This study indicates that the development of the vertebrae in the Atlantic salmon requires the orchestration of two sources of metameric patterning, derived from the notochord and the somite rows, respectively. Before segmentation of the salmon notochord, chordoblasts exhibit a well-defined cell axis that is uniformly aligned with the cranio-caudal axis. The morphology of these cells is characterised by a foot-like basal projection that rests on the notochordal sheath. Notochordal segments are initially formed within the chordoblast layer by metameric change in the axial orientation of groups of chordoblasts. This process results in the formation of circular bands of chordoblasts, with feet perpendicular to the cranio-caudal axis, the original chordoblast orientation. Each vertebra is defined by two such chordoblast bands, at the cranial and caudal borders, respectively. Formation of the chordoblast segments closely precedes formation of the chordacentra, which form as calcified rings within the adjacent notochordal sheath. Sclerotomal osteoblasts then differentiate on the surface of the chordacentra, using them as foundations for further vertebral growth. Thus, the morphogenesis of the rudiments of the vertebral bodies is initiated by a generation of segments within the chordoblast layer. This dual segmentation model for salmon, in which the segmental patterns of the neural and haemal arches are somite-derived, while the vertebral segments seem to be notochord-derived, contrasts with current models for avians and mammals.

The teleost backbone consists of amphicoelous vertebrae and intervertebral ligaments, both of which include notochord-derived structures. On the basis of a sequential developmental study of the vertebral column of Atlantic salmon ( Salmo salar L.) from the egg stage up to early fry stage (300-2500 day-degrees) we show that the vertebral body consists of four layers or compartments, two of which are formed through mineralization of preformed collagenous tissue (the notochordal sheath and the intervertebral ligament) and two of which are formed through ossification. The three inner layers have ordered lamellar collagen matrixes, which alternate perpendicularly from layer to layer, whereas the outer layer consists of cancellous bone with a woven matrix. The bone layers also differ in osteocyte content. In this study we describe the structural details of the layers, and their modes of formation.The results are compared with previous descriptions, and possible phylogenetic implications are discussed.

This study investigates the effect of different smolt production strategies on vertebral morphology (radiology), composition (mineral content) and mechanical strength (load-deformation testing) in Atlantic salmon (Salmo salar). Rapid-growing underyearling (0+) smolt were compared with slower-growing yearling (1+) smolt and a reference group of wild smolt (w). The underyearling and yearling smolt were transferred to seawater in October 2002 and May 2003, respectively. The underyearling smolt were reared under continuous light and the yearling smolt under natural light during the first twelve weeks in seawater, at ambient temperatures. Thus, the underyearling smolt hit seawater at 13 °C and were reared at 10-13 °C during the early seawater phase, whereas the yearling smolt hit seawater at 7 °C and were reared at 7-10 °C during the early seawater phase. All groups displayed increased longitudinal growth (up to 9% increase in relative length) of the caudal vertebrae during parr-smolt transformation. However, at transfer to seawater, the underyearling smolt had significantly lower vertebral mineral content (0+ 44%, 1+ 47%, w 50%) and higher incidence of deformed vertebrae (0+ 1.5%, 1+ 0%, w 0%), and at twelve weeks after transfer to seawater significantly lower vertebral mineral content (0+ 36%, 1+41%, w 43%), yield-load (0+6492 g, 1+8797 g, w 9150 g) and stiffness (0+7578 g/mm, 1+ 15,161 g/mm, w 20,523 g/mm), and significantly higher incidence of deformed vertebrae (0+ 2.5%, 1+ 0.3%, w 0%). There was a significant correlation between the mineral content and mechanical properties of the vertebrae. The underyearling smolt had significantly elevated plasma concentrations of total Ca, and P and Ca2+ during the parr-smolt transformation and in the early seawater phase.The results show that underyearling smolt may have an increased risk of developing vertebral deformities. It is possible that this risk can be reduced by postponing the start of the short-day treatment. This will reduce the temperature during smoltification, the temperature and daylength during the early seawater phase, and increase the age at smoltification.

Reared larval and juvenile Atlantic halibut H~ppoglossus hippoglossus collected during incidences of mass mortality in 2 hatcheries were studied by light and electron microscopy. A vacuolating encephalopathy and retinopathy as well as endocardial lesions were observed. By transmission electron microscopy isometric, spherical, unenveloped virus particles with mean capsid diameters of approximately 25 nm were detected. The virions were found intracytoplasmically in semicrystalline arrays or as membrane-associated aggregates and single particles. Infection could be observed in neurons, astrocytes, oligodendrocytes, microglia/macrophages, lymphocyte-like cells, vascular endothelium, endocardial endothelium, myocardial myocytes and in the mesothelium of the epicardium. Specific immunolabeling was obtained in affected tissues on immunohistochemistry applying primary antisera against nodaviruses isolated from the striped jack Pseudocaranx dentex and the sea bass Dicentrarchus labrax. We suggest that the mortality was caused by a nodavirus-like agent and that this disease of the Atlantic halibut is closely related to diseases termed viral nervous necrosis (VNN), viral encephalopathy and retinopathy, and fish encephalitis, which have been associated with mass mortality in numerous cultured marine teleost species. In Atlantic halibut, viral endocardial lesions were a constant finding, suggesting that viremia may be an important factor in the pathogenesis of the infection.

ABSTRACT. The susceptibility of the Atlantic halibut Hjppoglossus hippoglossus yolk-sac larvae to viral encephalopathy and retinopathy (VER) was investigated by waterborne challenge experiments with nodavuus. Transfer of VER was inhcated by several hnes of evidence. A sigluficantly higher cumulative mortality was observed after challenge with virus compared to mock challenge, and increasing doses of vlrus resulted in shorter incubation periods. When the challenge was performed on the day after hatchmg, the time from inoculation to the time when 50% of the larvae were dead (LT,,) ranged from 26 to 32 d. Postponement of challenge for 13 d reduced the LT,, to 14 d, inhcating that the susceptibility of the larvae to the present nodavirus strain was low during the first 2 wk after hatching. The progression of the infect~on was monitored by sequential unmunohistochemistry and electron mcroscopy. On Day 18 after hatching the initial signs of infection were observed as a prominent focus of immunolabelling in the caudal part of the brain stem. In the same larvae irnrnunolabelled single cell lesions were observed in the stratified epithelium of the cranial part of the intestine. The portal of entry into the larvae may thus have been the intestinal epithelium, while the route of infection to the CNS may have been axonal transport to the brain stem through cranial nerves such as the vagus nerves. Later in the infection, lesions became more severe and widespread and were also found throughout the b r a~n and spinal cord and in the retina, cranial ganglia, intestme, liver, olfactory epithelium, yolk-sac epithelium, gills and pectoral fins. The mortality in all virus-challenged groups was 100%. This study thus demonstrates that the present nodavirus strain is able to replicate and cause VER in Atlantic halibut yolk-sac larvae at temperatures as low as 6°C