Parasites as indicators of habitat preferences by hybrids between estuarine and freshwater congeneric silversides (Atherinopsidae)

Abstract

Silversides of the genus Odontesthes are characterized by their conserved morphology and vast phenotypic plasticity. They have developed complex population structures, promoting hybridization and speciation phenomena and often leading to intricate taxonomies. Within this genus, O. argentinensis, an originally marine species and O. bonariensis, a freshwater species, coexist in Mar Chiquita Coastal Lagoon (central Argentina). This lagoon has an inlet to the sea, creating a saline gradient that promotes highly variable conditions throughout the system. While O. argentinensis has established a resident population inside the lagoon, O. bonariensis is only sporadically found, inhabiting mainly tributary streams. Notably, hybrids between the two have been found in the lagoon. Parasites can significantly influence hybridization in host species, either promoting or hindering the process. From an evolutionary standpoint, parasites can impact hybridization by altering host immune responses, affecting host ecology and fitness. As a result, parasites can be valuable indicators of hybrid host ecology and their evolutionary trajectories. The present study analyzes the parasite communities of these sympatric Odontesthes and their hybrids, and determines their habitat preferences. While O. argentinensis showed evidence of marine parasites, O. bonariensis had a dominance of freshwater species as expected. And, although parasites of hybrids displayed similarities with both parental species, they indicated a preference of these hosts by freshwater habitats or longer periods of permanence in the oligohaline areas of Mar Chiquita. The presence of pure parental species, the differences in habitat use found between them and hybrids, and the conditioning effect of environmental heterogeneity over the hybridization event support a mosaic model for this hybrid zone. Finally, studies of parasitism in hybrid zones may contribute to understand the evolutionary history and stability of boundaries or possible biological barriers between host species, and they also provide insight on how natural selection and evolution drive speciation, colonization and habitat use in transitional environments.