Morphological and molecular characterisation of Cymothoa eremita (Brunnich, 1783) (Isopoda: Cymothoidae) from the South China Sea

A morphological description and molecular characterisation of the parasitic isopod Cymothoa eremita Brunnich, 1783, collected off the fish host Nemipterus furcosus (Valenciennes, 1830) from the South China Sea, is presented here. Since most cymothoid descriptions are primarily based on females, this paper highlights the morphological characteristics from 20 ovigerous female and 20 male C. eremita cymothoids, isolated from 300 fish individuals. Furthermore, our findings indicated significant variations in the morphological features of C. eremita, including: 1) average length and width dimensions, 2) the presence or absence of eyes, 3) the length of pleotelson, 4) the configuration of its posterior and lateral margins, and 5) the structure of the cephalon. These characteristics were compared among diverse fish host species reported in different geographical regions. Twenty one of 300 N. furcosus were infested with C. eremita, with a prevalence of 7%, and mean intensity of 1.9 (two cymothoids per individual fish) respectively. A total of 687 base pairs (bp) from the mitochondrial cytochrome oxidase subunit I (COI) gene were sequenced for C. eremita as a complementary measure to the morphological analysis undertaken in this study. These sequences were utilized to reconstruct a phylogenetic tree, incorporating our sequenced specimens and relevant sequences sourced from the GenBank database. The Maximum Likelihood (ML) phylogenetic tree comprised 45 cymothoid species, with partial COI gene sequences showing monophyletic clustering of most species within the same genus, which is in congruence with their morphological divergence. Continued efforts to conduct studies on the biology, taxonomy, ecology and genetics of both cymothoid and fish host are crucial to better understand the patterns of host-parasite specificity and co-evolutionary changes, which may indirectly contribute towards cymothoid speciation and diversification.