Most species of Nerita live on tropical rocky shores but a few occur in the cooler waters of southern Australia and the North Island of New Zealand. These temperate species have plain black shells, which has led to confusion about their identity. Is there only one species? Two species? Three?
Despite the abundance of Nerita atramentosa Reeve 1855 in southern Australia and New Zealand, it has been largely overlooked as a subject for study. It was assumed that it constituted a single species, which showed some variation in the colour of the operculum (the D-shaped plate that seals the aperture). The variation is not particularly unusual — the operculum ranges from black to tan — but it piqued the interest of researchers in New Zealand (Waters et al., 2005).
Using mitochondrial DNA, they looked at the population structure of N. atramentosa from SE Australia and NZ and found that the 'east' and 'west' populations are distinct. The eastern nerites tend to have tan-coloured opercula and western black opercula. The point where one dominates the other seems to be somewhere around Wilson's Promontory, the southernmost point on mainland Australia, which formed a barrier across Bass Strait at times of lower sea level. (But both types occur on either side of this point.)
Nerita atramentosa: A) Point Peron, Western Australia; B) Marino Rocks, South Australia; C) Wilson's Promotory, Victoria; D) Cape Maria van Diemen, New Zealand; E) Mallacoota, Victoria; F) Cape Conran, Victoria. From Waters et al., 2005
That the tan operculum Nerita also occurs in New Zealand suggests it has undergone at least one long distance dispersal event. But another black-shelled nerite, N. morio Sowerby 1833, from Easter Island, Pitcairn, Gambier and Austral Island in the eastern Pacific also has a tan operculum. Could it be this same species having made an even longer ocean crossing? The mtDNA says yes.
So there's a difference between the east and west populations. Are they different species?
The taxonomy of Nerita atramentosa is a little murky. Spencer et al. (2007) have cleared it up.
Reeve (1855) described N. atramentosa from Swan River, Western Australia and E.A. Smith (1884) proposed the name N. melanotragus for New Zealand specimens that had been misnamed by earlier authors. This latter name was widely applied to shells from both NZ and Australia and N. atramentosa was little used until Iredale and McMichael (1962) turned the tables. From then on, N. atramentosa gained currency.
Just about everyone assumed that there was only one species. They all looked the same, really, and no one was terribly interested in these plain black shells, so why not? As the earlier name, Nerita atramentosa had priority, it was the one that stuck.
When Spencer et al (2007) extended the earlier study of Waters et al (2005) they found that the two species are both valid. N. atramentosa is the black-operculum western species (from WA to central Vic) and N. melanotragus is the tan-operculum eastern species (from central Vic to southern Queensland, Lord Howe and Norfolk Islands, New Zealand and the Kermadec Islands).
Spencer et al. (2007) maintain N. morio as a separate species pending more information. They treat the very similar Easter Island endemic N. lirellata Rehder 1980 in the same way.
It all seemed so simple when we just had the one ...
Nerita species: A) N. atramentosa, Adelaide, South Australia; B) N. lirellata, Easter Island; C) N. melanotragus, Auckland, New Zealand; D) N. morio, Easter Island. From Spencer et al. 2007.
References
Rehder, HA. (1980). The marine mollusks of Easter Island (Isla de Pascua) and Salay y Gomez. Smithsonian Contributions to Zoology 289: 1 – 132.
Spencer, HG, Waters, JM & Eichhorst, TE. (2007). Taxonomy and nomenclature of black nerites (Gastropoda: Neritimorpha: Nerita) from the South Pacific. Invertebrate Systematics 21: 229 – 237.
Waters, JM, King, M, O'Loughlin, M & Spencer, HG. (2005). Phylogeographical disjunction in abundant high-dispersal littoral gastropods. Molecular Ecology 14: 2789 – 2802.
