Siriella cf. chaitiamvongae Murano & Fukuoka, 2008

Daneliya, Mikhail, Price, W. Wayne & Heard, Richard W., 2018, Revision of the Siriella brevicaudata species group (Crustacea: Mysida: Mysidae) from the West Indo-Pacific, European Journal of Taxonomy 426, pp. 1-80 : 71-76

Siriella cf. chaitiamvongae Murano & Fukuoka, 2008

Figs 32–34

Siriella chaitiamvongae Murano & Fukuoka, 2008: 149 , figs 76–78.

Siriella hanseni – O.S. Tattersall 1960: 167.

Material examined

SINGAPORE: 2 subadult ♂♂, 6 mm long, Singapore Strait, 7 miles off shore, st. 5, 1°14′ N, 103°55′ E, Sar 6, 15 Jul. 1955. Wickstead leg., previously identified by O.S. Tattersall as S. hanseni (O.S. Tattersall 1960) ( NHM 1964.1.21.6185). Badly damaged specimens; 1 ♂ with separate cephalothorax and abdomen, 1 ♂ partly dissected (slide NHM 1964.1.21.6185 ).

Description

Male ( Singapore specimens)

CARAPACE. Anterodorsal margin widely triangular, sharpened at apex, covering basal parts of eyes and antennular peduncles ( Fig. 32A ).

TELSON. Linguiform, posteriorly rounded, slightly shorter than last abdominal somite and 0.8 times as long as uropodal endopod, 2.6 times as long as wide ( Fig. 32B ). Lateral margins of telson with two or three anterior and 21 posterior spiniform setae; posterior spiniform setae form four or five gradation groups; terminal posterolateral pair of spiniform setae longest, 1.2–1.3 times as long as preceding spiniform setae and 0.1 times as long as telson. Central apical spiniform seta 1.6 times as long as flanking pair (all three spines of same length in another specimen) and 0.5–0.6 times as long as lateral terminal spiniform setae ( Fig. 32C ).

HEAD APPENDAGES. Eyes large, nearly rounded, produced beyond sides of anterior part of carapace, 1.1– 1.2 times as long as wide and 0.5–0.6 times as long as width of anterior part of carapace. Peduncle of antenna 1 longer than peduncle of antenna 2; segment 3 about as long as segments 1 and 2 together, and 1.2 times as long as wide ( Fig. 32A, D ). Segment 3 of antenna 2 peduncle 0.4 times as long as segment 2 ( Fig. 32E ). Antennal scale 1.5 times as long as peduncle of antenna 2, reaching about half of antenna 1 segment 3, 3.2 times as long as wide, with clear distal segment; scale length 1.2 times length from scale base to outer spine base. Labrum with long and strong articulated anterior spine, about half as long as rest of labrum ( Fig. 32G ). Left mandible only with lamellar single-toothed incisor and lamellar molar, lying in parallel planes ( Fig. 33A ). Right mandible ( Fig. 33B ): incisor with two lamellar cusps, with planes perpendicular to each other; lacinia mobilis single-toothed; molar lamellar, lying in plane of anterior cusp of incisor; four medial spines short, odontoid. Palp segment 2 with six setae along medial margin and four setae along lateral margin; palp segment 3 is 0.6 times as long as segment 2, with six long plumose proximal and eight short palmar setae on medial margin ( Fig. 32F ). Maxilla 1 ( Fig. 33C ): inner ramus with four setae; outer ramus with about 10 strong robust setae, two apical with slight distal serration. Maxilla 2 ( Fig. 33D ): exopod oviform, with 11 long plumose setae; endopod segment 1 with one short proximomedial and one long distomedial setae; endopod segment 2 is 1.1 times as long as exopod and 3.2 times as long as wide, with one distolateral seta and about 10 medial setae, interspersed by straight stronger setae; endites with three or four strong setae each.

MAXILLIPEDS. Maxilliped 1 ( Fig. 33 E–F): basis with one proximal and two distal setae; preischium with one distomedial seta; ischium with two distomedial setae; merus as long as preischium and ischium together, 1.8 times as long as wide, with five medial and one distolateral setae; carpopropodus 0.6 times as long as merus, with distomedial and distolateral bunch of setae; dactylus 0.6 times as long as carpopropodus, with long simple and strong finely serrated setae; dactylary unguis 1.1 times as long as dactylus. Maxilliped 2 ( Fig. 33 G–H): basis with three setae; preischium with two medial setae; ischium 1.7 times as long as wide, with four medial and one distolateral setae; merus 3.5 times as long as wide and 1.6 times as long as ischium, with two medial and one distolateral setae; carpopropodus as long as and as wide as merus, with two medial bunches of setae and two distolateral setae; dactylus conical, 0.3 times as long as carpopropodus, with five–six strong, serrated setae and smooth unguis; unguis about as long as dactylus.

PEREOPODS. Pereopodal exopods 10-segmented. Endopods thin and long; central pairs longer than anterior and posterior. Setae of endopod ischium and merus short, strong proximally and thin distally, distantly placed, not forming groups or bunches ( Fig. 34 A–B). Pereopod 1 endopod ( Fig. 34A ): basis with two setae; preischium without setae; ischium 2.7 times as long as wide, with six medial setae; merus 5.4 times as long as wide, with five medial and two distolateral setae; carpopropodus about as long as merus, 7.5 times as long as wide, with barely visible articulation between segments; carpopropodus segment 1 is 0.4 times as long as segment 2, with distomedial bunch of serrated setae, two lateral serrated falcate setae and one simple distolateral seta; carpopropodus segment 2 with medial bunch of serrated setae, one lateral serrated falcate seta, three simple lateral setae and bunch of paradactylary serrated setae, longer than unguis; dactylus 1.4 times as long as wide, with strong unguis and short seta; dactylary unguis 2.1 times as long as dactylus. Pereopod 2 longer than peropod 1 ( Fig. 34B ); basis with one proximal and two distal setae; preischium without setae; ischium 2.8 times as long as wide, with seven medial and one distolateral setae; merus 6.1 times as long as wide and 2.1 times as long as ischium, with five medial and one distolateral spiniform setae; carpopropodus slightly longer than merus, 10.5 times as long as wide, with barely visible articulation between segments; carpopropodus segment 1 is 0.3 times as long as segment 2, with distomedial bunch of serrated setae and 2 lateral serrated falcate setae; carpopropodus segment 2 with medial bunch of serrated setae, one lateral serrated falcate seta, two lateral simple setae and paradactylary bunch of serrated setae, as long as unguis; dactylus 2 times as long as wide, with strong unguis and short dactylary seta; unguis 1.9 times as long as dactylus.

PLEOPODS. Natatory; rami 10-segmented ( Fig. 34 C–D). Pseudobranchia biramous. Pleopod 1 uniramous. Pleopods 2–5 biramous.

UROPODS. Uropodal exopod 4.5 times as long as wide and 1.2 times as long as endopod; proximal segment 1.8 times as long as distal segment, with four distolateral spiniform setae ( Fig. 32H ). Uropodal endopod with 13 somewhat distantly placed medial spiniform setae; terminal spiniform seta extending to apex ( Fig. 32H ).

Distribution

Siriella chaitiamvongae is known from the Gulf of Thailand, South China Sea (type locality), Batbatan Island, Sulu Sea and Java Island, Java Sea ( Murano & Fukuoka 2008). If the specimens from this study belong to this species, it would constitute the first record from Singapore, and specifically from Singapore Strait.

Remarks

Siriella chaitiamvongae belongs to the brevirostris subgroup of the thompsonii group ( Murano & Fukuoka 2008). The specimens from Singapore have the following characters that are typical for S. chaitiamvongae : (1) pereopodal carpopropodus, lacking (or barely visible) proximal articulation, (2) paradactylary setae about as long as dactylar claw and (3) antennular peduncle segment 3 broad, 1.2 times as long as wide. Due to the immature and damaged condition of the specimens it is, however, not possible to identify such diagnostic characters as the presence of a median digitate process on the front of the carapace along its midline.

Several characters of the specimens from Singapore exhibit greater variation than reported in the original description (Gulf of Thailand). The apical spiniform setae of the telson are shorter, more closely resembling those of S. brevirostris Nouvel, 1944 , although in an immature female paratype ( Murano & Fukuoka 2008: fig. 78c), the apical setae were less than half as long as the terminal lateral spiniform setae, i.e., also similar to S. brevirostris . In one of the specimens from Singapore the central apical seta is 1.6 times as long as the pair of flanking setae, while in the other specimen all three setae are equal in length. Other morphological differences between specimens from Singapore and the Gulf of Thailand are as follows: (1) mandibular palp second segment with six medial setae (four in male from the type locality), (2) telson lateral margins with two or three anterior and 21 posterior spiniform setae (one or two and 21 to 27, respectively, at type locality), (3) telson with posterior spiniform setae forming groups of three to five (two to five at the type locality) graded setae, (4) antennal scale 3.2 times as long as wide (3.7 at the type locality) and (5) uropodal endopod with 13 spiniform setae (15 or 16 at the type locality).

Both specimens are not fully developed, particularly their penes, pleopods and probably other structures. The small differences mentioned above could partly be connected with the immature age of the specimens from Singapore and partly with the general variability of the species. Thus, the status of the specimens cannot be finally resolved; however, one can be certain that they are not S. hanseni as originally identified by O.S. Tattersall (1960).

Figures

Fig. 32. Siriella cf. chaitiamvongae Murano & Fukuoka, 2008, subadult ♂, Singapore. A. Anterior part of body. B. Telson. C. Posterior margin of telson, dorsal view. D. Antenna 1 peduncle, lateral view. E. Antenna 2 peduncle and antennal scale, dorsal view. F. Mandibular palp, lateral view. G. Labrum. H. Uropods, dorsal view. Scale bars: A = 1 mm; B, D–H = 0.25 mm; C = 0.1 mm.

Fig. 33. Siriella cf. chaitiamvongae Murano & Fukuoka, 2008, subadult ♂, Singapore. A. Left mandible. B. Right mandible. C. Maxilla 1, anterior view. D. Maxilla 2, anterior view. E. Maxilliped 1, posterior view. F. Distal part of maxilliped 1, posterior view. G. Maxilliped 2, posterior view. H. Distal part of maxilliped 2, posterior view. Scale bars: A–D, F, H = 0.05 mm; E, G = 0.1 mm.

Fig. 34. Siriella cf. chaitiamvongae Murano & Fukuoka, 2008, subadult ♂, Singapore. A. Pereopod 1 endopod, anterior view. B. Pereopod 6, anterior view. C. Pleopod 1, posterior view. D. Pleopod 4, posterior view. Scale bars: 0.25 mm.