Metabetaeus lapillicola, Yamashita & Komai & Kon, 2023

Yamashita, Ryunosuke, Komai, Tomoyuki & Kon, Koetsu, 2023, Metabetaeus lapillicola, a new species of alpheid shrimp (Decapoda: Caridea) from Japan, inhabiting unusual pebble beaches, Zootaxa 5258 (3), pp. 317-330 : 319-326

publication ID	https://doi.org/ 10.11646/zootaxa.5258.3.5
publication LSID	lsid:zoobank.org:pub:E7459C8A-4B2E-45CA-B8ED-5137A4708F87
DOI	https://doi.org/10.5281/zenodo.7781985
persistent identifier	https://treatment.plazi.org/id/29F2E00E-629D-4133-AB16-1148277902EE
taxon LSID	lsid:zoobank.org:act:29F2E00E-629D-4133-AB16-1148277902EE
treatment provided by	Plazi
scientific name	Metabetaeus lapillicola
status	sp. nov.

Treatment

Metabetaeus lapillicola n. sp.

[New Japanese name: Sazare-ohaguro-teppou-ebi]

( Figures 1–6 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 )

Material examined. Holotype. CBM-ZC 17113, ovigerous female, cl 3.8 mm, Higashi-makado , Numazu City, Shizuoka Prefecture, Japan, 35°05.45’N, 138°50.37’E, collected by R. Yamashita, 14 July 2022. GoogleMaps

Paratypes.A total of 13 specimens, cl 2.8–3.6 mm. Same data as holotype: CBM-ZC 17114, 1 ovigerous female, cl 3.4 mm, DNA voucher GoogleMaps ; CBM-ZC 17115, 1 female, cl 3.5 mm. Same locality as holotype: SPMN-CR GoogleMaps 64, 1 female, cl 2.8 mm, collected by R. Yamashita, 23 June 2020 ; CBM-ZC 17116, 1 male, cl 3.1 mm, collected by R. Yamashita, 30 June 2022 ; CBM-ZC 17117, 1 ovigerous female, cl 3.3 mm, collected by R. Yamashita, 30 June 2022 ; KPM-NH 4518, 1 female, cl 3.3 mm, collected by R. Yamashita, 29 July 2022 ; CBM-ZC 17118, 1 female, cl 3.6 mm, collected by R. Yamashita and T. Suematsu, 30 August 2022 , DNA voucher; CBM-ZC 17119, 4 ovigerous females, cl 3.1–3.5 mm, collected by R. Yamashita and T. Suematsu, 30 August 2022 . CBM-ZC 17120, 1 male, cl 2.9 mm, Oyama-ura , Kihoku Town, Mie Prefectures °06.04’N, 136°13.45’E, collected by T. Suematsu, 3 January 2022 GoogleMaps ; CBM-ZC 17121, 1 ovigerous female, cl 3.6 mm, Taso-ura , Minami-ise Town, Mie Prefecture, Japan, 36°17.22’N, 136°41.09’E, collected by H. Muto, 4 June 2022 GoogleMaps , DNA voucher.

Description. Body ( Figs. 1 View FIGURE 1 , 2 View FIGURE 2 ) moderately slender, not particularly compressed.

Carapace ( Figs. 2 View FIGURE 2 , 3A, B View FIGURE 3 ) smooth, glabrous, becoming slightly deeper posteriorly, subequal in length to anterior 3 pleomeres combined. Dorsum domed owing to development of ovary in females. Frontal margin with rostrum and orbital spines; rostrum triangular, twice as long as basal width, reaching midlength of basal article of antennular peduncle; orbital spines directed forward. Pterygostomial angle with small, anteriorly directed spine. Posterior margin with deep cardiac notch.

Pleon ( Fig. 2 View FIGURE 2 ) with anterior 4 pleura rounded; pleuron 5 with small, acute posteroventral tooth. Pleomere 6 without articulated plate at posteroventral angle; preanal plate rounded. Telson ( Fig. 3D View FIGURE 3 ) sub-rectangular, slightly tapering posteriorly; dorsal surface with 2 pairs of strong spiniform setae, first pair inserted at 0.4–0.5 of telson length, second pair at 0.7–0.8 length; posterior margin rounded medially, each lateral angle with 2 pairs of strong spiniform setae, mesial setae distinctly longer than lateral setae; median part with 4 long setae; anal tubercles absent.

Eyestalks ( Fig. 3A, B View FIGURE 3 ) completely concealed in dorsal and lateral views, but visible in anterior view; corneas relatively developed for genus, darkly pigmented, suboval, located in lateral position of eyestalk. Ocellar beak not visible even in anterior view.

Antennular peduncle ( Fig. 3A, B View FIGURE 3 ) half-length of carapace, distinctly overreaching distal margin of scaphocerite. Basal article with stylocerite reaching beyond midlength of penultimate peduncular article; mesioventral carina with anteriorly directed small tooth. Penultimate article shorter than visible portion of basal article, slightly longer than wide. Ultimate article subequal in length to penultimate article. Lateral flagellum stout, biramous, fused proximal portion consisting of 4–7 (usually 5) segments; shorter ramus consisting of 9–11 (usually 10) segments, bearing aesthetascs; longer ramus about 2 times as long as antennular peduncle. Mesial flagellum subequal in length to carapace.

Antennal peduncle ( Fig. 3A, B View FIGURE 3 ) with basicerite bearing strong ventrolateral distal spine. Carpocerite distinctly exceeding scaphocerite. Scaphocerite ovate; lateral margin nearly straight, terminating in strong spine slightly overreaching distal margin of rounded lamella. Flagellum about 2 times as long as carapace.

Epistomial sclerite almost flat, not projecting ventrally.

Mandible ( Fig. 4A–C View FIGURE 4 ) with two-segmented palp, well-developed molar process and incisor process; incisor process bearing 5 distal teeth and prominent dark-brown spot on outer surface proximally. Maxillule ( Fig. 4D View FIGURE 4 ) with distally bilobed palp, each lobe furnished with 1 stiff apical seta; basial endite with row of spiniform-like setae on distal margin; coxal endite simple, with several setae distally. Maxilla ( Fig. 4E View FIGURE 4 ) with simple endopod; basial endite divided into two rounded lobes; coxal endite simple, contiguous to basial endite; scaphognathite well developed. Maxilliped 1 ( Fig. 4F View FIGURE 4 ) with simple, non-segmented endopod; caridean lobe of exopod moderately broad. Maxilliped 2 ( Fig. 4G View FIGURE 4 ) without specific features, as illustrated.

Maxilliped 3 ( Fig.4H, I View FIGURE 4 )with rounded lateral plate on coxa.Antepenultimate article long, flattened dorsoventrally. Penultimate article about twice as long as wide. Ultimate article 1.6 times as long as penultimate article, tapering distally, with minute spiniform seta near blunt tip; mesial surface with transverse rows of finely serrulate setae.

Pereopods 1 (chelipeds) strongly unequal and dissimilar ( Figs. 1B–D View FIGURE 1 , 5A, B View FIGURE 5 ). Major cheliped ( Figs. 5A View FIGURE 5 , 6A, B View FIGURE 6 ) markedly elongated. Ischium slender, with 8–11 thick setae on dorsal margin, but without spiniform setae. Merus 2 times as long as ischium, fairly compressed dorsoventrally (cross section ovate), but not forming distinct facet on ventral side, without spiniform setae on ventral margin, distoventral and distodorsal angles blunt. Carpus elongate vase-shaped, widened distally. Chela elongate, twice as long as carpus; palm almost glabrous, elongate, not particularly swollen; fingers 0.3 times as long as palm, unarmed, slightly curved, not crossing for each other distally, bearing sparse long setae.

Minor cheliped ( Figs. 5B View FIGURE 5 , 6C, D View FIGURE 6 ) much shorter than major, reaching carpus of major cheliped. Ischium slender, with 4–11 thick setae on dorsal margin, but without spiniform setae. Merus 1.2 times as long as Ischium, fairly compressed dorsoventrally (cross section ovate), but not forming distinct facet on ventral side, without spiniform setae on ventral margin; distoventral and distodorsal angles blunt. Carpus vase-shaped, widened distally. Chela 1.5 times as long as carpus; palm almost glabrous, not particularly swollen; fingers slightly shorter than palm, slightly curved, not crossing for each other distally, bearing sparse long setae.

Pereopod 2 ( Fig. 5C View FIGURE 5 ) slender, distinctly overreaching distal margin of antennular peduncle. Ischium slightly longer than merus. Carpus composed of 5 segments, length ratio of segments equal to 1.0: 0.5: 0.7: 0.4: 0.7 (proximal to distal). Chela 0.2 times as long as carpus; fingers subequal in length to palm.

Pereopod 3 ( Fig. 5D, E View FIGURE 5 ) slender, reaching distal margin of antennular peduncle. Ischium with 1 or 2 (usually 1) spiniform seta(e) on lateral surface ventrally, rarely unarmed. Merus about 5 times as long as wide, with 2–4 (most frequently 3) spiniform setae on lateral surface ventrally. Carpus slenderer than merus, unarmed. Propodus slenderer than carpus, with 1 or 2 (usually 1) minute spiniform seta(e) or unarmed on flexor margin, flexor distal margin unarmed or with 1 minute spiniform seta. Dactylus simple, subconical, faintly curved, about 0.25 length of propodus, with minute setae arising at about midlength.

Pereopod 4 ( Fig. 5F, G View FIGURE 5 ) similar to pereopod 3 but slenderer, reaching distal end of basal article of antennular peduncle; merus with 1–3 (usually 2) spiniform setae on lateral surface ventrally. Pereopod 5 ( Fig. 5H, I View FIGURE 5 ) generally similar to third and fourth, reaching level of tip of rostrum; ischium unarmed or rarely with 1 spiniform seta on lateral surface ventrally; merus unarmed or armed with 1 or 2 (usually 1) spiniform seta on lateral surface ventrally; propodus with row of minute spiniform setae on distal half of flexor margin, forming grooming apparatus.

Male pleopod 1 endopod small, less than 0.2 length of exopod; female pleopod 1 endopod relatively long, more than 0.4 length of exopod. Male pleopod 2 with appendix masculina ( Fig. 3E View FIGURE 3 ) distinctly longer than appendix interna, bearing at least 3 short stiff setae distally. Pleopods 2–5 each with appendix interna on endopod.

Uropod ( Fig. 3C View FIGURE 3 ) with distolateral lobe of protopod obscurely bifid, distomesial lobe projecting into strong spine. Exopod with diaeresis bearing large subtriangular tooth adjacent to strong distolateral spiniform seta. Endopod unarmed or with rudimentary tooth on distolateral margin.

Coloration in life. Body and appendages generally semitransparent, with sparse red chromatophores, occasionally forming obscure band along posterior margin of carapace and on each pleomere 1–5; eggs at non-eyed stage grayish green ( Fig. 1A–D View FIGURE 1 ).

Distribution. Presently known only from three locations on the Pacific side of Honshu, Japan, viz., Higashimakado, Numazu City, Shizuoka Prefecture, Oyama-ura, Kihoku Town, Mie Prefecture, and Taso-ura, Minami-ise Town, Mie Prefecture.

Ecology. The examined specimens of Metabetaeus lapillicola n. sp. were collected exclusively with small and uniformly sized pebbles (the size is 1–3 cm in diameter) deposited in the lower intertidal to upper subtidal zones ( Fig. 7 View FIGURE 7 ), suggesting that they inhabit interstitial spaces among pebbles. The collection sites are exposed to strong waves at high tide. No other decapod species was seen in the areas where the present new species occurred. Other organisms sympatrically found with M. lapillicola n. sp. included:one unidentified species of lineid heteronemertean; two unidentified species of polychaetes, representing Gonidaididae and Amphinomidae ; two unidentified species of amphipods, Melitidae gen. sp. and Pontogeneia sp. (Pontogeneiidae) ; one species of gastropod, Caecum sp. (Caecidae) ; and five species of the earthworm goby genus Luciogobius (Teleostei: Gobiidae ).

Ovigerous specimens of M. lapillicola n. sp. were encountered between June and September at Numazu, Shizuoka Prefecture, and June at Minami-Ise, Mie Prefecture. Those carried rather few and large eggs (25–41 in number and 0.36–0.77 mm in diameter), suggesting abbreviated larval development as in the congeneric M. mcphersonae (cf. Anker 2010).

Etymology. The specific name lapillicola is derived from the Latin word meaning “dweller of pebbles”, in reference to the habitat of this new species. Used as a noun in apposition.

Remarks. The present new species is assigned to Metabetaeus because of the following diagnostic features (cf. Anker 2010): (1) carapace with orbital teeth; (2) pleomere 6 lacking articulated plate at posteroventral angle; (3) eyes concealed in dorsal and lateral views, visible in frontal view; (4) incisor process of mandible with prominent dark brown spot, visible through carapace; (5) pereopod 2 with 5-segmented carpus, first segment longest, third segment longer than second or fourth; (6) pereopods 3 and 4 meri bearing spiniform seta(e) on ventrolateral surface, and with simple, conical dactyli bearing stiff setae at about 0.7 length; (7) uropodal exopod with diaeresis bearing strong sharp tooth adjacent to strong distolateral spiniform seta. In particular, the features of the mandible (4) and the pereopod 2 carpus (5) are unique to Metabetaeus ( Anker 2010) . Nevertheless, the new species is distinctive in Metabetaeus in the following characters, requiring emendation to the generic diagnosis proposed by Anker (2010) (see below): (1) the carapace has a small pterygostomial spine in M. lapillicola n. sp., whereas in the other three congeneric species, the pterygostomial margin is unarmed ( Figs. 2 View FIGURE 2 , 3B View FIGURE 3 versus Anker 2010: figs. 1B, 9B); (2) the orbital spines on the carapace are directed forward in M. lapillicola n. sp., rather than directed mesially in the other three species ( Fig. 3A View FIGURE 3 versus Anker 2010: figs. 1A, 8A); (3) the pereopods 1 are greatly unequal and dissimilar, with the major one is substantially elongate in M. lapillicola n. sp., rather than subequal and similar in the other three species ( Figs. 5A, B View FIGURE 5 , 6 View FIGURE 6 versus Anker 2010: figs. 6A, B, 9C, 10A, C); (4) the uropodal endopod is devoid of a distinct distolateral spine (or at most a rudimentary spine is present) in M. lapillicola n. sp., while there is a distinct distolateral spine on the uropodal endopod in the other three species ( Figs. 2 View FIGURE 2 , 3C View FIGURE 3 versus Anker 2010: figs. 1F, 8F).

Metabetaeus lapillicola n. sp. further differs from M. mcphersonae , a congeneric species also occurring in the marine habitat, as follows: (1) the distolateral spine of the antennal scaphocerite clearly overreaches the distal margin of lamella in M. lapillicola n. sp., rather than falling short of it in M. mcphersonae ( Fig. 3A View FIGURE 3 versus Anker 2010: figs. 1A, 4A); an arthrobranch is absent on the maxilliped 3 in M. lapillicola n. sp., but present in M. mcphersonae ( Fig. 4H View FIGURE 4 versus Anker 2010: fig. 2G). the merus of the pereopod 1 has a rounded, unarmed ventral surface in both sides in M. lapillicola n. sp., rather than having a flat ventral surface with a row of spiniform setae on the ventromesial margin in M. mcphersonae ( Fig. 5A, B View FIGURE 5 versus Anker 2010: figs. 3A, 4B).

Other differentiating characters between the new species and M. lohena and M. minutus are as follows: (1) the cornea is better developed in the new species than in M. lohena and M. minutus , perhaps reflecting the habitat difference (the latter two species exclusively occur in aphotic anchialine environment) (cf. Fig. 3A, B View FIGURE 3 versus Anker 2010: figs. 8A, 9A, 10A–C); (2) the propodi of the pereopods 3 and 4 are unarmed or armed at most with two spiniform setae on the flexor margins in M. lapillicola n. sp., rather than armed with a row of numerous spiniform setae in M. lohena and M. minutus (cf. Fig. 5D, F View FIGURE 5 versus Anker 2010: figs. 8D, 9F). In addition, M. lohena and M. minutus attain larger size than M. lapillicola n. sp. does (the maximum cl is 6.0 and 5.8 mm in M. lohena and M. minutus respectively, while 3.8 mm in M. lapillicola n. sp.).

We sequenced partial fragments of the mitochondrial 16S rRNA and COI genes from three specimens of M. lapillicola n. sp. ( Table 1 View TABLE 1 ). Seven 16S rRNA gene sequences from the three known species of Metabetaeus were available in the GenBank database ( Table 1 View TABLE 1 ). With regard to the barcoding segments of the COI gene, as mentioned above, we selected one and two registered sequences from M. lohena and M. minutus respectively to reduce the number of sequences included in the genetic analysis ( Table 1 View TABLE 1 ). The topology of ML tree based on the 16S rRNA gene ( Fig. 8 View FIGURE 8 ) shows that M. lapillicola n. sp. is sister to the clade of M. minutus + M. lohena , although bootstrap support is low (49%). Genetic divergences between the new species and the other three species are high (15.8– 21.1%) ( Table 2 View TABLE 2 ), well supporting the status of the new taxon. Whereas genetic divergence among the three paratype specimens of M. lapillicola n. sp. is low (0–0.3%; Table 2 View TABLE 2 ), indicating that the voucher specimens represent a same species. Genetic divergences of the COI gene between the new species and M. lohena and M. minutes are also high (23.6–25.5%; Table 3 View TABLE 3 ), whereas intraspecific genetic divergence within the new species is also low (0.3–0.6%; Table 3 View TABLE 3 ). There is generally, but not totally defined, a threshold to delimit species of decapod crustaceans of about 3% genetic divergence for the barcoding segment of COI gene (e.g., Lefébure et al. 2006; Rodríguez-Flores et al. 2019), and the genetic divergence between the new species and the other three Metabetaeus species is substantially higher than the general threshold value.

It is remarkable that the present new species occurs exclusively in gravely beaches composed of small pebbles ( Fig. 7 View FIGURE 7 ). Although it is generally known that diverse and numerous micro invertebrates inhabit the interstitial spaces among pebbles ( Giere 2009), such interstitial habitats, which are influenced by strong waves, are generally considered too dynamic for macro- and mega-benthos to inhabit ( Kondo & Kato 2022). In fact, we have not encountered other decapods in the study sites during field surveys.