Paracalliope dichotomus Morino, 1991

Ariyama, Hiroyuki, 2021, Two Species of Paracalliopiidae from the Ryukyu Archipelago in Japan, with the Description of a New Species (Crustacea: Amphipoda), Species Diversity 26, pp. 79-91 : 88-90

Paracalliope dichotomus Morino, 1991

[Japanese name: Shiokawa-yokoebi] ( Figs 10–12 )

Paracalliope dichotomus Morino, 1991: 20 , figs 6–8; Aoyagi 2011: 109, fig. 1.

Material examined. Four males (OMNH-Ar-12085– 12088), 2.5, 2.4, 2.3 and 2.1 mm, respectively, 3 ovigerous females (OMNH-Ar-12089–12091), 1.9, 1.8 and 1.7 mm, respectively, and 1 ovigerous female (OMNH-Ar-12180, not dissected), 1.8 mm, Sashiki, Okinawa Island , Okinawa Prefecture, 26°10′31″N, 127°47′29″E ( Fig. 1C ), middle intertidal, sandy mud bottom in stream, 4 July 2015, coll . H . Ariyama.

Description of male (based on OMNH-Ar-12087, 2.3 mm). Body ( Fig. 10 ) relatively slender; head with medium-sized eyes (diameter: 5.2% of BL); pereonite 1 mediumsized, pereonites 2–6 narrow, pereonite 7 and pleonites 1–3 wide. Antenna 1 ( Fig. 11A, A 1 ), peduncular articles 3–5 with length ratio of 1.0: 0.7: 0.3, article 1 stout; accessory flagellum 1-articulate, with 2 setae on tip; flagellum with 12 articles, last article small, articles 1–4 each with calceolus medially, articles 7, 9–11 each with aesthetasc at distal end. Antenna 2 ( Fig. 11B) slightly longer than antenna 1; peduncular articles 3–5 with length ratio of 1.0: 2.65: 2.5; flagellum with 14 articles, last article small, articles 2–5 each with calceolus anteriorly. Gnathopod 1 ( Fig. 11C, C 1 ) with oval coxa, bearing 3 setae on ventral margin; basis bare; merus with 2 setae and many thin setae on posterior margin; carpus posterodistal lobe with 4 bifid setae distally and many thin setae on lateral surface; propodus about 1.3 times as long as carpus, anterodistal corner with 2 thick setae, palm oblique, weakly setose, without robust setae; dactylus slen- der, minutely bifurcate apically. Gnathopod 2 ( Fig. 11D, D 1 ), coxa longish ovoid, bearing 3 ventral setae; basis with single anterior long seta; ischium and merus each with slen- der seta and many lateral thin setae posteriorly; carpus posterodistal lobe with 4 distal and 5 medial setae and many thin setae on lateral surface; propodus about 1.8 times as long as carpus, palm slightly concave posterolaterally, with 2 posterolateral and 3 medial robust setae; dactylus slender, almost reaching end of palm, bifurcate apically.

Description of female (based on OMNH-Ar-12090, 1.8 mm, and OMNH-Ar-12180, 1.8 mm only for head). Body more inflated than that of male. Head ( Fig. 11E) smaller than that of male, with medium-sized eyes (diameter: 5.5% of BL). Antenna 1 ( Fig. 11F, F 1 ), peduncular articles 1–3 with length ratio of 1.0: 0.65: 0.5, article 1 stout; accessory flagellum 1-articulate, tip with 2 setae; flagellum with 9 articles, last article small, articles 6–8 each with aesthetasc at distal end. Antenna 2 ( Fig. 11G) slightly shorter than antenna 1; peduncular articles 3–5 with length ratio of 1.0: 2.0: 1.85; flagellum with 8 articles, last article small. Gnathopod 1 ( Fig. 11H) with oval coxa, bearing 3 setae on ventral margin; basis slender, with 1 anterior and 1 posterior long setae; merus bearing single seta and several thin setae on posterior margin; carpus posterodistal lobe with 4 bifid setae distally and many thin setae on lateral surface; propodus narrow, about 1.25 times as long as carpus, anterodistal corner with single thick seta, palm weakly oblique, without robust setae, posterolateral and anteromedial surfaces and posterior margin covered with thin setae; dactylus slender, not bifurcate. Gnathopod 2 ( Fig. 11I) slightly larger than gnathopod 1; coxa subrectangular, bearing 2 ventral setae; basis with 3 anterior and 1 posterior long setae; merus bearing single seta and several thin setae on posterior margin; carpus posterodistal lobe with 4 bifid setae distally and many thin setae on lateral surface; propodus narrow, about 1.1 times as long as carpus, palm weakly oblique, lacking robust setae, anteromedial surface and posterior margin covered with thin setae; dactylus slender, not bifurcate.

Coloration of specimen shortly after fixation ( Fig. 12 ). Eyes dark brown; posterior margins of pereonite 7, pleonites 1 and 2 dark red; other body and appendages pale orange with irregular dark red spots.

Remarks. Morphological characters of the male specimens almost agree with the original description and figures ( Morino 1991). But there are trifle differences; these specimens have (1) head with shallower antennal sinus, (2) wider peduncular article 1 of antenna 1, and (3) gnathopod 2 basis lacking long seta on posterior margin. The bifurcate dactyli of the male gnathopods 1 and 2 are distinctive to the genus. The morphology in female and the coloration are firstly clarified in the present study.

Habitat. Middle intertidal, sandy mud bottom in stream (present study); salty spring ( Morino 1991); waterway and small river near coast ( Aoyagi 2011).

Distribution. Japan: Okinawa Island ( Morino 1991; present study) and Iheya and Izena Islands ( Aoyagi 2011) in Okinawa Prefecture.

Paracalliope dichotomus is designated as a near-threatened (NT) species ( Okinawa Prefecture 2017; Ministry of the Environment, Government of Japan 2020); in my survey, this species was not collected from the other islands (Amami-ōshima, Kakeroma and Iriomote Islands).

Figures

Fig. 10. Paracalliope dichotomus Morino, 1991, male (OMNH-Ar-12087), 2.3mm, habitus, right lateral view.

Fig. 1. Map showing collecting sites of examined specimens in the Ryukyu Archipelago.

Fig. 12. Paracalliope dichotomus Morino, 1991. Photograph of a fixed specimen (male, 3.5 hours after fixation) collected together with examined specimens (OMNH-Ar-12085–12091), most part of pereopod 7 lost. Scale: 0.5mm.