The mystery of the missing neckbones!

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Head of a Serpent Black Dragonfish, Idiacanthus fasciola (Robin McPhee / NORFAN Founding Parties)

Dragonfishes of the deep-sea family Stomiidae are scary-looking fishes with rows of light organs and fearsome teeth. The family contains more than 290 species found in the oceanic’ twilight’ or mesopelagic zone – at depths between 200 and 1000 metres, or in even deeper waters where no light from the sun ever penetrates. Dragonfishes are voracious predators and most species have a bioluminescent “fishing lure” on their chin, thought to attract prey.

Like the larvae of all bony fishes, baby dragonfishes have a spinal support structure called a notochord – a flexible, rod-like structure that ossifies to become the backbone of vertebrates during larval development. As the larvae grow, the notochord is normally replaced by bone to become the vertebral column or backbone.

Head of a Sloane’s Viperfish, Chauliodus sloani, showing the enormous fangs (Robin McPhee / NORFANZ Founding Parties)

In most bony fishes, the backbone or vertebral column begins at the base of the skull. However, all dragonfishes have a puzzling boneless space between the back of the skull (the occiput) and the first vertebra – called the occipito-vertebral gap.

Scientists have long thought that this gap, or lack of rigid vertebrae, allows dragonfishes to bend their heads far back to swallow very large prey. Food is relatively scarce in the deep-sea, so these voracious predators take advantage of any meal that comes their way – sometimes even eating fishes larger than themselves.

In 2010, an international team of scientists solved the puzzle of these missing neckbones or vertebra. By studying larval and adult dragonfishes, Drs Nalani SchnellRalf Britz and Dave Johnson discovered that in 26 genera of stomiid fishes they studied, this gap is due to a remarkable extension of the notochord.

Head of a cleared and stained Black Loosejaw, Malacosteus niger, showing the cartilage in blue, and the bone in red (Credit: Nalani Schnell)

The team used a technique called ‘clearing and staining’ to examine specimens of 270 dragonfish species. First, an enzyme called trypsin was used to ‘clear’ the flesh of the specimens, making them almost invisible. The specimens were then stained with two dyes – alcian blue which stains cartilage, and alizarin red which stains bone.

The gap between the back of the skull and the first vertebra can be clearly seen in this image of a cleared and stained Black Loosejaw (Malacosteus niger). The notochord extension shows up as a bluish rod between the back of the skull and the first vertebra which are both stained red. This flexible extension of the notochord allows Black Loosejaws to bend their head far back while at the same time ‘throwing’ their jaws far forward to capture prey with their praying mantis-like jaws.

The team also discovered that the dragonfish notochord begins ossifying into bony vertebrae tail-first. This is completely in the opposite direction to the vertebral ossification in most other bony fishes where the backbone develops from the skull towards the tail.

Further reading:

Dingerkus, G. & Uhler, L.D. 1977. Enzyme clearing of alcian blue stained whole small vertebrates for demonstration of cartilage. Stain Technol. 52: 229-232.

Song, J. & Parenti, L.R. 1995. Clearing and staining whole fish specimens for simultaneous demonstration of bone, cartilage, and nerves. Copeia 1995(1): 114-118.

Schnell, N.K., Bernstein, P. & Maier, W. 2008. The pseudo-craniovertebral articulation of Stomias boa (Stomiidae, Teleostei). Journal of Morphology 269(5): 513-521.

Schnell, N.K., Britz, R. & Johnson, G.D. 2010. New insights into the complex structure and ontogeny of the occipito-vertebral gap in barbeled dragonfishes (Stomiidae, Teleostei). Journal of Morphology 271(8): 1006-1022.

Taylor, W.R. & van Dyke, G.C. 1985. Revised procedures for staining and clearing small fishes and other vertebrates for bone and cartilage study. Cybium 9: 107-119.

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