Caridina darwin, Klotz & von Rintelen & von Rintelen, 2024

Caridina darwin n. sp.;

Caridina sp. ‘NT2’ Wilson, 2008;

Caridina ‘sp. NT2’ Short, Page and Humphrey, 2019;

( Figures 1 View Figure 1 and 2 View Figure 2 ).

3.1. Material Examined

Holotype: ♂ cl 2.1 mm, Australia, New Territory«Provinz», Finnis River catchment, Howard Springs , leg. S. Choy, 29.06.1992, NTM Cr019569 .

Paratypes: One ♂ cl 2.4 mm, same data as holotype, NTM Cr019570 ; two ♀♀ cl 3.2 and 3.7 mm, one specimen lacking the pleon cl 1.8 mm, Australia, New Territory«Provinz», Darwin area, Blackmore River , cultured specimens provided by D. Wilson NTM Cr019571 ; two ov. ♀♀ cl 2.1 and 2.3 mm, Australia, Northern Territory, Howard Springs , S 12 ° 27.345 ′ E 131 ° 3.146 ′, leg. M. Glaubrecht, N. Brinkmann, T. von Rintelen 27.06.2004, ZMB 29211 . GoogleMaps

3.2. Comparative Material Examined

Paratypes. Caridina temasek Choy and Ng, 1991 . Five specimens, Singapore, freshwater stream near Sime road, MacRitchie catchment area , ca. N 1 ° 20.233 ′ E 103 ° 48.783 ′ leg. P.K.L. Ng and K.K.P. Lim Aug. 1990 GoogleMaps .

Other material. Caridina temasek Choy and Ng, 1991 . Two ♂♂ cl 2.1 and 2.3 mm, two ♀♀ cl 2.8 and 3.7 mm Singapore, Central catchment , N 1 ° 21.32 ′ E 103 ° 48.279 ′, leg. W. Klotz 08.12.2011, ZMB 29681 GoogleMaps .

Caridina excavatoides Johnson, 1961 . Three ♀♀ cl 2.3–3.2 mm, Indonesia, Sumatra, Jambi, Tributary of Batang Hari River, Danau Lamo Village , S 1 ° 28.003 ′ E 103 ° 38.262 ′, leg. M. de Bruyn 01.01.2009 ZMB 29583 GoogleMaps .

3.3. Description

Cephalothorax and cephalic appendages: Small shrimp with carapace lengths of 1.8–3.7 mm (median 2.3 mm, n = 7) in apparently adult specimens. Carapace ( Figure 1 A, B View Figure 1 ) smooth, antennal spine clearly separated from suborbital lobe, pterygostomial angle broadly rounded. Rostrum ( Figure 1 A, B View Figure 1 ) extending from midlength of second article of antennular peduncle to distal end of it, 0.70–0.85 (median 0.71, n = 5) times as long as carapace, nearly straight or slightly sinuous, terminating in acute tip; dorsal margin armed with 15–21 basally articulated teeth over entire length, including 4–5 postorbital, teeth above orbit larger than anterior and proximal teeth; ventral margin with 2–4 moderately large teeth located in distal half of the rostrum. Rostrum formula based on six specimens 4−5(4)+11−17/2−4. Eyes ( Figure 1 A, B View Figure 1 ) well developed with pigmented globular cornea. Antennular peduncle ( Figure 1 A, B, G View Figure 1 ), 0.86–0.93 (median 0.92, n = 4) times as long as carapace in females, 1.0 times as long as carapace in males, first segment 1.69–1.92 (median 1.81, n = 5) times as long as second segment, second segment 1.86–2.67 (median 1.88, n = 5) times longer as third segment; distolateral tooth on first segment well developed, acute, 0.15–0.38 (median 0.20, n = 5) times as longs as second segments; second and third segments unarmed, with few spiniform setae near distal margin. Stylocerite acuminate, reaching to 0.72–0.93 (median 0.80, n = 5) times the length of the basal segment of antennular peduncle. Antennal scaphocerite ( Figure 1 H View Figure 1 ) with well-developed distolateral tooth, 3.86–4.0 (median 3.80, n = 3) times as long as wide.

Pleon, telson and uropods: The sixth pleomere 0.60–0.67 (median 0.63, n = 4) times the carapace length, 1.63–1.83 (median 1.72, n = 4) times as long as the fifth pleomere, and 0.87–1.0 (median 0.92, n = 4) times as long as the telson. Telson ( Figure 1 C, D View Figure 1 ) slightly tapering distally, distal margin strongly convex to triangular, lacking median apex, with 3–4 (n = 3) pairs of dorsal short spiniform setae and one pair of short spiniform setae dorsolaterally; distal margin with 6–8 (n = 5) strong spiniform setae, where the lateral pair was the longest. Preanal carina ( Figure 1 E View Figure 1 ) rounded, low. Uropodal diaeresis ( Figure 1 F View Figure 1 ) with 10–13 movable spiniform setae, outermost one shorter than lateral angle.

Mouthparts and branchiae: Mandibles ( Figure 1 I View Figure 1 ) without palp, incisor process ending in irregular teeth, molar process truncated. Lower endite of maxillula ( Figure 1 J View Figure 1 ) broadly rounded with long pappose setae and few rows of serrate setae near margin and scattered cuspidate setae proximal, upper endite elongate, with numerous distinct cuspidate setae flanked by a row of serrate setae on inner margin and few pappose setae subterminal, palp slender with few simple setae and one conical spiniform seta near tip. Upper endites of maxilla ( Figure 1 K View Figure 1 ) subdivided, furnished with numerous pappose, simple and papposerrate setae arranged in rows, palp slender, scaphognathite tapering posteriorly, fringed with long, curved serrulate setae on truncate posterior margin and plumose setae inferiorly. Palp of first maxilliped ( Figure 1 L, M View Figure 1 ) terminating in finger-like extension, caridean lobe and flagellum with plumose setae, base with numerous pappose, papposerrate and serrate setae arranged in rows and tufts. Podobranch on second maxilliped ( Figure 1 N View Figure 1 ) well developed; exopod slender, protruding propodal segment, with long plumose setae distally; basis and merus with rows of pappose setae, distolateral margin of penultimate segment with numerous pappose and serrate setae; dactylus with numerous serrate setae flanked by a row of pappose setae. Third maxilliped ( Figure 2 A View Figure 2 ) slender, with two arthrobranchs; exopod slender, exceeding distal margin of antepenultimate segment, with few long plumose setae distally; antepenultimate segment of endopod slender, with few simple setae basally; penultimate segment slender, with straight lateral margins, with few simple setae distally, inner margin with rows of short serrate setae; ultimate segment slightly shorter than penultimate segment, tapering distally, ending in a claw-like curved spiniform seta (nail), proximal to nail with few cuspidate setae; inner margin densely covered with rows of serrate setae; epipod well developed, with distal hook. Branchial apparatus as shown in Table 2 View Table 2 ; well-developed (with distal hooks) epipods present on third maxilliped and first pereiopod ( Figure 2 B View Figure 2 ). Vestigial epipods (visible only after cleaning and staining of appendages) on second and third pereiopods.

Pereiopods: Chela of the first pereiopod ( Figure 2 C View Figure 2 ) was caridinoid in shape, slender, with well-developed palm, 2.44–2.90 (n = 2) times as long as wide, 1.30–1.32 (n = 2) times as long as carpus; fingertips rounded, without hooks but each with tuft of setae; dactylus 3.67–4.00 (n = 2) times as long as wide, 1.22–1.44 (n = 2) times as long as palm. Carpus slender, hardly excavated distally, 2.73–2.75 (n = 2) times as long as wide, 1.29–1.36 (n = 2) times as long as merus. Merus not inflated, 2.44–2.62 (n = 2) times as long as wide, shorter than ischium. Chela of second pereiopod ( Figure 2 D View Figure 2 ) very slender, caridinoid in shape, with well-developed palm, 3.65–3.75 (n = 2) times as long as wide, 0.73–0.84 (n = 2) times as long as carpus; fingertips rounded, without hooks but each with duft of seta, dactylus 6.17–6.67 (n = 2) times as long as wide, 1.42–1.67 (n = 2) times as long as palm. Carpus slender, 6.17–6.63 (n = 2) times as long as wide, 1.64–1.67 (n = 2) times as long as merus. Merus not inflated, 3.82–4.55 (n = 2) times as long as wide, shorter than ischium. Third pereiopod ( Figure 2 E, F View Figure 2 ) with dactylus ( Figure 2 F View Figure 2 ) 5.00–5.33 (n = 2) times as long as wide (including terminal claw and spiniform setae on flexor margin), terminating in one claw, with five or six accessory spiniform setae on flexor margin; propodus not dilated or distally curved, 12.0 (n = 2) times as long as wide, 3.0 (n = 2) times as long as dactylus; carpus 4.33–4.71 (n = 2) times as long as wide, 0.54–0.55 (n = 2) times as long as propodus, 0.45–0.49 (n = 2) times as long as merus, with one large spiniform seta near distal margin and three small spiniform setae proximal; merus 7.07–9.73 (n = 2) times as long as wide, 2.04–2.21 (n = 2) times as long as carpus, with three or four strong movable spiniform setae on externo-inferior margin; ischium with one spiniform seta. Fourth pereiopod was similar to third but slightly smaller. Fifth pereiopod ( Figure 2 G, H View Figure 2 ) slender; dactylus ( Figure 2 H View Figure 2 ) 4.0–5.50 (n = 2) times as long as wide (including terminal claw and spiniform setae on flexor margin), terminating in one large claw, with 21–49 spiniform setae on flexor margin; propodus with small spiniform setae on ventral margin, distal pair of spiniform setae not enlarged, propodus 12.50–16.40 (n = 2) times as long as wide, 2.27–4.10 (n = 2) times as long as dactylus; carpus 5.0–7.83 (n = 2) times as long as wide, 0.50–0.57 (n = 2) times as long as propodus, 0.61–0.67 (n = 2) times as long as merus, with one strong spiniform seta near distal margin and 2–5 small spiniform setae proximal; merus 9.11–9.33 (n = 2) times as long as wide, 1.49–1.64 (n = 2) times as long as carpus, bearing three strong appressed movable spiniform setae on externo-inferior margin. Ischium with or without one spiniform seta.

Pleopods: Endopod of male first pleopod ( Figure 2 I View Figure 2 ) elongated subtriangular, lacking appendix interna, 3.2 times as long as proximal width, 0.38 times as long as exopod, with long plumose setae on outer and distal margins, with few pappose setae on inner margin. Male second pleopod with appendix masculina rod-shaped ( Figure 2 J View Figure 2 ), 8.3 (n = 1) times as long as wide, 0.63 (n = 1) times as long as endopod, armed with strong spiniform setae on inner margin and distal margin; appendix interna originating from about 0.4 times the length of appendix masculina, reaching to about 0.75 times the length of appendix masculina.

Reproductive biology: Ovigerous females with medium-sized embryos; size of undeveloped embryos (without eyes) 0.59–0.64 × 0.37–0.39 mm.

Size: Postorbital carapace length of apparently adult specimens 1.8–3.7 mm.

Colouration: Transparent to brownish with cream-colored transversal stripes on the pleon (Wilson 2008; Short, Page and Humphrey 2019).

Distribution: The species occurs in rivers around Darwin, Northern Territory, Australia.

Etymology: The species name darwin is derived from the Darwin river catchment, the natural habitat of the proposed new species, and is used as a noun in apposition.

Common name. Blackmore River Caridina .

Remarks: Based on morphology, Caridina darwin n. sp. is related to a group of species that includes Caridina propinqua de Man, 1908 and Caridina laevis Heller, 1862 as the best known members. Within this group of species related to C. laevis , two species with large embryos— Caridina temasek Choy and Ng, 1991 and Caridina excavatoides Johnson, 1961 , both occurring in freshwaters of the Malayan peninsula—are most closely related to the proposed new species. Caridina darwin n. sp. differs from C. temasek by a higher number of teeth on the dorsal margin of the rostrum (15–21 vs. 14–16 in C. temasek ), the development of epipods (vestigial epipods (only visible after clearing and staining of the appendages) on the second and third pereiopods, the lack of an epipod on the fourth pereiopod vs. a well-developed (with distal hooks) epipod on the second pereiopod, a reduced epipod on the third and a vestigial epipod on the fourth pereiopod in C. temasek ). Furthermore, the proposed new species differs from C. temasek by lacking an appendix interna on the first pleopod of males (vs. with well-developed appendix interna in C. temasek ) and smaller embryo size (0.59–0.64 × 0.37–0.39 mm vs. 0.70–0.80 × 0.44–0.54 mm in C. temasek according to the original description). There is still some confusion about the shape of the preanal carina in C. temasek . Choy and Ng [ 23] did not give a drawing of this part, but in the verbal description, they wrote “Pre-anal carina with about 10 setae”, suggesting that they did not observe a distinct hook-like spine. In their paper on Atyidae from peninsular Malaysia and Singapore [ 24], Cai et al. wrote that the preanal carina in C. temasek is “triangular, no spine or with a very tiny spine”. Four of five paratypes of C. temasek observed for the present study show a prominent hook-like spine on the preanal carina, as do all four topotypical specimens from Singapore listed above. The holotype of C. temasek lacks a spine on the preanal carina (Y. Cai, pers. comm.). It seems likely that specimens assigned to C. temasek with and without a hook-like spine on the preanal carina might belong to different species. All specimens of Caridina darwin n. sp. lack a spine on the preanal carina.

Caridina darwin n. sp. is also similar to Caridina excavatoides Johnson, 1961 . Both species lack an appendix interna on the endopod of the first pleopod in males. The proposed new species differs from C. excavatoides by a longer sixth pleomere (0.60–0.67 times the length of the carapace vs. less than 0.6 times the length of the carapace in C. excavatoides ). Furthermore, Caridina darwin n. sp. differs from this species in the shape of the palp of the first maxilliped (with a distinct finger-like extension vs. ending in a triangular extension in C. excavatoides ). Caridina darwin n. sp. differs from C. excavatoides in the development of the epipods on the third maxilliped and pereiopods in the same way as C. temasek . The dactylus of the fifth pereiopod has a smaller number of spiniform setae on the flexor margin (21–49 vs. 50–60 in C. excavatoides ).