Elthusa rotunda, van der Wal, Serita, Smit, Nico J. & Hadfield, Kerry A., 2019

van der Wal, Serita, Smit, Nico J. & Hadfield, Kerry A., 2019, Review of the fish parasitic genus Elthusa Schioedte & Meinert, 1884 (Crustacea, Isopoda, Cymothoidae) from South Africa, including the description of three new species, ZooKeys 841, pp. 1-37 : 1

Elthusa rotunda sp. n. Figures 12, 13, Table 1

Material examined.

Holotype. SOUTH AFRICA • 1 ♀ (ovigerous, 29.0 mm TL; 20.0 mm W); Cape Town, Sea Point; 33°55'S, 18°23'E; January 1960; coll. G Branch; SAMC A11001.

Description

(ovigerous ♀). Figs 12-13. Body round, not twisted, 1.4 times as long as greatest width; dorsal surfaces smooth and polished in appearance, widest at pereonite 4, most narrow at pereonite 1; pereonite lateral margins mostly posteriorly ovate, medially indented. Cephalon 0.4 times longer than wide, visible from dorsal view, sub-triangular with blunt anterior point. Frontal margin thickened, ventrally folded. Eyes oval with distinct margins; one eye 0.2 times width of cephalon; 0.5 times length of cephalon. Pereonite 1 smooth, anterior border evenly concave; anterolateral angles rounded, extending to the medial region of eyes. Posterior margins of pereonites smooth, slightly curved laterally, posterior margins of pereonites 2-3 uneven. Coxae 2-3 wide; with posteroventral angles rounded; coxae 4-7 with rounded point, not extending past pereonite posterior margin. Pereonites becoming more progressively rounded posteriorly; pereonite 5 most narrow. Pleon 0.4 times as long as total body length; pleonite 1 largely concealed by pereonite 7, slightly visible in dorsal view; pleonites posterior margin slightly concave, smooth, slightly curved laterally. Pleonite 2 lateral margins overlapped by pereonite 7. Pleonites 3-4 similar in form to pleonite 2; pleonite 5 longest, overlapped by lateral margins of pleonite 4, posterior margin medially convex. Pleotelson broadly rounded, 0.7 times as long as anterior width, dorsal surface smooth; lateral margins convex; posterior margin evenly rounded.

Antennula shorter than antenna, consisting of eight articles; peduncle articles I and II distinct and articulated; extending to middle of eye. Antenna consists of ten articles, extending to past anterior margin of pereonite 1.

Pereopod 1 basis 1.7 times as long as greatest width; ischium 0.7 times as long as basis; merus proximal margin without bulbous protrusion; propodus 1.4 times as long as wide; dactylus slender, 1.3 times as long as propodus, 2.9 times as long as basal width. All pereopods without robust or simple setae. Pereopod 7 basis with carina, 2.1 times as long as greatest width; ischium with slight bulbous protrusion, 0.8 times as long as basis; merus proximal margin with bulbous protrusion, 0.6 times as long as wide, 0.3 times as long as ischium; carpus with bulbous protrusion, 0.7 times as long as wide, 0.3 times as long as ischium; propodus 1.2 times as long as wide, 0.9 times as long as ischium; dactylus slender, 1.7 times as long as propodus, 2.5 times as long as basal width.

Pleopods simple, exopod larger than endopod. Pleopod 1 exopod 1.3 times as long as wide, lateral margin weakly convex, distally broadly rounded, mesial margin weakly convex; peduncle 2.5 times as wide as long.

Uropod half the length of pleotelson, peduncle 0.9 times longer than rami, peduncle lateral margin without setae; rami not extending beyond pleotelson, marginal setae absent, apices broadly rounded. Endopod apically rounded, 2.6 times as long as greatest width, lateral margin weakly convex, mesial margin weakly convex. Exopod extending to end of endopod, 2.2 times as long as greatest width, apically rounded, lateral margin weakly convex, mesial margin straight.

Size.

Ovigerous female (29.0 mm TL, 20.0 mm W).

Etymology.

The epithet is a noun in the nominative singular. It is named after its most distinct, defining character, which is the rounded shape of the body. The Latin word for round is rotundus.

Distribution.

Currently only known from Sea Point, Cape Town, South Africa.

Hosts.

Not known.

Remarks.

The diagnostic characters of E. rotunda sp. n. include its circular body shape; a sub-triangular cephalon with blunt anterior margin; pereopod 7 merus and carpus with protrusions on the proximal and lateral margins; pereonite 7 lateral margins that extend to pleonite 4; pleonite 5 longest and medially convex; a broadly rounded pleotelson posterior margin; and uropod rami that are sub-equal in length to the peduncle.

When comparing E. rotunda sp. n. to the rest of the identified Elthusa species, its closest resemblance is to that of E. raynaudii . This is especially in regards to the shape of the uropods, pleon, and cephalon anterior margin. It can be distinguished from E. raynaudii in having a more rounded body shape compared to the ovoid body shape of E. raynaudii ; triangular cephalon as opposed to the narrowly truncate cephalon of E. raynaudii ; the broadly rounded pereonite 1 anterolateral margins of E. rotunda sp. n. compared to the narrowly rounded to pointed anterolateral margins of E. raynaudii pereonite 1; as well as the uropod rami and peduncles that are subequal in length, as opposed to the longer rami of E. raynaudii (see Table 1).

Elthusa rotunda sp. n. can be distinguished from E. xena sp. n. by the cephalon anterior margin which is more pointed in E. xena sp. n. and more rounded in E. rotunda sp. n.; broadly rounded uropod apices compared to the narrowly rounded ones from E. xena sp. n.; the shape of the pleotelson, which is broadly rounded for E. rotunda sp. n. and roughly quadrate for E. xena sp. n.; as well as the prominent presence of pereopod 7 protrusions on the merus and carpus of E. rotunda sp. n., that are less bulbous on E. xena sp. n.

The main differentiating characters between E. rotunda sp. n. and E. acutinasa sp. n. include the shape of the cephalon anterior margin (bluntly rounded versus produced point); and the uropod morphology, with E. rotunda sp. n. having broadly rounded, longer uropodal rami in comparison to the short, pointed uropodal rami of E. acutinasa sp. n. Elthusa rotunda sp. n. pleonite 5 is the longest, whereas E. acutinasa sp. n. pleonite 1 is the longest; the presence of pereopod 7 protrusions on E. rotunda sp. n. is more prominent and bulbous that those of E. acutinasa sp. n. pereopod 7 (see Table 1).