Limnadopsis minuta, Timms, 2009

Timms, Brian V., 2009, A Revision of the Australian Endemic Clam Shrimp Genus Limnadopsis Spencer & Hall (Crustacea: Branchiopoda: Spinicaudata: Limnadiidae), Records of the Australian Museum 61 (1), pp. 49-72 : 61-62

Limnadopsis minuta n.sp.

Figs. 2D , 5M , 10

Types. HOLOTYPE 3 MAGNT Cr15704; allotype ♀ MAGNT Cr15705 . PARATYPES 73, 10♀ MAGNT Cr15706 and 23 , 2♀, AM P76809. All types: Northern Territory, E of Kununurra (WA), Keep River National Park , c. 15°57'S and 129°03'E, 8.ii.1986, M.J. Tyler, M. Davies & G. Watson .

Distribution. Known only from the type locality in the far west of the Northern Territory near the border with Western Australia.

Etymology. This species is named for its small size. At just less than 10 mm, and about the size of a typical species of Limnadia , it is the smallest known species of Limnadopsis , a genus known for its relatively large size among spinicaudatans.

Male. Carapace ( Fig. 10A ) 8.8 mm by 4.6 mm, L:D ratio of 1.9. Dorsal margin slightly convex and smooth, with only very minor carinae at posterior of the growth line and dorsal margin junctions. Carapace half-oval shaped with both ventral corners well rounded. Dorsoanterior angle and dorsoposterior angle both about 110° and carapace slightly expanded dorsoposteriorly. Umbo present but inconspicuous. Eighteen well expressed growth lines. Carapace surface minutely granular and uniformly brown in preserved specimens.

Head ( Fig. 10B ) with pyriform frontal organ behind eye mound and subequal to latter in height. Anterior surface of head and rostrum meeting at sharp angle, c. 110°. Rostrum about as long as frontal surface of head, slightly downturned at apex and containing triangular naupliar eye lying in a similar axis. First antenna a little longer than peduncle of second antenna with 8 lobules. Second antenna with peduncle of 12 apparent segments, many with numerous spines dorsally, and with two flagella of 18–20 flagellomeres. Most flagellomeres, particularly those in middle region of each flagellum ( Fig. 10E ), with c. 4 spines on anterior margin, usually with 2 (rarely 3) crowded apically and with the proximal surface bare.

Trunk segments 24, each bearing a pair of thoracopods, these decreasing in size posteriorly and the anterior two pairs modified as claspers. Posterior dorsal armature of 3–5 spines on small protuberances on about last 6 segments, many long setae on the 5 or so segments preceding these and a few setae on the next 4 or so more anterior segments. Claspers ( Fig. 10D ) of normal structure for Limnadopsis , with one spine apically on movable finger and asymmetrical protrusion near the base of anterior edge of hand.

Telson ( Fig. 10C ) with two rows of 17 dorsal spines, all sharp and varying a little in size. First spine about 1.5 times larger than the next few spines. Dorsal surface between lateral spinous rows with anterior hump on which telsonic setae are inserted, followed posteriorly by marked depression as surface generally evenly slopes to caudal claw articulations. Caudal claw about 8 times longer than wide, with long, almost straight, basal section of even width and much shorter, curved, apical part narrowing tosharp apex. Caudal claw basally with about 12–15 setae mediodorsally, most a little longer than its width, followed by 3–5 short spines inserted almost dorsally, posteriormost of these being by far (3×) the largest. Curved tapering apical part bearing many short denticles dorsally.Ventroposterior corner of telson with a spinous projection.

Female. Very similar to male. Carapace slightly larger, 9.2 mm by 5.1 mm, L:D ratio of 1.86, dorsal margin slightly curved. Head ( Fig. 10F ) with typical short rostrum of female Limnadopsis . First antenna with 6 lobules. Telson ( Fig. 10G ) as in male and both the features peculiar to this species: depression in the dorsal surface posterior to telsonic filament mound, and spiniform projection at ventroposterior corner.

Eggs (Fig. 45K) rounded polyhedral, about 160 µm in diameter (range 153–168 µm; n = 5). Each surface of polyhedron consisting of 1–3 oval grooves, each groove containing low longitudinal ridge internally and surrounded by high, rounded ridge, these meeting end-to-end in rounded protrusions. Either series of such protrusions or ridges forming edges of each polygon.

Comments. Other specimens in the single collection available show little variation from the type material. No other species of Limnadiopsis is so small yet has 18–19 growth lines, nor such a high L:D ratio, only 24 trunk segments, a spinous projection on the ventroposterior corner of the telson, a depression in the dorsal surface of the telson, and the unique egg characteristics. Overall, L. minuta is reminiscent of L. tatei , but it lacks the dorsal carinae of the carapace of L. tatei (although sometimes these are absent in L. tatei , too), the carapace shape is similar in males and females in L. minuta , its telsonic spines are more numerous and less variable in size and position than those of L. tatei , and there are fewer spines on the caudal claw in L. tatei . It is perhaps tempting to consider L. minuta as comprising juveniles of L. multilineata n.sp. (see below), especially given the superficially similar telsons and the large number of growth lines, but this is not possible in light of the markedly convex dorsal margin of the carapace in females of L. multilineata ; the presence of a conspicuous ventroposterior expansion of the carapace and lack of a ventroposterior spinous outgrowth of the telson in that species; different spination of the antennal rami; and the completely different egg structure.

Figures

Fig. 2. Photographs of whole specimens of species of Limnadopsis. (A) L. birchii; (B) L. tatei; (C) L. parvispinus; (D) L. minuta; (E) L. multilineata; (F) L. occidentalis; (G) L. paradoxa; (H) L. pilbarensis. Scale bars 5 mm.

Fig. 5. SEMs of eggs. (A) Limnadopsis birchii, Twin Pans, Currawinya National Park, Qld; (B) L. birchii, Bluebush Bore Swamp, via Carnarvon, WA; (C) L. multilineata, Mitchell River Falls, Kimberley, WA; (D) L. tatei, Bloodwood Station, Paroo; (E) L. tatei, near Laverton, WA; (F) L. pilbarensis, Chichester Range, WA; (G) L. parvispinus, Bloodwood Station, via Bourke, NSW; (H) L. parvispinus, Coolringdon Station, via Cooma, NSW; (J) L. occidentalis, Boolathana Station, via Carnarvon, WA; (K) L. paradoxa, Sieda Farm, Grass Patch, WA; (L) L. paradoxa, East Lake Bryde, via Newdegate, WA; (M) L. minuta, Keep River National Park, NT. Scale bars 50 µm.

Fig. 10. Limnadopsis minuta n.sp., Keep River National Park, NT. Male: (A) carapace; (B) head; (C) telson; (D) a middle flagellomere of second antenna; (E) clasper. Female: (F) head; (G) telson. For clarity some growth lines of the carapace are incompletely shown anterior to umbo. Scale bars 1 mm.