Lacunae rostralis: A new structure on the rostrum of sailfish Istiophorus platypterus.

Recent comparative studies of billfishes (Istiophoridae and Xiphiidae) have provided evidence of differences in the form and function of the rostra (bill) among species. Here, we report the discovery of a new structure, lacuna rostralis, on the rostra of sailfish Istiophorus platypterus, which is absent on the rostra of swordfish Xiphias gladius, striped marlin Kajikia audax and blue marlin Makaira nigricans. The lacunae rostralis are small cavities that contain teeth. They were found on the ventral rostrum surface of all I. platypterus specimens examined and dorsally in half of them. Ventrally, the lacunae rostralis were most prominent in the mid-section of the rostrum. Dorsally, they occurred closer to the tip. The density of lacunae rostralis increased towards the rostrum tip but, because they are smaller in size, the percentage of rostrum coverage decreased. The teeth located within the lacunae rostralis were found to be different in size, location and orientation from the previously identified micro-teeth of billfish. We propose two potential functions of the lacunae rostralis that both relate to the use of the bill in feeding: mechanoreception of prey before tapping it with the bill and more efficient prey handling via the creation of suction, or physical grip.

Comparative morphology of the scales of roundscale spearfish Tetrapturus georgii and white marlin Kajikia albida.

The comparative morphology of the scales of roundscale spearfish Tetrapturus georgii and white marlin Kajikia albida was investigated. In addition, variation in scale morphology across different body regions within each species was analysed. Although considerable morphological variation was observed among scales from different body regions in both species, scales of K. albida generally have pointed anterior ends, fewer posterior points and are more heavily imbricated than those of T. georgii, which are frequently rounded anteriorly, often have many posterior points and are separated farther within the skin. In all sampled body regions and individuals, scales of T. georgii are significantly broader and have a lower length-to-width aspect ratio than those of K. albida. Superficial to the scales are denticular plates, which are ossified formations occurring on the surface layer of the epidermis; these were observed and described for T. georgii, K. albida and blue marlin Makaira nigricans. Detailed scale descriptions allow for a more accurate characterization of the variation within and differences between these two species and could potentially be a valuable tool for investigating istiophorid systematics.

Dynamic habitat suitability modelling reveals rapid poleward distribution shift in a mobile apex predator.

Many taxa are undergoing distribution shifts in response to anthropogenic climate change. However, detecting a climate signal in mobile species is difficult due to their wide-ranging, patchy distributions, often driven by natural climate variability. For example, difficulties associated with assessing pelagic fish distributions have rendered fisheries management ill-equipped to adapt to the challenges posed by climate change, leaving pelagic species and ecosystems vulnerable. Here, we demonstrate the value of citizen science data for modelling the dynamic habitat suitability of a mobile pelagic predator (black marlin, Istiompax indica) within the south-west Pacific Ocean. The extensive spatial and temporal coverage of our occurrence data set (n = 18 717), collected at high resolution (~1.85 km(2) ), enabled identification of suitable habitat at monthly time steps over a 16-year period (1998-2013). We identified considerable monthly, seasonal and interannual variability in the extent and distribution of suitable habitat, predominately driven by chlorophyll a and sea surface height. Interannual variability correlated with El Nino Southern Oscillation (ENSO) events, with suitable habitat extending up to ~300 km further south during La Nina events. Despite the strong influence of ENSO, our model revealed a rapid poleward shift in the geometric mean of black marlin habitat, occurring at 88.2 km decade(-1) . By incorporating multiple environmental factors at monthly time steps, we were able to demonstrate a rapid distribution shift in a mobile pelagic species. Our findings suggest that the rapid velocity of climate change in the south-west Pacific Ocean is likely affecting mobile pelagic species, indicating that they may be more vulnerable to climate change than previously thought.

How sailfish use their bills to capture schooling prey.

The istiophorid family of billfishes is characterized by an extended rostrum or 'bill'. While various functions (e.g. foraging and hydrodynamic benefits) have been proposed for this structure, until now no study has directly investigated the mechanisms by which billfishes use their rostrum to feed on prey. Here, we present the first unequivocal evidence of how the bill is used by Atlantic sailfish (Istiophorus albicans) to attack schooling sardines in the open ocean. Using high-speed video-analysis, we show that (i) sailfish manage to insert their bill into sardine schools without eliciting an evasive response and (ii) subsequently use their bill to either tap on individual prey targets or to slash through the school with powerful lateral motions characterized by one of the highest accelerations ever recorded in an aquatic vertebrate. Our results demonstrate that the combination of stealth and rapid motion make the sailfish bill an extremely effective feeding adaptation for capturing schooling prey.

The first articulated skeletons of enigmatic Late Cretaceous billfish-like actinopterygians.

Only few candidates of Mesozoic fishes with a similar body plan and ecological niche to the modern billfishes are suggested as their analogues. Several specimens were recovered from Cenomanian deposits in Germany and Lebanon and display a billfish-like fusiform body with elongated premaxillae. They are found close to the plethodids and show a unique combination of characters (rostrum pointed and extremely elongated, double articular head of the quadrate, anteroposteriorly elongated abdominal centra indicating a slender body and different types of scales on the body) allowing their inclusion in a new genus. Two 'Protosphyraena' species are also assigned to this new genus. This fish can be considered as an ecological analogue to the extant xiphioids sharing their feeding habits. This fish was abundant and roamed, as an apex predator, the Central Tethys and the Boreal realms during the Cenomanian.

Development and validation of multiplex PCR assay for differentiating tunas and billfishes.

Commercially available tunas and billfishes are generally processed as steaks, making it difficult to visually distinguish between the two. We developed and validated species-specific primers to prevent the adulteration of tunas by billfishes. Tunas and billfishes primers were designed on the cytochrome oxidase subunit I. Multiplex PCR bands obtained were 579 bp, 291 bp and 114 bp for tunas, billfishes and internal control. Sensitivity was determined to be 5 ng for tunas and billfishes. A total of 50 samples were monitored: 49 for tunas and 1 for billfish. As a result of the monitoring, the fake tunas did not show due to the agreement between product name and the raw material of the wrapping paper. Our results indicate that the species-specific primers developed in this study are suitable for differentiating tunas and billfishes. The newly developed multiplex PCR assay is a time and cost effective technique for determining the authenticity of tunas and billfishes.

Current status and potential contributions of fisheries statistics from artisanal fisheries for managing juvenile istiophorid billfishes in Southern Brazil.

Billfishes are considered important fishery resources and the identification of aggregation sites is imperative for proper management. Here we present evidence of a seasonal aggregation site for juvenile istiophorid billfishes in southern Brazil. We discuss the results as they relate with the need for management at a local scale and participatory monitoring with artisanal fishing communities as a way to ensure access to data on the occurrence and population status in the long term.

Feeding Biomechanics in Billfishes: Investigating the Role of the Rostrum through Finite Element Analysis.

Billfishes are large pelagic fishes that have an extreme elongation of the upper jaw bones forming the rostrum. Recent kinematic and biomechanical studies show the rostrum to be associated to feeding, however, it is less clear how the wide range of morphologies present among billfish may affect their striking behavior. In this study, we aim to assess the mechanical performance of different rostrum morphologies under loads that simulate feeding and to test existing hypotheses of species-specific feeding behaviors. We use finite element analysis (FEA)-a physics-based method that predicts patterns of stress and strain in morphologically complex structures under specified boundary conditions-to test hypotheses on the form and mechanical performance of billfish rostra. Patterns of von Mises stress and total strain energy suggest that distinct rostral morphologies may be functionally segregated. The rounder blue marlin rostrum may be better suited for a wide range of slashing motions to disable prey, whereas the more flattened swordfish rostrum appears to be more specialized for lateral swiping during prey capture. The almost homogenous stress distribution along each rostrum implies their possible use as a predatory weapon regardless of morphological differences between species. The mechanical implications of other less commonly reported behaviors such as spearing are discussed, as well as the potential impact of hydrodynamics in shaping the evolution of the rostrum in this lineage. Anat Rec, 2019. © 2019 American Association for Anatomy.