Molecular detection of apicomplexan blood parasites of coral reef fishes from free-living stages of ectoparasitic gnathiid isopods

A new and widespread group of fish apicomplexan parasites

Apicomplexans are obligate intracellular parasites that have evolved from a free-living, phototrophic ancestor. They have been reported from marine environmental samples in high numbers,1 with several clades of apicomplexan-related lineages (ARLs) having been described from environmental sequencing data (16S rRNA gene metabarcoding).2 The most notable of these are the corallicolids (previously ARL-V), which possess chlorophyll-biosynthesis genes in their relic chloroplast (apicoplast) and are geographically widespread and abundant symbionts of anthozoans.3 Corallicolids are related to the Eimeriorina, a suborder of apicomplexan coccidians that include other notable members such as Toxoplasma gondii.4Ophioblennius macclurei, the redlip blenny, along with other tropical reef fishes, is known to be infected by Haemogregarina-like and Haemohormidium-like parasites5 supposedly belonging to the Adeleorina; however, phylogenetics shows that these parasites are instead related to the Eimeriorina.6,7 Hybrid genomic sequencing of apicomplexan-infected O. macclurei blood recovered the entire rRNA operon of this apicomplexan parasite along with the complete mitochondrion and apicoplast genomes. Phylogenetic analyses using this new genomic information consistently place these fish-infecting apicomplexans, hereby informally named ichthyocolids, sister to the corallicolids within Coccidia. The apicoplast genome did not contain chlorophyll biosynthesis genes, providing evidence for another independent loss of this pathway within Apicomplexa. Based on the 16S rRNA gene found in the apicoplast, this group corresponds to the previously described ARL-VI. Screening of fish microbiome studies using the plastid 16S rRNA gene shows these parasites to be geographically and taxonomically widespread in fish species across the globe with implications for commercial fisheries and oceanic food webs.

Blood meal identification reveals extremely broad host range and host-bias in a temporary ectoparasite of coral reef fishes

Appreciation for the role of cryptofauna in ecological systems has increased dramatically over the past decade. The impacts blood-feeding arthropods, such as ticks and mosquitos, have on terrestrial communities are the subject of hundreds of papers annually. However, blood-feeding arthropods have been largely ignored in marine environments. Gnathiid isopods, often referred to as "ticks of the sea", are temporary external parasites of fishes. They are found in all marine environments and have many consequential impacts on host fitness. Because they are highly mobile and only associated with their hosts while obtaining a blood meal, their broader trophic connections are difficult to discern. Conventional methods rely heavily on detecting gnathiids on wild-caught fishes. However, this approach typically yields few gnathiids and does not account for hosts that avoid capture. To overcome this limitation, we sequenced blood meals of free-living gnathiids collected in light traps to assess the host range and community-dependent exploitation of Caribbean gnathiid isopods. Using fish-specific COI (cox1) primers, sequencing individual blood meals from 1060 gnathiids resulted in the identification of 70 host fish species from 27 families. Comparisons of fish assemblages to blood meal identification frequencies at four collection sites indicated that fishes within the families Haemulidae (grunts) and Lutjanidae (snappers) were exploited more frequently than expected based on their biomass, and Labrid parrotfishes were exploited less frequently than expected. The broad host range along with the biased exploitation of diel-migratory species has important implications for the role gnathiid isopods play in Caribbean coral reef communities.