Rhytida citrina Hutton, 1882

Brook, Fred J., Kennedy, Martyn, King, Tania M., Ridden, Johnathon, Shaw, Matthew D. & Spencer, Hamish G., 2020, Catalogue of New Zealand land, freshwater and estuarine molluscan taxa named by Frederick Wollaston Hutton between 1879 and 1904, Zootaxa 4865 (1), pp. 1-73 : 47-48

Rhytida citrina Hutton, 1882

Pl. 6, fig. D

Hutton, 1882. The New Zealand Journal of Science, 1: 282.

Type material. Three syntypes formerly in Canterbury Museum, Christchurch ( Hutton 1898 – 1900, Suter 1913: 772), but reported as missing by Powell (1946: 129), and not found during a search of the CMNZ molluscan collection in 2017. However, the collection at CMNZ does contain a radula mounted on a glass slide with the label details ‘ Rhytida citrina, Greymouth, XVI p. 167’, Hutton’s handwriting (i.e., CMNZ 2017.17.9), which is possibly primary type material (see radula descriptions by Hutton 1883d: 139, 1884b: 167, pl. 10, fig. R).

Type locality. Listed as ‘Greymouth (R. Helms)’ by Hutton (1882p: 282, 1883d: 139); designated here as Mt Davy, Rewanui (see neotype designation below).

Previous illustrations of type material. Radula teeth illustrated by Hutton (1884b: pl. 10, fig. R) possibly from type material; Suter (1915: pl. 30, fig. 14).

Remarks. Hutton submitted descriptions of two new species of Rhytida from Greymouth, R. citrina and R. patula , to the Transactions and Proceedings of the New Zealand Institute issue for 1882, but publication was delayed until May 1883 ( Hutton 1883d: 139), and was pre-empted by brief descriptions of both taxa in an account of a meeting of the Philosophical Institute of Canterbury ( Hutton 1882p: 282). A description by Hutton (1884b: 167, pl. 10, fig. R) of a radula of citrina was possibly based on CMNZ 2017.17.9 mentioned above. Suter (1913: 772) stated that the type material of R. citrina in the Canterbury Museum consisted of three shells, the largest of which had a maximum diameter of 7.75 mm. This type material was apparently subsequently lost. Powell (1946: 129) stated that he had “not seen the types which Dr. Falla has been unable to locate in the Canterbury collections”, but mentioned the existence of “two of Hutton’s Greymouth specimens [of citrina ] in the Canterbury Museum”, which he reported as having major diameters of 8.5 mm and 5.5 mm, respectively. These two specimens, both juvenile shells, correspond to lots CMNZ M1416 and M5456 [ex M1416], respectively. Freeman et al. (1997: 30) listed these two lots, and CMNZ M123 [ex ZS 818], as syntypes of R. citrina , but this does not stand scrutiny. The last-mentioned lot is from Buller River, which is not the type locality of citrina . The collector is not stated in the CMNZ molluscan catalogue but was probably Julius von Haast (see Hutton (1884c: 208). The catalogue indicates that lots M1416 and M5456 from Greymouth, along with M1417 from Balclutha, were all identified as citrina and were included in ‘old No. 122’ in Hutton’s collection. Hutton’s original labels for this material have been lost or destroyed. The register does not state who the material from Greymouth was collected by, and there is no indication that these two specimens are primary type material of citrina . The specimens from Balclutha (M1417) were evidently collected by Hutton himself, and were identified as R. patula by Hutton (1884b: 167, 1884c: 208), and R. otagoensis by Powell (1930).

Hutton (1882p: 282, 1883d: 139, 1884b: 167, 1884c: 208) described citrina and patula as differing from one another in shell and animal colouration, and radula features, but there has been confusion over the identity and distributions of these two taxa. Hutton (1884c: 208) recorded citrina from Greymouth and Buller River, and patula from Greymouth and Balclutha, respectively. Suter (1913: 772) treated them as separate species, but noted that the type material of R. citrina consisted of juvenile shells only, and observed that “it is not easy to separate this species from R. patula by shell characters alone”. Powell (1946: 129) considered that they were separate species with partly overlapping distributions, noting that “from Greymouth to the Buller River two forms occur, a reddish-brown one with a greatly accelerated last whorl, which is definitely patula , and a yellowish-olive one with closely coiled whorls, which is almost certainly the adult of citrina ”. His interpretation of citrina was based mainly on the specimen from Buller River in CMNZ M123, mentioned above. Powell (1946: 129, text fig. C1) described this specimen, which has since been badly damaged, as “an adult or nearly so, of 18.5 mm diameter, with “closely coiled whorls and the yellowish-olive coloration of citrina ”. He recorded R. citrina from Maruia Springs, Rewanui, and Lake Kaniere, in addition to Hutton’s records from Greymouth and Buller River. Parkinson (1979: 10), Powell (1979: 345), Spencer & Willan (1996) and Spencer et al. (2009) all listed R. citrina and R. patula as separate species, though the firstmentioned author noted that citrina was rather poorly known. Conversely, Efford (1998: 11) cast doubt on the putative differences in shell coloration and coiling between R. citrina and R. patula , noting that he was unable to detect these “once allowance is made for age-related changes in shell shape”. He suggested that most previous records of citrina were referable to R. patula , but that some may have been based on R. perampla Powell, 1946 .

Recent field surveys and preliminary results of a phylogenetic study (F. Brook & M. Kennedy unpub. data) indicate that there are two species of Rhytida in the vicinity of Greymouth that correspond to Hutton’s citrina and patula , respectively. Given that the whereabouts of the type material of the former is not known, and in order to prevent further confusion over the identity of this species and stabilise the nomenclature, we designate a specimen from Mt Davy, Rewanui, illustrated in pl. 6, fig. D (NMNZ M.329343), as the neotype of Rhytida citrina Hutton, 1882 . As interpreted here R. citrina is considerably more widely distributed in the northwestern South Island than was previously recognised (below). Assigned to genus Rhytida Albers, 1860 by Hutton (1882) and subsequent authors, but this placement requires re-evaluation (M. Kennedy & T. King unpub. data).

Current taxonomy. Rhytida citrina Hutton, 1882 — Hutton (1884b: 167, 1884c: 208), Hedley & Suter (1893: 631), Suter (1894b: 286, 1913: 772), Powell (1946: 129, 1979: 345), Parkinson (1979: 10), Spencer et al. (2009: 218).

Distribution. New Zealand, South Island from Cape Farewell south to Greymouth and Poplars Range, Lewis Pass (AIM and NMNZ collection records).