Review of the fish-parasitic genus Cymothoa Fabricius, 1793 (Crustacea: Isopoda: Cymothoidae) from Australia

Description of the Life Cycle Stages of the Parasitic Cymothoid, Cymothoa indica Schioedte and Meinert, 1884, and Its Prevalence in Commercial Fishes from Chilika Lagoon, India

PURPOSE

To study the morphology of all the life stages and the life cycle of the parasitic isopod Cymothoa indica Schioedte and Meinert, 1884, and its prevalence in different commercial fishes in the Chilika Lagoon, India.

METHODS

From April 2021 to December 2022, different life stages of the said parasites were collected from five different fish species from four localities of the Chilika Lagoon. The parasites were carefully removed from the host with a pair of forceps and then identified following standard literature and description. The morphology of all life stages, life cycle, and prevalence of the isopod C. indica were carried out as per the standard protocol.

RESULTS

A total of 1252 host fishes were analysed. The total prevalence was 44.5%. The host with the highest prevalence of C. indica was Glossogobius giuris, with 58.79% of the hosts parasitized by at least one parasite individual. Followed by the host Datnoides polota (50.98%), Johnius dussumieri (46.25%), Etroplus suratensis (28.45%) and the least prevalence was in Lates calcarifer (14.70%). The description of the life stages is based on the observation of 523 females, 116 males, 46 Juveniles, and 50 embryonic stages of the said parasites.

CONCLUSION

The description of all the life stages and life cycles certainly helps to identify this species with clarity and ease in understanding the species' biology. In future, this study may be helpful in the effective management towards the improvement of aquaculture productivity.

Unraveling the host-parasite interaction: immune response in Oreochromis mossambicus to Cymothoa eremita (Isopoda, Cymothoidae) infection

The host immune response in Oreochromis mossambicus during natural infection with the isopod Cymothoa eremita was investigated. The naturally infected fishes were examined for enzyme profile, viz., respiratory burst activity (RBA), myeloperoxidase activity (MPO), and expression of immune-related genes, viz., toll-like receptor 22 (TLR 22), interleukin-1β (IL-1β), tumor necrosis factor α (TNF-α), complement component (C3), chemokine (CXCa), and β-actin in tissues of various organs (buccal cavity, gills and anterior kidney). Significant reduction (P < 0.05) in RBA and MPO was observed in the parasite-infected fishes when compared to the uninfected control fishes. In the buccal cavity, the expression of the immune-related genes was significantly (P < 0.05) upregulated, whereas all the genes except IL-1β were significantly (P < 0.05) upregulated in the anterior kidney. In the case of gill tissue, the expressed genes showed a varied type of regulation. The immunological responses in O. mossambicus during isopod infection have not been investigated in detail so far, and this is the first study unveiling such insights. Hence, this study will help to improve our molecular understanding of the host-immune response to parasitic isopod infection.

Confirmation on the occurrence of Cymothoa indica, and first record of Norileca indica, with a note on new host records of Nerocila arres, and Nerocila depressa (Isopoda: Cymothoidae) from Odisha coast, India

The parasitic isopod Cymothoa indica Schiöedte and Meinert, 1884 was recorded from the buccal cavity of the host Glossogobius giuris (Hamilton, 1822) collected from Chilka Lake, Odisha in 1924, but its identification was doubtful as mentioned by the author in his book on "Fauna of Chilika Lake: Tanaidacea and Isopoda". The present report of parasitic isopod C. indica from the buccal cavity of the host Sphyraena obtusata Cuvier, 1833 collected from the water of Bay of Bengal, Gopalpur-on-Sea confirms its occurrence along the Odisha coast. The record of the isopod parasite Norileca indica (H. Milne Edwards, 1840) from the branchial cavity of the host Atule mate (Cuvier, 1833) collected from the water of Bay of Bengal, Gopalpur-on-Sea is the first record of this parasite from the coastal water of Odisha, India. The record of Nerocila arres Bowman and Tareen, 1983 from the caudal peduncle of the host fish species Terapon puta Cuvier, 1829, and the isopod N. depressa Milne Edwards, 1840 from the host fish species Megalaspis cordyla (Linnaeus, 1758) are the new host records for these respective parasites.

A Taxonomic Review of the Buccal-attaching Fish Parasite genus Lobothorax Bleeker, 1857 (Crustacea: Isopoda: Cymothoidae) with Description of a New Species from Southwestern India

The genus Lobothorax Bleeker, 1857 is revised with the description of a new species collected from the gempylidaen fish Promethichthys prometheus (Cuvier, 1832) from the southwestern coast of India. A revised generic diagnosis is provided based on the redescription of the type species. Lobothorax aurita (Schioedte and Meinert, 1883) is here synonymised with Lobothorax typus Bleeker, 1857 based on the original description. Lobothorax nicosmiti Aneesh, Bruce and Kumar sp. nov. is described from the female stage and it is characterized by: pereonite 1 anterolateral expansion not extending to the anterior margin of cephalon; coarsely pitted pereonites dorsal surfaces; pereonites without dorsal median longitudinal ridges; anteriorly truncate cephalon; pleotelson about 0.6 times as long as wide, posterior margin weakly emarginate, broadly sub-truncate, lateral margins convex; maxilliped palp article 3 with three RS; pereopods basis much wider with prominent carina. A key to the species of Lobothorax Bleeker, 1857 is presented.

Isopods Cymothoidae ectoparasites of fish from the Amazon

Most freshwater species of Cymothoidae are distributed in South America. They have mainly been recorded in the eastern and western regions of the Amazon River basin. However, in this ecosystem, the biodiversity of this group may be greater if the entire Amazon basin is considered. In this regard, the aim of the present study was to provide an updated list of isopod species of the family Cymothoidae that are found in fish in the Brazilian Amazon region and to report on new fish host occurrences and expanded geographical distributions for cymothoid isopods that parasitize fish in the southwestern Brazilian Amazon region. The parasites found in fish specimens were collected, fixed and identified later. We found eight species of Cymothoidae parasitizing different host fish species in the southwestern Amazon region. However, we found 14 species of Cymothoidae throughout the Brazilian Amazon region. Three additional species are thus reported here, which increases the number of species of Cymothoidae in this region to 17. These additional species are also new records for Brazil. Therefore, this study has contribute to expand the knowledge about the distribution and diversity of Cymothoidae in the Amazon basin.

Architectural instability, inverted skews and mitochondrial phylogenomics of Isopoda: outgroup choice affects the long-branch attraction artefacts

The majority strand of mitochondrial genomes of crustaceans usually exhibits negative GC skews. Most isopods exhibit an inversed strand asymmetry, believed to be a consequence of an inversion of the replication origin (ROI). Recently, we proposed that an additional ROI event in the common ancestor of Cymothoidae and Corallanidae families resulted in a double-inverted skew (negative GC), and that taxa with homoplastic skews cluster together in phylogenetic analyses (long-branch attraction, LBA). Herein, we further explore these hypotheses, for which we sequenced the mitogenome of Asotana magnifica (Cymothoidae), and tested whether our conclusions were biased by poor taxon sampling and inclusion of outgroups. (1) The new mitogenome also exhibits a double-inverted skew, which supports the hypothesis of an additional ROI event in the common ancestor of Cymothoidae and Corallanidae families. (2) It exhibits a unique gene order, which corroborates that isopods possess exceptionally destabilized mitogenomic architecture. (3) Improved taxonomic sampling failed to resolve skew-driven phylogenetic artefacts. (4) The use of a single outgroup exacerbated the LBA, whereas both the use of a large number of outgroups and complete exclusion of outgroups ameliorated it.

A new genus and species of fish parasitic cymothoid (Crustacea, Isopoda) from the Indian Ocean coast of South Africa, with a key to the externally attaching genera of Cymothoidae

Bambalocraintwalagen. et sp. nov. is described from Sodwana Bay, north-eastern South Africa. The monotypic genus is characterised by the broadly truncate anterior margin of the head with a ventral rostrum, coxae 2–5 being ventral in position not forming part of the body outline and not or barely visible in dorsal view, and the posterolateral margins of pereonites 6 and 7 are posteriorly produced and broadly rounded. The antennulae bases are widely separated, with both antennula and antenna slender. The species is known only from the type locality and the known hosts are species of Pomacanthidae (Angelfish). A revised key to the externally attaching genera of Cymothoidae is provided.

Mitochondrial Architecture Rearrangements Produce Asymmetrical Nonadaptive Mutational Pressures That Subvert the Phylogenetic Reconstruction in Isopoda

The phylogeny of Isopoda, a speciose order of crustaceans, remains unresolved, with different data sets (morphological, nuclear, mitochondrial) often producing starkly incongruent phylogenetic hypotheses. We hypothesized that extreme diversity in their life histories might be causing compositional heterogeneity/heterotachy in their mitochondrial genomes, and compromising the phylogenetic reconstruction. We tested the effects of different data sets (mitochondrial, nuclear, nucleotides, amino acids, concatenated genes, individual genes, gene orders), phylogenetic algorithms (assuming data homogeneity, heterogeneity, and heterotachy), and partitioning; and found that almost all of them produced unique topologies. As we also found that mitogenomes of Asellota and two Cymothoida families (Cymothoidae and Corallanidae) possess inversed base (GC) skew patterns in comparison to other isopods, we concluded that inverted skews cause long-branch attraction phylogenetic artifacts between these taxa. These asymmetrical skews are most likely driven by multiple independent inversions of origin of replication (i.e., nonadaptive mutational pressures). Although the PhyloBayes CAT-GTR algorithm managed to attenuate some of these artifacts (and outperform partitioning), mitochondrial data have limited applicability for reconstructing the phylogeny of Isopoda. Regardless of this, our analyses allowed us to propose solutions to some unresolved phylogenetic debates, and support Asellota are the most likely candidate for the basal isopod branch. As our findings show that architectural rearrangements might produce major compositional biases even on relatively short evolutionary timescales, the implications are that proving the suitability of data via composition skew analyses should be a prerequisite for every study that aims to use mitochondrial data for phylogenetic reconstruction, even among closely related taxa.

The complete mitochondrial genome of Cymothoa indica has a highly rearranged gene order and clusters at the very base of the Isopoda clade

As a result of great diversity in life histories and a large number of described species, taxonomic and phylogenetic uncertainty permeates the entire crustacean order of Isopoda. Large molecular datasets capable of providing sufficiently high phylogenetic resolution, such as mitochondrial genomes (mitogenomes), are needed to infer their evolutionary history with confidence, but isopod mitogenomes remain remarkably poorly represented in public databases. We sequenced the complete mitogenome of Cymothoa indica, a species belonging to a family from which no mitochondrial genome was sequenced yet, Cymothoidae. The mitogenome (circular, 14484 bp, A+T = 63.8%) is highly compact, appears to be missing two tRNA genes (trnI and trnE), and exhibits a unique gene order with a large number of rearrangements. High compactness and the existence of palindromes indicate that the mechanism behind these rearrangements might be associated with linearization events in its evolutionary history, similar to those proposed for isopods from the Armadillidium genus (Oniscidea). Isopods might present an important model system to study the proposed discontinuity in the dynamics of mitochondrial genomic architecture evolution. Phylogenetic analyses (Bayesian Inference and Maximum Likelihood) conducted using nucleotide sequences of all mitochondrial genes resolved Oniscidea and Cymothoida suborders as paraphyletic. Cymothoa indica was resolved as a sister group (basal) to all remaining isopods, which challenges the accepted isopod phylogeny, where Cymothoida are the most derived, and Phreatoicidea the most basal isopod group. There is growing evidence that Cymothoida suborder might be split into two evolutionary distant clades, with parasitic species being the most basal split in the Isopoda clade, but a much larger amount of molecular resources carrying a high phylogenetic resolution will be needed to infer the remarkably complex evolutionary history of this group of animals with confidence.