Austroassiminea Solem, Girardi, Slack-Smith and Kendrick, 1982

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 : 1996-2006

Austroassiminea Solem, Girardi, Slack-Smith and Kendrick, 1982

Type species: Austroassiminea letha Solem, Girardi, Slack-Smith and Kendrick, 1982 ; by original designation.

Diagnosis: Shell globose to conical, with open umbilicus, reddish-brown, with fine but distinct longitudinal ribs on teleoconch whorls. Operculum semicircular, thin, horny. Central radular teeth quadrangular, with no basal cusps; outer marginal teeth fan-like, wide. Gill of rudimentary filaments, consisting of both gf1+gf2. Rectum exhibiting conspicuous double S-shaped coils in middle of pallial roof. Penis large, very muscular, with triangular appendage at tip. Pallial vas deferens with very complex coils at anterior end of prostate. Renal oviduct strongly curved, with many small loops; with cluster of several tiny, brownish seminal receptacles in middle part of coiled renal portion of oviduct. Large, triangular chamber connecting oviduct and bursal duct. Right pleural and supraoesophageal ganglia separated but connective very short; left pleural ganglion completely fused with suboesophageal ganglion and forming single, large, flat ganglion.

Austroassiminea letha Solem, Girardi, Slack-Smith and Kendrick, 1982 (Figures 3D–F, 4C–D, 11–15)

Material examined

Cosy Corner, Hamelin Bay, near Augusta, WA, 34°15∞05◊S, 115°01∞E ( AMS, 5, C.401446, ex WAM 451-85 (topotypes)). Ellen Brook, adjacent to old Ellen Brook Homestead, Leeuwin-Naturaliste NP, NW of Margaret River, WA, 33°54.67∞S, 114°59.83∞E, 29 August 1985, swampy area in paddock, W. F. Ponder ( AMS, 20+, C.394139) .

Redescription

Shell (figure 11C). Bright reddish-orange to pale brown, small, globose to conical, thin, weakly shiny, translucent in young specimens, opaque in fully mature specimens. Protoconch with about 1.5 convex whorls with no distinct sculpture. Teleoconch of 3.5–3.8 weakly convex whorls, fine but distinct longitudinal ribs with regular interspaces (especially on early whorls); microscopic spiral threads irregularly found; suture shallow, impressed. Aperture wide, pyriform; peristome complete, sharp, with no apertural varix; outer lip prosocline, expanded outwardly at terminal growth; parietal lip rather long, nearly straight, thickened; columellar lip thick, gently curved, anterior end forming very weak projection crossing anterior part of outer lip, but with no canal. Umbilicus narrowly but distinctly open; umbilical area with dense axial growth lines, sometimes with a few spiral cords.

Shell dimensions: see table 3. Shells of males and females not distinguishable in shape or size (p>0.01).

Operculum (figure 11A–B). Semicircular with posterior end slightly projected, paucispiral, thin, horny, yellowish, transparent, with irregular growth lines on outer surface. Muscular scar elongate, wide, occupying about half of opercular minimum diameter.

Head-foot (figures 3D–F, 12, 13A,B). Majority of head dark grey to black except for colourless, translucent eye stalk, anterior end of snout (sn), sole and omniphoric groove (omg). Cephalic tentacles (ct) moderately long (although slightly shorter than extended snout), parallel-sided. Distinct black eyes (e) in lateral bulges at outer bases of cephalic tentacles; mass of tiny yellowish-white granules at proximal portion of eye bulge. Snout (sn) simple, rather long, distally bilobed. Foot (f) large, wide, rounded anteriorly and posteriorly. Opening of anterior mucous gland (ms) very wide, transverse slit at anterior edge of foot. Posterior mucous gland and metapodial tentacle absent. Omniphoric grooves (omg) run antero-ventrally on both sides of head; right groove wider than left. Right groove carries faecal pellets from inside pallial cavity to outside and both carry mucus to foot (observed in living animal). Sole (s) broad, flat. Mode of locomotion ‘step-like’; very active.

Pallial cavity (figures 4C, 12). Pallial cavity large, spacious, occupying about two thirds of body whorl (depth of cavity about 2.5 mm). Pallial roof unpigmented. Kidney opening (ko) conspicuous, in posterior-most corner of cavity; kidney (k) entirely behind pallial cavity, small, compact, consisting of mass of pale beige, translucent cells. Gill (g) rudimentary, row of four or five, blunt, finger-like filaments (gf1+gf2) lying on posterior portion of efferent vein (ef); gf1 portion of filaments rectangular, far larger than gf2; gf2 very small, reduced to peg-like projection on tip of gf1. Osphradium (os) small, short, elongate-oval, containing conspicuous osphradial ganglion (osg).

Digestive system (figures 4D, 11D–G, 12, 15A–C). Mouth (m) opens between pair of muscular lips into buccal cavity. Buccal mass (figure 15A–C: bm) large, rather long (1.3 mm in length), occupying most of snout. Radula protrudes from rather long radular sac extending about 0.5 mm behind and to right of buccal mass. Radula (figure 11D–G): central teeth (figure 11E, F) elongated rectangular, very tall; cutting edge with five long, narrow cusps, median cusp markedly longer (seven nearly equal-sized cusps shown in the figures of Solem et al., 1982); no basal cusps; lateral edges distinctly thickened, long and straight. Lateral teeth (figure 11E, G) subtriangular, slightly longer than central teeth, with six large cusps; central cusp longest; several small, blunt denticles on middle portion of inner edge (Solem et al., 1982: figure 6); short, oblique, duplicated accessory plates at outer side of lateral teeth (figure 11G). Inner marginal teeth (figure 11G) about 1.5 times as long as lateral teeth, rather wide, gently curved in distal portion, with eight to nine large, sharp cusps on cutting edge. Outer marginal teeth (figure 11D, E, G) long, wide, triangular, fan-shaped, rather flat, expanded in distal region; about ten long, wide primary cusps and about 50 small secondary cusps on cutting edge.

Oesophagus (figure 15A–C: oe) opens widely into buccal cavity. Pair of large, colourless glandular sacs [buccal pouches (bp)] open at posterior-most corner of dorso-lateral sides of buccal mass. Salivary glands (sg) short, simple, tube-like, lying together dorsally on buccal mass, do not pass through circum-oesophageal nerve ring. Oesophagus simple, with some distinct folds immediately posterior to nerve ring; enters stomach on left side at junction of anterior (figure 12: ast) and posterior (pst) chambers. Single opening to digestive gland lies posterior to oesophageal opening. Digestive gland (dg) pale green; consisting 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 (central parts of lobes darker than edges). Style sac large, finger-shaped, completely covered by ventroanterior arm of kidney (k). Origin of intestine at right anterior end of anterior chamber of stomach. Intestine tightly coiled over anterior tip of style sac before continuing to rectum (r) within pallial cavity. Rectum forms conspicuous double S-shaped coils (figure 4D) on middle part of pallial roof. Oval faecal pellets queued in single file in intestine and rectum. Anus (a) simple, about 0.2 mm posterior to anterior mantle edge (me).

Male reproductive system (figure 12B, 13). Testis (figure 12B: t) moderate in size (0.9 mm in length), pale lemon-yellow, consisting of about 20 bundles of slender lobes. Anterior end about 0.2 mm posterior to anterior end of digestive gland (dg). Coiled seminal vesicle arises from vas efferens in antero-ventral region of digestive gland. Tube of seminal vesicle narrow, highly convoluted. Posterior vas deferens (pvd) runs from seminal vesicle to middle part of prostate gland (pr), forming straight tube crossing over oesophagus (o). Prostate gland (figure 13D) large (1.6–2.0 mm in length), elongate, fan-shape, about half within posterior pallial cavity. Anterior vas deferens (avd) arises from middle part of prostate gland close to insertion of posterior vas deferens and passes across pallial roof as gently-curved tube, with complex coiling (usually at least four coils, variable) near anterior end of prostate (figure 12B; figure 13D); vas deferens then enters muscular wall of neck and continues to proximal end of penis (p). Penis (figure 13A–C: p) large, long (1.8–2.5 mm in length), highly muscular, attached to head about 0.5 mm behind right eye (e) and lies across head to left; proximal portion wide, thick, heavily bilobed; distal end divided into two triangular parts; one part forms long, sharplypointed triangular appendage (pa) turning anteriorly; another projection (ppn) located behind penial appendage, has penial vas deferens (pev) and genital opening (mgo) at tip. Penial vas deferens with one coil within proximal portion of penis; then straight in central part of penis.

Female reproductive system (figures 12A, 14). Ovary (figure 12A: ov) small (0.1–0.2 mm in length), covering ventro-lateral area of digestive gland (dg), consisting of tubules containing white oocytes. Posterior oviduct (pod) convoluted, runs along oesophagus (o) on right lateral edge; brown in proximal half; then bends toward posterior end of albumen gland (ag) before reaching end of pallial cavity. Beyond this latter bend, oviduct strongly curved making many distinct but variable small loops; typically anterior half of this part of oviduct with narrow duct and many smaller loops, posterior half with comparatively wider duct and two or three larger loops (figure 14: cod). Middle portion of looped region with cluster of three to eight small, slender, brownish or colourless bulbs [seminal receptacles (sr)], 0.15–0.20 mm in length, derived from slender seminal receptacle duct; these seminal receptacles and their ducts in middle part of coiled renal portion of oviduct (figure 14D, H). Anterior part of coiled renal oviduct opens to large, triangular chamber (odc) that opens to posterior end of albumen gland (ag). Bursal duct (bcd) rather long (0.8–1.0 mm in length), with few weak undulations. It rises from upper surface, and runs on latero-ventral side of, triangular chamber (odc) before entering ventral portion of bursa copulatrix (bc). Bursa copulatrix of moderate size (0.5–0.7 mm in length), triangular, clearly separated from posterior end of albumen gland by coiled renal oviduct (cod). Pallial oviduct long (1.9–2.6 mm in length), extends most of length of pallial cavity, clearly divided into albumen gland occupying posterior two thirds and anterior capsule gland (cg). 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 15). Circum-oesophageal nerve ring streptoneurous, epiathroid, with ganglia moderately concentrated. Cerebral ganglia (rcg, lcg) between posterior end of buccal mass (bm) and coil of mid-oesophagus (o); right ganglion (rcg; 0.38 mm in length, 0.20 mm in width) slightly larger than left (lcg; 0.30 mm in length, 0.18 mm in width); commissure (cc) rather long (0.2 mm). Right cerebral ganglion separated from right pleural ganglion (rpl) by constriction. Left cerebral ganglion connected with left pleural ganglion (lpl) by short, wide connective. Tentacular nerves (tn) thick, without distinct swellings at bases, not crossing optic nerves (opn). Optic nerves arise from portion of cerebral ganglion anterior to origins of tentacular nerves. Cerebro-pedal connectives (rcpc, lcpc) show asymmetry; right connective (0.3 mm in length) shorter than left one (0.4 mm). Pleuro-pedal connectives (rppc, lppc) about equal (0.4 mm) in length. Buccal ganglia (rbg, lbg) small, ovate, joined by relatively long commissures passing beneath buccal pouch (bp) at posterior end of buccal mass. Right pleural ganglion (rpl) about as long as right cerebral ganglion but much narrower (0.36 mm in length, 0.18 mm in width). Supraoesophageal ganglion (spg) slender, oval, separated from right pleural ganglion by weak constriction; about half length (0.18 mm) and width (0.08 mm) of right pleural ganglion. Osphradio-mantle nerve (omn) very long, crosses mid-oesophagus in anterior part of coil; osphradial ganglion (osg) very elongated, lies in mid part of osphradium (os), connected with osphradio-mantle nerve. Left pleural ganglion (lpl) completely fused with suboesophageal ganglion; slightly smaller than left cerebral ganglion (0.26 mm in length, 0.14 mm in width), very flat, just beneath right half of left cerebral ganglion and left half of cerebral commissure. Right and left pedal ganglia (rpg, lpg) about equal in size (both 0.4 mm in length, 0.18 mm in width), slightly smaller than cerebral ganglia, abutting each other, connected anteriorly to cerebral ganglia. Pedal commissure very short and barely visible. Metapodial (rmg, lmg) and propodial ganglia (rpr, lpr) distinct, globose, connected with pedal ganglia by short, heavy connectives; heavy nerves arise from these ganglia and innervate foot. Metapodial commissure not observed. Statocysts (st) rather large, posterior to pleuro-pedal connectives; containing single statolith. Visceral ganglion not examined.

Anterior aorta not expanded around circum-oesophageal nerve ring and anterior oesophagus.

Remarks

Austroassiminea letha has well-developed cephalic tentacles and the central radular teeth lack basal cusps. These characters are known in 14 genus-group assimineid taxa listed under Group 3 (=subfamily Omphalotropidinae ) in the Appendix. Besides these, other genus-group taxa are based on species with the central radular teeth lacking basal denticles (Group 2) but their head-foot and anatomical characters are unknown.

Amongst the genera of Group 3, Austroassiminea shares some characters in their reproductive systems with Allepithema Tomlin, 1930 and Paludinellassiminea Habe, 1994 and with some taxa attributed to Omphalotropis Pfeiffer, 1851 (Turner and Clench, 1972; H.F. unpublished). A penis with a large, triangular appendage is found in at least one species of Allepithema and two of Paludinellassiminea (including the type species) (see Appendix). Also, two species of Omphalotropis and two of Paludinellassiminea show complicated coils of the vas deferens on the pallial roof (Turner and Clench 1972, H.F. unpublished). These genera have generally dissimilar shells, apart from sharing a wide umbilicus.

Abbott (1949) and Turner and Clench (1972) illustrated a ‘cape’ on the posterior half of the snout in species of Omphalotropis , Allepithema and Pseudocyclotus . This character is not present in Austroassiminea .

It is of interest that the three genera most similar to Austroassiminea are terrestrial or marine (supratidal zone) inhabitants whereas Austroassiminea is found associated with coastal freshwater springs and seepages. The only other freshwater genus that is a definite member of Group 3 is found in eastern Africa—this being Eussoia Preston, 1912 . The radula and cephalic tentacles resemble other members of Group 3 and the umbilicate shell of Eussoia is somewhat similar to that of Austroassiminea . However, the penis of Eussoia (Brown, 1980b: figure 10d) is simple except for some weak swellings along one edge and lacks a distinct appendage. Other anatomical data have not been published to date. Brown (1980b: figure 11) reported ‘ Assimania ’ aurifera Preston, 1912 from Kenya and Zanzibar. This species is similar to Austroassiminea in the general appearance of the shell, but is thought to be terrestrial (Brown, 1980b). It has a generally similar radula, but the central teeth differ considerably in shape, those of Austroassiminea being broader with strong lateral wings (figure 11E).

The nervous system was not described in the original description of Austroassiminea . The most noticeable character in the nervous system is the right pleural and supraoesophageal ganglia being separated by a constriction, the derived condition within the Assimineidae . Another derived character is the fusion of the left pleural and suboesophageal ganglia to form a large, flat ganglion.

Apart from the difference in the number of cusps on the central teeth noted above, seven important differences have been noted between our observations and the account provided by Solem et al. (1982) that we interpret as observational errors rather than differences between populations (the material examined herein is not from the type locality). These are:

1. Gill. Solem et al. stated that assimineids, including this species, lack ‘any gill remnant’. We show that there is a row of four to six rudimentary gill filaments (figures 4C, 15B).

2. Anterior vas deferens. Solem et al. described and illustrated an ‘escape valve’ of the vas deferens that entered the rectum. The vas deferens is coiled (figures 12B, 13D) in the vicinity of the rectum but does not connect with it.

3. Pallial oviduct. Figure 11 of Solem et al. indicates the pallial oviduct having a tube running ventrally along the albumen and capsule glands. In fact the sperm tube and the central lumen of the albumen and capsule glands are fused with each other and form one duct in the middle and anterior region of the pallial oviduct (figure 14).

4. Seminal receptacles. Solem et al. make no mention of these structures but there is a cluster of several seminal receptacles opening to the renal oviduct (figure 14)

5. Position that the upper oviduct emerges from the ovary. Solem et al. illustrated the upper oviduct beginning at the ventro-central part of the ovary, but it actually begins from the anterior end of the ovary (figure 12A).

6. Alimentary canal. Solem et al. ignored the presence of the large style sac, and the rectum is illustrated as having irregular coils, but it is actually duplicated S-shaped coils.

7. Accessory plates in radula. These were stated to be absent (p. 124) but are clearly visible in their figures 4 and 6.

Solem et al. (1982) suggested that this species might be sexually dimorphic but our data does not support that view.

The small, blunt denticles on the middle portion of inner edge of the lateral radular teeth (Solem et al., 1982: figure 6) have not been previously described. This character may be important in considering omphalotropidine relationships as we have observed that Duritropis sp. from Norfolk Island has the same condition on the lateral teeth. We predict that this character will also be found in several other omphalotropidine genera.