Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

Further insight on carapid - holothuroid relationships

Carapidae (or pearlfish) are eel-like fishes that live inside different invertebrates, such as holothurians, sea stars, or bivalves. Those of the genus Carapus are commensal and use their host as a shelter, while Encheliophis species are parasitic and eat the host's gonads. In areas where they live in sympatry, C. boraborensis, C. homei, C. mourlani and E. gracilis are able to inhabit the same host species. Infestation is considered as monospecific when several conspecifics are observed in the same host. However, many aspects of this particular relation remain obscure, e.g. communication between carapids and the defence systems of the different protagonists (carapids and hosts). Experiments have been conducted in the field and laboratory to investigate several aspects of the carapids' relationships with their hosts. Sampling carried out in the Bay of Opunohu (Moorea, French Polynesia) determined the sex ratio of C. boraborensis (3:1) and C. homei (1:1) and their distribution rate within different Echinodermata. Our study showed that neither species was capable of determining whether a heterospecific already occupied a sea cucumber or not. They were, however, able to locate the sea cucumber's cloaca, due to the excurrent resulting from respiration. The sea cucumber's defence system (Cuverian tubules) minimises predator attacks, but is not effective against carapid intrusion. The Carapidae defence system is twofold. Due to a passive system related to the sea cucumber's low cloacal position, the Cuverian tubules are not expelled when fish enter the cloaca. Moreover, carapids resist sea cucumber toxins better than other reef fish. Their increased resistance might be related to their gills rather than to their mucus coating; however, the latter may assist the fish in resisting the sticky substances emitted by the Cuverian tubules

Phylogenetic analysis of the pearlfish Carapini (Ophidiiformes, Carapidae)

Fishes of the tribe Carapini (Encheliophis and Carapus) share a noteworthy peculiarity: they shelter in holothurian echinoderms or bivalve hosts. Some species are considered parasitic, others commensal. This study focuses on the phylogeny of the tribe, using two other Carapidae species as an outgroup (Snyderidia canina and Onuxodon fowleri). Insofar as possible, the selected anatomical and behavioural characters where chosen in an ecomorphological perspective, as features that could be responses to various lifestyle-related constraints. Our character selection also took into account the fact that some features are (presumably) linked. Such features were grouped together as a single trait to avoid their overvaluation. This methodology enabled us to separate commensals from parasites, the former belonging to Carapus and the latter to Encheliophis. Carapus species reflect in their morphology the constraints imposed by a diet of hard, mobile, elusive prey, showing predator-type features: a strong dentition, a wide mouth opening, a robust food intake apparatus. On the other hand, the endoparasitic Encheliophis species show a generally weaker buccal apparatus and narrow mouth opening, in relation to the different constraints of their lifestyle where the diet constraints are less pronounced: they eat body parts of their host. We propose changes in both generic diagnoses and transfer three species from Encheliophis to Carapus

Variations in the sound producing mechanism in the pearlfish Carapini (Carapidae)

Sound production in Carapus boraborensis results from the action of different sonic muscles terminating in complex tendons, which have hooks that fit over a tubercle on the swimbladder wall. The primary sonic muscles (PSM) draw progressively the forepart of the swimbladder until the hook releases the tubercle. This allows the swimbladder to snap back to its resting position, which initiates the onset of the sound. In the present study, the morphology of the C. boraborensis sound-producing apparatus and the resulting sounds were compared with Encheliophis gracilis and Carapus homei. The main difference concerns the direct insertion of the PSM on the swimbladder in C. homei and E. gracilis and, concurrent sonic characteristics. These morphological features also allow both fish to produce an additional kind of sound with more compact pulses, each being clearly composed of two parts and each having a different frequency. The sound producing system could be compared with a guitarist who makes a sound in releasing a guitar string and modulates it by moving his/her finger along the string. However, E. gracilis possess more filtered sounds than C. boraborensis and C. homei, probably because of the unusual shape of its swimbladder. This study highlights the diversity and plasticity of sonic mechanisms and their implication in the development of sonic repertoire in evolving species

Remodelling of the vertebral axis during metamorphic shrinkage in the pearlfish

Body shortening was observed in the pearlfish Carapus homei during metamorphosis. The tenuis larva at first possessed a suite of osseous vertebral bodies of similar length. The reduction in both the number and size of vertebrae followed increasing decalcification, degeneration of organic tissue and shortening. This involved a complete degradation and disappearance of the caudal tip vertebrae, and there was a reduction in the size of most of the remaining vertebrae. The further development of the vertebrae began with ossification of the neural and haemal arches before that of the vertebral body. This second part of the development followed a gradient: a gradual decreases towards the caudal tip in the size of the vertebrae and their completeness. (C) 2004 The Fisheries Society of the British Isles