Axianassa planioculus, Komai & Fujita, 2019

Komai, Tomoyuki & Fujita, Yoshihisa, 2019, A new species of the mud shrimp genus Axianassa Schmitt, 1924 (Decapoda: Gebiidea: Laomediidae) from the Ryukyu Islands, southern Japan, Zootaxa 4658 (3), pp. 452-460: 453-459

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Axianassa planioculus

n. sp.

Axianassa planioculus   n. sp.

[New Japanese name: Kabira-suna-syako-ebi]

( Figs. 1–5 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 )

Material examined. Holotype: male (cl 5.5 mm), southernmost part of Kabira Bay , Ishigaki Island , Yaeyama Islands , Ryukyu Islands (Monitoring Sites 1000 site number TFKBR-BU, 24°26'35"N, 124°08'21"E), intertidal sand/ mud flat, possibly from a probable echiuran borrow, steel suction pump (Alvey Co. Ltd), 20 August 2017, coll. Y. Fujita, CBM-ZC 15348. GoogleMaps  

Description of holotype. Body ( Fig. 1 View FIGURE 1 ) with sparse long, erect setae on dorsal surface; integument poorly calcified, soft.

Carapace ( Fig. 2 View FIGURE 2 A–C) with straight linea thalassinica extending along entire length and well-defined cervical groove passing posterior to midlength of carapace (at about 0.7 carapace length); anterolateral margin bilobed with shallow notch with ventral to anterior end of linea thalassinica; pterygostomial margin broadly rounded; gastric region smooth, without longitudinal carinae. Rostrum flattened dorsoventrally, triangular with rounded tip, wider than long, reaching midlength of article 2 of antennular peduncle; lateral margins nearly straight, unarmed; dorsal surface slightly convex transversely, with scattered short setae.

Pleon ( Fig. 1 View FIGURE 1 ) slightly compressed laterally. Pleomere 1 ( Fig. 2D View FIGURE 2 ) with subtriangular pleuron terminating in subacute point ventrally. Pleomeres 2–5 all lacking dense lateral fringe or patch of plumose setae; all pleura marginally rounded, unarmed. Pleomere 6 with posterolateral angle not markedly produced, ventrolateral margin sinuous. Telson ( Fig. 2E View FIGURE 2 ) suboval in general outline, 1.3 times longer than wide, greatest width at anterior one-fourth, narrowing posteriorly; posterior margin broadly rounded, unarmed, fringed with sparse long setae, merging into gently convex lateral margins; dorsal surface without spiniform setae but with scattered long setae.

Eyestalks ( Fig. 2B View FIGURE 2 ) depressed dorsoventrally, short, reaching midlength of rostrum, concealed by rostrum, thus not visible in dorsal view; cornea occupying entire terminal portion of eyestalk, lightly pigmented in alcohol-preserved condition.

Antennular peduncle ( Fig. 2 View FIGURE 2 A–C) slightly longer than half-length of carapace. Article 1 short, not visible in dorsal and lateral views. Article 2 also short. Article 3 elongate, subcylindrical, slender, slightly overreaching midlength of article 4 of antennal peduncle. Dorsal flagellum stouter and longer than ventral flagellum, each article of dorsal flagellum bearing some short setae on distal margin (about as long as 1 article length), subdistal 6 articles bearing aesthetascs.

Antennal peduncle ( Fig. 2 View FIGURE 2 A–C) stouter than antennular peduncle, with greatly elongate article 4, total length about 0.8 times as long as carapace. Article 1 stout, with somewhat produced ventrodistal margin with excretion pore. Articles 2 and 3 short. Article 4 subcylindrical, about 0.6 of carapace length, with sparse short to long setae. Article 5 less than 0.2 length of article 4. Scaphocerite small, slightly narrowing and bi-spined distally ( Fig. 2F View FIGURE 2 ), slightly reaching distal margin of peduncular article 2, about twice as long as wide; no additional spine on mesial margin. Flagellum reaching well beyond extended chelipeds, bearing sparse setae.

Mouthparts not dissected. Maxillipeds 1 and 2 each with well-developed epipod and associated podobranch.

Maxilliped 3 ( Fig. 4A View FIGURE 4 ) slender, pediform. Coxa unarmed. Basis short. Ischium widened distally in dorsal view; ventral margin carinate; crista dentata with row of 14 acute unequal teeth, middle tooth longest ( Fig. 5B View FIGURE 5 ). Merus slightly longer and wider than ischium, with small subdistal spine on carinate ventral margin. Carpus short, not particularly widened distally. Propodus about twice length of carpus. Dactylus 0.6 times as long as propodus, gradually tapering distally to blunt tip. Ventral margins of ischium to propodus with row of long setae. Exopod absent. Epipod ( Fig. 5A View FIGURE 5 ) large, distally bilobed, outer lobe broad, marginally multidenticulate, partially embracing podobranch, inner lobe slender, rod-like, longer than outer lobe, with sparse setae; podobranch well developed; mastigobranch slender, rod-like.

Left cheliped (pereopod 1) missing. Right cheliped ( Fig. 3A, B View FIGURE 3 ) compressed laterally. Ischium ( Fig. 3C View FIGURE 3 ) not particularly widened distally, with row of setae on ventral margin; ventromesial margin with row of minute granules. Merus 1.9 times as long as high; dorsal margin strongly convex, unarmed; ventral margin slightly convex, also unarmed, with shallow excavation in distal half to accommodate ventral margin of carpus when cheliped flexed. Carpus widened distally, subtriangular, unarmed. Chela about approximately as long as carapace. Palm elongate subrectangular with broadly convex proximoventral margin, about 2 times as long as high; dorsal surface rounded, almost smooth, with row of sparse short setae; lateral and mesial surfaces smooth, with few setae; ventral surface with low, flat keel with both lateral and mesial edges weakly delimited, each edge with row of long setae extending onto fixed finger. Fixed finger terminating in slightly curved, acute tip, with row of tufts of setae on lateral surface adjacent to ventral margin; lateral surface with blunt longitudinal carina adjacent to occlusal margin; occlusal margin compressed, with row of irregular, low teeth. Dactylus 0.9 times as long as palm, terminating in gently curved, acute tip crossing tip of fixed finger; dorsal surface with 2 blunt carinae and with 2 rows of long setae arising narrow space between carinae; occlusal margin compressed, faintly denticulate.

Pereopods 2–5 decreasing in length posteriorly. Pereopod 2 ( Fig. 4B View FIGURE 4 ) non-chelate, moderately robust, each article unarmed. Articulation between ischium and merus strongly oblique. Merus with slightly concave dorsal and gently convex ventral margins. Carpus widened distally, cup-shaped. Propodus with both dorsal and ventral margins gently convex. Dactylus ( Fig. 5C View FIGURE 5 ) about 0.6 times as long as propodus, gradually tapering distally to acute, corneous claw; dorsal margin gently convex, with row of numerous setae decreasing in length distally; lateral surface with 2 longitudinal rows of setae; ventral (flexor) margin nearly straight, bearing row of closely spaced minute spiniform setae. Dorsal and ventral margins of merus to propodus with row of long setae (ventral setae generally longer than dorsal setae).

Pereopod 3 ( Fig. 4C View FIGURE 4 ) moderately slender. Ischium, merus, and carpus unarmed, with sparse setae on margins (those on dorsal setae of merus very short). Merus with faintly sinuous dorsal and slightly convex ventral margins. Carpus and propodus combined subequal in length to merus. Carpus slightly widened distally. Propodus with both dorsal and ventral margins slightly convex, ventral margin with subdistal cluster of stiff setae, possibly forming grooming apparatus ( Fig. 5D View FIGURE 5 ). Dactylus ( Fig. 5D View FIGURE 5 ) slightly shorter than propodus, strongly compressed laterally, terminating in slender corneous unguis, blade-shaped with faintly sinus extensor and gently convex flexor margins; extensor margin bearing numerous spiniform setae arranged in irregular 3 rows and increasing in size distally; flexor margin with comb-like row of minute spiniform setae extending from proximal 0.3 to distal 0.4 of length, extending onto lateral surface proximally.

Pereopod 4 ( Fig. 4D View FIGURE 4 ) generally similar to pereopod 3. Propodus with ventrodistal cluster of stiff setae, but setae shorter and fewer than in pereopod 3 ( Fig. 5E View FIGURE 5 ). Dactylus ( Fig. 5E View FIGURE 5 ) with spiniform setae on extensor margins arranged in irregular 2 rows; flexor margin with comb-like microscopic setae not extending onto lateral surface.

Pereopod 5 ( Figs. 4E View FIGURE 4 , 5F View FIGURE 5 ) slightly more slender than pereopods 3 and 4. Propodus slightly widened distally, flexor distal margin not particularly produced. Grooming apparatus consisting of field of dense short setae on distal 0.7 of flexor margin extending onto distolateral and distomesial margins and row of longer setae in similar extent of field of short setae. Dactylus ( Fig. 5G, H View FIGURE 5 ) lanceolate, twisted, slightly recurved, terminating in slender, acuminate unguis; inner surface excavated; extensor margin unarmed; flexor margin convex, forming thin edge.

Gill/exopod/setobranch formula summarized in Table 1 View TABLE 1 . Pleurobranchs absent. Arthrobranchs present as following: 1 on first maxilliped, 2 each on second maxilliped through fourth pereopods, all well-developed, multilamellate; gills trichobranchiate, lamellae slender, rod-like. Epipods on maxillipeds 1 and 2 small. Epipod on maxilliped 3 as described above. Epipods on pereopods 1–3 similar in structure to that on maxilliped 3 (cf. Fig. 5A View FIGURE 5 ); mastigobranchs slender, also rod-like; podobranchs all multilamellate. Epipod on pereopod 4 smaller than preceding ones, consisting of 2 slender, unequal rod-like processes; no podobranch or mastigobranch. Setobranchs absent. No exopods on pereopods.

Pleopod 1 short, directed mesially, consisting of 2 articles ( Fig. 2D View FIGURE 2 ). Pleopods 2–5 ( Fig. 1 View FIGURE 1 ) similar to each other, slender, biramous, devoid of appendices internae. Uropod ( Fig. 2G View FIGURE 2 ) with short, unarmed protopod. Exopod oval, without transverse suture; margins with row of sparse, short to long setae, terminal margin with 3 minute spiniform setae; dorsal surface with weak median ridge. Endopod also oval, subequal in length to exopod, slightly overreaching posterior margin of telson; margins with row of short to long setae, some of them plumose, without conspicuous spine or spiniform setae; dorsal surface unarmed, with shallow excavation basally.

Colour in life. Body whitish, covered with brown particles derived from substrates; appendages generally white.

Distribution. At present, known only from Kabira Bay, Ishigaki Island, southern Ryukyu Islands; intertidal.

Remarks. The body surface of the holotype was partly covered with inorganic brownish matter apparently derived from the microhabitat sediments ( Fig. 1 View FIGURE 1 ). A similarly adherent, difficult-to-detach, brown-orange or darkbrown covering has been observed in other species of Axianassa   ( Anker 2010; Anker & Pachelle 2016).

The present new species appears morphologically most similar to the eastern Pacific A. mineri Boone, 1935   , known from Panama, Costa Rica and Mexico ( Anker & Pachelle 2016). The two species share the presence of an unarmed short rostrum; distally bidentate antennal scaphocerite; and the absence of a transverse suture on the uropodal exopod. Nevertheless, A. planioculus   n. sp. is easily distinguished from A. mineri   by the better developed rostrum concealing the eyestalks in the dorsal view (vs. broadly rounded, with the eyestalks exposed in the dorsal view), the suboval telson (vs. subtrapezoidal), the terminal position of the cornea on the eyestalks (vs. subterminal), the possession of only one spine on the ventral margin of the maxilliped 3 ischium (vs. three spines present), the unarmed carpus of the maxilliped 3 (vs. armed with one conspicuous spine at ventrodistal angle), the unarmed ventral margin of the cheliped merus (versus armed with one small spine), the better developed teeth on the occlusal margins of the pereopod 1 fingers, the unarmed uropodal endopod (vs. armed with one dorsal and one posterior marginal spines) and the uropodal exopod bearing only a few minute spiniform setae on the posterior margin (vs. some spiniform setae and spines present on posterolateral to posterior margin) ( Kensley & Heard 1990: 563: fig. 4; Anker & Pachelle 2016: fig. 1).

Two other species of Axianassa   are currently known from the western Pacific, viz., A. ngochoae Anker, 2010   and A. sinica Liu & Liu, 2010   , none of them yet recorded in Japanese waters.As mentioned above, Axianassa heardi   and A. japonica   were transferred to Heteroaxianassa   by Sakai (2016). Axianassa planioculus   n. sp. is readily distinguished from the two congeneric species by the short, bi-spined antennal scaphocerite and the possession of two rows of spiniform setae on the extensor margins of the pereopods 3 and 4 dactyli. In A. ngochoae   and A. sinica   , the antennal scaphocerite is elongate and dagger-like in the shape; the pereopods 3 and 4 dactyli each has a single row of spiniform setae on the extensor margin. All these three species are found in intertidal and shallow subtidal mudsand flats down to 33 m. The new species thus represents the third western Pacific species of Axianassa   , the first to be recorded from Japan.

Etymology. From the combination of the Latin, planus (flat) + oculus (eye), in reference to the flattened eyestalk of the new species. Used as a noun in apposition.