The rise of a Titan: a new species of the giant Titanolebias killifishes, and its phylogeny (Cyprinodontiformes: Rivulidae)

Abstract

The genus Titanolebias comprises the largest-bodied seasonal killifishes, inhabiting temporary wetlands of the La Plata Basin and adjacent areas in South America. Here, we describe a new species from the Eastern Chacoan region, representing the first record of the genus in this area, providing important biogeographic and evolutionary insights. Phylogenetic analyses confirm the monophyly of Titanolebias, placing Titanolebias monstrosus as the sister species to the remaining species of the genus and recovering the new species as sister to a clade composed of Titanolebias prognathus, Titanolebias elongatus, and Titanolebias cheradophilus. Titanolebias species, exhibits reduced sexual dimorphism, potentially associated with a trophic shift toward larger prey in the origin of this genus and presents several potential cases of aggressive mimicry. Megalebias genus represents a case of convergent evolution towards ichthyophagy with Titanolebias. Morphological analyses reveal that the new species present the diagnostic characters of the genus Titanolebias and can be distinguished by its unique coloration and other morphological traits. Behavioral and ecological data are also provided. Despite its restricted distribution and the vulnerability of ephemeral wetlands to habitat loss, the species is found within a National Park. This discovery provides valuable insights into convergent evolution linked to trophic shifts and contributes to the understanding of the evolution, phylogeny, ecology, and biogeography of Neotropical fishes, offering new avenues for future research and conservation policies for this threatened group of fishes.