Potamalpheops yamakawai, Yamashita & Komai & Koreeda, 2024

Yamashita, Ryunosuke, Komai, Tomoyuki & Koreeda, Reo, 2024, Potamalpheops yamakawai n. sp., representing the first record of the genus from Japan (Decapoda: Caridea: Alpheidae), Zootaxa 5555 (1), pp. 91-104 : 92-100

publication ID	https://doi.org/ 10.11646/zootaxa.5555.1.6
publication LSID	lsid:zoobank.org:pub:BCC8109C-19E8-424E-9131-3D15672A0DC8
persistent identifier	https://treatment.plazi.org/id/E75C87E8-FFCF-6769-FF2A-FA4969B037EC
treatment provided by	Plazi
scientific name	Potamalpheops yamakawai
status	sp. nov.

Treatment

Potamalpheops yamakawai sp. nov.

[New Japanese name: Uzumi-bi-yobi-mizu-teppou-ebi]

( Figures 1–3 View FIGURE 1 View FIGURE 2 View FIGURE 3 , 4D View FIGURE 4 )

Type material. Japan. Holotype: CBM-ZC 17507, ovigerous female (cl 3.9 mm), DNA voucher, mouth of Minami-no River, Ao-no River system, Teishi , Minami-izu Town , Shizuoka Prefecture, 34°38.50’N, 138°52.38’E, hand net, coll. R. Yamashita, 11 July 2021. GoogleMaps

Paratypes: i) CBM-ZC 17508, male (cl 3.8 mm), same data as holotype; CBM-ZC 17850, 1 female (cl 2.5 mm), Mizunari River, Beppu, Ei Town , Minami-kyushu City , Kagoshima Prefecture, 31°15.16’N, 130°26.05’E, yabby pump, coll. R. Koreeda , 28 September 2022 GoogleMaps ; ii) KAUM-AT 3422–3424 , 1 male (cl 3.2 mm), 2 ovigerous females (cl 3.8–4.4 mm), same locality as CBM-ZC 17850, yabby pump, coll. R. Koreeda , 22 July 2023 GoogleMaps ; iii) CBM-ZC 17851– 17867, 6 females (cl 3.5–4.6 mm), 8 ovigerous females (cl 3.6–4.9 mm), 3 males (cl 3.2–3.8 mm), 3 DNA vouchers, Arata River, Kotsuki River system, Simoarata , Kagoshima City , Kagoshima Prefecture, 31°34.23’N 130°33.20’E, hand net, coll. R. Koreeda , 05 August 2023 GoogleMaps ; iv) KAUM-AT 3425–3427 , 4 ovigerous females (cl 4.4–4.8 mm), same data as previous specimens GoogleMaps .

Diagnosis. Carapace glabrous, without distinct grooves on lateral surface. Rostrum almost as long as broad, short, not reaching or slightly exceeding distal corneal margins, ventral margin unarmed; extra-corneal spine directed slightly upward, located lateral to cornea. Telson with 2 spiniform setae on each posterolateral angle. Cornea well developed, darkly pigmented; anteromesial margin of eyestalk unarmed or with minute, sharply pointed tubercle, without setae or at most with 1 or 2 minute setae. Antennular peduncle with basal article serrated on dorsodistal margin; penultimate article slightly shorter than visible portion of basal article in dorsal view. Antennal carpocerite reaching 0.8–0.9 length of scaphocerite. Chelipeds equal in length, similar, not particularly enlarged; carpus with several rows of grooming setae ventromesially; chela subequal in length to carpus. Pereopod 5 ischium and merus unarmed. Uropodal exopod with 1 posterolateral tooth; diaeresis serrated with 20–25 teeth.

Description. Holotype female. Body ( Fig. 1 View FIGURE 1 , 2A, C View FIGURE 2 ) moderately stout, not particularly compressed.

Carapace ( Fig. 2A, B View FIGURE 2 ) smooth, glabrous, without distinct grooves on lateral surface, subequal in length to anterior 3 pleomeres combined. Dorsum slightly domed due to development of ovary. Rostrum triangular, nearly as long as broad, short, slightly exceeding distal margin of cornea, distally acute; dorsal surface non-carinate; ventral margin unarmed. Extra-corneal teeth directed slightly upward, located lateral to cornea; infra-corneal margin broadly rounded, reaching as far as extra-corneal teeth. Pterygostomial margin broadly rounded, slightly projecting anteriorly, without setae; ventrolateral margin without setae. Posterior margin with deep cardiac notch.

Pleon ( Figs. 1 View FIGURE 1 , 2C View FIGURE 2 ) with anterior 4 pleura rounded; pleuron 5 with posteroventral angle angular but not sharply pointed. Pleomere 6 with triangular articulated plate posteroventrally; posterolateral process rounded; anal tubercles absent. Telson ( Fig. 2C, D View FIGURE 2 ) subrectangular, slightly tapering posteriorly; dorsal surface with 2 pairs of strong spiniform setae, first pair inserted at about 0.3 of telson length, second pair at about 0.6 length; posterior margin with median part rounded, somewhat projecting, with about 15 plumose setae; each posterolateral angle with 2 strong spiniform setae, lateral distinctly shorter than mesial.

Eyestalks ( Fig. 2A, B View FIGURE 2 ) largely exposed in dorsal and lateral views; cornea darkly pigmented, oval-shaped, occupying most of eyestalk except for mesial and anteromesial portions; anteromesial margin of eyestalk with minute setae and minute tubercle. Ocellar beak not visible in lateral view.

Antennular peduncle ( Fig. 2A, B View FIGURE 2 ) moderately slender, about half-length of carapace, slightly exceeding distal margin of antennal scaphocerite. Basal article with distodorsal margin serrated ( Fig. 2B View FIGURE 2 ); dorsal surface with 2 spiniform setae ( Fig. 2A View FIGURE 2 ); stylocerite acute, slightly exceeding distal margin of basal article; ventromesial carina with anteriorly directed tooth. Penultimate article slightly shorter than visible portion of basal article in dorsal view. Ultimate article distinctly shorter than penultimate article. Lateral flagellum biramous, bearing 8 tufts of aesthetascs; fused portion consisting of 10 segments; accessory branch with 4 segments; mesial flagellum about twice as long as carapace.

Antenna ( Fig. 2A View FIGURE 2 ) with basicerite bearing acute ventrolateral distal tooth. Carpocerite moderately long, reaching 0.8 length of scaphocerite. Scaphocerite ovate, about 3 times as long as wide; lateral margin nearly straight, terminating distally in robust spine, latter slightly overreaching distal margin of rounded lamella. Flagellum not particularly thickened, slender, about 3 times as long as carapace.

Epistomial sclerite without visibly projecting process.

Mouthparts typical for genus (not illustrated). Maxilliped 3 ( Fig. 3A View FIGURE 3 ) slender. Ultimate article twice as long as penultimate article (= carpus), with 2 and 1 small spiniform setae on apex and anterolateral margin, respectively. Antepenultimate article depressed in proximal half. Coxal lateral plate somewhat ear-shaped, distally subacute. Exopod flagellum-like, slightly overreaching distal margin of antepenultimate article.

Pereopods 1 (chelipeds) ( Fig. 3B–D View FIGURE 3 ) symmetrical, not particularly enlarged or elongate, slightly exceeding distal margin of basal article of antennular peduncle. Ischium slightly widened distally. Merus distinctly longer than ischium and carpus. Carpus vase-shaped, widened distally, with 5 transverse rows of setae on ventromesial surface (= grooming apparatus). Chela subcylindrical, subequal in length to carpus, 3 times as long as wide; fixed finger with occlusal margin nearly straight, unarmed; dactylus slightly shorter than palm, with short setae distally, unarmed on occlusal margin.

Pereopod 2 ( Fig. 3E View FIGURE 3 ) slender, not exceeding distal margin of penultimate article of antennular peduncle. Ischium and merus subequal in length. Carpus divided into 5 segments; ratio of carpal segmentts approximately equal to (proximal to distal): 4.3: 1.2: 1.5: 1.0: 2.5. Chela almost as long as distalmost carpal segment; fingers subequal in length to palm, with short setae distally.

Pereopod 3 ( Fig. 3F View FIGURE 3 ) slender. Ischium armed with 1 spiniform seta on ventrolateral surface. Merus armed with 2 widely spaced spiniform setae. Carpus unarmed, distally with stiff seta. Propodus armed with 6 minute spiniform setae on flexor margin and 1 pair of spiniform setae on flexor distal margin. Dactylus 0.4 length of propodus, subconical, simple, slender, slightly curving distally.

Pereopod 4 ( Fig. 3G View FIGURE 3 ) similar to pereopod 3, slightly longer. Pereopod 5 ( Fig. 3H View FIGURE 3 ) longer than pereopods 3 and 4; ischium and merus unarmed; propodus with 4 minute spiniform setae on proximal half of flexor margin; grooming apparatus consisting of long distal setae and 6 rows of shorter stiff setae in distal half of flexor margin.

Pleopod 1 with endopod about 0.4 length of exopod. Pleopods 2–5 each with appendix interna on endopod.

Uropod ( Fig. 2C, E View FIGURE 2 ) with posterolateral lobe of protopod ending in strong tooth. Exopod longer than endopod; lateral margin nearly straight, with 1 small acute posterolateral tooth; posterolateral spiniform seta strong, distinctly longer than posterolateral tooth but not reaching posterior margin of exopod; diaeresis not reaching to mesial margin of exopod, serrated with 20 minute teeth between lateral margin and abrupt mesial incision at about one-third of diaeresis length.

Gill formula typical for genus (cf. Powell 1979; Anker 2003).

Eggs: 75 in number, 0.56–0.79 mm in diameter.

Paratypes. Body less stout in males than in ovigerous females. Carapace dorsum slightly domed in mature females, not in males and younger females. Rostrum varying in length, from not reaching distal margin of cornea to overreaching mid-length of basal article of antennular peduncle. Tubercle on anteromesial angle of eyestalk absent or present, if present, minute but sharp; 1 or 2 minute setae present or absent on anteromesial angle of eyestalk, sometimes barely visible. Antennular peduncle with basal article bearing 2 or 3 (usually 2) spiniform setae on dorsal surface; lateral flagellum bearing 7–13 tufts of aesthetascs, fused portion consisting of 8–12 segments, accessory branch constantly with 4 segments. Carpocerite reaching 0.8–0.9 length of scaphocerite. Pereopod 1 with ischium 0.6–0.9 length of merus. Pereopod 3 and 4 propodi armed each with 6–8 minute spiniform setae on flexor margin and 2 on flexor distal margin. Pereopod 5 propodus armed with 4–6 minute spiniform setae on flexor margin and grooming apparatus of 6–8 rows of short stiff setae on flexor margin. Male pleopod 1 with endopod small, less than 0.2 length of exopod. Male pleopod 2 with appendix masculina twice as long as appendix interna ( Fig. 3I View FIGURE 3 ), with 4 long distal setae and 1 subdistal seta on mesial margin. Uropodal exopod with diaeresis serrated with 20–25 minute teeth.

Size. Largest female cl 4.9 mm, ovigerous females cl 3.6–4.9 mm; largest male cl 3.8 mm.

Colour pattern in life. Body and appendages generally translucent, occasionally with pale orange tinge; anterolateral margin of carapace reddish; pleomere 1 with 2 red or purple transverse bands, pleomeres 2–6 each with red or purple transverse bands posteriorly; cornea dark grey; antennular peduncle with tinge of red; eggs yellowish brown or yellowish green at non-eyed stage ( Fig. 1 View FIGURE 1 ).

Distribution. Presently known only from three locations on two Japanese main islands: Minami-no River in Aono River system, Minami-izu Town, Shizuoka Prefecture, Honshu (type locality); Arata River in Kotsuki River system in Kagoshima City, and Mizunari River, Minami-kyushu City, Kagoshima Prefecture, Kyushu.

Ecology. Minami-no River: Two specimens (CBM-ZC 17507, 17508) of P. yamakawai sp. nov. were collected from submerged leaves of Phragmites australis in 0.2 m depth in the upper estuary ( Fig. 4A View FIGURE 4 ) during low tide. At this site, although tides caused fluctuations in the water level, the water remains fresh. In addition, a hot spring discharge has a constant input here, and the water temperature remains high even in winter ( Maruyama 2018). See Maruyama (2018) for a listing of sympatric animals occurring at this collection site.

Mizunari River estuary: One and three specimens (CBM-ZC 17850, KAUM-AT 3422–3424) of P. yamakawai sp. nov. were pumped, on two different dates, from burrows of the alpheid shrimp, Alpheus malabaricus ( Fabricius, 1775) sensu lato, and the eel-goby, Taenioides gracilis ( Valenciennes, 1837) , respectively, in the upper tidal zone of a muddy riverbank in the upper estuary ( Fig. 4B View FIGURE 4 ). In addition to these four examined specimens, 17 individuals of P. yamakawai sp. nov. were extracted from the burrow of T. gracilis , although they were not preserved to avoid overcollection. Other organisms observed at this site include: Macrobrachium formosense Spence Bate, 1868 (Caridea: Palaemonidae ), Orisarma dehaani (H. Milne Edwards, 1853) (Brachyura: Sesarmidae ), and several fish species (cf. Koreeda & Motomura 2022; Koreeda et al. 2022; Dewa et al. 2022). Noteworthy, no specimens were collected on a small tidal flat located 50 m downstream.

Arata River estuary: Eight and 13 specimens (CBM-ZC 17851–17867, KAUM-AT 3425–3427) of P. yamakawai sp. nov. were collected under cobbles and leaf debris, respectively on a sandy-muddy substrate deposited near culvert parts of the upper estuary ( Fig. 4C, D View FIGURE 4 ). Other organisms observed at this site included Ptychognathus ishii Sakai, 1939 (Brachyura: Varunidae ), Callogobius tanegasimae ( Snyder, 1908) and Luciogobius guttatus Gill, 1859 (Teleostei: Gobiidae ).

Etymology. The new species is dedicated to our friend and colleague Uchu Yamakawa (Tsukuba University), who helped the first author during fieldwork.

Remarks. The present new species plainly belongs to Potamalpheops by possessing the following features: frontal margin of carapace with triangular rostrum and extra-corneal teeth; pleomere 6 with articulated triangular plate posteroventrally; corneas almost completely exposed in dorsal view and largely so in lateral view; pereopods 1–4 with strap-like epipods; and diaeresis of uropodal exopod serrated with minute teeth ( Powell 1979; Holthuis 1993; Yeo & Ng 1997). Potamalpheops was subdivided into two informal species groups, based on the armature of the posterior margin of the telson ( Cai & Anker 2004). Potamalpheops yamakawai sp. nov. is assigned to the P. monodi ( Sollaud, 1932) species group, which is characterised by the presence of two spiniform setae on each posterolateral angle of the telson. Within the P. monodi group, the new species appears to be morphologically closest to P. miyai from the western Indonesian and Philippine mangroves ( Yeo & Ng 1997; Cai & Anker 2004), because of a short, triangular, ventrally unarmed rostrum, and a serrated distodorsal margin of the basal article of the antennular peduncle. However, P. yamakawai sp. nov. can be distinguished from P. miyai by the following minor morphological features: (1) the pterygostomial and ventrolateral margins of the carapace are devoid of setae in P. yamakawai sp. nov. vs. sparsely setose in P. miyai ( Fig. 2A View FIGURE 2 ; cf. Yeo & Ng 1997: 176); (2) the merus of pereopod 5 is unarmed in P. yamakawai sp. nov. vs. armed with one or two spiniform seta(e) in P. miyai ( Fig. 3H View FIGURE 3 ; cf. Yeo & Ng 1997: fig. 4i); (3) the uropodal exopod is armed with one posterolateral tooth in P. yamakawai sp. nov. vs. two teeth in P. miyai ( Fig. 2C, E View FIGURE 2 ; cf. Yeo & Ng 1997: figs. 3d, e). At our request, Dr. Darren C.J. Yeo kindly examined the type specimens of P. miyai , confirming that the posterolateral angle of the uropodal exopod is constantly bi-toothed, as described in Yeo & Ng (1997). In addition, the living colouration may be different between P. yamawakai sp. nov. and P. miyai . In P. yamakawai sp. nov., the body is generally translucent occasionally with red or purple transverse bands on the pleon ( Fig. 1 View FIGURE 1 ). On the other hand, Yeo & Ng (1997: 179) described the general colour of P. miyai as follows: "dark and striped over a background of dark purplish to purplish gray." None of the specimens of the new species matches the description by Yeo & Ng (1997).

The new species is also similar to P. amnicus , presently known from freshwater streams in southern peninsular Malaysia (Johor) and Singapore, in having a ventrally unarmed rostrum, a distodorsally serrated basal article of the antennular peduncle (as in P. miyai ), well-developed eyes with a darkly pigmented cornea, and ischium and merus of pereopod 5 being both unarmed. Nevertheless, the non-setose ventrolateral margin of the carapace and the extra-corneal teeth located laterally to cornea distinguish P. yamakawai sp. nov. from P. amnicus , in which the ventrolateral margin of the carapace bears a sparse row of short setae ( Yeo & Ng 1997: fig. 1a, c) and the extra-corneal teeth are located dorsolaterally to cornea ( Yeo & Ng 1997: fig. 1a, b).

In this study, 542 bp sequences of the 16S rRNA gene were generated from three paratypes of the new species ( Table 1). Nucleotide sequences of the 16S rRNA gene are currently available for only four other congeners, including P. miyai ( Table 1), although the sequence of P. miyai is substantially shorter (418 bp vs. 531–542 bp for other species). Genetic divergence between P. yamakawai sp. nov. and four other species from GenBank (including one possibly undescribed species from Taiwan) are summarised in Table 2. The genetic divergence between P. yamakawai sp. nov. and P. miyai is 1.2 %, which exceeds the range of intraspecific divergence observed in the new species from Japan (0–0.2%). In the 16S rRNA, there are known cases where genetic diversity is small even between different species (e.g., Cabezas et al. 2009; Ng et al. 2010; Lavery et al. 2014; Komai & Hibino 2019; Komai & Matsuzaki 2022), so this should not be underestimated. Since there are some, albeit subtle morphological differences between P. yamakawai sp. nov. and P. miyai , which we consider to be of specific significance (cf. Yeo & Ng 1997; Cai & Anker 2004), it seems unlikely that the newly proposed taxon is conspecific with P. miyai . Much greater genetic divergences in the 16S rRNA sequences (18.4–27.8%) were observed between other species of Potamalpheops ( P. amnicus , P. tigger , and Potamalpheops sp. ).

Bruce (1991) considered that species of Potamalpheops were relict from an originally wider Tethyan distribution, based on the disjunct pattern of the geographical distribution exhibited by the species known at that time. However, recent findings have shown that this genus is more widely distributed across tropical and subtropical regions in both the Indo-West Pacific and Atlantic ( Soledade et al. 2014; Christodoulou et al. 2019; Chow et al. 2021; Marin 2021). Potamalpheops yamakawai sp. nov. marks the first occurrence of the genus in Japan, extending the distribution of the genus into a warm-temperate mainland area. Many species in this genus live in cryptic habitats and are small in size, which is probably why the new species has not been discovered until now. As noted by Anker (2005), it is highly probable that further undiscovered species of this genus await discovery.

Potamalpheops yamakawai sp. nov. is the northern-most species of Potamalpheops penetrating into a warm-temperate region, and the first representative of the genus in Japan. Of the 16 named species only, P. amnicus and P. stygicola are restricted to freshwater, completely free of tidal influence, whereas five other species occur in transitional zones between freshwater and brackish conditions (see above). The present new species appears to inhabit both freshwater areas upstream of the tidal limit as well as estuaries, suggesting that it is a euryhaline shrimp that can penetrate into freshwater habitats.

Possibly facultative “commensalism” with other invertebrates has been reported for two species of Potamalpheops : (1) P. tyrymembe Soledade, Santos & Almeida, 2014 , which is sometimes found in burrows of the mangrove “uça” crab, Ucides cordatus ( Linnaeus, 1763) ( Decapoda : Ocypodidae ), and the snapping shrimp Alpheus pontederiae de Rochebrune, 1883 in Brazil ( Soledade et al. 2014; Almeida et al. 2023); and (2) P. kisi Marin, 2021 , with some individuals collected from burrows of unknown hosts in mangroves in Vietnam ( Marin 2021). The vast majority of specimens of P. yamakawai sp. nov. were collected from piles of dead leaves deposited on the bottom, however, some were collected from burrows of the eel-goby, Taenioides gracilis and the alpheid shrimp, Alpheus malabaricus sensu lato. It is possible that P. yamakawai sp. nov. occasionally these burrows as temporary shelter and do not live in a close symbiotic relationship with their owners.