The full understanding of the parasite fauna of a host species requires sampling individuals across their entire distribution range. However, very few studies include an analysis considering such geographical coverage. The Ocellated killifish, Floridichthys polyommus, is an endemic estuarine fish whose distribution spans from the southwestern Gulf of Mexico to Belize. Here, we aimed at describing the metazoan parasite fauna of F. polyommus in localities across the Yucatán Peninsula to assess the parasite species richness at regional and local scales. From 2022 to 2024, 409 specimens of F. polyommus were collected from six localities. Data from three localities from a previous study were also included. Individuals were dissected, and examined for ecto and endoparasites. Twenty-four metazoan parasite taxa were identified using morphological characters and/or molecular data. Thirteen parasite taxa correspond to larval stages. Nine parasite taxa represent new host records, increasing to 35 the total number of metazoan parasites known for F. polyommus. The nematode Contracaecum sp. was found in all sampled localities, whereas the cestode Glossocercus caribaensis and the copepod Ergasilus sp. were found in seven, with variable prevalence and mean intensity of infection values. Two general patterns were uncovered irrespective of the geographical scale of the analyses (local or regional): (1) Digeneans were the most species-rich parasite group and (2) F. polyommus harboured more larval parasite taxa than adults. It seems that the parasite fauna of the Ocellated killifish in each sampling site represent a subsample of the total species richness at the regional level.

Clinostomids are a group of digeneans in which substantial diversity has been recently discovered, with some metacercariae specific to their fish hosts. Genetic analysis has been instrumental in elucidating species diversity within this genus. Recently, seven COI lineages were reported in Argentina, along with three metacercarial morphotypes lacking molecular data. Two of these were found parasitising Rivulidae fishes. The discovery of Clinostomum metacercariae in Trigonectes aplocheiloides and Titanolebias monstrosus from temporary ponds in the western Chacoan region allowed us to redescribe the metacercariae previously identified by other authors and provide the first sequences of this lineage. The morphology of the metacercariae in both hosts matched previously reported descriptions. Genetic analysis clustered the new lineage with Clinostomum detruncatum, Clinostomum sp. 7, Clinostomum L1, and Clinostomum CRA. This discovery leaves only two morphological records of metacercariae to be characterised using DNA sequencing: one in another Rivulidae (Neofundulus paraguayensis) and one in a Loricaridae (Hypostomus sp.). The present results represent the eighth clinostomid lineage sequenced from Argentina, highlighting the extensive diversity in South America and the many lineages yet to be described or identified, considering that only one of these lineages is formally described based on adult specimens found in the heron Ardea cocoi.

Host behavioural modification by parasites is a common and well-documented phenomenon. However, knowledge on the complexity and specificity of the underlying mechanisms is limited, and host specificity among manipulating parasites has rarely been experimentally verified. We tested the hypothesis that the ability to infect and manipulate host behaviour is restricted to phylogenetically closely related hosts. Our model system consisted of the brain-encysting trematode Euhaplorchis sp. A and six potential fish intermediate hosts from the Order Cyprinodontiformes. Five co-occurring cyprinids were examined for naturally acquired brain infections. Then we selected three species representing three levels of taxonomic relatedness to a known host to experimentally evaluate their susceptibility to infection, and the effect of infection status on behaviours presumably linked to increased trophic transmission. We found natural brain infections of Euhaplorchis sp. A metacercariae in three cyprinids in the shallow sublittoral zone. Of the three experimentally exposed species, Fundulus grandis and Poecilia latipinna acquired infections and displayed an elevated number of conspicuous behaviours in comparison with uninfected controls. Euhaplorchis sp. A was able to infect and manipulate fish belonging to two different families, suggesting that ecological similarity rather than genetic relatedness determines host range in this species.