Hybridization is increasingly being recognized as a widespread process, even between ecologically and behaviorally divergent animal species. Determining phylogenetic relationships in the presence of hybridization remains a major challenge for evolutionary biologists, but advances in sequencing technology and phylogenetic techniques are beginning to address these challenges.Here we reconstruct evolutionary relationships among swordtails and platyfishes (Xiphophorus: Poeciliidae), a group of species characterized by remarkable morphological diversity and behavioral barriers to interspecific mating. Past attempts to reconstruct phylogenetic relationships within Xiphophorus have produced conflicting results. Because many of the 26 species in the genus are interfertile, these conflicts are likely due to hybridization. Using genomic data, we resolve a high-confidence species tree of Xiphophorus that accounts for both incomplete lineage sorting and hybridization. Our results allow us to reexamine a longstanding controversy about the evolution of the sexually selected sword in Xiphophorus, and demonstrate that hybridization has been strikingly widespread in the evolutionary history of this genus. K E Y W O R D S :Hybridization, incomplete lineage sorting, premating isolation, sexually selected traits, transcriptome sequencing.

This study tested the hypothesis that in Montezuma swordtail Xiphophorus montezumae, bearers of longer swords would face higher costs in terms of swimming speed than males with shorter swords, as a result of differences in total body length when natural swords were present. Variation in natural sword length, however, did not appear to be correlated with the cost in terms of critical swimming speed, U crit . On the other hand, the mean U crit of males with the sword excised was 21% higher than that when natural sword was present. This study suggests that the sword of Montezuma swordtail represents a disadvantage in terms of swimming performance regardless of the differences in sword length.

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Hermit crabs recognize different shell attributes and choose shell size, type, or species to maximize their individual fitness. Aquatic hermit crab species commonly prefer relatively heavy shells species in contrast to semi-terrestrial crabs which prefer lighter shells. Lighter shells favor rapid growth and increase individual fitness because larger individual achieve higher success in intra-and inter-individual competition. In this study, we experimentally measured the daily growth of hermit crabs Calcinus californiensis that were forced to occupy four different shell species with different internal volume to mass ratios (IV/M ratio), and whose sequence in preference is known from previous studies (Cantharus [ Stramonita [ Columbella [ Nerita). The shell with the highest IV/M ratio (Nerita) favored body growth rate, while the relatively heaviest (lowest IV/M ratios) and most preferred shells (Stramonita and Cantharus) hindered body growth. Our results suggest that in this aquatic hermit crab, the benefits of carrying heavy shells that give better predatory protection exceeded the benefits of rapid growth that can be obtained using lighter, thinner, and less protective shells.

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