Trophic evolution in African citharinoid fishes (Teleostei: Characiformes) and the origin of intraordinal pterygophagy

Divergent karyotypes in five genera of the African endemic fish family Distichodontidae (Cithariniformes, Osteichthyes)

The African family Distichodontidae comprises 109 species in 16 genera. Up-to-date cytogenetic information was available for the only distichodontid species Distichodusaffinis Günther, 1873. Here we report chromosome number and morphology in: Distichodusengycephalus Günther, 1864 (2n = 52, FN = 104), Ichthyborusbesse (Joannis, 1835) (2n = 46, FN = 92), Nannocharaxniloticus (Joannis, 1835) (2n = 54, FN = 106) and three taxa, Nannaethiopsbleheri Géry et Zarske, 2003, Nannaethiops sp., and Neolebiasunifasciatus Steindachner, 1894, that exhibit the same karyotypes (2n = 50, FN = 98). To confirm the Nannaethiops Günther, 1872 and Neolebias Steindachner, 1894 species identification, mt-DNA sequences of the two markers (COI and 16S rRNA) were obtained from karyotyped specimens and compared with the relevant sequences accessible from GenBank. The great prevalence of biarmed chromosomes (the karyotypes of most species contain exclusively biarmed chromosomes) is a distinctive characteristic of Distichodontidae and Cithariniformes as a whole.

A novel intramandibular joint facilitates feeding versatility in the sixbar distichodus

The intramandibular joint (IMJ) is a secondary point of movement between the two major bones of the lower jaw. It has independently evolved in several groups of teleost fishes, each time representing a departure from related species in which the mandible functions as a single structure rotating only at the quadratomandibular joint (QMJ). In this study, we examine kinematic consequences of the IMJ novelty in a freshwater characiform fish, the herbivorous Distichodus sexfasciatus. We combine traditional kinematic approaches with trajectory-based analysis of motion shapes to compare patterns of prey capture movements during substrate biting, the fish's native feeding mode, and suction of prey from the water column. We find that the IMJ enables complex jaw motions and contributes to feeding versatility by allowing the fish to modulate its kinematics in response to different prey and to various scenarios of jaw-substrate interaction. Implications of the IMJ include context-dependent movements of lower versus upper jaws, enhanced lower jaw protrusion, and the ability to maintain contact between the teeth and substrate throughout the jaw closing or biting phase of the motion. The IMJ in D. sexfasciatus appears to be an adaptation for removing attached benthic prey, consistent with its function in other groups that have evolved the joint. This study builds on our understanding of the role of the IMJ during prey capture and provides insights into broader implications of the innovative trait.

Adaptation to herbivory and detritivory drives the convergent evolution of large abdominal cavities in a diverse freshwater fish radiation (Otophysi: Characiformes)

Convergent evolution is often interpreted as evidence of natural selection favoring an optimal phenotype during adaptation. Morphological convergence is frequently found among lineages that converge on diet, but most studies have focused on morphological traits that relate exclusively to food handling and processing. In vertebrates, there is a strong inverse relationship between intestine length and trophic level. However, little is known about whether adaptation to a low trophic level influences the evolution of abdominal cavities that can accommodate larger intestines. Here, I reconstruct the evolutionary history of trophic ecology and examine abdominal cavity shape across 157 species of the fish order Characiformes to determine whether adaptation to an herbivorous-detritivorous diet drives convergent evolution of large abdominal cavities. Herbivorous-detritivorous species evolved significantly larger abdominal cavities than other trophic groups and repeatedly converged on a similar abdominal cavity morphology. Other trophic groups evolved abdominal cavity morphologies either stochastically or by selective pressures from an untested ecological character. These findings demonstrate that the selective demands of a larger intestinal tract promote the repeated convergence of a large abdominal cavity within herbivorous-detritivorous characiform fishes, while allowing other lineages to evolve randomly or adapt in response to other selection pressures, contributing to the overall body shape diversity of the order.

Phylogenetic relationships of the family Tarumaniidae (Characiformes) based on nuclear and mitochondrial data

ABSTRACT Characiformes is an order of freshwater fishes that includes many commercially important and emblematic species from South America and Africa, such as the popular piranhas, hatchetfishes, African tiger fishes and tetras. The order is split into two suborders with a total of 24 families, 282 genera and ca. 2,100 species. Here, we present an expanded phylogeny of characiform fishes, including data for 520 species and three genes (12S, 16S and RAG1), and the recently described family Tarumaniidae, which has not been examined by previous molecular analysis. Although our genetic coverage is limited to three gene fragments, the tree inferred based on maximum likelihood and Bayesian inference supports the monophyly of all characiform families and is largely congruent with results from recent studies that sampled less taxa but more genes. Also in agreement with a morphological hypothesis, our results strongly support the sister-group relationships between the family Tarumaniidae and Erythrinidae. Based on our results and that of the other molecular analyses, we propose a revised circumscription of the superfamily Erythrinoidea to include the families Tarumaniidae and Erythrinidae only.