Abstract

A new earthworm, Acanthodrilus ker madecensis is described, and the distribution of the genus Acanthodrilus is discussed.

While making a soil survey of Raoul (Sunday) Island, in the Kermadec group, during March and April, 1949, Mr. A. C. S. Wright collected 99 specimens of earthworms from 11 localities on the island.

Previous knowledge of the earthworm fauna of Raoul Island is limited to two papers published by Benham (1904 and 1914). In the first of these papers Benham described a species, Rhododrilus kermadecensis, and mentioned that other specimens in the collection belonged to a species of Allolobophora. In his second paper, apart from describing two new species and one new variety of “microdrilous” Oligochaetes, Benham recorded the European earthworm species Eisenia foetida, Allolobophora caliginosa and Helodrilus (Bimastus) constrictus.

Mr. Wright's collection includes two of the species mentioned by Benham (1914), namely Allolobophora caliginosa and Eisenia foetida. Both of these species were found to be widespread on the island, occurring mostly under rotting leaves in forest litters. E. foetida was also found living in fairly warm soil under a cover of moss growing close to a fumarole in the main crater. Three other introduced species were also collected: Lumbricus rubellus from three localities under forest litter and Allolobophora longa and Pheretima campestris, Lee (*), both from the Ministry of Works Camp garden. P. campestris has previously been recorded only from North Auckland, New Zealand, and it is likely that the species has been introduced with seeds and plants from New Zealand. The only endemic worms found were three small specimens which proved to belong to the genus Acanthodrilus. The specimens were collected from hot soil at the mouth of a fumarole, near the locality recorded for the specimens of Eisenia foetida, mentioned above. I have named this species Acanthodrilus kermadecensis, n.sp., and a description of the species is given below. Mr. Wright says that the soil from which A. kermadecensis was collected was so hot that it was difficult to handle, and that steam was rising from it. It is a well known fact that hot water (or even warm water) is extremely injurious to most earthworms, and the presence of A. kermadecensis in this soil is therefore the more remarkable.

The earthworms described by Benham (1904) were collected by Captain Bollons, who mentioned that it was only after a great deal of searching that any earthworms were found. In contrast, Mr. Wright comments that most soil types on the island now carry a remarkably dense earthworm population. These are mostly European species which have apparently spread out from areas that from time to time have been under cultivation; in 45 years they have spread into every

corner of the island. The rate of decomposition of forest litter is high, and would account for the widespread occurrence of Eisenia foetida, a species found in New Zealand principally in compost heaps and in situations where the rate of decomposition of organic matter is abnormally high. The comparative poverty of endemic species on the island is probably related to the isolated geographic position of Raoul Island.

Acanthodrilus kermadecensis, n.sp. (Pl. 5, Figs. 1–3.)

The specimen on which this description is based is 23 mm. in length and 1·5 mm. in diameter, with 88 segments.

The prostomium is tanylobous. There are eight chaetae on each segment, arranged in four pairs. On xxiv their arrangement is as follows. —

ab = cd = 0·25 mm.; aa = 1 mm.; bc = 0·5 mm.; dd = 2 mm. The clitellum is buff in colour and very prominent, covering the dorsal and lateral aspects of xiii-xx, down to the level of chaeta a.

There are two pairs of spermathecal pores, a pair at 7/8 and a pair at 8/9, in line with chaeta b on each side. A single pair of female pores occurs on xiv, one on each side, 0·25 mm. from the ventral midline, in the centre of a small pale area. There are two pairs of prostatic pores, a pair on xvii and a pair on xix. They are not situated on raised papillae, as is usual in Acanthodrilus, and each pore is 0·25 mm. from the ventral midline. The two prostatic pores of each side are joined by a narrow groove which passes across xviii and the male pores are on xviii, one on each side, in the grooves.

Nephridiopores commence on ii in line with chaeta d on each side, and they occur in a single series on each side in line with that chaeta on each segment posterior to ii. Dorsal pores commence at 6/7.