Elthusa Schioedte & Meinert, 1884

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

Genus Elthusa Schioedte & Meinert, 1884

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).

The original description by Bleeker (1857) did not specify any host species, genus or even family ("the skin of various species of fish") and Trilles and Randall’s (2011) redescription is not supported by or based on specimens being from the same host species or genus. Trilles and Randall (2011) did not examine Bleeker’s remaining syntype, and comparison of the two accounts suggest that there are some differences between the Bleeker (1857) figures and those of Trilles and Randall (2011); most notably being the shape of the cephalon, which is truncate or subtruncate in the syntype but anteriorly concave in Trilles and Randall’s redescription; and the pleotelson in the syntype is broadly rounded ( “semi-circular”) while distally narrowed in Trilles and Randall’s redescription. Trilles and Randall (2011) made no direct reference to Bleeker’s (1857) description and did not comment on any perceived character difference. These differences suggest that direct comparison to Bleeker’s syntype is needed to confirm conspecificity of the specimens identified by Trilles and Randall (2011) as E. emarginata .