# Mitochondrial Genomes of the Blood Flukes Cardicola forsteri and Cardicola orientalis (Trematoda: Aporocotylidae)

## TL;DR

This paper reports the first mitochondrial genomes of two blood fluke species that infect tuna, providing a valuable resource for studying these economically important parasites.

## Contribution

The study presents the first mitochondrial genomes for any aporocotylid species, revealing gene content and diversity insights.

## Key findings

- Both Cardicola species have mitogenomes with 12 protein coding, 2 ribosomal, and 22 tRNA genes.
- A control region with distinct repeat patterns was identified in each species.
- High nucleotide diversity was observed between the two species.

## Abstract

Parasitic diseases can be a significant constraint on aquaculture industries, which continue to develop in response to the rise in global demand for sustainable protein sources. Blood flukes, Cardicola forsteri and Cardicola orientalis, are economically significant parasites of Southern bluefin tuna (Australia), Pacific bluefin tuna (Japan), and Atlantic bluefin tuna (Mediterranean) as they are responsible for blood vessel obstruction in the gills leading to branchitis and mortalities when untreated. Here, we have defined the mitochondrial genomes for these species—the first for any aporocotylids. Oxford nanopore long-read sequencing was used to sequence C. orientalis from a single individual. The mitochondrial genome of C. forsteri was assembled and curated from available sequence data. Both Cardicola spp. mitogenomes contained 12 protein coding, 2 ribosomal and 22 tRNA genes, with the gene order matching that of Asian schistosomes. A control region was identified for each species which contained long and short repeats; the region for C. forsteri was longest, and the overall pattern differed between the two species. A surprisingly high nucleotide diversity was observed between the two species, generating interest into the mitochondrial genes of related species. This paper provides a useful resource for future genetics-based research of aporocotylids and other flatworm parasites of socioeconomic significance.