Manaxius paullus, Komai & Anker, 2020

Komai, Tomoyuki & Anker, Arthur, 2020, A new species of the axiid shrimp genus Manaxius Kensley, 2003 (Decapoda Axiidea) from shallow coral reefs of the western Pacific, Zootaxa 4858 (4), pp. 507-520 : 508-519

Manaxius paullus n. sp.

[New Japanese name: Sango-ana-ebi]

( Figs. 1–5 )

Type material. Holotype: FLMNH-UF 16344 , female (pocl 3.5 mm), French Polynesia, Society Islands , Moorea, between Cook’s Bay and Sheraton Hotel, 17.4764°S, 149.8327°W, depth 4–7 m, outer reef slope, inside dead coral head ( Acropora sp.), 10 November 2008, coll. S. McKeon & J. Moore (extracted by A. Anker & S. Ivanenko).

Paratype: CBM-ZC 16165, 1 male (pocl 4.2 mm), Japan, Ryukyu Islands , Yaeyama Islands , Kuro-shima Island, Fuki Beach, 24.2542°N, 123.9936°E, lagoon on reef flat, depth 2 m, 3 September 1998, coll. K. Nomura.

Diagnosis. Rostrum triangular in dorsal view, reaching distal margin of article 2 of antennular peduncle, each lateral margin bearing 1 small spine at its mid-length. Carapace surface glabrous on lateral sides; gastric region with median carina bearing 1 small anterior spine followed by 1 small denticle, 1 obsolete (or trace of) tubercle and 1 minute denticle; submedian carinae each consisting of 3 spinules; lateral carinae each divided in 2 parts, anterior part with 1 conspicuous spine (supraocular spine), posterior part ending in 1 small spine and sometimes followed by 1 minute denticle; 1 pair of additional small spines present between first spine of submedian carinae and median carina; no trace of postcervical median carina. Pleura 2–4 rounded, posteroventral margin of pleuron 5 slightly angular. Telson with 2 pairs of small spines on dorsal surface; lateral margins each with 1 conspicuous proximal lobe, latter with 1 minute spine, 3 or 4 pairs of small spines and 1 pair of strong spiniform setae; posterolateral angles each with 2 unequal submarginal spiniform setae. Eyestalks reaching mid-length of rostrum, cornea pigmented. Chelipeds subequal in length, slightly dissimilar in armature and stoutness; each ischium with 1 small subdistal spine on dorsal margin and 1 subdistal spine followed by some small tubercles on ventral margin; each merus with 2 small subdistal spines on dorsal margin, ventromesial margin armed with 1 strong distal spine followed by serration consisting of 5 or 6 spinules; each carpus with small distodorsal spine; dorsal and lateral surfaces of chelae densely covered with tufts of short and long setae; dorsal margin of each palm armed with 4 or 5 spines, each lateral surface with 3 or 4 small spines adjacent to dorsal margin, but without spine near base of fingers; dactylus slightly shorter than palm on major cheliped side, subequal in length to palm on minor cheliped side.

Description. Holotype female. Rostrum ( Figs. 1A , 2A, B ) moderately narrow, triangular in dorsal view, reaching distal end of article 2 of antennular peduncle, 0.3 times as long as distance between rostral base and cervical groove or 0.17 times as long as carapace, tapering into acute tip; lateral margins elevated, each with 1 spine at midlength and supraocular spine, continuous with lateral gastric carina on carapace; dorsal surface shallowly concave. Carapace ( Figs 1A , 2A ) with smooth surface, with either tufts of or individual short to long setae arranged in longitudinal rows on gastric region and scattered short setae on postcervical dorsum; dorsal surface nearly straight in lateral view, gastric region slightly sloping down to rostrum; median gastric carina distinct, extending anteriorly to level of supraocular spines, not reaching cervical groove, with 1 small anterior spine followed by 1 small denticle, 1 trace of minute tubercle and denticle; submedian carinae consisting of 3 spinules; lateral gastric carinae falling far short of midway between rostral base and cervical groove, divided into 2 widely separated parts, anterior part without armature other than supraocular spine, posterior part represented by 1 small spine; area between median and submedian carinae at level of anterior end of submedian carina with 1 small spine; no trace of postcervical carina; cervical groove distinct, but restricted to dorsum; suborbital lobe obtuse; pterygostomial margin slightly produced, broadly rounded; cardiac notch deep.

Thoracic sternite 7 with shield ( Fig. 2C ) deeply depressed medially, divided by deep median groove, lateral margins strongly elevated, subtriangular with sharply pointed tip.

Pleon ( Fig. 1B ) with scattered tufts of short to long setae on each somite. Pleomere 1 widening posteriorly in dorsal view, anterodorsal margin excavate; pleuron triangular with subacute tip, produced slightly beyond ventral margin of pleuron 2. Pleomere 2 with pleuron slightly asymmetrical; ventral margin unarmed, rounded at either angle. Pleomeres 3–5 with all pleura unarmed; pleura 3 and 4 rounded; pleuron 5 slightly angular on posterolateral margin. Pleomere 6 with pleuron bluntly pointed ventrally; posterolateral projection triangular, with blunt tip.

Telson ( Fig. 2D ) subrectangular, 1.2 times as long as maximal width at anterolateral lobes; lateral margin with conspicuous blunt anterolateral lobe latter bearing 1 submarginal spinule, 4 (left) or 3 (right) spines in posterior 0.4 of telson length and 1 strong spiniform seta adjacent to first spine; posterior margin convex, without posteromedian spine, fringed with numerous long setae, each posterolateral area with 2 unequal spiniform setae (lateral seta much smaller than mesial); dorsal face shallowly sulcate medially, armed with 2 pairs of small spines, anterior spine lo- cated at mid-length and posterior spine at 0.6 of telson length; long setae arising at bases of dorsal spines.

Eyestalk ( Fig. 2B ) reaching mid-length of rostrum; cornea hemispherical, subequal in length and width to eyestalk, darkly pigmented.

Antennular peduncle ( Fig. 2B ) reaching distal margin of article 4 of antennal peduncle; statocyst lobe on article 1 unarmed; flagella subequal in length to carapace. Antennal peduncle ( Figs 1A , 2B ) with article 1 bearing 1 small spine at ventromesial distal angle; article 2 with straight dorsolateral distal spine reaching beyond mid-length of scaphocerite; scaphocerite simple, acuminate, reaching almost to mid-length of article 4; article 3 with 1 small spine at ventromesial distal angle; article 4 slightly longer than article 2 (except for dorsolateral distal spine); article 5 about 0.6 length of article 4; flagellum twice as long as carapace, each article distally with some setae of various length (longest setae about twice length of 1 article).

Mouthparts not dissected. Maxilliped 3 ( Fig. 5A ) moderately slender. Coxa with small spine at distoventral angle. Basis with 1 small distoventral spine. Ischium with distomesial angle produced into strong projection; crista dentata consisting of row of sharp teeth diminishing in size proximally ( Fig. 5B ); ventral margin with 2 small spines in proximal half. Merus bearing 2 unequal prominent spines on ventral margin (distal spine much stronger). Carpus with 1 small spine at ventrolateral distal angle. Dactylus shorter than propodus. Exopod almost reaching distal margin of merus, its distal half with multiple subdivisions.

Chelipeds ( Figs 3 , 4 ) subequal in length, right slightly stouter than left; mero-carpal articulation with about 30° of torsion (left with clockwise torsion, right with counter-clockwise torsion); chelae with dense setation particularly on lateral surface ( Fig. 3 ).

Right (major) cheliped ( Fig. 4 A–D) coxa bearing 1 spinule on distolateral margin. Basis unarmed. Ischium with 1 small subdistal spine on dorsal margin; ventral margin also with 1 subdistal spine followed by 2 small tubercles. Merus widening distally; dorsal margin strongly arcuate, sharply carinate, with 1 small subdistal spine followed by 1 somewhat distant small spine and row of setae in distal half; lateral face glabrous, generally convex, distoventrally with deep excavation delimited by distinct ridge and fitting flexor margin of carpus, when cheliped is flexed; distolateral angle with prominent articular condyle; mesial face smooth, somewhat flattened; ventral margin carinate, bearing 1 small but conspicuous spine subterminally, followed by 6 small spines decreasing in size proximally. Carpus short, much higher than long, dorsal margin sharply carinate, with 1 small subdistal spine and numerous tufts of long setae; lateral face with several tufts of short to long setae on dorsal half, almost glabrous on ventral half, with sharp keel along ventral margin; mesial and ventral surfaces glabrous; mesial surface deeply excavated proximally to accommodate distoventral margin of merus when cheliped is flexed. Palm 1.3 times as long as high; dorsal surface non-carinate, armed with 4 widely spaced, relatively small spines, obscured by numerous tufts of long plumose setae; lateral surface flattened, with 3 small spines adjacent to dorsal margin, obscured by numerous tufts of short to long plumose setae; finger base unarmed; ventrolateral margin sharply keeled, this keel not extending onto fixed finger; mesial face inflated, almost glabrous except for some tufts of short setae on distal part. Fingers crossing, each terminating in acute tip; leaving narrow hiatus proximally when closed. Fixed finger with slightly arcuate ventral margin; occlusal margin with 3 obtuse teeth; lateral surface partially obscured by numerous tufts of long plumose setae; mesial face with tufts of short setae adjacent to ventral margin. Dactylus 0.8 times as long as palm, gently curved, with numerous tufts of long plumose setae on non-carinate, spineless dorsal margin and lateral surface; both lateral and mesial surfaces slightly elevated in midline, appearing as blunt ridge in proximal half; occlusal margin without conspicuous teeth.

Left (minor) cheliped ( Fig. 4 E–H) with coxa to carpus similar to those of major cheliped; ischium with 1 subdistal spine on dorsal margin, ventral margin with 1 subdistal spine followed by 2 small tubercles. Merus with 1 small subdistal spine and 1 additional spine on dorsal margin; ventral margin with 1 large distal spine followed by row of 5 small spines diminishing in size proximally. Carpus with 1 small subdistal spine on dorsal margin. Palm and fingers slightly narrower than those of major cheliped, otherwise generally similar; palm 1.3 times as long as wide, dorsal margin with 5 well-spaced spines; lateral face with 4 small spines adjacent to dorsal margin. Fixed finger with 2 widely spaced small subacute teeth. Dactylus 1.2 times as long as palm; occlusal margin without tooth.

Pereopod 2 ( Fig. 5C ) moderately slender. Ischium unarmed. Merus with 1 small spine on ventral margin located distally to mid-length. Chela 1.1 times as long as carpus; fixed finger straight, occlusal margin with row of minute spiniform setae; dactylus 1.2 times as long as palm.

Pereopod 3 ( Fig. 5D ) moderately slender. Ischium unarmed. Merus with 2 minute spines on ventral margin. Propodus with 5 transverse rows of spiniform setae (each consisting of 2 or 3 unequal spiniform setae) on flexor lateral surface; distal margin with row of several spiniform setae, longest seta on flexor side reaching mid-length of dactylus; flexor margin with cluster of stiff grooming setae in its distal half. Dactylus 0.6 times as long as propodus, gently curving, terminating in minute, basally demarcated unguis; lateral surface with longitudinal row of small spiniform setae adjacent to extensor margin and row of minute spiniform setae on flexor margin; mesial face unarmed.

Pereopod 4 ( Fig. 5E ) noticeably slenderer than pereopod 3. Ischium to carpus unarmed. Propodus with 4 transverse rows of spiniform setae on flexor lateral face (each consisting of 2 or 3 unequal spiniform setae) and grooming setae on distal half of flexor margin; distal margin with row of several spiniform setae, longest seta on flexor side reaching mid-length of dactylus. Dactylus 0.5 times as long as propodus, gently arcuate, terminating in basally demarcated unguis; lateral surface with row of 3 spiniform setae adjacent to extensor margin and row of minute spiniform setae on flexor margin; mesial face unarmed.

Pereopod 5 ( Fig. 5 F–H) most slender and shortest among pereopods, neither chelate nor semichelate. Ischium to carpus unarmed. Propodus with grooming apparatus consisting of 3 subterminal spiniform setae and field of stiff setae extending to mid-length of propodus. Dactylus lanceolate, somewhat twisted, terminating in basally demarcate, acute unguis; extensor and outer surface with thin setae, but without spiniform setae; inner surface excavated proximally; flexor margin with 3 minute spiniform setae followed by some microscopic spiniform setae; mesial surface excavated proximally, with 3 spiniform setae adjacent to extensor margin.

Pleopod 1 uniramous, consisting of 2 articles; pleopods 2–5 narrow, each with well-developed, rod-like appendix interna, 0.3–0.4 length of endopod.

Uropod ( Fig. 2E ) with endopod about twice as long as wide, lateral margin with row of 6 small spines on posterior half of lateral margin (posterolateral spine largest); median ridge on dorsal surface with row of 4 spines, posterior marginal spine strongest. Exopod subequal in length to endopod; lateral margin slightly convex, with 5 small spines on posterior 0.6 of exopod length (including posterolateral spine) and with 1 strong spiniform seta mesial to posterolateral spine; mid-dorsal ridge unarmed, obsolete lateral ridge with 2 small spines; transverse suture serrate, with row of 6 small spines.

Paratype male. Generally similar to holotype. Rostrum as in holotype. Carapace median gastric carina bearing 2 small spines, 1 obsolete tubercle and 1 minute tubercle; submedian carinae each with 1 small spine followed by 2 minute tubercles; posterior part of lateral gastric carinae each with 1 spinule and 1 minute tubercle. Telson almost as in holotype, with 2 pairs of posterolateral spiniform setae. Maxilliped 3 armature as in holotype. Cheliped meri with ventral margins each armed with 1 strong subdistal spine followed by smaller 5 or 6 spines; occlusal margin of fixed finger with 2 rounded teeth, that of dactylus with 1 rounded tooth proximally; dorsal margins of palm each armed with 4 or 5 moderately large spines. Pereopodal armature almost as in holotype; pereopod 2 merus with 2 small spines on ventral margin; pereopod 3 merus with 2 or 3 small spines on ventral margin; pereopod 5 coxae each with gonopore. Pleopod 1 absent; pleopod 2 devoid of appendix masculina, perhaps not yet differentiated. Uropodal endopod with row of 5 or 6 small spines on lateral margin; median ridge on dorsal surface with row of 4 spines, posterior marginal spine strongest. Exopod lateral margin with 5 small spines on posterior 0.6 of its length (including posterolateral spine) and with 1 strong spiniform seta mesial to posterolateral spine; mid-dorsal ridge unarmed; dorsolateral ridge obsolete, with 2 or 3 spinules; transverse suture serrate, with 6 or 7 small spines.

Gill formula examined on paratype. Arthrobranch on maxilliped 2 rudimentary, simple; 2 arthrobranchs on maxilliped 3 to pereopod 4, all well lamellate. Pleurobranch above each base of pereopods 2–4 small, lamellate, partially hidden by arthrobranchs. Epipods present on maxilliped 1 to pereopod 4, those on maxilliped 3 to pereopod 3 slender, located inside gill chamber, each with simple or poorly lamellate (with few papillae) podobranch, those on maxillipeds 1 and 2 and pereopod 4 devoid of podobranch. Setobranchs on pereopods 1–4 well developed, each consisting of tuft of long setae located inside gill chamber.

Colour in life. Anterior part of carapace, including rostrum, whitish semi-translucent; dorsal part brownish, becoming paler laterally, with scattered red-brown spots on cardiac and branchial regions (spots on cardiac area being larger and with more intense red-brown colour); pleon semi-translucent, with tinge of brown laterally, including pleura, pleuron 2 with 2 brown spots anteriorly and posteriorly, pleura 3–5 each with brown spot posteriorly; ocular peduncle whitish semi-translucent, cornea darky pigmented; articles 1 and 2 of antennular peduncle whitish, article 3 brown, flagella pale brown; articles 1–4 of antennal peduncle whitish, article 5 brown, flagellum pale brown; chelipeds generally whitish, with tinge of yellow-brown on basal half of fingers, ventrolateral keel on palm conspicuous yellow-brown; pereopods 2–5 pale brown ( Fig. 6 ).

Etymology. From the alternative form ( paullus ) of the Latin adjective paulus, for small, little, referring to the new species’ small size.

Type locality. Moorea, Society Islands, French Polynesia.

Distribution. Presently known only from two distant localities, viz. Society Islands, French Polynesia, and Ryukyu Islands, Japan; presumably more widely distributed in the western Pacific Ocean.

Ecology. Shallow coral reefs, both on outer slope and reef flats, presently known depth range: 2–7 m; the holotype was extracted from a dead coral head ( Acropora sp.).

Remarks. The two presently available specimens of Manaxius paullus n. sp. agree well with each other in almost every diagnostic detail. The minor differences between the holotype and the paratype mentioned above all appear to be within the new species’ range of variation.

As discussed by Kensley & Hickman (2001; under Calaxius ), species of Manaxius can be divided into two groups, viz. those with ventrally or posteroventrally acute pleura and those with ventrally rounded pleura. However, in M. kensleyi , only the pleuron 2 is posteroventrally acutely pointed, whereas the pleura 3–5 are rounded; thus, this species appears to be somewhat intermediate between the two groups. Manaxius paullus n. sp. is characterised by the rounded pleura 2–5 and in therefore can be grouped together with M. andamanensis , M. euophthalmus , M. galapagensis , M. inaequalis , M. iro , M. izuensis , M. jenneri , M. mimasensis , M. pailoloensis , M. pitatucensis , M. poupini , and M. supia (cf. Kensley & Hickman 2001; Clark et al. 2007; Komai et al. 2002; Komai 2011, 2016; Poore 2020). Manaxius inaequalis and M. jenneri are very different from the other species in this group by their long ocular peduncles reaching the rostral apex (cf. Rathbun 1901; Williams 1974); in addition, M. jenneri is distinguishable by its markedly dissimilar chelipeds ( Williams 1974). Manaxius andamanensis is diagnostic in having unarmed gastric carinae, of which the median one extends posteriorly to the cervical groove ( Sakai 2015). Manaxius iro is the only species in the genus characterised by the absence of spines on the dorsal (upper) margin of the cheliped palms ( Poore 2020). Manaxius galapagensis and M. pitatucensis are characteristic in the rostrum presenting a strongly tridentate apex, as well as in the highly spinose gastric carinae on the carapace in the former ( Kensley & Hickman 2001; Sakai 2011). Manaxius pailoloensis is also unique within the genus, for instance, in having a pubescent body, a granulate carapace and only two distodorsal spines on the cheliped palm ( Rathbun 1906).

Differentiating characters among M. paullus n. sp. and the remaining five species, i.e. M. euophthalmus , M. izuensis , M. mimasensis , M. poupini and M. supia , are summarised in Table 1 . Interspecific differences are seen in the rostral shape and armature, the armature of the gastric median and submedian carinae, the development of the antennal scaphocerite and the armature of the chelipeds. The main distinguishing characters of the new species are the relatively broad rostrum, with one pair of lateral spines in addition to supraocular spines (vs. usually two or three pairs of lateral spines, often arranged asymmetrically, in other species); the presence of a subdistal spine on the dorsal margin of the cheliped ischium (vs. spineless on the dorsal margin in other species); the presence of additional three or four small spines on each cheliped palm adjacent to the dorsal margin (vs. without spines in this area in other species); and the lack of a spine near the base of fingers of each cheliped palm (vs. with one conspicuous spine in other species). Furthermore, the new species occurs in shallow waters on coral reefs, whereas the other five species are found at lower continental shelf to upper bathyal depths (see Table 1 ).

Only one species of Manaxius , M. poupini , has been hitherto recorded from French Polynesia ( Komai 2016; Poore 2020), with the present new species being the second species of the genus known from the area. Poore (2020) suggested that specimens from French Polynesia identified as Calaxius sibogae (de Man, 1925b) by Ngoc- Ho (2005) might actually represent M. poupini . Four species of Manaxius , viz. M. izuensis , M. mimasensis , M. ohsumiensis and M. pitatucensis , have been recorded so far from Japanese waters ( Sakai 1967; Komai et al. 2002; Komai 2011; Poore 2020), with M. paullus n. sp. now representing the fifth species. Poore (2020) concluded that M. ohsumiensis was conspecific with Manaxius pitatucensis sensu Kensley (2003) (= Manaxius aganaensis Sakai, 2011 ; Nomen nudum).

Figures

FIGURE 1. Manaxius paullus n. sp., holotype, female (cl 3.5 mm), FLMNH-UF 16344.A, cephalothorax and cephalic appendages, lateral view (antennal flagellum partially omitted); B, pleon, telson and left uropod, lateral view.

FIGURE 2. Manaxius paullus n. sp., holotype, female (cl 3.5 mm), FLMNH-UF 16344. A, carapace, dorsal view (setae omitted); B, anterior part of carapace and cephalic appendages, dorsal view; C, coxae of pereopods 3 and 4 and thoracic sternite 7; D, telson, dorsal view (setae omitted); E, left uropod, dorsal view (perpendicular; setae omitted).

FIGURE 3. Manaxius paullus n. sp., holotype, female (cl 3.5 mm), FLMNH-UF 16344, showing setation of chelae. A, right major chela, lateral view; B, same, mesial view; C, left minor chela, lateral view; D, same, mesial view.

FIGURE 4. Manaxius paullus n. sp., holotype, female (cl 3.5 mm), FLMNH-UF 16344, chelipeds (setae omitted except for A, F). A, right (major) cheliped, ischium and merus, lateral view; B, same, carpus and chela, lateral view; C, same, ischium and merus, mesial view; D, same, carpus and chela, mesial view; E, left (minor) cheliped, ischium and merus, lateral view; F, same, carpus and chela, lateral view; G, same, ischium and merus, mesial view; H, same, carpus and chela, mesial view.

FIGURE 5. Manaxius paullus n. sp., holotype, female (cl 3.5 mm), FLMNH-UF 16344. A, left maxilliped 3, lateral view; B, same, crista dentata on ischium, ventral view; C, left pereopod 2, lateral view; D, left pereopod 3, lateral view; E, left pereopod 4, lateral view; F, left pereopod 5, lateral view; G, same, propodus and dactylus, ventral view; H, same, dactylus, dorsal view.

FIGURE 6. Manaxius paullus n. sp., holotype, female (cl 3.5 mm), FLMNH-UF 16344. A, habitus, dorsolateral view; B, same, lateral view. Photographs by A. Anker.