Elthusa Schiöedte & Meinert, 1884

Ravichandran, S., Vigneshwaran, P. & Rameshkumar, G., 2019, A taxonomic review of the fish parasitic isopod family Cymothoidae Leach, 1818 (Crustacea: Isopoda: Cymothooidea) of India, Zootaxa 4622 (1), pp. 1-99 : 24

Genus Elthusa Schiöedte & Meinert, 1884

Elthusa Schiöedte & Meinert, 1884: 337 .— Bruce, 1990: 254.— Trilles 1994: 164.— Trilles & Randall 2011: 453–454.— Hadfield, Tuttle & Smit, 2017b: 127 .

Type species. Livoneca emarginata Bleaker, 1857 , by monotype ( Schiöedte & Meinert 1884).

Diagnosis. A revised diagnosis of the genus was provided by Bruce (1990) as well as Trilles & Randall (2011).

Remarks. Elthusa can be identified are the weakly vaulted body shape; pleonite 1 as wide as or slightly narrower than pleonite 2; posterior margin of cephalon not trilobed; wide pleon; cephalon posterior argin not trilobed; a usually wide pleon, antennula shorter than antenna and all pleopods lamellar.

Elthusa is currently one of the most varied and species-rich isopod genera parasitizing marine fishes. The genus was recently redefined and revised by Bruce (1990), and now contains about 33 species, most of which were previously housed in Livoneca Leach, 1818 (see Bruce, 1990 for list of species that have at any time been placed in Livoneca ; for the official spelling Livoneca see: Opinion 1849, Bull. zool. Nomencl., 53(3): 210–212 (1996)).

Elthusa can be regarded as one of the most morphologically varied cymothoid genera, with many Elthusa species still requiring detailed study and redescription. The antennula position (being close together or far apart), shape of the cephalon anterior margin, as well as the width of the pleon and pleonite 5 are some of the varying characters noted within species of this genus ( Bruce 1990). This variation can cause confusion in genus identifications and may lead to incorrect species placement if not taken into account with all of the other genus characters.

Elthusa seems to be closely allied to the genus Rhexana Schiöedte & Meinert, 1883 replaced with Rhexanella by Stebbing (1911), but later placed in synonymy with Ceratothoa by Bruce & Bowman (1989). However, Trilles (1979) and Bruce & Bowman (1989) did not have the opportunity of seeing the type material of Rhexana verrucosa Schiöedte & Meinert, 1883 , for which the genus was founded, deposited in the collection of the RMNH. A male specimen held in the collection of the MNHN was only reported by Trilles (1972). So, further studies are necessary to conclude really on that point.