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

True grit? Comparative anatomy and evolution of gizzards in fishes

Gut morphology frequently reflects the food organisms digest. Gizzards are organs of the gut found in archosaurs and fishes that mechanically reduce food to aid digestion. Gizzards are thought to compensate for edentulism and/or provide an advantage when consuming small, tough food items (e.g., phytoplankton and algae). It is unknown how widespread gizzards are in fishes and how similar these structures are among different lineages. Here, we investigate the distribution of gizzards across bony fishes to (1) survey different fishes for gizzard presence, (2) compare the histological structure of gizzards in three species, (3) estimate how often gizzards have evolved in fishes, and (4) explore whether anatomical and ecological traits like edentulism and microphagy predict gizzard presence. According to our analyses, gizzards are rare across bony fishes, evolving only six times in a broad taxonomic sampling of 51 species, and gizzard presence is not clearly correlated with factors like gut length or dentition. We find that gizzard morphology varies among the lineages where one is present, both macroscopically (presence of a crop) and microscopically (varying tissue types). We conclude that gizzards likely aid in the mechanical reduction of food in fishes that have lost an oral dentition in their evolutionary past; however, the relative scarcity of gizzards suggests they are just one of many possible solutions for processing tough, nutrient-poor food items. Gizzards have long been present in the evolutionary history of fishes, can be found in a wide variety of marine and freshwater clades, and likely have been overlooked in many taxa.

Diadromy Drives Elevated Rates of Trait Evolution and Ecomorphological Convergence in Clupeiformes (Herring, Shad, and Anchovies)

AbstractMigration can have a profound influence on rates and patterns of phenotypic evolution. Diadromy is the migration between marine and freshwater habitats for feeding and reproduction that can require individuals to travel tens to thousands of kilometers. The high energetic demands of diadromy are predicted to select for ecomorphological traits that maximize swimming and locomotor efficiency. Intraspecific studies have shown repeated instances of divergence among diadromous and nondiadromous populations in locomotor and foraging traits, which suggests that at a macroevolutionary scale diadromous lineages may experience convergent evolution onto one or multiple adaptive optima. We tested for differences in rates and patterns of phenotypic evolution among diadromous and nondiadromous lineages in Clupeiformes, a clade that has evolved diadromy more than 10 times. Our results show that diadromous clupeiforms show convergent evolution for some locomotor traits and faster rates of evolution, which we propose are adaptive responses to the locomotor demands of migration. We also find evidence that diadromous lineages show convergence into multiple regions of multivariate trait space and suggest that these respective trait spaces are associated with differences in migration and trophic ecology. However, not all locomotor traits and no trophic traits show evidence of convergence or elevated rates of evolution associated with diadromy. Our results show that long-distance migration influences the tempo and patterns of phenotypic evolution at macroevolutionary scales, but there is not a single diadromous syndrome.

Molecular phylogenetics of the Clupeiformes based on exon-capture data and a new classification of the order

The Clupeiformes, including among others herrings, anchovies, shads and menhadens are ecologically and commercially important, yet their phylogenetic relationships are still controversial. Previous classification of Clupeiformes were based on morphological characters or lack of synapomorphic characters. More recent studies based on molecular data as well as new morphological evidence are keeping challenging their phylogenetic relations and there is still no consensus on many interrelationships within the Clupeiformes. In this study, we collected nuclear sequence data from 4,434 single-copy protein coding loci using a gene-capture method. We obtained a robust phylogeny based on 1,165 filtered loci with less than 30 % missing data. Our major findings include: 1) reconfirmation of monophyly of the Clupeiformes, that is, Denticipitidae is sister to all other clupeiforms; 2) the polyphyletic nature of dussumieriids and early branching of Spratelloididae from all other clupeoids were confirmed using datasets curated for less missing data and more balanced base composition in the respective taxa. The next branching clade is the monophyletic Engraulidae. Pristigasteridae also is monophyletic, but it was nested in the previously defined "Clupeidae". Within Pristigasteridae there is no support for monophyletic Pelloninae. Chirocentrus is close to Dussumieria and not to engraulids. The miniaturized Sundasalanx is placed close to the ehiravine Clupeonella, however, with a relatively deep split. The genus Clupea, is not part of the diverse "Clupeidae", but part of a clade containing additionally Sprattus and Etrumeus. Within the crown group clades, Alosidae and Dorosomatidae are retrieved as sister clades. Based on new fossil calibration points, we found that major lineages of the clupeiforms diverged in the late Cretaceous and early Paleogene. The extinction event at the end of the Cretaceous may have created ecological niches, which could have fueled the diversification of clupeiform fishes. Based on the strong evidence of the present study, we propose an updated classification of Clupeiformes consisting of ten families: Denticipitidae; Spratelloididae; Engraulidae (Engraulinae + Coiliinae); Clupeidae; Chirocentridae; Dussumieriidae; Pristigasteridae; Ehiravidae; Alosidae, Dorosomatidae.

Landscape Evolution as a Diversification Driver in Freshwater Fishes

The exceptional concentration of vertebrate diversity in continental freshwaters has been termed the “freshwater fish paradox,” with > 15,000 fish species representing more than 20% of all vertebrate species compressed into tiny fractions of the Earth’s land surface area (<0.5%) or total aquatic habitat volume (<0.001%). This study asks if the fish species richness of the world’s river basins is explainable in terms of river captures using topographic metrics as proxies. The River Capture Hypothesis posits that drainage-network rearrangements have accelerated biotic diversification through their combined effects on dispersal, speciation, and extinction. Yet rates of river capture are poorly constrained at the basin scale worldwide. Here we assess correlations between fish species density (data for 14,953 obligate freshwater fish species) and basin-wide metrics of landscape evolution (data for 3,119 river basins), including: topography (elevation, average relief, slope, drainage area) and climate (average rainfall and air temperature). We assess the results in the context of both static landscapes (e.g., species-area and habitat heterogeneity relationships) and transient landscapes (e.g., river capture, tectonic activity, landscape disequilibrium). We also relax assumptions of functional neutrality of basins (tropical vs. extratropical, tectonically stable vs. active terrains). We found a disproportionate number of freshwater species in large, lowland river basins of tropical South America, Africa, and Southeast Asia, under predictable conditions of large geographic area, tropical climate, low topographic relief, and high habitat volume (i.e., high rainfall rates). However, our results show that these conditions are only necessary, but not fully sufficient, to explain the basins with the highest diversity. Basins with highest diversity are all located on tectonically stable regions, places where river capture is predicted to be most conducive to the formation of high fish species richness over evolutionary timescales. Our results are consistent with predictions of several landscape evolution models, including the River Capture Hypothesis, Mega Capture Hypothesis, and Intermediate Capture Rate Hypothesis, and support conclusions of numerical modeling studies indicating landscape transience as a mechanistic driver of net diversification in riverine and riparian organisms with widespread continental distributions.

Digestion and assimilation of benthic biofilm by the Sábalo, Prochilodus lineatus

The Sábalo, Prochilodus lineatus (Valenciennes, 1837), is one of 270 fish species in the Rio Paraná system, and it comprises >50% of the fish biomass. Its diet is flocculant benthic biofilm comprised of algae, bacteria and non-living organic matter: a food resource apparently of little value to other fishes. Digestion and assimilation of key nutrients from its biofilm diet by P. lineatus were described and quantified in an attempt to discover how this species is so successful. The fish begin a feeding period with empty digestive tracts, accumulate food during the feeding period and then void the gut content at the end of the feeding period. Early in a feeding period, sand accumulates in the gizzard-like pyloric stomach where it serves as both a grinding medium and a sieve. After sufficient sand has been acquired, food particles passed from the pyloric stomach to the intestine are dramatically reduced in size to <20 μ maximum dimension, whereas larger particles including mineral matter and plant fibres are retained. Total ash, hydrolysis-resistant-ash and hydrolysis-resistant-organic-matter were tested as reference materials against which to measure assimilation and hydrolysis-resistant-organic matter best met the assumptions of the technique. Comparison of the first food ingested to food ingested later in the feeding period shows that grinding of food and selective retention of larger particles results in a three-fold increase in the assimilation of ash-free-dry-mass (to 56%) and hydrolysis-labile-organic-matter (to 67%), and a six-fold increase in the assimilation of amino acids (AAs; to 74%). When food quality is assessed in terms of g AA assimilated · kJ^-1 energy assimilated, the quality of food ingested by P. lineatus ranges from a maintenance level of 5 to 12 mg AA · kJ^-1 , a level expected to produce near maximum growth. Thus, the processing of food in the pyloric stomach is integral to the success of P. lineatus in establishing large populations on a diet of flocculent benthic biofilm.

Phylogeny, body morphology, and trophic level shape intestinal traits in coral reef fishes

Trait-based approaches are increasingly used to study species assemblages and understand ecosystem functioning. The strength of these approaches lies in the appropriate choice of functional traits that relate to the functions of interest. However, trait-function relationships are often supported by weak empirical evidence.Processes related to digestion and nutrient assimilation are particularly challenging to integrate into trait-based approaches. In fishes, intestinal length is commonly used to describe these functions. Although there is broad consensus concerning the relationship between fish intestinal length and diet, evolutionary and environmental forces have shaped a diversity of intestinal morphologies that is not captured by length alone.Focusing on coral reef fishes, we investigate how evolutionary history and ecology shape intestinal morphology. Using a large dataset encompassing 142 species across 31 families collected in French Polynesia, we test how phylogeny, body morphology, and diet relate to three intestinal morphological traits: intestinal length, diameter, and surface area.We demonstrate that phylogeny, body morphology, and trophic level explain most of the interspecific variability in fish intestinal morphology. Despite the high degree of phylogenetic conservatism, taxonomically unrelated herbivorous fishes exhibit similar intestinal morphology due to adaptive convergent evolution. Furthermore, we show that stomachless, durophagous species have the widest intestines to compensate for the lack of a stomach and allow passage of relatively large undigested food particles.Rather than traditionally applied metrics of intestinal length, intestinal surface area may be the most appropriate trait to characterize intestinal morphology in functional studies.