Cymothoidae Leach, 1818

Kawanishi, Ryota & Ohashi, Shinpei, 2020, First Record of the Rare Parasitic Isopod Elthusa splendida (Cymothoidae) from the Pacific Ocean, Based on a Specimen Found in a Museum Shark Collection, Species Diversity 25, pp. 343-348 : 344-347

publication ID	https://doi.org/ 10.12782/specdiv.25.343
persistent identifier	https://treatment.plazi.org/id/03C387D2-B726-D535-D0CE-FAFB7008F947
treatment provided by	Felipe
scientific name	Cymothoidae Leach, 1818
status

Treatment

Family Cymothoidae Leach, 1818 View in CoL

[Standard Japanese name: Uonoe-ka] Genus Elthusa Schioedte and Meinert, 1884 [Standard Japanese name: Eru-uonoe-zoku] Elthusa splendida ( Sadowsky and Moreira, 1981) [New standard Japanese name: Oo-uonoe] ( Figs 1–3 View Fig View Fig View Fig , 4A View Fig , 5 View Fig )

Lironeca splendida Sadowsky and Moreira, 1981: 143 View in CoL (original description; type locality: Oc/ S Prof. W. Besnard GoogleMaps St. 2403. Lat. 23°27′S, Long. 43°20′W. 104–108 m depth).

Elthusa splendida: Bruce 1990: 254 View in CoL (revision of the genus; no new specimens).

Description of ECS specimen. Ovigerous female (57.9 mm in total length, TL; 29.1 mm in maximum width; Figs 1 View Fig , 2 View Fig ). Body elongated oval, almost symmetrical, weakly vaulted dorsally, dorsal surfaces smooth except for pereonite 1, widest at pereonite 4 ( Figs 1 View Fig , 2A, B View Fig ); TL 1.99 times maximum width. Cephalon 0.73 times as long as width, visible in dorsal view, obtuse sub-triangular; posterior margin not trilobed ( Fig. 2A View Fig ). Frontal margin roundly pointed, ventrally folded ( Fig. 2C View Fig ). Eyes elongated oval with indistinct margins; one eye 0.19 times width of cephalon, 0.52 times length of cephalon. Pereonite 1 smooth except for medial four pits; the anterior two pits (0.2–0.3mm in depth) were slightly deeper than the posterior two pits (0.1 mm); the distance between the anterior two pits was slightly greater than that between the posterior two pits ( Fig. 3 View Fig ); anterior end of anterolateral margin roundly pointed, extending to posterior third of eyes.

Coxae 2–3 narrow, with posteroventral angles rounded; coxae 4–7 with rounded point, not extending past respective pereonites ( Fig. 2A, B View Fig ). Pereonites 1–7 in width ratio 1.00: 1.28: 1.44: 1.47: 1.37:1.27: 1.14. Pleon length 0.42 times TL. Pleonites similar and gradually increasing in width towards posterior ( Fig. 2A, D View Fig ). Pleonite 1 posterior margin concave; lateral margins concealed by pereonite 7 in dorsal view. Pleonite 2 not overlapped by pereonite 7; posterior margin concave; posterolateral angles narrowly rounded. Pleonites 3–4 similar in form to pleonite 2. Pleonite 5 longest, lateral margins not concealed by pleonite 4. Pleotelson length 0.65 times its width, 0.27 times TL; pleotelson width 1.13 times width of pereonite 7; dorsal surface with two shallow depression, without setae; anteromedial margin slightly emarginated, thickened; lateral margins convex; posterior margin evenly rounded.

Antennule consists of 8 articles, shorter than antenna, reaching to anterolateral angle of pereonite 1; bases not in contact ( Fig. 2C View Fig ). Antenna consists of 12 (but the left one consists of 11, due to the lack of the tip) articles, reaching to pereonite 1 ( Fig. 2C View Fig ).

Pereopod 1 basis with very weak carina, 1.59 times its greatest width; ischium 0.50 times as long as basis, distal margin with subacute protrusion; propodus 1.51 times its width; dactylus slender, 0.86 times as long as propodus, 2.31 times its basal width ( Fig. 2E View Fig ). Pereopod 7 basis with strong carina, 1.90 times its greatest width; ischium 0.63 times as long as basis, distal margin with obtuse protrusion; merus distal margin produced; carpus with bulbous protrusion; propodus 1.30 times its width, 0.54 times as long as ischium; dactylus slender, as long as propodus, 1.77 times its basal width ( Fig. 2F View Fig ).

Pleopods all lamellar, gradually decreasing in size posteriorly; exopod larger than endopod, but the difference gradually decreases from pleopod 1 to 5 ( Fig. 2G–K View Fig ). Pleopod 1 exopod length 1.16 times its width, with crenate margin, distally broadly rounded, lateral and mesial margins strongly convex; endopod length 1.51 times its width, with crenate margin, lateral margin convex, distally broadly rounded, mesial margin straight; peduncle length 0.37 times its width, without retinaculae ( Fig. 2G View Fig ). Proximomedial angle of endopod produced into a roundly pointed process, especially prominent at pleopods 4 and 5 ( Fig. 2J, K View Fig ).

Uropod without setae, subequal to half length of pleotelson; peduncle 1.18–1.28 times longer than rami; endopod apically rounded, 2.51 times its greatest width, lateral margin weakly convex, mesial margin straight; exopod not extending beyond end of endopod, 2.07 times its greatest width, apically rounded, lateral margin weakly convex, mesial margin straight ( Fig. 2D View Fig ).

Remarks. According to Bruce (1990) and van der Wal et al. (2019), the ECS specimen clearly belongs to Elthusa in having the following combination of characters: body weakly vaulted dorsally; antennule shorter than antenna, bases not in contact; posterior margin of cephalon not trilobed; pereopods with relatively short dactylus; wide pleon with all lamellar pleopods.

The presence of four pits on pereonite 1 is known as a unique character of E. splendida ( Sadowsky and Moreira 1981) . Using a 3D measurement scanner, we confirmed that the pit arrangement, as well as its presence, of the ECS specimen is also well consistent with that of the holotype ( Sadowsky and Moreira 1981). Furthermore, despite the larger size of the ECS specimen (57.9 mm TL) than any known specimens of the species (up to 48.0 mm TL; Sadowsky and Moreira 1981), the ECS specimen is fully consistent with the original description of E. splendida : elongated oval body, with the TL/maximum width ratio of approximately 2; obtuse sub-triangular cephalon with ventrally folded frontal margin; antennule bases not in contact; the four well marked pits on pereonite 1; broadly rounded posterolateral angles of pereonite 7 concealing lateral margins of pleonite 1; lateral margins of pleonite 4 not concealing those of pleonite 5; large pleotelson accounting for approximately one quarter (24–27%) of TL and wider than pereonite 7 (not including coxae); pleopods with crenate margins, particularly evident in exopods; uropod with peduncle longer than rami.

As Sadowsky and Moreira (1981) stated, E. splendida is similar to E. raynaudii . These two species are easily distinguished by differences in pereonite and pleotelson morphology other than the presence/absence of the four dorsal pits on pereonite 1 ( Fig. 4 View Fig ). Elthusa splendida has broadly round- ed posterolateral angles of pereonite 7 and large pleotelson accounting for approximately 25% of TL and wider than pereonite 7, while E. raynaudii has roundly pointed posterolateral angles of pereonite 7 and relatively small pleotelson accounting for approximately 20% of TL and narrower than pereonite 7 ( Fig. 4 View Fig ).

Williams et al. (2010) reported, as an accidental case in Taiwan, Elthusa raynaudii from the palate of a lantern shark specimen that was later designated as the holotype of Etmopterus joungi Knuckey, Ebert, and Burgess, 2011 . Howev- er, the attachment site, host taxon (i.e., elasmobranch fish), and the large pleotelson shown in a photograph of Williams et al. (2010) suggest that their specimen might also be E. splendida rather than E. raynaudii . This specimen (California Academy of Science; CASIZ 180277) needs to be further examined in future research.

We herein propose a new standard Japanese name “Oouonoe” based on the present specimen (HUMZ-C 2400). “Oo” means “large” in Japanese and “uonoe” is a common Japanese name for the family Cymothoidae .