Axianassa microlepis, Komai & Watanabe & Matsui & Tamego, 2020

Komai, Tomoyuki, Watanabe, Tetsuya, Matsui, Shoko & Tamego, Tomoyasu, 2020, A new species of the mud shrimp genus Axianassa Schmitt, 1924 (Decapoda Gebiidea: Laomediidae) from Japan, Zootaxa 4852 (4), pp. 461-474: 462-470

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Axianassa microlepis

n. sp.

Axianassa microlepis   n. sp.

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

( Figs. 1–7 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 View FIGURE 7 )

Material examined. Holotype: CBM-ZC 16144, female (cl 9.5 mm), Yura , Sumoto , Awaji Island , eastern part of Seto Inland, Sea, Japan, depth 8 m, 7 April 2018, airlift suction sampler, coll. Shoko Matsui and Tetsuya Watanabe.  

Paratype: CBM-ZC 16145, 1 female (cl 9.9 mm), same locality and depth as in holotype, 21 April 2018, airlift suction sampler, coll. Tomoyasu Tamego.  

Diagnosis. Rostrum roundly triangular, faintly crenulate marginally. Carapace with cervical groove passing about midlength. Pleomere 1 pleuron produced ventrally into blunt process; pleomeres 3–6 without dense pubescence on pleura. Telson evenly tapering to rounded posterior margin. Ocular peduncles almost concealed dorsally by rostrum, somewhat depressed dorsoventrally; cornea subterminal lateral in position. Antennular peduncle not reaching midlength of antennal peduncle article 4. Antennal peduncle 0.7 times as long as carapace; scaphocerite small, tapering, about twice as long as wide, distal margin rounded or with distolateral tooth and rounded or obliquely truncate distal blade. Maxilliped 3 ischium with crista dentata consisting of row of unequal sharp teeth, distalmost tooth stronger than others; merus unarmed on ventral margin; carpus also unarmed. Cheliped ischia each armed with conspicuous ventrodistal spine, followed by row of spinules; meri unarmed on ventral margins; palms granulate on mesial faces dorsally. Pereopods 3 and 4 dactyli each with single row of 4–6 spiniform setae on extensor margin. Uropodal endopod with small notch on lateral margin, without marginal or dorsal spines; exopod with tiny posterolateral spine and spiniform seta, dorsal surface unarmed.

Description. Holotype. Body ( Figs. 1A, B View FIGURE 1 ; 2A, B View FIGURE 2 ) moderately robust, integument poorly calcified, soft, surface almost glabrous.

Carapace ( Figs. 2A View FIGURE 2 ; 3A, C, D View FIGURE 3 ) with straight linea thalassinica extending along entire length and well-defined cervical groove passing 0.55 of carapace length; anterolateral margin slightly bilobed with shallow notch somewhat ventral to anterior end of linea thalassinica; pterygostomial margin broadly rounded; gastric region smooth, without longitudinal carinae, with few short setae. Rostrum ( Fig. 3 View FIGURE 3 A–C) flattened dorsoventrally, triangular with rounded tip, slightly wider than long, not reaching midlength of article 2 of antennular peduncle; lateral margins faintly sinuous, slightly crenulate; distal margin also slightly crenulate; dorsal surface slightly convex transversely, with few short setae and minute granules.

Pleon ( Figs. 1A, B View FIGURE 1 ; 2B View FIGURE 2 ) subcylindrical, pleomeres 1–5 each with 1 pair of dorsal setae posteriorly, pleomere 6 with 1 pair of tufts of dorsal setae posterior to midlength. Pleomere 1 with narrow, subtriangular pleuron terminating in blunt apex ventrally. Pleomeres 2–5 all lacking dense lateral fringe or patch of plumose setae; all pleura marginally rounded, unarmed. Pleomere 6 with posterolateral angle produced into blunt process, ventrolateral margin gently concave.

Telson ( Fig. 3E View FIGURE 3 ) roundly subtriangular in general outline, 1.3 times as long as wide, greatest width at base; lateral margins parallel in anterior one-third, tapering towards rounded posterior margin; posterior margin unarmed, without definite posterolateral angles, fringed with sparse long setae; dorsal surface without spiniform setae but with few long setae.

Ocular peduncle ( Figs. 2A View FIGURE 2 ; 3C, F View FIGURE 3 ) slightly depressed dorsoventrally, short, reaching beyond midlength of rostrum, almost concealed by rostrum, barely visible in dorsal view; distomesial portion slightly produced into blunt process; cornea located subterminal and lateral position of eyestalk, darkly pigmented in preservative.

Antennular peduncle ( Figs. 2A View FIGURE 2 ; 3C, D View FIGURE 3 ) about 0.4 times as long as carapace. Article 1 short, only partially visible in dorsal and lateral views. Article 2 short, slightly longer than wide. Article 3 subcylindrical, slender, longer than articles 1 and 2 combined, falling far short of 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, subdistal 9 articles bearing aesthetascs.

Antennal peduncle ( Figs. 2A View FIGURE 2 ; 3C, D View FIGURE 3 ) stouter and much longer than antennular peduncle, with greatly elongate article 4, total length about 0.7 times as long as carapace. Article 1 stout, with somewhat produced ventrolateral distal portion bearing excretion pore. Articles 2 and 3 short, obliquely articulated on dorsal side. Article 4 longest, subcylindrical, about half of carapace length, with sparse short to long setae ventrally. Article 5 about 0.15 length of article 4. Scaphocerite distinctly shorter than article 2 of antennal peduncle, reaching proximal 0.1 of article 4; about twice as long as wide, tapering distally towards rounded apex; lateral margin unarmed; mesial margin gently convex ( Fig. 3G View FIGURE 3 ). Flagellum reaching well beyond extended chelipeds, bearing sparse setae on distal margin of each article.

Mouthparts not dissected.

Maxilliped 3 ( Fig. 4A View FIGURE 4 ) moderately slender, pediform. Coxa armed with small subdistal spine on dorsomesial margin ( Fig. 7A View FIGURE 7 ). Basis short, unarmed. Ischium widened distally in dorsal view; ventral margin carinate; crista dentata with row of 12 unequal, acute teeth, distalmost tooth strongest ( Fig. 7B View FIGURE 7 ). Merus slightly longer than ischium, unarmed on ventral margin. Carpus short, cup-shaped, widened distally. Propodus twice as long as carpus. Dactylus 0.6 times as long as propodus, gradually tapering distally into blunt tip, with brush of dense stiff setae on distal half of flexor margin. Ventral (flexor) margins of ischium to propodus with row of long setae. Exopod absent. Epipod large, distally unequally bilobed, outer lobe broad, triangular, outer margin multidenticulate, inner lobe slender, rodlike, slightly longer than outer lobe, with sparse setae ( Fig. 4A View FIGURE 4 , inset); podobranch well developed; mastigobranch slender, rod-like.

Chelipeds (pereopods 1) slightly unequal and dissimilar, compressed laterally. Right major cheliped ( Figs. 1A, B View FIGURE 1 ; 5A, B View FIGURE 5 ) large, stout. Ischium widened distally; ventrodistal angle with conspicuous spine, followed by row of spinules on ventrolateral margin; ventromesial margin weakly delimited, with row of minute spinules not reaching distal end ( Fig. 6A View FIGURE 6 ). Merus 1.7 times as long as high; dorsal margin noticeably convex, unarmed; ventral margin strongly sinuous, unarmed, with shallow excavation in distal 0.3 to accommodate ventral margin of carpus when cheliped flexed. Carpus widened distally, subtriangular, unarmed. Chela approximately as long as carapace, 2.4 times as long as high, with slightly sinuous ventral margin in lateral view; ventral surface with low, flat keel extending from proximal to midlength of palm to near tip of fixed finger, margins of keel smooth distally, granulate and becoming obsolescent proximally on palm, each with row of sparse, distolaterally directed setae ( Fig. 6B View FIGURE 6 ). Palm 1.3 times as long as high, somewhat inflated; dorsal surface carinate in proximal 0.3 and rounded in distal 0.7, with sparse minute granules and short setae, extending onto dorsal parts of lateral and mesial surfaces; lateral and mesial surfaces each with patch of minute granules near base of fixed finger, otherwise almost smooth. Fixed finger terminating in slightly upturned, acute tip; lateral surface with distinct median carina and 2 rows of short setae adjacent to occlusal margin; mesial surface depressed below along occlusal margin; occlusal margin compressed, with row of blunt teeth of unequal size. Dactylus subequal in length to palm, terminating in gently curved acute tip, crossing tip of fixed finger; dorsal surface with 2 irregular rows of moderately long setae, dorsolateral margin distinctly carinate over entire length; dorsomesial margin bluntly carinate in proximal half, mesial surface with obsolescent median carina, lower part along occlusal margin depressed, forming excavation; occlusal margin armed with 2 conspicuous, bluntly triangular teeth, interspersed by row of small blunt teeth.

Left minor cheliped ( Fig. 5C, D View FIGURE 5 ; 6C, D View FIGURE 6 ) subequal in length to right major cheliped, but carpus and chela somewhat slender, palm less inflated. Ischium and merus similar to those of major cheliped. Carpus similar to that of major cheliped, but about 0.8 height of major cheliped carpus. Palm about 0.8 height of major cheliped palm, ornamentation and setation of palm similar to those of major cheliped palm. Fingers also slender than those of major cheliped, but similarly carinate; occlusal margin of fixed finger armed with row of small teeth, of them 2 teeth around midlength slightly larger and sharper than others. Dactylus 1.2 times as long as palm; occlusal margin with row of tiny, obsolescent teeth.

Pereopod 2 ( Fig. 4B View FIGURE 4 ) non-chelate, moderately robust, each article unarmed. Articulation between ischium and merus strongly oblique. Merus with nearly straight dorsal and gently convex ventral margins; few setae on dorsal margin, while fine row of numerous long setae on ventral margin. Carpus widened distally, cup-shaped. Propodus with slightly convex extensor and nearly straight flexor margins. Extensor and flexor margins of carpus and propodus with fine row of moderately long setae. Dactylus ( Fig. 7C View FIGURE 7 ) 0.6 times as long as propodus, slightly curved, gradually tapering to acute, corneous unguis; extensor margin gently convex, with row of numerous setae decreasing in length distally; lateral surface with sparse setae; flexor margin with row of minute spiniform setae.

Pereopod 3 ( Fig. 4C View FIGURE 4 ) moderately slender, longer than pereopod 2. Ischium, merus, and carpus unarmed, with few short setae on margins. Merus with faintly sinuous dorsal and slightly convex ventral margins. Carpus and propodus combined subequal in length to merus. Carpus widened distally. Propodus-carpus articulation somewhat twisted; propodus ( Fig. 7D View FIGURE 7 ) with both extensor and flexor margins faintly convex, flexor margin with subdistal cluster of stiff setae, possibly forming grooming apparatus. Dactylus ( Fig. 7E, F View FIGURE 7 ) 0.8 times as long as propodus, strongly compressed laterally, slightly recurved, terminating in slender corneous unguis, blade-shaped with faintly sinus extensor and gently convex flexor margins and deep constriction on flexor side near base; extensor margin with single row of 4 or 5 spiniform setae, becoming longer and more widely spaced distally; flexor margin sharply edged, extending onto mesial face subterminally, bearing comb-like row of minute spiniform setae extending from proximal 0.2 to distal 0.8 of length, extending slightly onto lateral face proximally.

Pereopod 4 ( Fig. 4D View FIGURE 4 ) generally similar to, but about 0.8 length of pereopod 3. Articulation between carpus and propodus somewhat twisted. Propodus ( Fig. 7G View FIGURE 7 ) with slightly arcuate extensor margin and nearly straight flexor margin; flexor distal cluster of stiff setae present, but setae shorter and fewer than in pereopod 3. Dactylus ( Fig. 7H View FIGURE 7 ) with single row of 5 or 6 spiniform setae on extensor margin, becoming longer and more widely spaced distally; flexor margin extending onto mesial face subdistally, with comb-like row of minute spiniform setae.

Pereopod 5 ( Fig. 4E View FIGURE 4 ) slightly more slender than pereopods 3 and 4, subequal in length to pereopod 4. Carpus subequal in length to propodus; extensor margin gently arcuate, flexor margin straight. Propodus ( Fig. 7I View FIGURE 7 ) slightly widened distally, flexor distal margin not particularly produced; grooming apparatus consisting of field of dense short setae on distal 0.8 of flexor margin extending onto distolateral and distomesial margins and row of longer setae in similar extent of field of short setae. Dactylus ( Fig. 7J View FIGURE 7 ) lanceolate, twisted, terminating in slender, acuminate unguis; inner surface excavated proximally; extensor margin unarmed; flexor margin gently convex, forming thin edge, unarmed.

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

Pleopod 1 ( Fig. 2B View FIGURE 2 ) small, directed mesially, consisting of protopod and single ramous. Pleopods 2–5 ( Fig. 1A View FIGURE 1 ) similar to each other, slender, biramous, devoid of appendices internae.

Uropod ( Fig. 3H View FIGURE 3 ) with short protopod, dorsodistal margin bilobed, distolateral margin blunt. Exopod oval, devoid of diaeresis (transverse suture); outer margin with minute spiniform seta just mesial to tiny posterolateral spine; margins fringed with numerous setae. Endopod also oval, 1.7 times as long as wide, subequal in length to exopod, overreaching posterior margin of telson; margins fringed with row of short to long setae, outer margin with small but distinct notch somewhat distal to midlength; dorsal surface unarmed, with shallow excavation basally.

Paratype. Generally similar to holotype. Scaphocerite having distolateral tooth and obliquely truncate distal blade (left; Fig. 3I View FIGURE 3 ), or having tiny distolateral tooth and rounded distal blade (right, Fig. 3J View FIGURE 3 ); lateral margin slightly concave, with small proximal tubercle ( Fig. 3I, J View FIGURE 3 ).

Variation. Notable variation is seen in the shape of the antennal scaphocerite, as described above. In the paratype, the shape of the antennal scaphocerite is different from the left to the right ( Fig. 3I, J View FIGURE 3 ). Other than the structure of the scaphocerite, the paratype is almost identical with the holotype. It seems reasonable to consider that the observed differences in the scaphocerite represent intraspecific variation.

Colour in life. Body whitish semitranslucent; hepatopanclea visible inside of cephalothorax yellow; ovary, extending from posterior cephalothorax to pleomere 5 orange; antennae and pereopods 2–5 whitish semitranslucent; chelipeds ivory white ( Fig. 1A, B View FIGURE 1 ).

Type locality. Yura , Sumoto, Awaji Island, at depth of 8 m.

Distribution. Presently known only from Yura, Sumoto, Awaji Island.

Habitat. Muddy bottom. Four specimens of the alpheid shrimp Automate awaji Komai, Tamego & Hanano, 2020   were collected sympatrically ( Komai et al. 2020), but not together with the two type specimens of the new species. A parallel case of sympatric occurrence of Automate   ( A. cf. evermanni Rathbun, 1901   ) and Axianassa   ( A. australis Rodrigues & Shimizu, 1992   ) was reported by Dworschak & Coelho (1999), although no evidence of association between the two species was found.

Etymology. From the combination of the Greek, mikros (= small) and lepis (= scale), in reference to the characteristic small antennal scaphocerite (= antennal scale) of the new species.

Remarks. Because of the absence of a diaeresis (transverse suture) on the uropodal exopod and the non-marginally dentate rostrum (only crenulate), the new species is assigned to Axianassa   ( Sakai 2016; Komai & Fujita 2019). The shape of the antennal scaphocerite of A. microlepis   n. sp. is unique within the genus, although its shape is rather variable, as described above: it is distinctly shorter than the antennal peduncle article 2, and never dagger-shaped. In all but A. mineri Boone, 1931   and A. planioculus   , the antennal scaphocerite is slightly to distinctly longer than the article 2 of antennal peduncle, and is dagger-like in the shape (cf. Kensley & Heard 1990; Anker 2010; Anker & Lazarus 2015; Anker & Pachelle 2016). In A. mineri   and A. planioculus   , the antennal scaphocerite is short as in the new species, but distally bifid into two sharp teeth (cf. Kensley & Heard 1990; Komai & Fujita 2019). Other unique features of the new species include: rostrum weakly crenulate marginally; and uropodal endopod with small notch on the lateral margin. In other congeneric species, the rostral margins are smooth; and there is no notch on the lateral margin of the uropodal endopod ( Kensley & Heard 1990; Rodrigues & Shimizu 1992; Anker 2010; Liu & Liu 2010; Anker & Lazarus 2015; Anker & Pachelle 2016; Komai & Fujita 2019).

In Japanese waters, only A. planioculus   is known as a member of Axianassa   , originally described from Ishigaki Island, southern Ryukyu Islands ( Komai & Fujita 2019). In addition to the features above mentioned, A. microlepis   n. sp. differs from A. planioculus   in the almost glabrous carapace and pleon (versus bearing scattered erect setae on the dorsal surface in A. planioculus   ), the more anteriorly passing cervical groove (0.5 of the carapace length versus 0.7 in A. planioculus   ), the roundly triangular telson (versus suboval in A. planioculus   ), the unarmed ventral margin of the maxilliped 3 merus (versus armed with a subdistal spine on the ventral margin in A. planioculus   ), the presence of a distinct ventrodistal spine on the cheliped ischium (versus no spine in A. planioculus   ), the possession of a single row of spiniform setae on the extensor margins of the pereopods 3 and 4 dactyli (versus spiniform setae are arranged in two rows in A. planioculus   ) and the presence of a small spine and minute spiniform setae on the outer margin of the uropodal exopod (versus unarmed in A. planioculus   ) (cf. Komai & Fujita 2019).

The other western Pacific congeners are A. ngochoae   and A. sinica   . In addition to the above mentioned features, the new species differs from A. ngochoae   and A. sinica   in the following characters (cf. Anker 2010; Liu & Liu 2010): each cheliped ischium is armed only with a distinct ventrodistal spine in A. microlepis   n. sp. (versus there are two or three conspicuous subdistal spines in A. ngochoae   and A. sinica   ); the major cheliped merus is unarmed on the ventral margin in A. microlepis   n. sp. (versus armed with a row of spinules in A. ngochoae   , with one conspicuous subdistal spine followed by a row of much smaller spines in A. sinica   ). Axianassa ngochoae   further differs from the new species in having three small spines in addition to the posterolateral spine on the outer margin of the uropodal exopod (versus only with the posterolateral spine in A. microlepis   n. sp.). Axianassa sinica   also further differs from the new species in the triangular rostrum with an acute tip (versus rounded in A. microlepis   n. sp.).

TABLE 1. Axianassa microlepis n. sp., gill formula. Gills/appendages Maxillipeds Pereopods

Gills/appendages   Maxillipeds       Pereopods    
  12312345 12312345 12312345 12312345 12312345 12312345 12312345 12312345
Pleurobranchs 0 0 0 0 0 0 0 0
Arthrobranchs 1 2 2 2 2 2 2 0
Podobranchs 0 1 1 1 1 1 0 0
Epipods 1 1 1 1 1 1 1 0
Mastigobranchs 0 0 1 1 1 1 0 0
Exopod 1 1 0 0 0 0 0 0
Setobranchs 0 0 0 0 0 0 0 0