# Effects of Genetic Diversity on Health Status and Parasitological Traits in a Wild Fish Population Inhabiting a Coastal Lagoon

## TL;DR

This study shows that fish with medium genetic diversity in a coastal lagoon have worse health and more parasites, suggesting parasites drive disruptive selection.

## Contribution

The study identifies parasite-mediated disruptive selection in a wild fish population and reveals two genetic strategies for controlling parasitism.

## Key findings

- Fish with medium genetic diversity had worse health and higher parasite prevalence.
- Parasites exert greater pressure on genetically intermediate individuals, indicating disruptive selection.
- High homozygosity and high heterozygosity both reflect better immune responses to parasitism.

## Abstract

Parasites impose selective pressures on wild fish, depending on host genetic variability. Coastal lagoons are transitional ecosystems where no study exists on fish genetic diversity and their parasites. Black-striped pipefish were collected in summer from the Mar Menor (a Mediterranean lagoon, SE Spain). The frequency of individuals with a medium level of genetic diversity was lower in the sampled population. For this same category, both internal and external health indices indicated a worse status, as well as a higher number of parasitised fish. A particular type of natural selection appears to be acting: disruptive selection, where parasites exert a greater disease pressure against the genetically intermediate individuals. This study demonstrated two clear genetic strategies displayed by hosts to better control parasitism: (1) low diversity, and (2) high diversity. Both categories may reflect a stronger immune response. Thus, parasites can change genetic diversity within animal populations, which will affect the evolution of host species.

Host genetic variability is relevant to understanding how parasites modulate natural selection in wild fish populations. Coastal lagoons are transitional ecosystems where knowledge lacks on relationships between genotypic diversity with parasitism. The aim of this study was to assess the effect of genetic diversity on host health and parasitological traits in fish inhabiting a Mediterranean lagoon. Black-striped pipefish Syngnathus abaster were collected in August 2023 and 2024 from the Mar Menor (Iberian lagoon, SE Spain). Genetic diversity was measured as Internal Relatedness (IR: a homozygosity index from microsatellite markers). Population frequency was lower for the medium IR level. For this same category, both health indices (external body condition and internal organs) indicated a worse status. Parasite prevalence, abundance and an index of life-cycle complexity (heteroxenous species) were greater for the medium level of genetic diversity. Such results are explained under a scenario of parasite-mediated disruptive selection: a higher disease pressure against the phenotypically intermediate individuals. Two contrasting strategies were detected to better control parasitism at the host genotypic level: (1) high homozygosity, and (2) high heterozygosity, which probably reflects better immuno-competence as a phenotypic trait. From an evolutionary perspective, parasites play a crucial role in shaping genetic diversity within host populations.