Alpheus ikedosoma, Komai, Tomoyuki, 2015

Komai, Tomoyuki, 2015, A new species of the snapping shrimp genus Alpheus (Crustacea: Decapoda: Caridea: Alpheidae) from Japan, associated with the innkeeper worm Ikedosoma elegans (Annelida: Echiura: Echiuridae), Zootaxa 4058 (1), pp. 101-111 : 102-109

Alpheus ikedosoma n. sp.

[New Japanese name: Yume-yumushi-teppou-ebi] Figs. 1–5

Material examined. All the specimens examined were extracted from burrows of echiuran worm near water edge at low tide by using yabby pump. Holotype: Hojyo Beach, Tateyama, Boso Peninsula, central Japan, 35°00.11’N, 139°51.30’E, coll. T. Komai, 30 July 2015, male (cl 8.8 mm), CBM-ZC 13128.

Allotype: spouse of male holotype, ovigerous female (cl 11.1 mm), CBM-ZC 13129.

Paratypes: Tatara Beach, Minami-Boso, Boso Peninsula, 35°02.17’N, 139°49.29’E, coll. T. Komai, 22 August 2013, 1 male (cl 8.0 mm), 1 ovigerous female (cl 9.6 mm), CBM-ZC 13130; same data as holotype and allotype, 1 male (cl 8.4 mm), 1 female (cl 11.2 mm), CBM-ZC 13131; same locality as holotype and allotype, 31 August 2015, coll. M. Tanaka, 1 male (cl 9.6 mm), 1 ovigerous female (cl 9.7 mm), CBM-ZC 13132; Hondo flat, Amakusa, Kumamoto Prefecture, Ariake Sea, Kyushu, 32°27.09’N, 130°12.19’E, 11 May 2013, 3 males (cl 6.5–8.2 mm), 6 females (cl 6.9–10.5 mm), 1 ovigerous female (cl 9.4 mm), CBM-ZC13133; same data, 1 male (cl 8.0 mm), 1 ovigerous female (cl 9.8 mm), KMNH IvR 500845; same locality, 12 May 2013, 2 females (cl 9.7, 10.7 mm), CBM-ZC 13134; Okoshiki Beach, Uto, Kumamoto Prefecture, Ariake Sea, 32°39.55’N, 130°32.13’E, 13 May 2013, coll. T. Komai, 2 ovigerous females (cl 11.7, 12.2 mm), CBM-ZC 13135.

Description. Males. Body ( Fig. 1 A) relatively slender; integument rather soft, surface glabrous.

Rostrum ( Fig. 2 A–D) very short, falling far short of midlength of first segment of antennular peduncle, triangular in dorsal view, slightly to strongly curved downward, distally acute or subacute; dorsal surface strongly sloping, non-carinate. Carapace ( Fig. 2 A, B, D) smooth, glabrous, moderately compressed laterally; postrostral carina and adrostral grooves absent; dorsum nearly straight in lateral view; orbital hoods not inflated, anteriorly unarmed; anterior margin between rostrum and orbital hoods nearly straight; pterygostomial angle (Fig.) broadly rounded; cardiac notch minute ( Fig. 2 E).

Pleon ( Fig. 1 A) without distinctive features; pleura broadly rounded marginally; sixth pleomere without articulated flap, preanal area not protruding as tooth. Telson ( Fig. 2 F) relatively broad, subtrapezoidal, narrowing posteriorly; lateral margins without concavity; dorsal surface with 2 pairs of minute dorsolateral spines, situated at some distance from lateral margins, first pair located at 0.6 of telson length, second pair at 0.8; posterior margin broadly rounded, with 2 pairs of unequal spines at posterolateral angles (mesial spine much longer than lateral spine), and with row of long plumose setae between posterolateral spines.

Eyes ( Fig. 2 A–D) concealed in dorsal view, partially visible in lateral view, anteromesial margin produced, with minute sharp tubercle; cornea small, darkly pigmented. Ocellar beak not conspicuously protruding between eyes.

Antennular peduncle ( Fig. 2 A, B, D) moderately stout, overreaching distal margin of scaphocerite (antennal scale). First segment with ventromesial carina forming large crest-like lamina; stylocerite somewhat inflated, terminating in blunt or subacute tip, slightly falling short of or reaching distal margin of first peduncular segment. Second segment about 2.0 times as long as broad. Lateral flagellum with numerous tufts of aesthetascs on about 10 articles; secondary ramus rudimentary.

Antenna ( Fig. 2 A, B, D) with basicerite unarmed at ventrolateral distal angle or dorsolateral distal angle.

Carpocerite stout, overreaching lamella of scaphocerite. Scaphocerite ( Fig. 2 G) short, 0.2–0.3 times as long as carapace and about 2.6 times as long as wide, greatest width at proximal 0.2 of length; lateral margin faintly sinuous, terminating in small distolateral tooth slightly falling short of distal margin of lamella; cleft between lamella and distolateral tooth slit-like, deep; lamella broadly rounded distally.

Mouthparts typical for Alpheus (only external observation made, not figured). Third maxilliped ( Fig. 3 A–C) moderately stout, slightly overreaching distal end of antennal carpocerite. Lateral plate on coxa extending to base of exopod, with subacute point. Antepenultimate segment strongly flattened dorsoventrally in proximal half, with sharply carinate lateral and mesial margins; ventral surface with obtuse ridge adjacent to mesial margin. Penultimate segment (= carpus) slightly flattened dorsoventrally, about 2.6 times as long as wide in dorsal or ventral view, with 3 long spiniform setae on distal half of mesial surface. Ultimate segment tapering, trigonal in cross section, tip blunt with tufts of long setae.

Major cheliped ( Figs. 1 , 4 A–E) much larger than minor cheliped. Ischium short, unarmed. Merus moderately stout, broadened distally, trigonal in cross-section; dorsal margin nearly straight, dorsodistal margin not produced; ventrolateral margin faintly sinuous, unarmed; ventromesial margin straight or faintly sinuous, armed usually with small subdistal tooth and 1 minute movable spinule (entirely unarmed in holotype). Carpus very short, cup-shaped. Chela strongly compressed, oval in cross section, slightly elongate, about 3.0–3.2 times as long as wide, 1.1–1.4 times as long as carapace; lateral margin nearly straight and mesial margin slightly sinuous, greatest width at proximal 0.4 of palm. Palm about 2.2 times as long as wide, without any grooves or depressions on surfaces; surfaces smooth, glabrous, ventral surface slightly inflated. Pollex less than half-length of palm, terminating in upturned, acute tip; dorsal side of socket forming convexity bearing row of prominent setae. Dactylus slightly arcuate in longitudinal plane, not double-headed, tip acuminate, crossed with pollex; plunger much reduced, obliquely truncate, defined only by proximal angle.

Minor cheliped ( Figs. 1 , 4 F–I) not balaeniceps form, reaching level of midlength of left major chela. Ischium similar to that of major cheliped. Merus with dorsal margin faintly sinuous, dorsodistal margin not produced; ventrolateral margin faintly convex; ventromesial margin also faintly convex, small subterminal tooth present or absent, no movable spinules. Carpus short, cup-shaped, similar to that of major cheliped. Chela slender, subcylindrical, about 4.7 times as long as wide, without any depressions or grooves; mesial margin faintly sinuous. Palm about 2.0 times as long as wide, surfaces almost glabrous, mesial face slightly inflated proximally; lateral margin slightly convex. Fingers slender, about 1.5 times as long as palm, with strongly curved, crossing tips; ventral side forming longitudinal excavation; occlusal margins each bordered with thin, chitinous blade.

Second pereopod ( Fig. 3 D) slender, overreaching distal end of antennal carpocerite by length of chela. Ischium subequal in length to merus; carpus divided into five articles, with approximate ratio 1: 0.4: 0.2: 0.2: 0.4; chela about 0.25 times as long as carpus, fingers longer than palm.

Third pereopod ( Fig. 3 E–H) moderately slender, overreaching distal end of antennal carpocerite by length of dactylus. Ischium with 1 small movable spine on lateral surface ventrally. Merus unarmed, about 7.0 times as long as wide. Carpus unarmed. Propodus 1.1–1.2 times as long as carpus, armed with 2 widely spaced, slender movable spines in proximal half of ventral (flexor) margin and with pair of unequal movable spines at ventrodistal margin. Dactylus subspatulate, terminating in acute tip, about half as long as propodus; extensor surface with only few short setae, flexor surface glabrous.

Fourth pereopod ( Fig. 3 I, J) similar to third pereopod, but slightly shorter, not reaching distal end of antennal carpocerite.

Fifth pereopod (Fig. K) shorter and slenderer than third and fourth pereopods. Ischium unarmed or armed with 1 movable spine on lateral face ventrally. Propodus with grooming brush consisting of several transverse rows of stiff setulose setae on distal 0.6 length, one small spiniform seta present at ventrodistal angle (usually concealed by grooming setae, not visible in lateral view); dactylus narrower than those of third and fourth pereopods.

Male second pleopod with appendix masculina ( Fig. 2 H) subequal in length to appendix interna, furnished with numerous stiff setae on dorsal surface to apex.

Uropod ( Fig. 2 I) with protopod armed with 2 large acute distal teeth. Exopod with slightly convex lateral margin, terminating in small posterolateral tooth, with adjacent 2 movable spinules; diaeresis faintly trilobate; distal margin with row of minute spinules dorsal to main setal fringe. Endopod with scattered distal margin with row of minute spinules dorsal to main setal fringe.

Gill/exopod formula typical for Alpheus .

Females. Non-spawning females generally similar to males. In spawning molt, rostrum acute to blunt; dorsum of carapace becoming weakly convex along with development of ovary. Pleon fairly widened ( Fig. 1 B). Major cheliped ( Fig. 5 A, B) comparatively small, with chela 0.8–1.0 times as long as carapace; tip of fingers blunt. Minor chela ( Fig. 5 C, D) stouter than in males, with palm about 1.7 times as long as wide; fingers 1.3–1.4 times as long as palm.

Size. Largest male cl 8.8 mm, largest female cl 12.2 mm, ovigerous females cl 9.6–12.2 mm.

Colouration in life. Males: Entirely semitranslucent, with tinge of pink on posterior part of each pleomere and distal part of major cheliped; eyes dark gray; yellowish hepatopancrea visible through integument.

Females: More pinkish than in males, particularly in anterior part of carapace, pleon and chelipeds; ovary orange; eggs also orange.

Variation. As is apparent from the above description, sexual dimorphism influences the stoutness of the body, the shape of the dorsum of the carapace and the relative length of the chelipeds. Females seem to attain larger size than males do.

Symbiotic association. All the specimens of shrimp examined were extracted from burrows of echiuran worm constructed on easily accessible intertidal sand beaches or sand flats, with the aid of a bait suction pump. Specimens of the host worm, collected at Boso Peninsula, were identified with Ikedosoma elegans ( Ikeda, 1904) by Dr. Masaatsu Tanaka (Faculty of Science, Toho University) ( Fig. 6 A). Five heterosexual pairs of the present new species were collected from two nearby locations in Boso Peninsula (Tatara and Hojo beaches). One pair was found to inhabit one burrow.

Specimens collected at Hondo flat, Amakusa, and Okoshiki, Uto, both in Kumamoto Prefecture, were not always paired, but this may be due to sampling condition. The host worms collected at these two localities in Kumamoto Prefecture were not preserved, but the specimen photographed at Hondo flat is very similar to I. elegans ( Fig. 6 B).

Distribution. Presently known only from Japanese main islands: Boso Peninsula (Honshu, Chiba Prefecture), and Amakusa and Uto, Ariake Sea (Kyushu, Kumamoto Prefecture); intertidal. The host innkeeper worm Ikedosoma elegans has been recorded only in Japan, viz., Misaki, Sagami Bay (Kanagawa Prefecture), Hamana Lake (Shizuoka Prefecture), and Takasu, Seto Inland Sea (Okayama Prefecture) ( Tanaka et al. 2014), and it could be expected that the present new species possibly occurs in those localities with records of the host worm.

Remarks. The present new species is tentatively placed in the Alpheus brevirostris species group because of the compressed major chela without conspicuous sculpture or armature and the subspatulate dactyli of the third and fourth pereopods ( Banner & Banner 1982; Chace 1988; Bruce 1994). The species group is currently represented by 44 species ( Bruce 1994; Hayashi & Nagata 2000, 2002; Anker & Dworschak 2004; Anker et al. 2007), several of them burrowing soft sediments or associated with gobiid fishes ( Banner & Banner 1982; Karplus 1987; Bruce 1994). In spite of the group assignment, A. ikedosoma n. sp. does not exhibit close similarities to any of the 44 species in this informal species group. It is characterized by a suit of characters, including the very short, often down-curved rostrum without a dorsal ridge, the absence of adrostral grooves on the frontal region of the carapace, the strongly reduced dorsolateral spines on the telson, the lack of a ventrolateral distal tooth on the antennal basicerite, and the almost glabrous major chela without setal rows on margins. The real relationship of the present new species remains unclear.

In the general shape of the major cheliped, A. ikedosoma n. sp. is superficially similar to A. mitis Dana, 1852 and A. paracrinitus Miers, 1881 , currently referred to the A. diadema Dana, 1852 species group, but the latter two species have a less compressed major chela and subconical, simple dactyli of the third and fourth pereopods (cf. Banner & Banner 1982).

As mentioned above, association with echiuran burrows has been reported for the five species of Alpheus , and the present new species represents the sixth example of the association. Of the five species, four are morphologically similar for one another within the A. brevirostris group (constituting the A. barbatus species complex; see Anker et al. 2007). Nevertheless, A. ikedosoma n. sp. is not closely related to the A. barbatus species complex, because it is primarily distinguished from the latter species complex by the lack of a transverse groove or cleft on the palm of the major cheliped proximal to the dactylar articulation (versus a distinct groove or cleft is present) and the subspatulate dactyli of the third and fourth pereopods (versus the dactyli are subconical in the shape) ( Anker et al. 2007). Alpheus echiurophilus , another representative of the echiuran associate, belongs to the separate species group, A. leviusculus species group, suggesting a remote phylogenetic relationship to the other echiuran associates.

Etymology. The new species is named after the host animal, Ikedosoma elegans. Used as a noun in apposition.

Figures

FIGURE 1. Alpheus ikedosoma n. sp. A, holotype, male (cl 8.8 mm), CBM-ZC 13128; B, allotype, female (cl 11.1 mm), spouse of male holotype, CBM-ZC 13129.

FIGURE 2. Alpheus ikedosoma n. sp., holotype, male (cl 8.8 mm), CBM-ZC 13128. A, carapace and cephalic appendages, lateral view; B, anterior part of carapace and left antennular and antennal peduncle, lateral view; C, close up of rostrum and left eye, lateral view; D, anterior part of carapace, and antennular and antennal peduncles, dorsal view (marginal setae on left antennal scale omitted); E, posterior margin of carapace, lateral view; F, telson, dorsal view (setae omitted); G, left antennal scale, dorsal view (setae omitted); H, appendices internal and masculina on left second pleopod, dorsal view; I, right uropod, dorsal view (perpendicular) (marginal setae on rami omitted). Scale bars: 2 mm for A; 1 mm for B, D – G, I; 0.5 mm for C, H.

FIGURE 3. Alpheus ikedosoma n. sp., holotype, male (cl 8.8 mm), CBM-ZC 13128, left thoracic appendages. A, third maxilliped, lateral view; B, same, coxa, exopod and antepenultimate segment of endopod, ventral view (setae omitted); C, same, penultimate segment (= carpus), ventral view; D, second pereopod, lateral view; E, third pereopod, lateral view; F, same, ischium, ventral view; G, same, propodus and dactylus, lateral view; H, same, distal part of propodus and dactylus, flexor view; I, fourth pereopod, lateral view; J, same, ischium, ventral view; K, fifth pereopod, lateral view. Scale bars: 1 mm for A – F, I – K; 0.5 mm for G, H.

FIGURE 4. Alpheus ikedosoma n. sp., holotype, male (cl 8.8 mm), CBM-ZC 13128. A, left major cheliped (first pereopod), ischium to carpus, lateral view; B, same, mesial view; C, same pereopod, chela, dorsal (outer) view; D, same, mesial (upper) view; E, same, lateral (lower) view; F, right minor cheliped, lateral view; G, same, mesial view; H, same pereopod, chela, upper view; I, same, lower view. Scale bars: 2 mm.

FIGURE 5. Alpheus ikedosoma n. sp., paratype, ovigerous female (cl 12.2 mm), CBM-ZC 13135. A, left major cheliped, ischium and merus, mesial view; B, same pereopod, chela, lateral view; C, right minor cheliped, mesial view; D, same pereopod, chela, mesial view. Scale bars: 2 mm.

FIGURE 6. Host innkeeper worm of Alpheus ikedosoma n. sp. A, Ikedosoma elegans (Ikeda, 1904), CBM-ZW, Hojo Beach, Tateyama, Chiba Prefecture, Japan, 30 July 2015, scale circle: 33 mm; B, unidentified specimen, not preserved, Hondo flat, Amakusa, Kumamto Prefecture, 11 May 2013.