Cymothoa Fabricius, 1793

Martin, Melissa B., Bruce, Niel L. & Nowak, Barbara F., 2016, Review of the fish-parasitic genus Cymothoa Fabricius, 1793 (Crustacea: Isopoda: Cymothoidae) from Australia, Zootaxa 4119 (1), pp. 1-72 : 5-6

Genus Cymothoa Fabricius, 1793

Cymothoa Fabricius, 1793: 503 .— Milne Edwards, 1840: 264.— Schioedte & Meinert, 1884: 223.— Kussakin, 1979: 289.— Brusca, 1981: 185.— Brusca & Iverson, 1985: 45.— Trilles, 1994: 137.— Hadfield, Bruce & Smit, 2011: 58 .— Hadfield, Bruce & Smit, 2013: 153 .

Type species. Oniscus oestrum Fabricius, 1793 ; by subsequent designation ( Kussakin 1979).

Remarks. The genus diagnosis has been comprehensively revised by Hadfield et al. (2011, 2013). Cymothoa can be identified by the strongly vaulted body; slender antennae with widely separated bases, antenna longer than antennula; subtruncate rostrum; pereopods 5–7 basis with broad blade-like carina; coxae partially visible dorsally; pereonite 7 posterolateral margins extending past pleonite 1; and pleopods 3–5 with large fleshy folds.

Brusca (1981) presented a phylogeny of the Cymothoidae family, proposing three evolutionary linages based on host site attachment. Brusca (1981) suggested that cymothoids evolved first as externally-attaching parasites and then further evolved to using the host’s buccal cavity and gills.

The molecular analyses of Jones et al. (2008) (using 16S mtDNA) and Ketmaier et al. (2008) (using 16S rRNA and cytochrome oxidase I) did not support Brusca’s (1981) evolutionary hypothesis for the Cymothoidae . Neither of these analyses revealed distinct clades for the buccal, gill or externally-attaching genera. Jones’s et al. (2008) tree topology showed a basal division, with Nerocila Leach, 1818 sister to the buccal-attaching Cymothoa indica Schioedte & Meinert, 1884 and Olencira praegustator ( Latrobe, 1802) ; the other larger clade showed Ceratothoa as sister to the Anilocra clade. Ketmaier’s et al. (2008) analysis showed Nerocila as sister to Ceratothoa and the gill-attaching taxa within the larger clade. The analyses of Ketmaier et al. (2008) and Jones et al. (2008) did not resolve the relationships between the cymothoid genera and their results are not congruent with the results of Hadfield’s (2012) morphological analysis. However, it was emphasized that their results were based on small datasets of 11 and 6 species (excluding outgroup) respectively, and were not regarded as conclusive.

Hadfield (2012) presented the first morphological cladistics analysis of the Cymothoidae based on 23 genera and 40 multistate characters. The 50% majority rule tree showed that the external-attaching Anilocra group formed a morphologically well-supported clade that also included the gill-attaching Livoneca Leach, 1818 , the buccalattaching Smenispa and Paracymothoa Lemos de Castro, 1955 ( Hadfield 2012) . Sister to the Anilocra clade was the buccal-attaching Ceratothoa clade, with the remaining genera basally unresolved ( Hadfield 2012). The Ceratothoa group formed a cohesive clade, with Cymothoa sister to the Cinusa + Lobothorax and Ceratothoa + Glossobius upheld by pereonite 1 anterolateral margins encompassing the cephalon (developed into lobes in Glossobius ), pereopods 5–7 basis with large blade-like carina (except Lobothorax which has no carina), and maxilla medial lobe partly fused ( Hadfield 2012).

Smit et al. (2014) showed that the highest cymothoid diversity is found within the tropics and rapidly drops as latitude increases. This is relevant to Cymothoa , where nine species are each found in the central Indo-Pacific and tropical Atlantic region and eight species in the western Indo-Pacific. Thirteen species of Cymothoa were known from Australian waters, and this number has not changed despite the current review for the genus, suggesting that perhaps different cymothoid genera have different levels of diversity or distribution.