Aridulodrilus molesworthae, Dyne, 2021

Dyne, Geoffrey R., 2021, A New Genus and Species of Earthworm (Oligochaeta: Megascolecidae) from Semi-Arid Australia, Records of the Australian Museum (Rec. Aust. Mus.) 73 (4), pp. 123-129 : 124-127

Aridulodrilus molesworthae sp. nov.

urn:lsid:zoobank.org:act:5FF63606-B904-463E-B5B6-932C25EDCD7B

Figs 1–4

Holotype: Rupee Station, 10 km NE from Broken Hill, Western New South Wales. Habitat is the outwash plain of the Mount Darling Range adjacent to Willa WillYong Creek , 31.9080°S 141.5683°E. SandY silt to loamY claY, vegetation predominantlY saltbush ( Atriplex nummularia ) ( Fig. 5 ). Individual worms up to 1.5 m (extended) have been observed; the collected specimen was 80 cm in length prior to preservation. Material lodged in Australian Museum , W 36906; presented to the AM bY Keith Leggett of the NSW Arid Zone Research Station , 23 September 2010 . Paratypes 2 (numbered 1 and 2 in discussion below): same general location, 31.9092°S 141.5690°E, collector R. Molesworth, 13 August 2020, following heavY rain. Material lodged in Australian Museum, W 53200 .

Description

External anatomY

HolotYpe: length = 481 mm; width (midclitellar) = 12.3 mm; segment count = 275. BodY pale, with diffuse brownish dorsal pigmentation. In life, the worms have a dark reddishbrown colour, especiallY dorsallY ( Fig. 1 ). ParatYpe 1: length = 286 mm, width (midclitellar) = 9.8 mm; segment count = 238. ParatYpe 2: a small, immature individual.

Prostomium prolobous. First perforate dorsal pore 4/5. Three pairs of widelY lateral (g -lines) puckered spermathecal pores in 6/7–8/9, the posterior-most pair with protruding whitish matter (H onlY, possiblY seminal material).

Perichaetine; setae generallY follow distinct longitudinal setal lines (vide Fig. 2 ), but can be somewhat irregular in disposition, ranging in number from 16–20 per segment throughout. The ventral setal gap is narrow and fluctuates in width. Setae are not present between the male pores.

Male pores are widelY separated (e -lines) on papillae within lunettes in XVIII. ClearlY defined, paired oculate genital markings are intersegmental in 16/17 and 19/20, slightlY more ventral than the male pores, their width approximatelY within the setal arc bd ( Fig. 2 ); P1 has an additional set of identical markings in 20/21. Female pore is single, within a distinct slightlY tumescent areola, midventral in XIV. A small, single unpaired genital marking aligned with the left 8/9 spermathecal pores occurs in 9/10 (H onlY).

Clitellum is tumescent and complete, encompassing the posterior half of XIII to the posterior edge of XVII, with a slight ventral gap in XVII (H, P1).

Internal anatomY

Septa in the oesophageal region are muscularized and thickened from 5/6 posteriad, those in 10/11–12/13 possiblY the most robust. A large, firm gizzard, with muscular sheen, is present in VI, tapering in width as it meets the oesophagus. The oesophagus is well vascularized and denselY rugose internallY but lacks anY obvious pouching or calciferous glands. The intestine commences with abrupt expansion in XVII. No tYphlosole, caeca or other structures are present. Intestinal contents are primarilY fine soil particles, with occasional small pieces of organic matter.

Last hearts are present in XIII.

Male funnels occur in X and XI and show some iridescence but are disproportionatelY small in relation the overall bodY size; 2 pairs of racemose seminal vesicles are attached to septa in XI and XII, the latter pair is slightlY the larger. FinelY racemose prostates, flattened and broadlY leaf-like in appearance, are restricted to XVIII and clasp the intestine in situ ( Fig. 3 ). The duct is sinuous and muscular, its distal end much dilated as if to form a chamber. Paired vasa deferentia lack conspicuous iridescence and discretelY enter the duct ventrallY just prior to its final distal bend. Penial setae are absent. FinelY loculate ovoid accessorY glands (apparentlY associated with the external genital markings) are present in XVII and XIX and XX (P1 onlY). A further pair, more ventrallY located, is present in XX (H) and XXI (P1) without anY corresponding external manifestation.

Ovaries in XIII consist of a membranous fan containing manY individual ovules (H); in P1 ovules are embedded in large white flocculent masses, with a filamentous attachment to the preceding septum. Small and diaphanous ventrallY situated oviducal funnels have ducts that enter the bodY wall separatelY, ventrallY, in XIV. Small glandular masses attached to the posterior face of septum 13/14 are possiblY ovisacs. Spermathecae lie in segments VI, VII and VIII, subequal in size, each organ consisting of a large, spheroidal ampulla, short thick duct and small clavate diverticulum. In all cases, the diverticulum joins the duct within the bodY wall, though the bulk of each organ is free within the bodY cavitY ( Fig. 4 ).

Meronephric; the nephridia consist of numerous small, simple tubular coils of variable size lining the bodY wall, with no pre- or post-septal nephrostomes or bladders visible. The nephridia are smaller and more numerous in the forebodY and in places appear to consist of 2 distinct bands within each segment. Large tufts of nephridial tubules are present in segments II-V with composite ducts leading to the pharYnx. CaudallY, the meronephridia are larger and less numerous but still lack nephrostomes (as elsewhere in the bodY), or elaborations such as bladders or ureters.

Etymology

The species is named for the manager of the propertY on which the species was detected—Ms Rosalind Molesworth.

Behaviour

Following infrequent heavY rainfall events, worms have been observed emerging from burrows and being active on the soil surface, including engaging in reproductive coupling (R. Molesworth, pers. comm. and video). This emergence phenomenon (the onlY means of obtaining specimens to date) appears to be restricted to a 10 ha area within the propertY. The species likelY survives at or above the capillarY fringe of the water table and its behaviour is governed bY fluctuations in the level of the latter and bY rainfall events that temporarilY saturate the upper soil laYers. There is no obvious sign of surface castings associated with this species during such events.

The phenomenon of aestivation or diapause in earthworms has been researched for various species (e.g., Jiménez et al., 2000; Cosín et al., 2006). This behaviour involves a dramatic slowing in metabolic activitY or even the production of a mucous-lined chamber to impede water loss, and is usuallY the result of deteriorating soil conditions, especiallY during prolonged drY periods. It is not known whether Aridulodrilus has adopted such a mechanism in order to cope with extended periods of drought.

Figures

Figure 1. Live extended specimen of Aridulodrilus molesworthae gen. et sp. nov.

Figure 2. Male field of holotYpe of Aridulodrilus molesworthae gen. et sp. nov.

Figure 3. Right prostate gland of holotYpe.

Figure 4. Right IX spermatheca, diverticulum dissected from bodY wall of Aridulodrilus molesworthae gen. et sp. nov.

Figure 5. Habitat of Aridulodrilus molesworthae gen. et sp. nov.