Elthusa acutinasa, 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 acutinasa sp. n. Figures 8, 9, 10, 11, Table 1

Material examined.

Holotype. SOUTH AFRICA • 1 ♀ (ovigerous, 39.0 mm TL, 19.0 mm W); Indian Ocean, south coast of South Africa, RV Africana (fish sorting table); 34°38'S, 25°38'E; April 2003; coll. Nico J Smit; SAMC-A089960.

Paratypes. SOUTH AFRICA • 3 ♀♀ (ovigerous, 28.0-30.0 mm TL, 15.0-17.0 mm W); same data as holotype; SAMC-A089961.

Other material. SOUTH AFRICA • 1 ♀ (ovigerous, 29.0 mm TL, 17.0 mm W); same data as holotype; dissected; in the collection of the authors at NWU • 4 ♀♀ (non-ovigerous, 19.0-24.0 mm TL, 10.0-14.0 mm W); same data as holotype; in the collection of the authors at NWU • 9 ♀♀ (three ovigerous, six non-ovigerous, 15.0-40.0 mm TL, 8.0-19.0 mm W); Indian Ocean, south coast of South Africa, RV Africana (fish sorting table); 30°29'S, 16°0'E; 213 m depth; January 1999; SAMC-A091307 • 1 ♀ (ovigerous, 40.0 mm TL, 19.0 mm W); same data as preceding; 30°25'S, 16°9'E; 259 m depth; SAMC-A091308 • 1 ♀ (ovigerous, 30.0 mm TL, 15.0 mm W); same data as preceding; 31°8'S, 15°20'E; 234 m depth; SAMC-A091309.

Description

(ovigerous ♀). Figs 8-11. Body slightly twisted to the right, elongated ovoid, 2.1 times as long as greatest width. Body 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 narrowly rounded anterior point. Frontal margin thickened, ventrally folded. Eyes oval with distinct margins; one eye 0.2 times width of cephalon, 0.4 times length of cephalon. Pereonite 1 smooth, anterior border with medially produced point, with two indentations; anterolateral angle rounded, extending to posterior margin of eyes. Posterior margins of pereonites smooth and slightly curved laterally. Coxae 2-3 wide; with posteroventral angles rounded; 4-7 with rounded point. Coxae 7 extending slightly past pereonite posterior margin. Pereonites 2-5 subequal, becoming more progressively rounded posteriorly. Pleon 0.4 times as long as total body length, with pleonite 1 longest, lateral margins concealed by pereonite 7, visible in dorsal view; pleonites posterior margin smooth and slightly curved laterally. Pleonite 2 partially overlapped by pereonite 7; posterolateral angles of pleonite 2 rounded. Pleonites 3-5 similar in form to pleonite 2; pleonite 5 overlapped by lateral margins of pleonite 4, posterior margin straight, with slight medial point. Pleotelson 0.7 times as long as anterior width, dorsal surface smooth; lateral margins weakly convex; posterior margin rounded, with slight medial indent.

Antennula shorter than antenna, consisting of eight articles; antennula peduncle articles I and II distinct and articulated; article II 0.9 times as long as article 1; article III 1.4 times as long as wide, 0.5 times as long as combined lengths of articles I and II; antennula flagellum with five articles, extending to middle of eye, with tufts of setae on articles I–III and article VIII. Antenna consists of twelve articles. Antenna peduncle article III 1.3 times as long as article II; article IV 1.3 times as long as wide, 1.2 times as long as article III; article V 1.5 times as long as wide, 1.1 times as long as article IV. Antenna flagellum with six articles, terminal article terminating in 1-5 short simple setae, extending to past anterior margin of pereonite 1. Mandible palp article II with five distolateral setae, and article III with three simple setae. Maxillula simple with four terminal robust setae. Maxilla mesial lobe not fused to lateral lobe; lateral lobe without simple setae, two recurved robust setae; mesial lobe without simple setae, and two large recurved robust setae. Maxilliped consists of III articles, with lamellar oostegite lobe or second, smaller oostegite lobe on basal part of article, palp article II without simple setae, article III with three recurved robust setae. Oostegites margin covered in numerous plumose setae.

Pereopod 1 basis 1.9 times as long as greatest width; ischium 0.7 times as long as basis; merus proximal margin with slight bulbous protrusion; carpus with rounded proximal margin; propodus 1.1 times as long as wide; dactylus slender, 1.3 times as long as propodus, 3 times as long as basal width. Pereopod 3 similar to pereopod 2, all pereopods without robust or simple setae. Pereopod 7 basis 1.9 times as long as greatest width; ischium with slight bulbous protrusion on distal margin, 0.9 times as long as basis; merus proximal margin with slight 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 times as long as wide, 0.3 times as long as ischium; dactylus slender, 1.9 times as long as propodus, 3.3 times as long as basal width.

Pleopods simple; exopod larger than endopod, with 4-7 simple setae on peduncle of pleopods 2-5. Pleopod 1 exopod 1.3 times as long as wide, lateral margin weakly convex, distally broadly rounded, mesial margin straight; peduncle 3 times as wide as long. Endopod 1.6 times as long as wide, lateral margin convex, distally narrowly rounded, mesial margin straight, peduncle 2.4 times as wide as long. Pleopods 2-5 similar to pleopod 1, mesial margins becoming more strongly produced, peduncle lobes absent.

Uropod less than half the length of the pleotelson, peduncle 0.7 times longer than rami, peduncle lateral margin without setae, marginal setae absent, apices narrowly rounded. Endopod apically slightly pointed, 3.4 times as long as greatest width, lateral margin weakly convex, mesial margin straight, terminating without setae. Exopod extending to end of endopod, 2.3 times as long as greatest width, apically rounded, lateral margin distally convex, mesial margin straight, terminating without setae.

Variations. Intra-specific variation was observed among the examined specimens of Elthusa acutinasa sp. n. The size of the medial point formed at the anterior margin of pereonite 1 may vary. Some specimens portrayed an obvious, sharp medial point, while others only had a weak medial projection of the anterior margin of pereonite 1. Variation in the length of the uropods are slight, but one specimen had uropod rami extending to half the length of the pleotelson, while all the others specimens’ uropods were remarkably short. The overlapping of pleonite 5 lateral margins by pleonite 4 was consistent, except with one of the other examined paratype females, where pleonite 5 lateral margins were slightly visible. Some variation was also noted in the width of pleonite 1.

Etymology.

The epithet is a noun in the genitive singular. The species name acutinasa was derived by the son of one of us (NJS) from a combination of the two Latin words acute and nasus . The word acute translates to a feature that is pointy or ends with a sharp point; while nasus translates to nose. The combined word, acutinasa , therefore means pointy nose, and appropriately describes one of the characters of this species, which is its pointed anterior margin of the rostrum.

Size.

Ovigerous females (28.0-40.0 mm TL, 15.0-19.0 mm W), non-ovigerous females (19.0-24.0 mm TL, 10.0-14.0 mm W).

Distribution.

Known from the Indian Ocean, off the south coast of South Africa.

Hosts.

Not known (type material was collected from the fish sorting table following a trawl and not from a specific fish species).

Remarks.

Elthusa acutinasa sp. n. can be identified by its elongate, ovoid body shape; pointed anterior margin of the cephalon; anterior margin of pereonite 1 with short medial point; short, apically pointed uropod rami, which extend to less than half of the length of the pleotelson; coxae 7 that extends past the posterior margin of pereonite 7; pleonite 5 lateral margins that are largely concealed by pleonite 4; pleonite 5 posterior margin with a slight medial point; pleonite 1 the longest of the pleonites; and pleopod 5 endopod approximately half the size of the exopod.

Several characters differentiate between E. acutinasa sp. n. from E. raynaudii (see Table 1). Elthusa acutinasa sp. n. has a prominent, pointed cephalon anterior margin with a medially pointed pereonite 1 anterior margin compared to the straight anterior margin of E. raynaudii cephalon and pereonite 1. Pleon differences include the longer pleotelson of E. acutinasa sp. n. with pleonite 1 widest and pleonite 5 lateral margins concealed by those of pleonite 4 (not seen in E. raynaudii ). Elthusa acutinasa sp. n. also has short uropods that do not extend to the half of the pleotelson length, whereas those of E. raynaudii reach to, or extend past, the half of the pleotelson length.

Elthusa acutinasa sp. n. can also be distinguished from E. xena sp. n. by its short uropods and coxae 7 that extend past the posterior margin of pereonite 7. Further differences are found within pleon morphology, where E. acutinasa sp. n. pleonite 5 lateral margins are largely concealed by pleonite 4, whereas those of E. xena sp. n. are visible. Pleonite 1 in E. xena sp. n. is as wide as the other pleonites, whereas pleonite 1 in E. acutinasa sp. n. is narrower than the other pleonites. The pleotelson shape of E. acutinasa sp. n. is evenly rounded, compared to the roughly quadrate pleotelson of E. xena sp. n. (see Table 1).