The High-Tidal Mollusca of Rangitoto Island, Auckland; with Descriptions of a New Genus and Two New Species.

By A. W. B. Powell, Conchologist, Auckland Museum.

a minute taenioglossid, Assiminea vulgaris. Associated with these molluscs are a few amphipods, isopods, and the crabs Hemigrapsus sexdentatus (M. Ed.) and Heterozius rotundifrons (M. Ed.).

In depressions among the rocks, masses of decaying wrack accumulate, made up for the most part of remnants of low-tidal brown algae Carpophyllum maschalocarpum (Turn.) Grev. and C. plumosum (A. Rich.) J. Ag., with intermingled Zostera nana.

This accumulation of decaying vegetative matter supports large numbers of half to full-grown individuals of the pulmonate mollusc Marinula filholi, while the decaying Zostera in particular is favoured by numerous Assiminea vulgaris.

Beneath the decaying algae are loose stones which rest upon or are partly imbedded in mud, and upon these occur large numbers of the young of Marinula filholi, together with Assiminea vulgaris and a second taenioglossid, Suterilla neozelanica, while underneath in contact with the mud a further three species are to be found. Two of these (one a new genus of the pulmonates) are organic-mud feeders which receive their sustenance by retrieving from the mud minute particles of decaying algae and animal detritus. The third species, a new Kellia, occurs in vast numbers, but the two pulmonates are by no means common.

At Rangitoto the food of Melarhaphe consists for the most part of an obscure dark greenish, almost black lichen, which is rendered quite inconspicuous owing to the dark-coloured lava upon which it grows.

In other areas Melarhaphe is often found in numbers feeding upon the sea-lettuce, Ulva, and also upon Enteromorpha, a small alga which forms a pale green coating upon high-tidal rocks wherever fresh-water seepage occurs. Of all the high-tidal molluscs only one, Melarhaphe, is able to withstand long periods of desiccation, and on sunny days the stones upon which it occurs often become quite hot.

Contact with the sea at the high-tidal levels is very brief, so the remaining seven species owe their existence primarily to the moisture retained in the mud shaded by the Salicornia and to the dampness caused by the more or less permanent accumulation of decaying wrack, which is retained owing to the roughness of the rocks. Very little of the debris that once touches the shore at Rangitoto is ever washed away again by succeeding tides, so the conditions are rendered more or less permanent.

This stability of the high-tidal conditions, which means an ever-present abundance of food, can be taken as the main reason why the Rangitoto high-tidal fauna is so much richer than that of any other high-tidal area so far studied in New Zealand.

The Salicornia zone at Hobson Bay, Auckland Harbour, in sharp contrast to that of Rangitoto Island, has only two species of mollusca, Ophicardelus costellaris and Melarhaphe oliveri, the species Marinula filholi, Suterilla neozelanica, Assiminea vulgaris, Kellia maoria, and Rangitotoa insularis all being absent. However, this paucity of species may be due to some extent to the fact that the salinity of the waters of Hobson Bay is known to be slightly lower than at Rangitoto.

Summary of the Feeding Habits of the High-Tidal Mollusca.

In brackish situations Potamopyrgus antipodum is often found living upon the mud in association with mangroves. Caecum digitulum, abundant elsewhere, has not yet been observed at Rangitoto; it seems to require smooth, soft rock, of which there is none at this locality.

Assimineidae.

The first reference of the New Zealand shells to their correct family was in Finlay (1926, p. 379), where on Iredale's authority our species was referred to the widely distributed Polynesian Assiminea nitida (Pease). Finlay also mentioned that this species has been recorded from the Kermadecs by Oliver.

The species nitida, however, is not an Assiminea, as has been demonstrated by Thiele (1927, p. 128), for the dentition and open umbilicus show that it belongs to the genus Paludinella.

The writer has examined the dentition of Rangitoto specimens, which are topotypes of Brookes's Tatea hedleyi, and it is almost inseparable from that of grayana, the English genotype (as figured by Thiele 1. c.), having the characteristic hinder denticles on the central tooth, and moreover the shell has the umbilicus closed, which is a characteristic feature of the genotype, but not of nitida, in which it is quite open. The open umbilicus in nitida was mentioned in Pease's original description, and this feature is clearly shown in specimens from Tonga in the Auckland Museum collection.

Furthermore, upon shell characters, Kermadec Island specimens are inseparable from New Zealand examples.

If Iredale's statement (l.c., 1913, p. 370) is correct, that the Loyalty Island Barleeia chrysomela, Melville and Standen (1896, J. Conch., vol. 8, p. 309), is identical with the Kermadec Island shells, then it may be necessary later to change the specific name of the New Zealand shell once more. However, in the absence of Loyalty Island specimens, and taking into account both the poor condition of the holotype (as mentioned by Iredale, l.c., p. 370) and the fact that Iredale misidentified the Kermadec specimens as nitida, the wisest course meanwhile is to select an available New Zealand name for the New Zealand shell. This name comes from an unexpected source, as was revealed by a comparison between Webster's holotype of Rissoia vulgaris and topotypes of Brooke's Tatea hedleyi, which proved to be identical. It is possible that the combination Assiminea vulgaris has been used before, but I have not succeeded in tracing any such combination, so Webster's name may be employed for the New Zealand and Kermadec Island Assiminea, the type locality for which becomes Waipipi, Manukau Harbour.

Dentition (Fig. 7): The dental formula of Assiminea vulgaris is 1 + 1 + 1 + 1 + 1 + 1 + 1. Central tooth with five cusps, and three denticles on each side, below. Inner lateral with six cusps and outer lateral with five. Marginal teeth with 15 pectinate cusps. The central tooth is a little broader, and the cusps more regular in size, otherwise the dentition of vulgaris is almost identical with that of the genotype.

Subfamily Omphalotropidinae.
Genus Suterilla Thiele 1927.

Type (original designation): Cirsonella neozelanica Murdoch.

Suterilla neozelanica (Murdoch).

• 1899. Cirsonella? neozelanica Murdoch. Pro. Mal. Soc., vol. 3, pt. 6, p. 320.

Ellobiidae.

Rangitotoa insularis n. sp. (Fig. 1).

Shell minute, semi-transparent, imperforate, thin, and colourless. Outline ovate-cylindrical, spire conical, less than half height of aperture. Whorls seven, slowly increasing at first, including a minute smooth protoconch of one flattened whorl. Aperture long and narrow. Outer lip thin and sharp, without a ridge or any processes within. Inner lip with three plaits, uppermost weak and not plainly visible from without, situated at a little above half the height of aperture. Central plait large and prominently projecting; lower plait less prominent, merging below into the thickened narrowly rounded basal lip. Surface smooth except for faint irregular axial growth lines.

Dentition (Figs. 5 and 6): Radula very minute, the detail of the teeth being difficult to see even with a sixteenth oil immersion objective. Dental formula small (12–16) + 1 + (16–12). Central tooth with one large cusp and an obsolete denticle on each side of it; laterals 16, each with a mesocone and an ectocone; marginals 12, each with an entocone, a mesocone, and from two to four ectocones.

• Height, 3.7 mm.; diameter, 1.9 mm. (holotype)

• " 2.9 mm.; " 1.4 mm. (average size specimen).

Holotype: In Auckland Museum (collected by A. W. B. P., March 12, 1930.

Habitat: Rangitoto Island; type from near the “Beacon.” Found living only near high-water mark, on the under sides of stones which are partly in contact with mud. The writer has found this species to be distributed around the southern and western sides of Rangitoto Island, but so far has failed to discover it elsewhere.

Genus Marinula King 1831.
Type (by monotypy): Marinula pepita King.

Marinula filholi Hutton 1878.

This species shows considerable adaptability, and this accounts for its wide distribution around New Zealand shores. At Rangitoto filholi lives under stones and among decaying algae in muddy situations within the high-tidal zone, but it has been recorded by Oliver (1923, p. 499) from “between tide marks” at Island Bay, Cook Strait, and the writer has observed it on the open coast at Cape Foulwind, Manukau North Head, and Muriwai, living on cliff faces at above high-tide mark in situations where fresh-water seepage occurs.

Dentition: The dental formula (not previously recorded) is 121 + 1 + 121. The central tooth has one simple cusp, the laterals from two to three and the marginals from four to five cusps. The radula is very small, and the individual teeth are crowded and difficult to see.

It is also of interest to record that Marinula striata Odhner occurs abundantly at Mason's Bay, Stewart Island. Formerly this species was known only from the Auckland and Antipodes Islands. The Mason's Bay specimens were found alive and in numbers by Mrs R. H. Harrison, of Stewart Island.

Genus Leuconopsis Hutton 1884.
Type (original designation): Leuconia obsoleta Hutt.

Leuconopsis obsoleta (Hutton 1878).

This species is found in the sheltered harbour bays towards high-tide, usually in company with Caecum digitulum and Onchidella patelloides. It occurs sometimes on the open coast, and at Muriwai it is associated with Marinula filholi, and lives on cliffs just above high-tide mark, in situations where fresh-water seepage occurs.

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Dentition (Figs. 3 and 4): Hutton, 1884, Trans. N.Z. Inst., vol. 16, p. 213, gave the formula 55 + 1 + 55/150 but he was unable to make out the form of the teeth. The radula is certainly very minute, the detail of the teeth being very difficult to see even with a sixteenth oil immersion objective. Dental formula small (16 + 39) + 1 + (39 + 16). Central tooth with one large elongated cusp, on each side of which is an obsolescent base of a former side cusp. The base of the central tooth is shaped like an inverted T. The laterals, which number 39, are each prominently bicuspid; both cusps, of which the mesocone is the larger, being long and slender and inclined outwards. Marginals 16, pectinate, each with an entocone, a mesocone, and an ectocone, all three of which are moderately long and of almost equal length.

Although the dental formula is smaller than is usual in the Pedipedinae, the character of both the teeth and the shell in Leuconopsis is most in accord with that subfamily.

Erycinidae.
Genus Kellia Turton 1822.

Type (subsequent designation, Herrmannsen 1846): Cardium rubrum

Mont.

Kellia, W. Turton. Conch. Insul. Brit., pp. 19, 56, 1822. Examples: Kellia suborbicularis (Montagu) and K. rubra (Montagu). Kellia, Turton, Herrmannsen, Indicis Gen. Malac., vol. 1, p. 568, 1846. “Typus: Cardium rubrum Mont.” Lasaea, Leach in Brown, Ill. Conch. Gt. Brit. Ed. 1, Pl. 20, Figs. 17–18, 1827. Sole example, L. rubra (Montagu).

References.

Finlay, H. J., 1926. A Further Commentary on New Zealand Molluscan Systematics. Trans. N.Z. Inst., vol. 57.

Finlay, H. J., 1928. The Recent Mollusca of the Chatham Islands. Trans. N.Z. Inst., vol. 59.

Grant, U. S., and Gale, H. R., 1931. Pliocene and Pleistocene Mollusca of California. Mem. San Diego Soc. Nat. Hist., vol. 1.

Odhner, N. H., 1925. Marinula juanensis, n. sp., nebst Bemerkungen über die Systematik der Ellobiiden. Arkiv for Zoologi, Stockholm. Band 17, h. 2, pp. 1–15.

Oliver, W. R. B., 1923. Marine Littoral Plant and Animal Communities in New Zealand. Trans. N.Z. Inst., vol. 54.

Thiele, J., 1927. Uber die Schneckenfamilie Assimineidae. Zool. Jahrb., Jena. Band 53, pp. 113–146.