Species richness of Myxobolus (Cnidaria, Myxozoa) parasites infecting thicklip grey mullet Chelon labrosus in the Douro River, Portugal

Abstract

Background: Myxozoans are widespread cnidarian parasites that mostly infect fish and annelids as temporary and definitive hosts, respectively [1]. Mullets are a diverse fish group, which ubiquitous nature leaves vulnerable to parasitic infections [2]. Objective: This study aimed to acknowledge the myxozoan diversity infecting thicklip grey mullet Chelon labrosus in the Douro River estuary in northern Portugal. Methods: The internal and external organs of 13 specimens were macro- and microscopically examined. Cysts and infected tissues were individually photographed and processed for sequencing of the small subunit ribosomal gene (18S rDNA). Phylogenetic reconstructions were performed using maximum likelihood and Bayesian inference methodologies. Results: Eleven potentially new Myxobolus spp. were morphologically described and molecularly characterized. Additionally, a novel host, geographic region and morphometric profile was reported for Myxobolus pupkoi Gupta et al., 2022. Molecular comparisons further matched two of the novel Myxobolus sequences with sphaeractinomyxon types previously reported from marine oligochaetes in another Portuguese estuary. Phylogenetic analyses revealed the novel sequences clustering according to host affinity, with tree topologies resolving well-supported lineages of myxobolids infecting mullets from the genera Chelon, Mugil, Crenimugil and Planiliza. Conclusions: The elevated number of potentially novel Myxobolus spp. found in C. labrosus confirmed the successful hyperdiversification of these myxozoans in mullets [2], further reinforcing molecular-based comparisons as imperative for taxonomic descriptions. Morphometrical divergence between geographical isolates of M. pupkoi was hypothesized to correlate with adaptation to distinct abiotic factors and annelid communities. The formation of more than one Chelon-infecting lineage revealed that myxobolids entered this genus multiple times during their evolution. Lastly, the matching of two novel Myxobolus sequences with sphaeractinomyxon types reinforces the latter as specific life-cycle counterparts of mugiliform-infecting Myxobolus [3]. The large number of unmatched sphaeractinomyxon sequences positioned within the Chelon lineages suggests that Myxobolus diversity in Portuguese estuaries remains underestimated.