A Second Species of Planorbis from New Zealand.
[Issued separately, 23rd May, 1931.]
The discovery of another species of Planorbis in New Zealand is of more than usual interest. Since P. corinna Gray was described in 1850, it has been accepted that there is only one species here, rather widely spread in the North Island, and recorded by Suter also from the River Avon and Lake Wakatipu. We can now announce, however, that there is a second species, apparently restricted to the Hawke's Bay district, differing at sight from corinna in its coiling.
Early in 1927, some fresh-water molluses were forwarded to one of us (Finlay) for determination by Mr. C. S. M. Hopkirk, B.V.Sc., Officer in Charge of the Wallaceville Veterinary Laboratory. The species were rather important, for it was suspected that one of them acted as host for the notorious liver-fluke of sheep. This proved in the end to be Potamopyrgus zelandiae Gray, and Mr. Hopkirk has recorded this in the N.Z. Journal of Agriculture, vol. 35, No. 3, pp. 175–177; Sept. 20, 1927. He has also shown (l.c., pp. 141–150) that the path opened up by the fluke in the sheep's body also allows the entry, establishment, and rapidly fatal spread of the gas gangrene gacillus, B. oedematiens. Thus the extermination of fluke carriers in the district becomes a matter of importance.
Amongst the fresh-water shells sent down were several specimens of a Planorbis which at once appeared distinct from corinna, no examples of which occurred. On being informed of the interest of his discovery, Mr. Hopkirk searched diligently for more specimens, and sent down a full range from adult to juvenile, though he stated that the draining of the swamps then in progress was making this species increasingly rare. We are perhaps fortunate to have received it before it was exterminated. It may be described as follows.
Planorbis kahuica n. sp. (Text-Figs. 1, 2, 3).
Shell very similar to P. corinna, but more rapidly expanding. No sculpture beyond fine oblique growth lines. Colour dark greenish-brown, whitish in places, hardly shining; some are light greenish-horn, and this is probably the true colour, the dark layer being a fresh-water deposit. Spire relatively more sunken than in corinna. Protoconeh indistinct, but apparently larger and less regularly globular than in corinna. Whorls three (corinna has nearly four—or perhaps more), more convex than in that species, especially the inner ones, and increasing in width more rapidly; each whorl is twice the width of its predecessor at the same spot, but only one and a third times in corinna; this makes the two species reach about the same size in the end, in spite of the difference of a whorl. Suture deeper, sub-canaliculate, bordered by a distinct narrow flat space before the convexity of the whorl begins (this is absent in corinna). Base much less
excavated, the difference in the width of the whorls being very noticeable from below. Aperture much as in corinna, but more rectangularly oval, with a slight tendency to an upper carina (not well shown in the figure).
Major diameter, 3.9 mm.; minor, diameter, 3.5 mm.; height, 1 mm. (type, 3 whorls).
Corresponding dimensions for paratype, 2¾ whorls; 2.7 × 2.4 × 0.8 mm.
Corresponding dimensions for corinna (4 whorls); 3.3 × 2.6 × 0.8 mm.
Corresponding dimensions for corinna (2¾ whorls); 1.6 × 1.4 × 0.3 mm.
Locality—Raupu swamp and dam, Rissington, Hawke's Bay (type, and numerous specimens). Also Poututu, Gisborne, a few specimens collected by C. S. M. Hopkirk, also Lake Tutira, Hawke's Bay, a number of dead shells collected with Potamopyrgus, Myxas, Isidora, Gundlachia, and Sphaerium in shore-drift by Dr. R. S. Allan.
Type in Finlay collection.
Suter gives the dimensions of a specimen of corinna of four whorls as 4.5 × 1.1 mm.; if these figures are correct, his shell must have had more than four whorls. A topotype of corinna (4 whorls) is here figured (Text-Figs. 4, 5, 6) for comparison with kahuica.
Apparently this species is limited to the Hawke's Bay district, to the exclusion of corinna. Suter gives “Petane” amongst the localities for the latter, but he probably had unknowingly specimens of this new species. Perhaps the reason for this restriction is supplied by Mr. Hopkirk's remarks on the pH (hydrogen ion concentration) of the Hawke's Bay water (l.c., p. 176). “The Hawke's Bay water is 7.2 or thereabouts, while water at Wallaceville is at least as acid as 6.4; again, just outside the range of fluke-infestation of sheep, the pH is 6.8.” Neutral water has a pH of 7.0–7.1, so that the swamp water in which this Planorbis lives is very faintly alkaline. We suggest that corinna cannot live in neutral or alkaline water, while kahuica depends for its existence on the absence of acidity. Actual experiments to prove or disprove this would be interesting. It is well known that Planorbis in other parts of the world is very sensitive to local conditions, and this is probably the case here.
“Kahu” is a Maori personal name, and was part of the tribal name of the principal Maori tribe in the Hawke's Bay district.