Review of the fish parasitic genus Elthusa Schioedte & Meinert, 1884 (Crustacea, Isopoda, Cymothoidae) from South Africa, including the description of three new species

Abstract The branchial-attaching cymothoid genus, Elthusa Schioedte & Meinert, 1884 is a genus with a worldwide distribution of 36 species, including the three species described here. Elthusaraynaudii (Milne Edwards, 1840) is the only species that has been described from southern Africa. All South African material held at the National Museum of Natural History, Paris, France (MNHN) and the Iziko South African Museum, Cape Town (SAMC) identified as, or appearing to belong to, Elthusa was examined. Four species were identified, Elthusaraynaudii and three species that proved to be undescribed. Elthusaxenasp. n. can be distinguished by an evenly rounded pereonite 1 anterior margin, a roughly rectangular pleotelson, and narrowly rounded uropod apices that extend to more than half the length of the pleotelson. Elthusaacutinasasp. n. is identified by the produced and narrowly rounded cephalon anterior margin, acute uropods that are shorter than half the length of the pleotelson, and pereonite 1 anterior margin with medial projection. Elthusarotundasp. n. is characterised by the round body shape, broadly rounded uropod apices, and protrusions on the proximal and lateral margins of the merus and carpus of pereopod 7. A key to the South African Elthusa species is provided, together with a table summarising the hosts and localities of the 33 previously known species of Elthusa.


Introduction
Elthusa Schioedte & Meinert, 1884 is a branchial cavity-inhabiting cymothoid genus that was described as a monotypic genus for Elthusa emarginata (Bleeker, 1857). Elthusa was subsequently largely overlooked until Bruce (1990) provided a new diagnosis based on one of Bleeker's (1857) syntypes and the Australian species of the genus. Most species of Elthusa were originally described and placed within the genus Livoneca before their revision and redescription by Bruce (1990).
Currently, there are 33 known and accepted Elthusa species (Öktener et al. 2018a). Elthusa is one of the more speciose genera within the family Cymothoidae Leach, 1818, however many species of Elthusa still need to be studied and redescribed due to their original descriptions being inadequate, lacking morphological detail and illustrations. The high morphological intraspecific variability that exists within this genus (Hadfield et al. 2017) has also contributed, in some cases, to misidentifications and confusion regarding the placement of species.
Most species of Elthusa inhabit the branchial cavities of their fish hosts (Smit et al. 2014), with the exception of two species. Elthusa neocytta (Avdeev, 1975) ovigerous females have been recorded from the buccal cavity of the spiky oreo, Neocyttus rhomboidalis Gilchrist, 1906(see Stephenson 1987, and Elthusa splendida (Sadowsky & Moreira, 1981) has been described from the buccal cavity of the spiny dogfish Squalus cubensis Rivero, 1936 (see Sadowsky and Moreira 1981).
Elthusa is considered to be cosmopolitan, except for polar waters (Bruce 1990, Bruce et al. 2002, Rocha-Ramírez et al. 2005, Hadfield et al. 2017, and is predominantly recorded from the Indo-West Pacific (see Bruce 1990, Trilles andJustine 2006) with only occasional records of species from the Eastern Pacific (Brusca 1978, Espinosa-Pérez andHendrickx 2001), the Atlantic (Kensley and Brusca 2001) and the Mediterranean Justine 2006, Öktener et al. 2018a). Elthusa raynaudii (Milne Edwards, 1840) is the only species of Elthusa that has been described from sub-Saharan Africa. The lack of species records is most likely due to the lack of studying cymothoid isopods from this region and is not a true representation of the biodiversity of this genus. This paper forms part of a detailed study on the cymothoids from sub-Saharan Africa and confirms this postulation with the identification of three new species from the region.

Materials and methods
Twenty-seven specimens of Elthusa were examined. Material loaned from the National Museum of Natural History, Paris, France (MNHN) and the Iziko South African Museum, Cape Town (SAMC) was included in the examination. These specimens were collected as early as 1840 (MNHN) and 1960 (SAMC). Non-museum material was collected during 1993 in the intertidal zone of Alexander Bay, as well as from deep-sea trawlers during January 1999 and April 2003 off the south coast (RV Africana), and during February 2010 off the west coast of South Africa (RV Dr Fridtjof Nansen).
Specimens were identified by illustrating all body parts and appendages using a Nikon SMZ1500 Stereo Microscope and a Nikon Eclipse80i Compound Microscope, both equipped with drawing tubes. The position of specimens and dissected parts were manipulated to obtain the most accurate direct and complete view in order to minimise errors in illustrated ratios of segments. Material loaned from national museums was not dissected. Species descriptions were made with the aid of the taxonomy software package DELTA (Descriptive Language for Taxonomy) (see Coleman et al. 2010), following a general Cymothoidae character data set originally developed by Hadfield et al. (2010) and recently updated for other genera (Hadfield et al. 2013(Hadfield et al. , 2016b. Ratios and measurements for the descriptions were made using the maximum values at the middle of the specific measured segment, and all proportional measurements were rounded to one decimal place. Higher-level classification follows that of Brandt and Poore (2003). Host authorities are not included in the text or references; host nomenclature and distribution were sourced from FishBase (see Froese and Pauly 2018) and Catalog of Fishes (see Eschmeyer 2018 Elthusa: Schioedte and Meinert 1884: 337;Bruce 1990: 254;Trilles and Randall 2011: 453;Hadfield et al. 2017: 3. Type species. Livoneca emarginata Bleeker, 1857; by monotypy (Schioedte and Meinert 1884). The original number of type specimens that were available to Bleeker (1857) is unknown. A single female syntype, examined by Bleeker (1857), is deposited at the Naturalis Biodiversity Center (previously the Rijksmuseum von Natuurlijke Historie), Leiden (RMNH.CRUS.I.66). Another type specimen from the latter museum has been lost. The specimen examined by Schioedte and Meinert (1884) is held at the Natural History Museum in Paris (MNHN241) (Trilles 1976). Remarks. Species from Elthusa can be distinguished from other genera by having a weakly vaulted dorsum with a wide pleon; antennulae that are shorter than, or subequal in length to antennae, bases not in contact; a cephalon posterior margin that is not trilobed; and lamellar pleopods. Other diagnostic characters include a slender maxilliped palp article 3, with setae present; as well as pereopods with relatively short dactyli (see Bruce 1990 for a revised diagnosis of the genus). Trilles and Randall (2011) redescribed the type species for the genus, E. emarginata. This redescription provided a more detailed description and more accurate drawings of the species that had previously not been possible due to the fragility of the syntype. It also allows for a diagnosis and description of the genus based on the type material. However, Trilles and Randall (2011) designated one of the examined specimens [material deposited by Schioedte and Meinert (1884) into the Natural History Museum in Paris, MNHN No. 241] as the lectotype for the species. This does not follow the ICZN rules (Article 74.1) for lectotype designation as there is extant type material (RMNH.CRUS.I.66). Furthermore, no figures were provided of the designated lectotype material to ensure recognition of the specimen designated (ICZN Article 74.7.2). As such this lectotype designation is invalid and set aside (ICZN Article 74.2).
Antennula shorter than antenna, consisting of eight articles; antennula peduncle articles I and II distinct and articulated, extending to anterior of pereonite 1. Antenna consists of eleven articles, extending to middle of pereonite 1.
Pereopod 1 basis 1.6 times as long as greatest width; ischium 0.7 times as long as basis; merus proximal margin without bulbous protrusion; carpus with rounded proximal margin; propodus 1.4 times as long as wide; dactylus slender, 1.6 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, 2.5 times as long as greatest width; ischium without protrusions, 0.5 times as long as basis; merus 0.7 times as long as wide, 0.4 times as long as ischium; carpus without bulbous protrusion, 0.7 times as long as wide, 0.3 times as long as ischium; propodus 0.8 times as long as wide, 0.3 times as long as ischium; dactylus slender, 2.3 times as long as propodus, 3.5 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 narrowly rounded, mesial margin straight; peduncle 2.3 times as wide as long.
Uropod more than half the length of pleotelson; peduncle 0.5 times longer than rami, lateral margin without setae; rami not extending beyond pleotelson, apices broadly rounded. Endopod apically rounded, 2.7 times as long as greatest width, terminating without setae. Exopod extending to end of endopod, 2.2 times as long as greatest width, apically rounded, terminating without setae.
Variations. Intra-specific variations can cause difficulty in identification and should be taken into consideration. One of the more obvious variations is the overall body shape of examined individuals, as seen from the dorsal view. While the syntype (MNHN-IU-2016-9885) has weakly convex, symmetrical lateral margins, specimen SAMC-A089957 is not as symmetrical, with the right margin being strongly convex and that of the left margin, weakly convex. The latter specimen therefore appears to be less symmetrical. Bruce (1990) mentioned this occasional asymmetrical body shape as an observed variation, as a result of slightly twisted individuals. The body shape of the South African specimen (SAMC-A089957) accords to the shape of individuals illustrated and described by Bruce (1990). In addition, the widest part of this species may vary between pereonite 4 and pereonite 5. This variation may also cause individual body shapes to appear dissimilar. The anterior margin of the cephalon of the syntype (MNHN-IU-2016-9885) appears to be rounded rather than subtruncate. The posterior margin of pleonite 5 can be roughly straight (AM G2181 from Bruce 1990), have a slight medial point, or be weakly concave (Bruce 1990, present study). Although   Bruce (1990) described the uropods as being short, most measure more than half the length of the pleotelson.
Remarks. Elthusa raynaudii can be distinguished by the cephalon having a narrowly truncate rostrum; pereonite 1 with anterior margin straight; pleonites subequal in shape and width; and broadly rounded uropod apices that extend to more than half the length of the pleotelson.
Originally described in 1840, from the Cape of Good Hope in South Africa, from an unknown host, Elthusa raynaudii has been recorded numerous times from a wide range of localities within the Indo-Pacific region. It is the only species of Elthusa that has been described from sub-Saharan Africa. It has been recorded from an unknown host from the Cape of Good Hope (see Milne Edwards 1840); from the rocksucker, Chorisochismus dentex (Pallas, 1769) near Cape Town (Table Bay) (see Barnard 1920); from a wrasse in Durban (see Barnard 1955); as well as from the striped trumpeter, Latris lineata (Forster, 1801) (see Kensley 1976).
Elthusa sigani Bruce, 1990, which is only known from its type locality in Queensland, Australia, seems to be most similar to E. raynaudii. Elthusa sigani can be distinguished from E. raynaudii by having an evenly concave pereonite 1 anterior margin; a flat, straight cephalon anterior margin; and coxae 7 that extend past the posterior margin of pereonite 7. In addition, E. sigani is a much smaller species in overall body length range (9.0-13.0 mm), compared to E. raynaudii (20.0-26.7 mm).
Antennula shorter than antenna, consisting of eight articles; peduncle articles I and II distinct and articulated, extending to anterior of pereonite 1. Antenna consists of eleven articles, extending to past anterior margin of pereonite 1.
Pereopod 1 basis 1.8 times as long as greatest width; ischium 0.7 times as long as basis; merus proximal margin without bulbous protrusion; carpus with rounded proximal margin; propodus 1.8 times as long as wide; dactylus slender, 0.8 times as long as propodus, 2.3 times as long as basal width. Pereopods 2-3 similar to pereopod 1, all pereopods without robust or simple setae. Pereopod 7 basis with carina, 1.5 times as long as greatest width; ischium without protrusions, 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, 0.9 times as long as wide, 0.5 times as long as ischium; propodus as long as wide, 0.4 times as long as ischium; dactylus slender, 1.9 times as long as propodus, 3.1 times as long as basal width. Pleopods simple, exopod larger than endopod. Pleopod 1 exopod 1.1 times as long as wide, lateral margin strongly convex, distally broadly rounded, mesial margin weakly convex; peduncle 2.8 times as wide as long.
Uropod more than half the length of pleotelson, peduncle 0.8 times longer than rami, peduncle lateral margin without setae; rami not extending beyond pleotelson, apices narrowly rounded. Endopod apically rounded, 2.5 times as long as greatest width, lateral margin weakly convex, mesial margin straight, terminating without setae. Exopod extending beyond end of endopod, twice as long as greatest width, apically rounded, lateral margin weakly convex, mesial margin straight, terminating without setae.
Description (paratype intermoult ♂). Figs 6, 7. Male similar to female but smaller. Specimen mid-moult. Body rectangular, not twisted, twice as long as greatest width. Pereonite lateral margins mostly subparallel. Cephalon 0.7 times longer than wide. Frontal margin rounded to form blunt rostrum. Eyes oval with distinct margins; one eye 0.2 times width of cephalon; 0.5 times length of cephalon. Pereonite 1 smooth, anterior border concave, extending past base of cephalon. Posterior margins of pereonites smooth and straight, except pereonite 4 and 5. Coxae 2-3 wide, with posteroventral angles rounded; coxae 4-7 rounded. Pereonites 6 and 7 narrower, becoming more progressively rounded posteriorly. Pleon 0.3 times as long as total body length, with pleonite 1 largely concealed by pereonite 7, slightly visible in dorsal view; pleonites 1-3 posterior margin posteriorly concave, smooth and slightly curved laterally. Pleonite 5 overlapped by lateral margins of pleonite 4, with posterolateral angles narrowly rounded, posterior margin straight. Pleotelson 0.8 times as long as anterior width, lateral margins straight or weakly convex, posterior margin broadly truncate. Antennula shorter than antenna, consisting of eight articles. Antenna consists of ten articles, extending to middle of pereonite 1.
Pereopod 1 basis twice as long as greatest width; ischium 0.6 times as long as basis; propodus 1.6 times as long as wide; dactylus 1.1 times as long as propodus, 3 times as long as basal width. Pereopod 7 twice as long as greatest width; ischium 0.7 times as long as basis; merus proximal margin without bulbous protrusion, 0.7 times as long as wide, 0.4 times as long as ischium; carpus without bulbous protrusion, 0.7 times as long as wide, 0.4 times as long as ischium; propodus 1.3 times as long as wide, 0.6 as long as ischium; dactylus slender, 1.4 times as long as propodus, 2.7 times as long as basal width.
Pleopod 1 exopod 1.2 times as long as wide, lateral margin weakly convex, distally broadly rounded, mesial margin straight; endopod 2.1 times as long as wide, lateral margin weakly convex, mesial margin straight, peduncle 2.2 times as wide as long. Pleopod 2 appendix masculina with parallel margins, 1.1 times as long as endopod, distally narrowly rounded.
Uropod same length or slightly longer than the pleotelson, peduncle 0.4 times longer than rami, rami extending slightly beyond pleotelson, apices narrowly rounded. Endopod apically slightly pointed, 3 times as long as greatest width. Exopod 2.6 times as long as greatest width.
Penes medially adjacent; penial process 0.7 times as long as basal width. Etymology. The epithet is constructed in a possessive form of a personal name. This species is named after Xena, the warrior princess, in reference to the strong nature of the female cymothoid isopod.
Size. Ovigerous female (34.0 mm TL, 17.0 mm W), male (8.0 mm TL, 4.0 mm W). Distribution. Currently only known from the mouth of the Orange River, Alexander Bay, South Africa (Atlantic Ocean).
Hosts. Clinus superciliosus (Linnaeus, 1758). This is the first record of a klipfish (of the genus Clinus Cuvier, 1816), and of the intertidal super klipfish, Clinus supercilious, as a fish host of a species of Elthusa. This host belongs to the fish order Perciformes, and is endemic to the Southeast Atlantic Ocean, from northern Namibia to the Kei River of South Africa (Smith and Heemstra 1986).
Remarks. Elthusa xena sp. n. female can be identified by the elongate, ovoid body shape; coxae 7 that do not extend past the posterior margin of pereonite 7; a bluntly pointed anterior margin of the cephalon; evenly rounded, slightly concave anterior margin of pereonite 1; uropod rami with apices narrowly rounded and more than half the length of pleotelson; pleonite 5 posterior margin with indentations; and the pleotelson is short, roughly quadrate, with margins that curl upward.
Two other Elthusa species have been recorded from related perciform fish hosts from the family Clinidae Swainson, 1839 (blennies). Elthusa californica (Schioedte & Meinert, 1884) was noted from the striped kelpfish Gibbonsia metzi Hubbs, 1927; and Elthusa menziesi (Brusca, 1981) from both the spotted kelpfish Gibbonsia elegans (Cooper, 1864) and the crevice kelpfish Gibbonsia montereyensis Hubbs, 1927. However, this is the first record of Elthusa collected from a Clinus sp.  Elthusa xena sp. n. can be distinguished from E. raynaudii by having a bluntly pointed cephalon anterior margin, compared to the narrowly truncate margin of E. raynaudii. Other differences include the shape of the pleotelson (which is quadrate, wide and short for E. xena sp. n., and evenly rounded for E. raynaudii); pleonite 1 is the same length as the other pleonites in Elthusa xena sp. n. but narrower in E. raynaudii; and the uropod apices of E. xena sp. n. are narrowly rounded compared to the broadly rounded apices of E. raynaudii uropods. See Table 1   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.
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. 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).  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.
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.
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).

Conclusions
From previous collections across South Africa, four Elthusa species were recognised. Elthusa raynaudii, the only known Elthusa species from South Africa, was identified along with three new species from this genus. These new species, E. xena sp. n., E. acutinasa sp. n., and E. rotunda sp. n., more than double the known records of Elthusa from this region. Descriptions were provided for the three new Elthusa species along with an identification key with diagnostic characters to distinguish between the sub-Saharan Elthusa species (Table 1). A summative table was provided with currently known information on all species from the genus Elthusa, including host and location records of each (Table 2).

TH:
Unknown See in text.

OL:
See text

OL:
North-western Pacific; Australia; Japan and Pacific coast

OH:
Species from the families Synaphobranchidae;