Aviassiminea palitans, Fukuda & Ponder, 2003

Fukuda, Hiroshi & Ponder, Winston F., 2003, Australian freshwater assimineids, with a synopsis of the Recent genus-group taxa of the Assimineidae (Mollusca: Caenogastropoda: Rissooidea), Journal of Natural History 37 (16), pp. 1977-2032 : 1980-1995

Aviassiminea palitans n. sp.

(Figures 1, 2, 3A–C, 4A–B, 5–8)

Derivation: palitans —Latin—wandering about, a reference to this taxon being widespread in northern Australia.

Type material: H (AMS, C.395093), and 100+ paratypes (AMS, C.394683); six paratypes ( WAM, S.30244) ; six paratypes ( NTM, P18556) .

Type locality: Great Sandy Desert, SSW of Lagrange, Pilbara, WA, behind Sandfire, first spring, 19°47.25∞S, 121°20.5∞E, 18 September 1987, W. F. Ponder and W. Edgecombe.

Other material examined: (all material in Australian Museum, Sydney, unless otherwise stated).

Millstream, Millstream NP, Pilbara, WA: Crystal Pool, 21°35.6∞S, 117°4∞E, 23 September 1987, W. B. Edgecombe (20+, C.394685); Lily Pond/Crystal Pool, edge of pool, 21°35.5∞S, 117°4∞E, 14 September 1987, W. F. Ponder and W. Edgecombe (many, C.394145, C.394673, C.394688, C.394687, C.394672); Lily Pond, litter under paperbarks and palms, around edge, 14 September 1987, W. F. Ponder (20+, C.394709); Deep Reach Pool, 21°37.25∞S, 117°6.5∞E, 12–13 September 1987, W. F. Ponder and W. Edgecombe (20+, C.394740, C.394676); top end of Palm Pool, concrete crossing, 21°34.33∞S, 117°3.17∞E, 14 September 1987, W. F. Ponder and W. Edgecombe (5, C.394689).

SSW of Lagrange, Pilbara, WA, Great Sandy Desert: near first spring, behind Sandfire, 19°47.25∞S, 121°20.75∞E, 18 September 1987, W. F. Ponder and W. Edgecombe (20+, C.394147); Eil Eil South, c. 200 km SSW of Lagrange, 19°48∞S, 121°26.83∞E, 18 September 1987, W. F. Ponder and W. Edgecombe (20+, C.394670); N of Eil Eil, 19°47.75∞S, 121°26.5∞E, 19 September 1987, W. F. Ponder and W. Edgecombe (20+, C.394678).

Cassini Island, N. of Admiralty Gulf, Bonaparte Archipeligo, WA, 13°57∞23◊S, 125° 38∞13◊E, in vine thicket at base of laterite cliff in damp crevices (pools apparently present in wet season, V. Kessner, pers. com.), 17 July 1988, V. Kessner and A. Longbottom (Field Museum, Chicago, many, 219798/N).

Northern end of Ningbing Ranges, N. of Kununurra, eastern Kimberley, WA: Brolga Spring, 14°53.64∞S, 128°34∞E, on wood and damp litter, 23 May 1986, P. H. Colman (20+, C.151085); same locality, 28 June 1996, W. F. Ponder, A. C. Miller, V. Kessner and D. Beechey (5, C.401408); large unnamed spring, 2 km N of Brolga Springs, 14°52.86∞S, 128°33.65∞E, 28 June 1996, W. F. Ponder, A. C. Miller, D. L. Beechey and V. Kessner, pH 6.18 (20+, C.318699); near Long Spring, 14°53.54∞S, 128°38.53∞E, 29 June 1996, washed from dead vegetation around edge of pool, W. F. Ponder, A. C. Miller, D. L. Beechey and V. Kessner, pH 6.23 (20+, C.327890); Haleys Spring, 14°52.83∞S, 128°40.63∞E, 29 June 1996, on mud on side of small pools, W. F. Ponder, A. C. Miller, D. L. Beechey and V. Kessner, pH 6.13 (20+, C.394719).

Mataranka, NT: Mataranka Thermal Springs Reserve, 14°55∞S, 133°8.5∞E, 9 May 1987, V. Kessner (16, C.394712); Mataranka Springs, 14°55∞S, 133°8.5∞E, 18 May 1986, on edge of spring against dense palm stands, P. H. Colman (18, C.150820); same locality, near bath, 14°55.52∞S, 133°8.11∞E, 19 August 1994, W. F. Ponder, G. Wilson and V. Kessner (40+, C.327892; C.327896); Roper R, just below crossing at Mataranka, 14°55∞S, 133°7∞E, 9 May 1987, in seep under palms, V. Kessner (20 +, C.394708); same location, 18 May 1986, P. H. Colman (20+, C.151986); Elsie Falls at Roper River, main pool, 14°55.09∞S, 133°7.06∞E, 20 August 1994, 1– 8 cm above water on roots etc., W. F. Ponder, G. Wilson and V. Kessner (20 +, C.394722); 1.4 km S of Elsie Falls on road between limestone sink holes, 14°58∞S, 133°16∞E, 19 May 1986, P. H. Colman (2, C.151235); Elsie Cemetery, 11 km S of Mataranka Springs, 15°5.15∞S, 133°7.44∞E, 23 June 1996, in recently dried up pool/swamp, W. F. Ponder and D. L. Beechey (1, C.371631); NW of airstrip to Elsey Stn, E of Mataranka, 14°57∞S, 133°17.5∞E, 18 May 1986, along creek intermittently appearing and disappearing in limestone, P. H. Colman (20 +, C.151073); Roper Creek near junction with Waterhouse Creek, near Mataranka, 14°55.74∞S, 133° 7.06∞E, 19 August 1994, W. F. Ponder, D. Beechey and V. Kessner (5, C.401410); Salt Creek, at Roper highway, 15°0.74∞S, 133°14.31∞E, 19 May 1997, W. F. Ponder, G. Wilson and V. Kessner (20+, C. 401409).

Description

Shell (figure 1). Small, ovate–conic to conic, thin, shiny, translucent in young specimens, opaque in mature specimens. Protoconch (figure 1G–I) of 1.5–1.75 convex whorls, sculptured with fine wrinkles (figure 1H). Teleoconch bright reddishorange to pale brown, of 4.0–5.5 weakly convex whorls, sculptured with weak growth lines and irregular microscopic spiral threads; suture shallow, impressed. Aperture wide, pyriform; peristome complete, sharp, with no apertural varix; outer lip prosocline, slightly expanded outwardly at terminal growth; parietal lip (note: we use the term parietal lip for that part of the inner lip of the aperture lying against the parietal wall and the term columellar lip for the part of the inner lip anterior to this) rather long, slightly curved, thin, transparent; columellar lip thin, gently curved, anterior end forming a weak projection crossing anterior part of outer lip, but lacking canal. Umbilicus narrowly but distinctly open; umbilical area with dense axial growth lines, surrounded by a few obscure spiral ridges.

Shell dimensions. See table 1. Shells of males and females are similar in shape and size (p>0.01).

Operculum (figure 2A, B). Paucispiral, thin, horny, yellowish, transparent, pyriform, with posterior end slightly elongated. Outer surface with irregular growth lines. Muscular scar on inner surface long, elongate-oval, occupying about 4/5 of surface.

Head-foot (figures 3A–C, 5, 6A, B). Majority of head covered with thin black pigment in many specimens, except for colourless, translucent eye stalk (=bulge; figure 3A: b), anterior end of snout (sn) in some specimens, sole (s) and omniphoric groove (omg). Eyes (e) conspicuous, black, at middle of rather long eye stalk (b); mass of tiny white granules (wg) at proximal portion of eye stalk. No distinct cephalic tentacle present (see Nervous system). Snout moderately long, bilobed. Foot (f) large, wide, anterior and posterior ends simple and rounded. Opening to anterior mucous gland (ms) wide, transverse slit at about 0.2 mm posterior to anterior end of foot. Anterior mucous gland (amg) composed of large white cells visible through translucent skin of sole and arranged as two large sacs that open at middle part of transverse slit. No posterior mucous gland or metapodial tentacle present. On both sides of head, wide omniphoric grooves (omg) run ventrally; right groove wider than left. Right groove carries faecal pellets from inside pallial cavity to outside (observed in living specimens). Sole broad, flat. Mode of locomotion ‘step-like’ (similar to Pomatiopsis, Davis, 1967 ); animals very active, crawling powerfully and rapidly on mud and solid objects.

Pallial cavity (figures 4A, 8B). Pallial cavity large, spacious, occupies about two thirds of body whorl (depth of cavity about 2.0– 2.2 mm). Pallial roof with black pigmentation, particularly in dorsal half and area along anterior mantle edge (figure 4A: me). Kidney opening (ko) conspicuous, in posterior-most corner of cavity; kidney (k) entirely behind pallial cavity, compact, consisting of mass of colourless, transparent cells. Gill (g) very rudimentary, a row of four (six in one specimen, figure 8B) blunt, finger-like filaments consisting of only gf1; on posterior portion of efferent vein (figure 4A: ef) on left side of cavity. Osphradium (figures 4A, 8B: os) small, short, elongate-oval, containing conspicuous osphradial ganglion (osg).

Digestive system (figures 2C–G, 4B, 5, 8A–C). Mouth (m) opens between pair of muscular lips into buccal cavity. Buccal mass (figure 8A–C: bm) large, rather long, occupying most of snout. Radular sac very short and not visible laterally. Radula (figure 2C–G) taenioglossate (cusp counts given in table 2); central teeth (figure 2C, D) quadrangular, rather elongated; cutting edge with five to seven distinct cusps and two to three pairs of distinct basal cusps laterally; triangular basal process long, arising from basal face of tooth, tip sharply pointed. Lateral teeth (figure 2E, F) rectangular, slightly longer than central teeth, with prominent, sharp cusps; central cusp longer than others. Long, wide accessory plates at outer side of lateral teeth. Inner marginal teeth long, about twice as long as central teeth, slender, generally narrower than central and lateral teeth in width, distinctly curved in distal portion, with five to seven large cusps on cutting edge. Outer marginal teeth long, wide, somewhat fan-shaped, expanded in distal region, with 11–18 tiny, uniform cusps on cutting edge; outer edge recurved distally (figure 2E–G).

Oesophagus (figure 8A–C: o) opens widely into buccal cavity. Pair of large, glandular, black buccal pouches (bp) at posterior-most corner of dorso-lateral sides of buccal mass. Salivary glands (sg) long, simple, club-like, located at dorso-lateral sides of buccal mass, not passing through nerve ring. Oesophagus simple, with a few strong folds just behind nerve ring; enters stomach on left side at junction of anterior (figure 5A: ast) and posterior (pst) chambers. Stomach with single opening to digestive gland posterior to oesophageal opening. Digestive gland (dg) pale lemonyellow; composed of two parts; anterior portion mass of small cells covering part of anterior and posterior stomach, posterior part row of large, round lobes in upper whorls of visceral coil) (figure 5) (central parts of lobes darker than edges). Style sac (ss) large, finger-shaped, completely covered by ventro-anterior arm of kidney (k). Origin of intestine at right anterior end of anterior chamber of stomach. Intestine tightly looped over anterior tip of style sac before continuing to rectum; rectum (r) forms conspicuous S-shaped coil in middle of pallial roof (figure 4B). Oval faecal pellets queued in single file in intestine and rectum. Anus (figure 5: a) simple, about 0.2 mm posterior to anterior mantle edge (me).

Male reproductive system (figures 5B, 6). Testis (figure 5B: t) large, bright lemon-yellow, consisting of about ten bundles of wide lobes. Anterior end about 0.2 mm posterior to anterior end of digestive gland (dg). Coiled seminal vesicle (figure 6C: sv) arises from vas efferens in antero-ventral region of digestive gland. Tube of seminal vesicle narrow, highly convoluted, pale orange in middle region. Posterior vas deferens (pvd) runs from seminal vesicle, forms straight tube that crosses over oesophagus and runs to middle part of prostate gland (pr) just within pallial cavity. Prostate gland elongate, about half within posterior pallial cavity. Anterior vas deferens (avd) arises from middle part of prostate gland (close to inlet of posterior vas deferens), passes straight across pallial roof and enters muscular wall of neck. Within neck vas deferens slightly curved before reaching proximal end of penis (p). Penis (figure 6A, B: p) long, usually showing right-handed coil, highly muscular, attached to head about 0.5 mm behind right eye (e). Distal end forms long, tubular projection (penial papilla: pp) inclined to right; genital opening (mgo) at tip of papilla. Flat, thin, semi-circular appendage (pa) present on right edge behind papilla. Whole outer surface of penis (except papilla) with many fine, distinct wrinkles. Penial vas deferens (pev) with few coils in base, straight within remainder of penis where it is located centrally.

Female reproductive system (figures 5A, 7). Ovary (figure 5A: ov) small (about 0.2 mm in length), covering ventro-lateral area of digestive gland (dg), consisting of mass of many large white oocytes. Posterior oviduct (od) convoluted, runs along oesophagus (o) on right lateral edge; widens near style sac (ss); then bends toward posterior end of albumen gland (ag) before reaching end of pallial cavity. Beyond this latter bend, oviduct strongly curved making two to three distinct loops; shape of loops variable (figure 7E–H: cod). Seminal receptacle duct short, thick, arises slightly anterior to end of loops. Seminal receptacle (sr) small (about 0.16 mm in length), elongately-ovate brown sac. Seminal receptacle duct usually runs beside dorsal part of bursa copulatrix duct (bcd) and seminal receptacle lies between bursa copulatrix (bc) and loops of oviduct. Bursa copulatrix large (0.7–1.0 mm in length), bean-shaped, posterior to albumen gland (ag) and readily separated from it in dissection. Bursal duct (bcd) rather short (0.3 mm in length), arises from oviduct where it lies on left hand side of posterior end of albumen gland; opens to anterior portion of bursa. Bursal duct and oviduct united slightly anterior to posterior end of albumen gland. Pallial oviduct long (1.8–2.0 mm in length; figures 7A–D), runs nearly straight forward to opening of pallial cavity; clearly divided (in gross dissection) into two approximately equal parts, albumen gland (ag) posteriorly and capsule gland (cg) anteriorly. Muscular sperm duct completely surrounded by glandular pallial oviduct and fused with lumen of oviduct (=median egg channel) within ventral region of oviduct. Genital opening (fgo) at anterior end of capsule gland.

Nervous system (figure 8). Circum-oesophageal nerve ring streptoneurous, epiathroid, with ganglia weakly concentrated. Cerebral ganglia (rcg, lcg) about 0.34 mm in length, 0.14 mm in width, between posterior end of buccal pouch (bp) and a coil of mid-oesophagus (o); cerebral commissure (cc) short (about 0.14 mm in length, 0.06 mm in width). Pleural ganglia (rpl, lpl) separated from each cerebral ganglion by a constriction. Tentacular nerves (tn) thick, without distinct swellings at their bases, reach inner side of body wall just behind eye stalk after crossing optic nerves (opn); each divided into two nerves distally. Optic nerves arise from portion of cerebral ganglion beneath origins of tentacular nerves. Cerebro-pedal (rcpc, lcpc) and pleuro-pedal (rppc, lppc) connectives rather short (0.16 mm in length), about equal in length; asymmetry not observed. Buccal ganglia (rbg, lbg) small, ovate, joined by long commissures passing along oesophagus near posterior end of buccal mass. Right pleural ganglion (rpl) about half size of cerebral ganglion, oval, partially fused with right cerebral ganglion, being separated by distinct constriction. Supraoesophageal ganglion (spg) small (0.2 mm in length, 0.1 mm in width), slender cylindrical; right pleural and supraoesophageal ganglia completely separated by long (0.24 mm in length), distinct pleural-supraoesophageal connective (spc). Left pleural ganglion (lpl) completely fused with suboesophageal ganglion, connectives between these ganglia not visible; combined ganglion slightly smaller than, and abutting, left cerebral ganglion. Pedal ganglia (rpg, lpg) connected posteriorly to cerebral ganglia. Left pedal ganglion (0.26 mm in length, 0.16 mm in width) slightly longer than right pedal ganglion (0.22 mm in length, 0.16 mm in width). Pedal commissure not visible, possibly rudimentary, both pedal ganglia pressed together; left pedal ganglion slightly above right ganglion giving asymmetry to nerve ring. Metapodial (rmg, lmg) and propodial ganglia (rpr, lpr) distinct, globose, connected with pedal ganglia by very short, heavy connectives; heavy nerves arising from these ganglia to foot. Metapodial commissure not observed. Small statocysts (st) in posterior part of pleuro-pedal connectives. Osphradial ganglion (osg) small, elongated, in osphradium (os). Visceral ganglion not examined.

Anterior aorta narrow, not expanded around circum-oesophageal nerve ring and anterior oesophagus as in Assiminea s.s. and Angustassiminea (H.F. pers. obs.).

Distribution and habitat

This species is known from four discrete areas across the western half of northern Australia (Pilbara Region, WA: Millstream and springs in the Great Sandy Desert; mid and eastern coastal Kimberleys, WA (Bonaparte Archipeligo and northern end of the Ningbing Ranges); and Mataranka (Northern Territory) and nearby areas) (figure 9). The snails live around the damp edges of permanent seepages and springs where they can be found crawling on dead vegetation and mud.

Remarks

The new species does not closely resemble any other Australian assimineid in shell characters other than Austroassiminea letha , which is redescribed in detail below and contrasted with the new taxon. There is no other gastropod living in the area occupied by this species that can be easily confused with it other than perhaps some populations of the hydrobiid Posticobia brazeri (Smith, 1882) that lives in the Roper River and some other rivers to the east of the Roper. This species has a row of pegs on the inside of the operculum and often has the periphery angled or even keeled. Some currently undescribed species of Bithyniidae are also very superficially similar but mostly larger and have a calcareous operculum.

Ponder and De Keyzer (1998: 757–758) stated that ‘several small aquatic [undescribed] species are found only in isolated spring habitats in north-western and northern Australia’. However, comparisons of several populations of Aviassiminea from throughout its range failed to reveal any consistent differences in the shell (figure 10), radula (figure 2C–G, table 2) or anatomy (figures 6A, B, 7). Nevertheless subtle differences between the widely separated populations exist because a discriminant function analysis of populations (using the shell measurements summarized in table 1) produced an overall discrimination success rate of 78% (see figure 10 for plots of scores). This discrimination largely results from small differences in overall size, rather than shape (cf. figure 10A showing the plots of scores on the first two axes, the first axis being influenced by size differences, and figure 10B which shows the plots of scores on the second and third axis). Given that we can find no observable, consistent morphological differences and pending genetic studies, we interpret all these populations as belonging to a single, widespread species.

The wide distribution seen in this taxon is unusual for small caenogastropod snails associated with freshwater (cf. for example many hydrobiids that are similar in size, e.g. Ponder, 1994). The populations of this species show virtually no differences in shell morphology or anatomy, suggesting that they have not been isolated for long periods of time and/or that dispersal may be successfully introducing new immigrants at a sufficiently high rate to prevent population differentiation. This species is associated with the permanently damp edges of springs and seepages and, because it is amphibious, is presumably able to be dispersed accidentally by wading birds, many of which frequent these habitats. In addition, the widespread wet conditions during the monsoonal wet season may also facilitate dispersal.