Metacrimenes fenestra Komai & Sakiyama 2023, n. sp.
Creators: name: Komai, Tomoyuki
name: Sakiyama, Hiroki
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Isopoda
Palaemonidae
Metacrimenes
Metacrimenes fenestra

description: Metacrimenes fenestra n. sp.
[New Japanese name: Torinoashi-yadori-ebi]
(Figs. 1–6, 7B)
Material examined. Holotype. CBM-ZC 17530, ovigerous female (cl 3.2 mm), off Numazu, Suruga Bay, Japan, 35°05.03’N, 138°48.18’E, 131.5 m, ROV manipulator, associated with the crinoid Metacrinus rotundus, 27 April 2023.
Non-type. CBM-ZC 17531, 1 juvenile (cl 1.3 mm), off Numazu, Suruga Bay, Japan (close to type locality), 150–200 m, ROV manipulator, collected together with the crinoid Metacrinus rotundus, 24 February 2023.
Description. Holotype (ovigerous female).Body(Figs.1, 2)small(cl 3.2mm),moderately slender, subcylindrical, somewhat elongate in general form; integument surface glabrous.
Rostrum (Figs. 2A, 3A, B) 0.7 times as long as carapace, reaching beyond distal margin of antennular peduncle, falling short of distal margin of antennal scaphocerite, directed forward, nearly straight, with moderately developed dorsal and ventral blades; dorsal margin faintly convex, armed with 11 moderately small teeth, including 9 on rostrum proper, becoming smaller distally, and 2 on carapace posterior to orbital margin, none having complete or incomplete basal suture, posterior-most tooth not distinctly separated from remainder of series, arising at 0.12 of carapace length; interspaces between teeth with short setae; ventral margin slightly convex, unarmed; lateral carina low but sharply delimited in proximal half, merging into orbital margin. Carapace (Fig. 2A, 3A, B) with low postrostral ridge extending to mid-length; dorsal profile in lateral view almost straight; orbital margin evenly concave; supraorbital spine absent; suborbital lobe triangular, well exceeding beyond antennal spine; antennal spine slender, marginal; hepatic spine moderately small, slender, arising below level of antennal spine and level of posterior-most rostral tooth, fixed (without basal suture); pterygostomial angle not produced, rounded.
First to fifth pleomeres (Fig. 2B) with all pleura rounded marginally; posterolateral margin of fourth and fifth pleomeres moderately produced posteriorly. Third pleomere tergum moderately produced posterodorsally. Sixth pleomere (Fig. 2B) 1.8 times as long as fifth pleomere, approximately twice as long as deep; posterolateral process terminating in acute tip, posteroventral angle subacute.
Telson (Figs. 2B, 3C, D) equal in length to sixth pleomere, approximately 4 times longer than anterior width. Lateral margins with anterior third subparallel, posterior two-thirds straight, convergent. Posterior margin produced as subtriangular lobe. Dorsal surface with 2 pairs of spiniform setae, first pair arising at mid-length of telson, second pair at 0.75 telson length. Posterior margin with 3 pairs of spiniform setae; lateral setae short, similar to dorsolateral setae; intermediate setae long, slender, exceeding 5 times as long as lateral setae; submedian setae slightly longer than lateral setae.
Epistome with 1 pair of low protuberances. Thoracic sternum narrow; fourth and sixth sternites (first and third pereopods) each with transverse ridge having distinct median notch; seventh sternite with low median ridge; eighth sternite slightly raised medially.
Eyestalk (Fig. 3A, B) subcylindrical, somewhat inflated proximally, wider than cornea. Cornea moderately large, subglobular (corneal width 0.15 of carapace length), shorter than eyestalk, well pigmented, without ocellus; boundary between cornea and eyestalk oblique.
Antennular peduncle (Fig. 3B) reaching mid-length of antennal scaphocerite. Proximal article with slender, acute stylocerite, reaching nearly to mid-length of article; anterolateral margin produced with 2 slender spines, reaching mid-length of intermediate article; statocyst small; mesial margin with sparse setae; ventromesial margin unarmed. Distal two articles combined subequal in length to proximal article. Intermediate article with distomesial and distolateral margins bluntly produced. Distal article longer than intermediate article. Left lateral flagellum (Fig. 3E) biramous, with 6 proximal segments of ramus fused; shorter free ramus with about 5 segments, shorter than fused portion; total length of flagellum about 0.8 of carapace length; longer ramus, slender, filiform. Mesial flagellum slender, filiform, subequal in length to lateral flagellum. Both right flagella abnormally short, probably in process of regeneration.
Antenna (Fig. 3A, B, F) with basicerite moderately robust, with small ventrolateral distal spine. Carpocerite reaching 0.3 length of scaphocerite, subcylindrical. Scaphocerite well developed, 0.9 times as long as carapace, 3.4 times longer than wide, greatest width at about mid-length; lateral margin faintly concave, terminating distally in slender distolateral spine; distal lamella strongly produced, subtriangular, considerably overreaching distolateral spine. Flagellum broken on both sides.
Mandible (Fig. 4A) without palp; molar process normal, with 4 strong, blunt teeth; incisor process slender, gently curved, obliquely truncate distally, with 4 acute teeth, central teeth smaller than outer teeth. Maxillule (Fig. 4B) with well-developed, distally bilobed palp, inner lobe strongly curved, non-setose, outer lobe simple; distal endite subovate, with row of spiniform setae on truncate margin; proximal endite reaching as far as distal endite, tapering distally, with several setae. Maxilla (Fig. 4C) with endopod rather stout, with strongly sinuous mesial margin; distal endite well produced, deeply bilobed, with setae on margin; proximal endite poorly developed; scaphognathite moderately broad. First maxilliped (Fig. 4D) with basial endite broad, with numerous simple setae mesially; coxal endite unknown (damaged during dissection); endopod nearly straight, falling slightly short of base of exopodal flagellum; exopod well developed, flagellum-like, with 5 long terminal setae; caridean lobe moderately large, broad; epipod large, distinctly bilobed. Seccond maxilliped (Fig. 4E) with normal endopod; dactylus about 4 times wider than long, with numerous serrulate setae on and along margin; propodus with slightly produced distomesial angle, with several stiff setae; carpus with mesial margin strongly produced into tooth-like projection; ischiomerus and basis without special features; exopod flagellum-like, with 4 long terminal setae; epipod small, simple, without podobranch (damaged during dissection, not figured).
Second pereopods (Figs. 5B, C, 6C) not particularly elongate, rather slender, symmetrical, overreaching distal margin of antennal scaphocerite by length of chela; all surfaces smooth. Chela approximately half as long as carapace, 0.8 times as long as carpus, almost straight; palm smooth, slightly compressed, 4.7 times longer than maximal depth; fingers gradually tapering, crossing distally, each terminating in acute tip; dactylus 1.2 of palm length, slightly curved, occlusal margin unarmed; fixed finger nearly straight, occlusal margin unarmed. Carpus elongate, 9.1 times as long as distal width, unarmed. Merus subequal in length to chela, 9.1 times as long as distal depth, unarmed. Ischium 1.1 of merus length, unarmed. Basis and coxa without special features.
Ambulatory pereopods (third to fifth pereopods) relatively short, moderately slender. Third pereopod (Figs. 5D, 6D) not reaching distal margin of scaphocerite. Dactylus short, 0.15 times as long as propodus, compressed, gently curved, biunguiculate; unguis clearly demarcated at base; subterminal accessory spine slender, subequal in length to unguis, slightly upcurved; flexor margin smooth, unarmed proximal to subterminal accessory spine. Propodus with some long setae distally; flexor margin with 1 pair of spiniform setae on distal margin followed by 2 widely spaced spiniform setae and row of sparse simple setae. Carpus about half of propodus length, slightly widened distally, unarmed. Merus subequal in length to propodus, unarmed; ischium unarmed. Basis and coxa without special features.
Fourth pereopod (Fig. 5E) similar to third pereopod.
Fifth pereopod (Figs. 5F, 6E) generally similar to third and fourth pereopods, but slightly longer.
Uropod (Figs. 2B, 3G) with posterolateral angle of protopod short, rounded. Exopod distinctly overreaching posterior end of telson; lateral margin almost straight, with minute spiniform seta just mesial to base of small posterolateral denticle; diaeresis obliquely sinuous. Endopod elongate subovate, about 0.9 times as long as exopod.
Juvenile. Generally similar to holotype. Rostrum reaching distal end of proximal article, with 9 teeth, including 2 postrostral, on dorsal margin; ventral margin unarmed. Second pereopod proportionally shorter, reaching to distal margin of antennal scaphocerite by tip of fingers.
Third maxilliped (Fig. 4F, G) with endopod moderately slender, short, reaching level of distal margin of fourth article of antennal peduncle. Ultimate article subequal in length to penultimate article (= carpus), tapering distally to subacute tip, with numerous transverse rows of coarsely serrulate setae on mesial face. Penultimate article subcylindrical, with sparse setae on mesial face. Antepenultimate article gently bowed in dorsal view, somewhat compressed dorsoventrally; mesial margin with row of sparse setae; lateral margin with row of 5 stiff setae in distal 0.3 of its length. Exopod greatly reduced to rudimentary bud. Coxa with rounded lateral plate. Arthrobranch absent.
First pereopod (Figs. 5A, 6A, B) reaching base of distolateral spine of antennal scaphocerite by tip of fingers. Chela 0.75 times as long as carpus, 4.9 times as long as wide, gradually tapering distally; palm slightly compressed, 2.3 times as long as maximal width at about mid-length, without grooming apparatus; fingers 1.2 times as long as palm, slender, tapering, each with tufts of short setae distally; occlusal margins unarmed. Carpus slightly widened distally, 6.7 times as long as distal width, with grooming apparatus consisting of subdistal tuft of setae on mesial face. Merus subequal in length to carpus, 8.8 times as long as central width, straight. Ischium about half-length of merus. Basis without special features. Coxa without ventromesial process.
Colouration in life. Carapace with dorsal mid-line translucent; reddish brown longitudinal stripes running on either side of mid-line; lateral surface with alternating longitudinal stripes of reddish brown and white; rostrum reddish brown. Pleon with mid-line bearing yellowish longitudinal stripe running over entire length, in continuation with mid-line band of carapace, flanked by translucent and reddish brown stripes; lateral surface largely translucent over most pleomeres; ventral parts of pleura 1–5 with alternating reddish brown and whitish stripes, reddish brown stripe extending onto pleomere 6. Corneas grey. Antennal scaphocerite whitish along lateral margin, reddish brown along mid-lin. First and third to fifth pereopods translucent; chela and distal half of carpus of second pereopod with tinge of reddish brown and whitish dots, otherwise translucent (Fig. 1).
Distribution. Presently known only from Suruga Bay, Honshu, central Japan, at depths of 131– 200 m.
Ecology. The holotype of Metacrimenes fenestra n. sp. was found clinging to the arm of the stalked crinoid Metacrinus rotundus (Fig. 7A, B) at a depth of 131 m, off Numazu, Suruga Bay. The juvenile non-type specimen was collected together with specimens of M. rotundus also from off Numazu, Suruga Bay, at a depth ranging from 150 to 200 m. Although M. rotundus has been widely reported from Japanese waters (e.g., Fujita et al. 1987; Imaoka et al. 1991), there have been no previous records of symbiotic associations with shrimps or other decapod crustaceans.
Remarks. As discussed above, the new taxon is morphological fairly distinctive within the Palaemonidae. The K2P divergences of the 16S rRNA and COI gene sequences between M. fenestra n. sp. and putative closest palaemonid taxa are presented in Tables 2 and 3, respectively. The pairwise genetic divergences between M. fenestra n. sp. and other palaemonids range from 11.4 to 31.8% for the 16S rRNA gene and from 21.1 to 35.5% for the COI gene.
Etymology. From the Latin noun fenestra (= window), in reference to the window-like, translucent area on the lateral surface of the pleon of the new species. Used as a noun in apposition.
description: Published as part of Komai, Tomoyuki & Sakiyama, Hiroki, 2023, A new genus and new species of palaemonid shrimp (Decapoda: Caridea), associated with the deep-sea crinoid Metacrinus rotundus (Echinodermata: Isocrinidae), from Suruga Bay, Japan, pp. 227-246 in Zootaxa 5380 (3) on pages 230-243, DOI: 10.11646/zootaxa.5380.3.2, http://zenodo.org/record/10224718