Metacrangon rau, Komai & Ahyong, 2010

Komai, Tomoyuki & Ahyong, Shane T., 2010, The crangonid shrimp genus Metacrangon (Crustacea: Decapoda: Caridea) from New Zealand, with descriptions of four new species, Journal of Natural History 45 (1 - 4), pp. 77-111 : 91-95

Metacrangon rau sp. nov.

( Figures 7–10)

Material examined

Holotype. Stn E 0880, west of Northland, North Island , 35 ◦ 19.99 ′ S, 172 ◦ 19.99 ′ E, 1029–1074 m, 22 March 1968, male (CL 5.4 mm), NIWA 6329 .

Paratypes. Stn P 0927, Challenger Plateau, 40 ◦ 50.09 ′ S, 168 ◦ 14.80 ′ E, 1009– 1005 m, 18 April 1980, male (CL 4.9 mm), two females (CL 6.7–7.9 mm), NIWA 11799 .

Description

Body ( Figures 7, 8A,C ) moderately robust. Rostrum ( Figures 8A,B , 10A,C ) dorsoventrally flattened, triangular with acute or subacute apex in dorsal view, slightly ascending, about 0.2 times as long as carapace; dorsal surface nearly flat; lateral margin nearly straight in lateral view, merging into orbital margin; midventral carina low, ventral margin sinuous. Carapace ( Figures 7, 8A,B , 10B,D ) not widened posteriorly, distinctly longer than wide postorbitally; surface covered with very short setae; dorsal midline with two prominent, laterally compressed teeth; anterior tooth very large, blade-like, with sinuous to straight dorsal margin, arising distinctly posterior to rostral base, strongly ascending (angle against horizontal plane of carapace about 55 ◦), overlapping rostrum, tip reaching rostral tip; posterior (cardiac) tooth hooked, arising at 0.49–0.52 of CL; submedian tooth small (in paratypes) or reduced to rudimentary protuberance (in holotype); hepatic tooth small, followed by epibranchial ridge; postorbital carina low, blunt, accompanied by suture; antennal tooth relatively large, acuminate, reaching or under-reaching rostral tip, directed forward in dorsal view, slightly ascending in lateral view; orbital cleft absent; anterolateral margin between antennal and branchiostegal teeth unarmed; branchiostegal tooth strong, curved slightly laterally in dorsal view, slightly ascending in lateral view, distinctly over-reaching dorsodistal margin of antennal basicerite; pterygostomial tooth small, not visible in lateral view.

Abdomen ( Figures 7, 8C ) slightly sculptured; anterior three somites with trace of mid-dorsal carina, fourth somite with low, blunt, but clearly delimited mid-dorsal carina. First pleuron with subacute tooth ventrally; second pleuron with subacute tooth at middle of ventral margin; ventral margin of first and second pleura flared laterally, so ventral teeth visible in dorsal view; third pleuron with subacute tooth at anteroventral angle; fourth pleuron unarmed. Fifth somite with low, but distinct middorsal carina; posterodorsal margin barely produced medially, posterolateral margin unarmed; pleuron shallowly concave on lateral surface; ventral margin sinuous, posteroventral angle rounded. Sixth somite with distinct, slightly curved submedian

left eye, basal parts of antennae, obliquely lateral view; (C) abdomen and telson, dorsal view. Scale bars: 2 mm for A, C; 1 mm for B.

carinae, not reaching posterodorsal margin; dorsolateral carina distinct; posterodorsal margin faintly bilobed; pleuron flared laterally, posteroventral tooth strong; posterolateral process elongate, directed somewhat laterally, terminating in sharp tooth. Telson ( Figure 8C ) with three pairs of dorsolateral spines, first pair located at about posterior 0.4; tip subacute.

Male fifth to eighth thoracic sternites each with sharply pointed median tooth; first to fourth abdominal sternites each with long, anteriorly curved median tooth; fifth sternite with short median carina. In spawning moult of female, fifth thoracic sternite with thin, triangular carina with acute apex directed ventrally; sixth sternite with low, thin median carina; seventh and eighth sternites unarmed, without median tooth or carina; first to third abdominal sternites unarmed; fourth and fifth sternites with low median carina. In non-spawning moult, fifth thoracic sternite bearing sharp, procurved tooth; sixth to eighth sternites each with low, thin median carina; first to fourth abdominal sternites with ventrally directed tooth; fifth sternite with blunt tubercle. Sixth abdominal sternite medially depressed.

Eye ( Figure 8A,B ) about as long as wide; cornea as wide as eyestalk, darkly pigmented, corneal width 0.17 of CL; eyestalk with papilla-like dorsal tubercle.

Antennular peduncle ( Figure 8A ) reaching midlength of antennal scale. First segment with prominent, subacutely pointed distolateral process, distomesial margin unarmed; stylocerite directed forward, terminating in acute tooth slightly falling short of tip of distolateral process of first segment, lateral margin angular. Second segment distinctly widened distally, with blunt distolateral process. Third segment wider than long. Outer flagellum broken off in distal part, but preserved part over-reaching distal margin of lamella, with 12 articles.

Antennal basicerite ( Figure 8A ) stout, with sharply pointed dorsodistal lateral angle, ventrolateral tooth small, slightly over-reaching dorsodistal lateral angle. Antennal scale about 0.60 times as long as carapace and 2.6 times longer than wide; lateral margin very slightly concave; distolateral tooth relatively narrow, distinctly over-reaching rounded distal margin of lamella.

Third maxilliped relatively slender, over-reaching antennal scale by half length of ultimate segment; ultimate segment ( Figure 9A ) gradually tapering distally, about 7.7 times longer than wide; penultimate segment ( Figure 9A ) about 3.3 times longer than wide; antepenultimate segment with pair of greatly unequal movable spinules subdistally on ventral surface.

First pereopod ( Figure 9B,C ) moderately stout, reaching distal margin of antennal scale; palm 3.3 times longer than wide, not widened proximally or distally, lateral and mesial margins faintly sinuous, cutting edge oblique; thumb relatively slender; carpus with small ventrolateral tooth; merus with small dorsodistal tooth, ventral margin sinuous, crested. Second pereopod ( Figure 9D ) reaching nearly to midlength of antennal scale; dactylus about 0.4 times as long as palm; length ratio of chela to merus 1: 1.6: 1.7. Third pereopod ( Figure 9E ) slender, nearly reaching distal margin of antennal scale by tip of dactylus; length ratio of dactylus to ischium 1: 1.6: 2.6: 2.1: 1.9. Fourth pereopod ( Figure 9F ) moderately stout for genus, slightly falling short of distal margin of antennal scale by dactylus; dactylus ( Figure 9G ) narrowly spatulate, about 0.7 times as long as propodus, margins naked; dactylus– propodus articulation about 45 ◦; propodus about 4.5 times longer than wide. Fifth pereopod ( Figure 9H ) slightly shorter than fourth pereopod, reaching proximal 0.3 of antennal scale; dactylus subspatulate, more slender than that of fourth pereopod, about 0.9 times as long as propodus; setation much less than in fourth pereopod.

Second pleopod with appendix masculina reaching midlength of endopod, bearing about 10 spiniform setae.

Posterolateral tooth of uropodal exopod truncate with minute lateral denticle ( Figure 9I ); spinule mesial to posterolateral tooth present.

Colouration in life

Not known.

Size

Largest male CL 5.4 mm; largest female (spawning moult) CL 7.9 mm.

Remarks

Metacrangon rau sp. nov. is known from males and females, which are similar in most respects. Apart from sexual differences (i.e. differences in thoracic and abdominal sternal ornamentation), the chief variation noted is in the rostral apex and condition of the submedian teeth of the carapace. The rostrum varies from reaching as far as, to slightly beyond the apices of the antennal teeth, and is usually apically spiniform, being subacute in one female (pre-spawning moult, CL 6.7 mm). The submedian teeth on the carapace are absent in the male holotype but present in the three paratypes (minute in the male; small but distinct in the females). Variability in the presence of the submedian carapace teeth is unusual in Metacrangon ; the absence of submedian teeth is otherwise known only for M. trigonorostris (Yokoya, 1933) (Komai, unpublished data).

Distribution

Known from northwest North Island, off Northland, and the Challenger Plateau; 1005–1074 m.

Etymology

From the Maori word “ rau ”, meaning blade, in reference to the prominent, blade-like anterior mid-dorsal tooth of the carapace; used as a noun in apposition.

Figures

Figure 8. Metacrangon rau sp. nov., holotype, male (CL 5.4 mm), NIWA 6329. (A) Carapace and cephalic appendages, dorsal view (setae partially omitted); (B) anterior part of carapace,

Figure 9. Metacrangon rau sp. nov., holotype, male (CL 5.4 mm), NIWA 6329. (A) Distal two segments of right third maxilliped, ventral (flexor) view; (B) right first pereopod, lateral view; (C) same, subchela, dorsal (extensor) view; (D) right second pereopod, lateral view; (E) right third pereopod, lateral view; (F) right fourth pereopod, lateral view; (G) dactylus of left fourth pereopod, flexor view; (H) right fifth pereopod, lateral view; (I) posterolateral part of uropodal exopod, dorsal view (setae omitted). Scale bars: 1 mm for A–F, H; 0.5 mm for G, I.

Figure 10. Metacrangon rau sp. nov. (A,B) Paratype, female, non-spawning moult (CL 6.7 mm), NIWA 11799. (C,D) Paratype, female, spawning moult (CL 7.9 mm), NIWA 11799. A, C, anterior carapace, dorsal view. B, D, carapace, left lateral view. Scale bar 1 mm.