Elamena similis, Rahayu & Nugroho, 2019

Rahayu, Dwi Listyo & Nugroho, Dharma Arif, 2019, Three new species of the genus Elamena H. Milne Edwards, 1837 (Crustacea: Decapoda: Hymenosomatidae) from Lombok Island, Indonesia, Raffles Bulletin of Zoology 67, pp. 171-182 : 176-180

Elamena similis new species

( Figs. 2C, D , 3B , 4 )

Material examined. Holotype, male (2.9 × 2.9 mm) ( MZB Cru 4897), Kecinan , Lombok Island, 9 June 2009 . Paratype, ovigerous female (3.5 × 3.3 mm) ( ZRC2019.0593 ), same locality as holotype .

Description. Carapace subpentagonal, as broad as long in male ( Fig. 4A ), slightly broader than long in female ( Fig. 4G ); dorsal surface slightly concave, gastro-cardiac groove present but faint in male, regions well demarcated in female. Rostrum truncated, anterior margin slightly convex medially, with sparse bristles, laterally continuous with anterolateral margin of carapace; ventral keel deep dorsoventrally, triangular in lateral view, partially visible from dorsal view, posteriorly continuous with prominent interantennular septum. Postocular tooth moderately broad, triangular, not visible from dorsal view. Anterolateral margin slightly sinuous, slightly concave near base of rostrum; first anterolateral tooth replaced by slight convexity, second moderately produced. Posterolateral margin slightly projecting dorsally, slightly convex, not well demarcated from posterior margin; posterior carapace margin slightly convex. Pterygostomian region with 2 lobes ( Fig. 4C ), first lobe broadly triangular, positioned just below first anterolateral tooth, second lobe smaller, placed at anterior to base of cheliped coxa. Epistome length subequal to breadth, posterior margin sinuous, with median notch.

Antennules well developed, basal antennular segment placed between bases of eyes, medially separated by strong septum (proepistome) with short disto-external protuberance. Antennae directed anteriorly; basal antennal article developed from below base of eye. Eyes well developed, short, stout, partially visible from anterolateral margins in dorsal view.

Third maxillipeds ( Fig. 4B ) subquadrat, covering entire buccal cavity when closed; exopod narrow, distal end not reaching disto-external angle of merus, with distinct flagellum; ischium subquadrat, external margin slightly shorter than internal margin; merus subtriangular, inner margin slightly convex, shorter than outer margin.

Male thoracic sternum wide, glabre; sternites 1–3 fused, partially hidden by third maxillipeds, sunken toward buccal cavity; strong transverse ridge at junction of sternites 3/4; sternoabdominal cavity moderately deep, anterior tip reaching anterior border of sternite 4, without sutures, prominent press-button situated at level between sternites 5, 6.

Chelipeds ( Fig. 4A ) subequal, slender; merus rounded in cross-section, with protrusion on ventro- distal end; carpus rounded, distal inner angle blunt; chela ( Fig. 4D ) with fingers equal in length, with pointed tips, spoon-shaped, no wide gape when closed, cutting edges lined with small teeth interspersed with much smaller teeth, with larger subproximal tooth on dactylus.

P2−P5 ( Fig. 4A, E ) long, slender, all surfaces with short bristle, extensor and flexor margins lined with short setae except for flexor margin of dactyli which lined with longer setae; P3 longest, P5 shortest; meri longer than propodi; meri, carpi of all legs with distal acute extension on extensor margins; distal one-third of dactyli gently curving inward, terminating in sharp claw, flexor margin with 2 subdistal teeth, directed backwards.

Male pleon with 5 somites, including pleotelson ( Fig. 4F ). Third, fourth pleonal somites fused, gradually narrowed over distal half with slightly convex lateral margins; lateral margins of fifth somite straight; pleotelson triangular, apex rounded, 1.6 times as long as fifth somite. G1 ( Figs. 2C, D , 3B ) slightly twisted, curved, forming C-shaped, apex tapering to pointed tip directing slightly upward, with row of 4 stiff, subterminal setae.

Female pleon wide ( Fig. 4H ), 6 somites free including pleotelson, subcircular, covering entire thoracic sternum; pleotelson subquadrat, apex concave, terminal margin with tufts of short setae. Vulvae small, placed on medial fused plate of thoracic sternum, at level of sternite 4.

Etymology. The name comes from Latin similis , resemble, alluding to the likeness of this species to “ E. truncata ” sensu Gordon (1940) and Lucas (1940) from Australia. The name is used as a noun.

Type locality. Kecinan , northern Lombok Island, Indonesia .

Remarks. The new species resembles E. truncata sensu Gordon, 1940 and Lucas, 1980 in the shape of the male pleon and G1. Ng & Chuang (1996) placed E. truncata of Gordon (1940) and Lucas (1980) from Australia tentatively in synonymies with E. vesca , and elucidate the differences with E. truncata s. str. ( Ng & Chuang, 1996: 73). Ng & Richer de Forges (1996) indicated the differences between E. vesca and “ E. truncata ” from Australia, in the shape of male pleon and pleotelson, and suggested to review the taxonomic status of those Australian species. It is not in the scope of this study to discuss the status of those Australian species. Nevertheless E. similis , new species differs from Australian “ E. truncata ” in having carapace as long as broad or slightly broader than long, anterolateral and lateral angle faintly defined ( Fig. 4A, G ), P2−P5 with dactyli proportionally longer ( Fig. 4A, E ), the lateral margins of fused 3-4 somites of the male pleon is only slightly convex, with the pleotelson 1.6 times as long as the fifth somite ( Fig. 4F ), the G1 strongly bent with four subdistal long setae ( Figs. 2C, D , 3B ). In the Australian species the carapace is continuous from lateral margin of rostrum to anterolateral, and posterolateral margins, without any angle (cf. Lucas 1980: fig. 2D), dactyli of P2−P5 proportionally stouter (cf. Gordon, 1940: fig. 5c; Lucas, 1980: fig. 6D), the lateral margins of fused 3-4 somites of the male pleon is strongly convex, the male pleotelson 1.5−2 times as long as the fifth somite (cf. Gordon, 1940: fig. 5D; Lucas, 1980: fig. 8B), and the G1 is directed more upward or more strongly curved, with four to five subdistal long setae (cf. Gordon, 1940: fig. 5a, b; Lucas, 1980: fig. 10H).

Elamena similis differs from E. vesca in the shape of the third maxilliped [more elongate in E. vesca (cf. Ng & Richer de Forges, 1996: fig, 8C), squarish in E. similis ( Fig. 4A )], the G1 is more strongly curved with 8 long subterminal setae in E. vesca (cf. Ng & Richer de Forges, 1996: fig, 8H, I), versus strongly bent, tip directed laterally, with 4 subterminal long setae in E. similis ( Figs. 2C, D , 3B ). The female pleon is much wider in E. vesca (cf. Ng & Richer de Forges, 1996: fig. 8F compared to more elongate in E. similis ( Fig. 4H ). This difference in shape is more likely not size related although the female of E. similis is much smaller, but it is fully mature as indicated by the presence of eggs in the pleon.

While the carapace of E. truncata s. str. and E. similis is slightly broader than long (1.08 versus 1.06 as long as broad respectively), the outline of the carapace is less pronounced in E. similis (the lateral margin of E. truncata with visible broad angle [cf. Ng & Chuang, 1996: fig. 30A, B] versus very slight angle in E. similis [ Fig. 4A, G ]), the G1 is long, slender, the tip pointed upward with nine subdistal setae in E. truncata (cf. Ng & Chuang, 1996: fig. 30J) while in E. similis the G1 stout, bent, tip pointed laterally with four subdistal setae ( Figs. 2C, D , 3B ).

Elamena similis similar to E. samalensis in having ambulatory legs surfaces and margins lined with short bristly setae, dactyli shorter than propodi with one third distal curving, but in E. samalensis the dactyli have one subdistal tooth on the flexor margin (cf. Husana et al., 2013: fig. 1B) while in E. similis two teeth are present ( Fig. 4E ). Furthermore the male pleotelson is acutely triangular and the G1 is twisted with row of seven subterminal setae in E. samalensis (cf. Husana et al., 2013: figs. 1C, 2), while in E. similis the pleotelson is bluntly triangular and the G1 is not twisted and has four subterminal setae ( Figs. 2C, D , 3B , 4F ).

Although E. castanea and E. sundaica have similar C-shaped G1 as the new species, but these three species can be differentiated in the tapering tip of G1 that turned outward with 11 subdistal setae in E. sundaica (cf. Ng & Chuang, 1996: fig. 12K, J), the more slender G1, its distal half directed ventrally, with 6 subdistal setae in E. castanea (cf. Naruse et al., 2008: fig. 3e, f), while in the new species the G1 is shorter and stouter, distal half is directed ventrally with 4 subdistal setae ( Figs. 2C, D , 3B ). Moreover, in E. similis the male pleon is narrower, the lateral margins of the fused somites 4–5 are slightly convex, the pleotelson is subtriangular, 1.6 times as long as the fifth somites ( Fig. 4F ), while in E. sundaica the male pleon is broad, the lateral margins of the fused somites 4–5 are more clearly convex, the pleotelson semicircular, 1.9 times as long as the fifth somite (cf. Ng & Chuang, 1996: fig. 12I). In E. castanea the male pleon relatively broad with the lateral margins of the fused somites 4–5 strongly convex, slightly swollen proximally, and the pleotelson is longer, subtriangular, 2 times as long as the fifth somite (cf. Naruse et al., 2008: fig. 3d).

Other species in the truncated rostrum group of Elamena have different shape of G1. In E. cristatipes the G1 is strongly curved with the tip pointed upward and three subterminal setae (cf. Ng & Chuang, 1996: fig. 7F, G); the G1 expanded near tip with numerous setae in E. mathaei (cf. Gordon, 1940: fig. 3a, c); long, S-shaped G1 with numerous setae at the tip in E. abrolhensis (cf. Gordon, 1940: fig. 7a); S-shaped G1 with twisted base and oblique row of setae subdistally in E. panglao (cf. Naruse et al., 2008: fig. 5e, f). In addition, E. cristatipes and E. panglao have meri of ambulatory legs clearly shorter than propodi (cf. Ng & Chuang, 1996: fig. 7D; Naruse et al., 2008: fig. 5c) versus shorter propodi than the meri of ambulatory legs in E. similis ( Fig. 4E ); in E mathaei and E. abrolhensis the anterolateral and the lateral angle of the carapace is more clearly defined compare to only slightly defined in E. similis .

The male and female of E. nara is slightly larger than E. similis , both species has white transparent color, and collected from the same island. Although both species is adult as it shown in the fully developed G1 and ovigerous females, careful examinations are necessary to ascertain that the morphological differences were not related to the size of the specimens. The carapace of E. similis is more elongate with longer anterolateral margin, the chelipeds are more slender, the propodi of ambulatory legs are clearly shorter than the meri, the third maxilliped is subquadrate and the female pleon is more elongate ( Fig. 4 ). In E. nara the carapace is rounded with shorter anterolateral margin, the chelipeds are slightly swollen, the propodi and meri of ambulatory legs are subequal, the third maxilliped is more elongate and the female pleon is dome-shape (Fig, 1).

Habitat and distribution. Intertidal, sandy and gravel substrate in seagrass bed. Lombok Island, Indonesia.

Figures

Fig. 2. G1 of Elamena species. A, B, Elamena nara, new species, holotype, male (3.8 × 3.5 mm) (MZB Cru 4895), Teluk Nara, Lombok I., Indonesia, May 2010; C, D, Elamena similis, new species, holotype, male (2.9 × 2.9 mm) (MZB Cru 4897); E, F, Elamena sira, new species, holotype, male (3.3 × 3.0 mm) (MZB Cru 4898). A−D, right G1; E, F, left G1; A, C, F, ventral view; B, D, E, dorsal view. Scale bars = 0.25 mm.

Fig. 3. G1 of Elamena species. A, Elamena nara, new species, holotype, male (3.8 × 3.5 mm) (MZB Cru 4895), Teluk Nara, Lombok I., Indonesia, May 2010; B, Elamena similis, new species, holotype, male (2.9 × 2.9 mm) (MZB Cru 4897), Kecinan, Lombok I., Indonesia, 9 June 2009; C, D, Elamena sira, new species, holotype, male (3.3 × 3.0 mm) (MZB Cru 4898) Sira, Lombok I., Indonesia, 21 July 2009. A, B, right G1,ventral view; C, left G1, dorsal view; D, tip of left G1, ventral view. Not to measure.

Fig. 4. Elamena similis, new species. A−F, holotype, male (2.9 × 2.9 mm) (MZB Cru 4897), Kecinan, Lombok I., Indonesia, 9 June 2009; G, H, paratype, ovigerous female (3.5 × 3.3 mm) (ZRC2019.0593), same locality as holotype. A, habitus, dorsal view, left P4 missing; B, left third maxilliped; C, anterior region, ventral view; D, right chela; E, left P3, bristle partially omitted; F, H, somites 1−5 of pleon and pleotelson; G, carapace. Scale bars: A, F, G = 1 mm; B–E = 0.5 mm.