus

Nerita (Lisanerita) melanotragus,
Pilot Bay, 27/9/16

Nerita (Lisanerita) melanotragus,
Pilot Bay, 27/9/16

These are black neritas, Nerita (Lisanerita) melanotragus *. They are very common in the north of New Zealand in a variety of habitats, from exposed coasts to mangrove forests, but aren't found much south of Cook Strait. Neritas are mostly tropical and there are far more species found in those warmer waters.

They are also quite interesting because their distribution is clumped. That is to say, they seem to like hanging out together, while you commonly see isolated individuals, you also see them tightly grouped together.

There have been a few reasons put forward to try and explain why they might clump together in this way. One reason is to avoid dehydration. Clumped together they increase the surface area around them, so water doesn't evaporate as quickly. Also, by being a in a group like this, the ones in the middle have some protection from predation. For example I've seen the large black rock shell Haustrum haustorium (Gmelin, 1791) attempting to bore into a large solitary nerita. Also, one other reason could be to find a breeding partner: it's presumably a lot easier to do this if you're surrounded by your own species.

* E.A. Smith, 1884

More info:

https://www.ncbi.nlm.nih.gov/pubmed/16029478

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