A Note on the Earthworm Fauna of the Kermadec Islands
Soil Bureau, Department of Scientific and Industrial Research,
[Read before the Wellington Branch, November 28, 1951; received by the Editor, March 26, 1952.]
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.
Internal Anatomy (Pl. 5, Fig. 2)
Alimentary Canal. The pharynx is rounded and muscular and occupies the first four segments. There is a short, thin-walled proventriculus in v, opening into an elongate gizzard with thick, muscular walls in vi and vii. The oesophagus extends from viii to xvii and lacks oesophageal glands. The intestine commences in xviii, is thin walled, and has a typhlosole.
Vascular System. The dorsal blood vessel is unpaired throughout its length. There are four pairs of hearts, a pair in each of x, xi, xii and xiii.
Reproductive System. There are two pairs of testes, a pair in x and a pair in xi, and a pair of ovaries in xiii. Two pairs of very minute spermathecae are situated in viii and ix and open to the exterior at 7/8 and 8/9. Each is a small, laterally flattened thin-walled, ovoidal sac, opening into a round, thick-walled muscular atrium which opens to the exterior by a short, narrow duct. There is a very small, rounded diverticulum, with a narrow duct, opening into the lateral aspect of the atrium. (Pl. 5, Fig. 3.) There are two pairs of uncoiled tubular prostates, a pair in xvii and a pair in xix, each pair confined to the one segment. I could not see any penial chaetae, but the very small size of the worm makes it very difficult to see many of the organs. There are two pairs of small, racemose vesiculae seminales, a pair in xi and a pair in xii.
The nephridia are minute, convoluted, tubular structures which open into a small, transparent vesicle before discharging to the exterior.
Fig. 1 — Acanthodrilus kermadecensis Ventral aspect, segments xii–xxi Fig. 2.—A. kermadecensis Dissection from the dorsal aspect. Fig. 3.—A. kermadecensis. Right posterior spermatheca, lateral aspect.
at.= atrium of the spermatheca; e.g. = cerebral ganglion, cl. = elitellum, d.b.v. = dorsal blood vessel; div. = diverticulum of the spermatheca, f.p. = female pore; g. = gizzard; h. = heart; int. = intestine; m = mouth; m.p. = male pore; n. = nephridium; o. = ovary, oe. = oesophagus, p. = prostate; ph. = pharynx, p.p. = prostatic pore; s. = septum, sp. = spermatheca; sp.d. = spermathecal duct; t. = testis; t.f. = testicular funnel; v.s. = vesicula seminalis.
Remarks. Pickford (1937) divided the genus Acanthodrilus into two parts, according to whether the species had or had not nephridial vesicles. For those which had vesicles she retained the name Acanthodrilus, while those without vesicles were grouped with some species of Microscolex under the generic name Eodrilus (a name which had formerly been used and subsequently discarded, for some species of Acanthodrilus). The present species has the typical “J-shaped” nephridial vesicles of Pickford's Acanthodrilus.
All the species referred by Pickford to Acanthodrilus are confined to New Caledonia, and were described by Michaelsen (1913). The present species is very small and, of Michaelsen's species, only four, A. coneensis, A. vateensis, A. cavaticus and A. ungulatus, are small species. The present species may be distinguished from these four species by the following characters:—
The prostatic pores of A. coneensis are on xviii and xx; those of A. kermadecensis are on xvii and xix.
The gizzard in A. coneensis and A. vateensis is confined to vi; in A. kermadecensis it extends through vi and vii.
The spermatheca of A. cavaticus has a relatively large diverticulum of a peculiar five-lobed form, opening directly into the spermatheca at the junction of the duct and sac; that of A. kermadecensis has a small, rounded diverticulum, opening by a long, narrow duct into the atrium of the spermatheca.
A. ungulatus has two pairs of vesiculae seminales, a pair in ix and a pair in xii; A. kermadecensis has two pairs, a pair in xi and a pair in xii.
These are the more obvious differences; a number of other differences may be noted in each case when the descriptions given by Michaelsen are examined.
Benham, W. B., 1904. Earthworms from the Kermadecs. Trans. N.Z. Inst., vol. 37, pp. 298–9.
— 1914. Oligochaeta from the Kermadec Islands. Trans. N.Z. Inst., vol. 47, pp. 174-85.
Lee, K. E., —* Studies on the Earthworm Fauna of New Zealand—III. Trans. Roy. Soc. N.Z. (in press).
Michaelsen, W., 1913. Die Oligochaten von Neu-Caledonien und den benachbarten Insel-gruppen. F. Sarasin and J. Roux, Nova Caledonia, Zoologie, vol. 1, 1. 3, no. 5, Wiesbaden, 1913.
Pickford, G. E., 1937. A Monograph of the Acanthodriline Earthivorms of South Africa. W. Heffer and Sons Ltd., Cambridge, England, 1937.