The art of lighting up

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Dr Julien Claes, of the Laboratory of Marine Biology, Earth and Life Institute, at Université Catholique de Louvain in Belgium, explains how and why lantern sharks light up...

What is bioluminescence?

This corresponds to the emission of light by an organism. It’s a phenomenon, based on chemical reaction, and common in the permanent darkness of the deep where numerous species use light to find mates or food and avoid predation.

Although more than 50 of the 500 currently described shark species are able to emit light, these were the least investigated group for luminescence. This is why my team decided to focus attention on one of them — the Velvet belly lantern shark (Etmopterus spinax). It’s a common mesopelagic shark found almost everywhere in

the East Atlantic and Mediterranean Sea.

Our study site was the Raunefjord, a southern fjord in Norway known to shelter an easy-access population of E. spinax close to the marine research station of Espeland where sharks can be maintained alive and where experiments can be easily carried out.

The results allowed us to better understand not only why they are luminous but also how they control light emission.

How are these sharks able to emit light?

It comes from thousands of tiny epidermal photogenic organs called photophores, composed of photogenic cells in a pigmented sheath and topped by at least one lens cell.

These organs form different luminous zones that appear separately during embryogenesis, mainly on the ventral part but also on the lateral parts.

These organs are essentially under hormonal control, contrary to luminous teleost fishes whose control was strictly nervous.

What is it used for?

We analysed the physical characteristics of this emission and found them similar to those in the fjord in the depth range of this shark, rendering it almost invisible from below by cloaking its silhouette against the background.

Apparently not able to adjust intensity, this shark probably daily migrates in the water column to remain hidden from predators and preys. This camouflage mechanism is called counter-illumination and common in mesopelagic organisms.

This item first appeared in the September 2010 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.