Gabbia microcosta, Ponder, 2003

Ponder, Winston F., 2003, Monograph of the Australian Bithyniidae (Caenogastropoda: Rissooidea), Zootaxa 230 (1), pp. 1-126 : 90-92

Gabbia microcosta n. sp.

Etymology Micro (Gr.) – small; costa (L.) – rib.

Type material

1.5 km N of Bridge Ck , along Stuart Hwy, ca . 32 km SE of Adelaide R TS, NT, 13° 25.450'S, 131° 18.300'E, 28 APR 1996, V.Kessner (ex VKC 24293) (Holotype, AMS C.417677; paratypes AMS C.318641, 17 wet, 11 dry, 5 on SEM stub; VKC 24293, 16; NTM P21374, 5) .

Additional material examined

Northern Territory: Stuart Hwy , 3 km S of Adelaide R TS, 13° 14.210'S, 131° 6.330'E, in mud among grass and reeds, 24 JUN 1996, W.F.Ponder & D.L.Beechey ( AMS C.202810, 20+); 25.5 km N of Hayes Ck, along Stuart Hwy (ca. 32 km SE of Adelaide R TS) NT, 13° 25.450'S, 131° 18.300'E, 16 APR 1988, V.Kessner ( VKC 12811, 20 +; AMS C.318564, 6; AMS C.307279, 2; AMS C.307861, 9); 4.2 km W of Mt Ringwood Stn HS, E of Howley Ck, 13° 7.820'S, 131° 20.000'E, 10 AUG 1996, V.Kessner ( VKC 24263, 2 ); ca. 33 km S Adelaide R TS at waterhole on side of Stuart Hwy, 13° 26'E, 131° 19'E, in dried up waterhole, 25 JUN 1996, W.F.Ponder, A.C.Miller, D.L.Beechey & V.Kessner ( AMS C.202807, 9); seasonal ponds 17.7 km NW of Hayes Ck, along Stuart Hwy, ca. 40 km SE of Adelaide R TS, NT, 13° 28.680'S, 131° 21.360'E, 16 APR 1988, V.Kessner ( VKC 12812, 5 ; AMS C.318628, 1) .

Description

Shell ( Figs 1K , 19 E­G, 31B,C) moderate to large (up to 10.6 mm in length), conical, with up to 5.5 convex whorls. Protoconch of about smooth 1.5 whorls. Teleoconch sculptured with moderately distinct, regular collabral, linear­spaced micro­costae crossed by very fine (but visible with light microscope) spiral threads; base evenly convex; umbilicus small to very small, closed. Aperture broadly ovate; peristome thin; outer lip slightly to moderately prosocline. Opaque to semi­translucent; periostracum very thin, transparent, yellowish­white to pale brown.

Dimensions. See Table 23 for dimensions of holotype and Appendix, Table 29 , for summary shell dimensions and whorl counts.

Operculum ( Fig. 27 J­L) typical of genus. Ovate, yellow­brown to brown, with distinct concentric growth ridges; inner surface with muscle scar sculptured with brain­like sculpture to irregular pits and ridges.

Radula (Appendix Table 30 ; Fig. 33 D­G) typical of genus. Central teeth with 3­4 cusps on either side of median cusp which is about 1.3­1.4 times longer than adjacent cusps and its base about twice as wide; median cusp triangular, base about 0.7­0.8 length, with angulation at about half to one third of length often bearing weak denticle. Face of central tooth with 1­3 pairs of cusps that extend to lateral margin forming simple to denticulate ridge, inner pair much larger than others, large (about half total height of tooth); lateral margins straight to very slightly concave, at about 30­37º; basal tongue narrowly triangular, with rather pointed end. Lateral teeth with cusp formula 3 + 1 + 3; with cutting edge about 0.3­0.4 length of lateral part of tooth; median cusp up to nearly twice as long as adjacent cusps, bluntly pointed to sharp; upper edge of lateral part of tooth at about 60º to cutting edge, lateral edge straight to slightly concave. Inner marginal teeth with 16+ cusps, outer marginals with 9­15 cusps.

Head­foot and anatomy not examined.

Distribution ( Fig. 22 ) and habitat. This species appears to have a restricted distribution, being known only from a small area south of Adelaide River township where it lives in seasonal swamps and pools.

Remarks

This species is most similar to G. affinis from Queensland, but is much larger ( Fig. 30B ) and has more distinct and regular micro axials than that species and fine spiral threads that are easily visible under a light microscope. As with G. affinis , the shells, radula and opercula are generally similar to those of G. vertiginosa , but the shells differ in being larger and in having fine, regular axial microcostae rather than just fine growth lines. The present species also differs from G. vertiginosa in the same radular characters outlined for G. affinis . The basal cusps in this species are variable in strength, with specimens from one sample having a single pair of distinct cusps, although others were usually represented by minute denticles.

The radulae of the specimens attributed to G. affinis differ in detail from G. microcosta . These differences include: The basal denticle on lateral teeth projects beyond the base; the lateral edges of the central teeth are at a steeper in most specimens; there is usually at least one fewer basal cusps on the central teeth and these extend to the lateral edges so that the cusps and lateral edge converge distally (they diverge in G. affinis ); the median cusp on the central teeth is relatively wider, with the denticles usually placed more distally (half or more down the tooth compared with half to further back in G. affinis ) and the sides of the central tooth behind the denticles are oblique (the entire tooth is triangular) but are approximately parallel­sided in G. affinis ; and the basal tongue of the central teeth is triangular and pointed (rounded and blunt in G. affinis ).

Figures

FIGURE 1. Shells of type specimens: A, Bithynia vertiginosa Frauenfeld 1862, holotype, NHMV, shell length 6.79 mm; B, Gabbia iredalei Cotton 1942, holotype, SAM, D14098, shell length 6.97 mm; C, Gabbia australis suspecta Iredale 1944, syntype, AMS, C.51675, shell length 6.54 mm; D, Gabbia australis suspecta Iredale 1944, syntype, AMS, C.100600, shell length 4.65 mm; E; Bithynia schraderi Frauenfeld 1862, holotype, NHMV, shell length 6.46 mm; F, Bithinia hyalina Brazier 1875, syntype, AMS, C.110159, shell length 4.98 mm; G, Bithinia hyalina Brazier 1875, syntype, MMS A69, shell length 6.63 mm; H, Gabbia australis Tryon, 1865, probable paralectotype, AMS, C.110160, shell length 5.43 mm; I, Gabbia affinis Smith 1882, topotype, AMS, C.2515, shell length 4.44 mm; J, Bithinia australis Smith, 1882 (= Gabbia smithii Tate 1882), lectotype, BMNH, 197748, shell length 3.20 mm; K, Gabbia microcosta n.sp., holotype, AMS, C.417677, shell length, 8.51 mm; L, Gabbia spiralis, n. sp., holotype, AMS, C.417682, shell length 4.56 mm.

FIGURE 19. Protoconchs and microsculpture: A, B, Gabbia beecheyi, A, paratype, AMS, C.331852, B, AMS, C.332843; C, D, Gabbia affinis, AMS, C.308008; E ­ G, Gabbia microcosta, paratype, AMS, C.318641; H ­ R, Gabbia adusta, H, I, AMS, C.333080, J ­ L, AMS, C.331871, M, N, Q, R, AMS, C.203381, O, AMS, C.318589, P, AMS, C.331876; A, E, H ­ K, Q, protoconchs, B ­ D, F ­ G, L ­ P, R, microsculpture; Scales: A, G, N, 500µm, B ­ C 20µm, D, L, R, 50µm, E ­ F, H ­ K, P, 100µm, M, O, Q, 200 µm.

FIGURE 22. Distribution of Gabbia obesa (o), G. affinis (), G. microcosta (), G. clathrata () and G. spiralis ().

FIGURE 27. Opercula: A, B, Gabbia kessneri, paratype, AMS, C.322308; C ­ E, Gabbia beecheyi, paratype, AMS, C.331852; F ­ I, Gabbia affinis, AMS, C.308008; J ­ L, Gabbia microcosta, paratype, AMS, C.318641. A, C, E ­ H, J, K, outer side, B, D, I, L, inner side. Scales: 500 m.

FIGURE 30. A, B ­ Plots of shell dimensions. A, shell length (SH) and aperture length (AH); x G. kessneri; O G. beecheyi. B, shell length (SH) and shell width (SW); x G. microcosta; O G. affinis..

FIGURES 33. Radulae: A ­ C, Gabbia affinis, AMS, C.308008; D ­ G, Gabbia microcosta, D, E, paratype, AMS, C.318564, F, G, paratype, AMS, C.318641; H ­ K, Gabbia adusta, H, AMS, C.331869, I, AMS, C.346419, J, K, AMS, C.331876; L, M, Gabbia lutaria, paratype, AMS, C.318599. A, D, E, F, I, M, central teeth, B, J, marginal teeth, C, G, lateral teeth, H, K, central and lateral teeth, L, half row of teeth; Scales: 20 m.