The serrasalmids: piranhas, pacus, and their relatives, are ubiquitous Neotropical fishes with diverse diets, ecologies, and behaviors. Serrasalmids have a bony, serrated keel which lines the underbellies of these fishes, the structure for which the family is named. We examined the diversity and structure of the keel in piranhas and allies using micro-computed tomography scanning in over 30 species of serrasalmids, a third of the species richness for the family, and for 95 total characiform specimens. The keel is highly diverse across serrasalmids, with serrae shape dictating the overall form of the keel. Serrae shape varies considerably among different species and even within keels themselves. The keel morphology can be divided into distinct anterior and posterior regions, as separated by the pelvic fins. Compared to other characiform fishes, serrasalmid skeletons are frequently damaged. Gouging perforations and signs of healing (serrae fusion) are common on the keel. We propose the keel is a defensive structure based on the high incidence of injury (>50%) in our dataset. This is the highest incidence of damage ever recorded in the skeletons of bony fishes. The loss of the anterior keel region in rheophilic taxa suggests competing performance demands and selective pressures on this structure. Competition and aggression among conspecifics or confamilials is a frequently invoked phenomenon for explaining animal weaponry and armor in terrestrial vertebrates. The keel in serrasalmids and other instances of armor in fishes could be complementary study systems for examining competitive rivalry in vertebrates. Anat Rec, 303:30-43, 2020.

Environmental DNA (eDNA) has attracted interest in relation to fisheries, with its possibilities for species identification and promises for species quantification. In the context of fisheries catches, eDNA can be most useful for the estimation of bycatch proportions. The assessment of species mixtures in large catches (>1000 t) is challenging, especially when morphologically similar species are to be differentiated. We used an experimental set-up to simulate industrial sprat fishery catches, and tested two types of water (blood water and discharge water) derived from this simulated fishery for their suitability in reliable species quantification. We analysed nine mixtures of sprat and herring—the main bycatch species. Species-specific quantitative PCR was used for species identification and quantification. Species-to-species weight fractions and eDNA fractions in mixtures showed a strong correlation. Accounting for size-based differences in DNA abundance (allometrically scaled weight) reduced the estimated standard error on weight fraction prediction from 0.064 to 0.054 in blood water, and from 0.080 to 0.075 in discharge water when comparing the weight-based model with the allometrically scaled weight model, respectively. Accounting for allometric scalling in genetic analyses of fisheries process water can serve as a more precise method for the assessment of bycatch, thus in a wider sense improve the quality of fisheries-dependent data.

In recent years, the analysis of environmental DNA (eDNA) has significantly improved, allowing for high‐resolution species identification and possible biomass quantification from water samples. Fisheries management typically requires monitoring of catches, including precise information about bycatch quantities to make sound assessments of exploitation rates. Bycatch assessment is particularly challenging in large catches (>500 T), and the current practice of visual assessment of subsampled catches is time‐consuming, requires extensive labor, and often has low precision. We explored the feasibility for applying eDNA‐based methods for studying catch composition using the pelagic North Sea herring fishery with bycatch of mackerel as a case. First, we experimentally simulate a series of catches using a range of herring and mackerel weight proportions to establish relationships under real fisheries scenarios. The relationship is subsequently used to estimate the biomass of mackerel bycatch from eDNA from three herring catches, by sampling and comparing processing water both onboard ships and at the processing factory. All samples are analyzed using species‐specific quantitative PCR (qPCR). The experiments reveled a strong correlation between DNA and weight fractions characterized by a constant overrepresentation of mackerel DNA compared to expected mackerel weight. We found that eDNA‐based and visual methods applied to the same landing reflect the within catch variability in species composition alike, however, the methods can show disparity in total estimates of mackerel biomass. Accounting for haul mixing within total landed catches increases the precision of the factory and ship eDNA‐based estimates for the same catch. We show that eDNA‐based bycatch estimates provide coherent quantitative data, and likely improve quality and reduce costs of collecting fisheries‐dependent data and thereby contribute to securing sustainable fisheries.

Information about the dietary composition of a species is crucial to understanding their position and role in the food web. Increasingly, molecular approaches such as DNA metabarcoding are used in studying trophic relationships, not least because they may alleviate problems such as low taxonomic resolution or underestimation of digestible taxa in the diet. Here, we used DNA metabarcoding with universal primers for cytochrome c oxidase I (COI) to study the diet composition of the northern shrimp (Pandalus borealis), an Arctic keystone species with large socio‐economic importance. Across locations, jellyfish and chaetognaths were the most important components in the diet of P. borealis, jointly accounting for 40%–60% of the total read abundance. This dietary importance of gelatinous zooplankton contrasts sharply with published results based on stomach content analysis. At the same time, diet composition differed between fjord and shelf locations, pointing to different food webs supporting P. borealis in these two systems. Our study underlines the potential of molecular approaches to provide new insights into the diet of marine invertebrates that are difficult to obtain with traditional methods, and calls for a revision of the role of gelatinous zooplankton in the diet of the key Arctic species P. borealis, and in extension, Arctic food webs.

Some consider that the first winged insects had living tissue inside the wing membrane, resembling larval gills or developing wing pads. However, throughout the developmental process of the wing membrane of modern insects, cells and tracheoles in the lumen between dorsal and ventral cuticle disappear and both cuticles become fused. This process results in the rather thin rigid stable structure of the membrane. The herewith described remarkable case of the dragonfly shows that in some highly specialized wings, the membrane can still be supplemented by tracheae. Such a characteristic of the wing membrane presumably represents a strong specialization for the synthesis of melanin-filled nanolayers of the cuticle, nanospheres inside the wing membrane and complex arrangement of wax crystals on the membrane surface, all responsible for unique structural coloration.