Due to the difficulty in accurately identifying cymothoids, these parasitic isopods are often incorrectly named or confused with other species. Within the genus Ceratothoa, a number of recent studies have aimed at clarifying some of the problematic species; however, several of the less studied species still require revision. This paper redescribes, from type material, several poorly known Ceratothoa species including C. angulata, C. capri, C. carinata, C. collaris, C. gilberti, C. gobii, C. guttata, C. italica, C. oestroides, and C. verrucosa, further resolving taxonomic uncertainties within the genus.

The genus Cymothoa Fabricius, 1793 is revised for southwestern Indian Ocean waters. Cymothoa borbonica Schioedte & Meinert, 1884 and C. eremita Brünnich, 1783 are redescribed. Cymothoa rotundifrons Haller, 1880, from Mauritius lacks type material and the host is unknown, therefore it is here relegated to nomen dubium. Cymothoa sodwana sp. nov., from Trachinotus botla (Carangidae), collected from the Kwazulu-Natal coast of South Africa, is described and is distinguished by the large, ovoid, hunched body with rugose dorsal surfaces; the anterolateral angles of pereonite 1 are narrow and rounded reaching half the length of the cephalon; the ischium of pereopod 7 has a large protrusion and pereonite 7 which laterally overlaps the pleon margins, extending posteriorly to the pleotelson.

Redescription of the monotypic genus Cinusa Schioedte and Meinert, 1884 (Isopoda, Cymothoidae), a buccal-cavity isopod from South Africa

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.

The genus Ceratothoa Dana, 1852 is revised for South African waters and re-diagnosed. Ceratothoa retusa (Schioedte & Meinert, 1883) is recorded from the eastern coast, and Ceratothoa africanae sp. n. and C. famosa sp. n. are described; C. imbricata (Fabricius, 1775) and C. trigonocephala (Leach, 1818), are redescribed, revised and excluded from the South African fauna. Ceratothoa africanae sp. n. can be distinguished by the stout body shape of the female; triangular cephalon with a pointed rostrum; short uropods which do not extend past the pleotelson; large carinae on the pereopod basis; a broad pleon; and large medial lobes on female pleopods. Ceratothoa famosa sp. n. is characterised by the long rectangular body shape; pereonite 1 with a raised medial protrusion; narrow antenna with antennule article 1 expanded; uropods which reach the posterior margin of the pleotelson; narrow rami on uropods; and no appendix masculina on pleopod 2 of the male specimens.

Three species of Mothocya are reported from the east coast of southern Africa: Mothocya plagulophora (Haller, 1880) from Maputo, Mozambique, from the gills of Hemiramphus far (Forsskål, 1775); Mothocya renardi (Bleeker, 1857) from diverse localities in South Africa and Mozambique, from the hosts Strongylura leiura (Bleeker, 1850) and Tylosurus choram (Rüppell, 1837); and Mothocya affinis sp. nov. from Sodwana Bay, South Africa, from the gills of Hyporamphus affinis (Günther, 1866). Mothocya affinis sp. nov. is characterised by relatively small size (maximum 16 mm); large, wide coxae on pereonite 7 that overlap the pleon; uropods that do not extend past the pleotelson posterior margin; produced anterolateral margins on pereonite 1; and a twisted pleon and pleotelson. Mothocya katoi Nunomura, 1992 and Mothocya toyamaensis Nunomura, 1993 are both transferred to the genus Ceratothoa, with M. katoi being placed into junior synonymy with Ceratothoa guttata (Richardson, 1910). Irona ogcocephalus Avdeev & Avdeev, 1974 and I. callionymus Avdeev & Avdeev, 1974 are both transferred to Elthusa, and Irona trillesi Rokicki, 1986 is synonymised with Mothocya longicopa Bruce, 1986. A key to the south-western Indian Ocean species of Mothocya is given, and a table summarising recent and new nomenclatural acts in the genus is provided.