Weird and wonderful animals
Sive Finlay introduces us to the Stoplight Loosejaw fish. Inhabiting the depths it produces two colours of light and can snap large prey with enormous hinged jaws
Stoplight Loosejaw, Malacosteus niger.
From Ocean Ichthyology by G Brown Goode and Tarleton H Bean, 1896.
V
enture beyond the placid surface waters around our coast and you will find a world of truly weird animals. Deep sea creatures spend their lives in a dark, high-pressure, cold and often lonely world. Clearly very specific adaptations are needed to survive in such seemingly inhospitable environments. The mesopelagic zone (2001000m deep) marks the transition between sunlit surface waters and the impenetrable darkness of the deep. Among the many monstrous creatures which call this zone home, the Stoplight Loosejaw fish are some of the strangest. Comprised of two species of dragonfish belonging to the Malacosteus genus, Stoplight Loosejaws are found worldwide with the exception of the polar regions. The clues to their weirdness lie in their name… Stoplight Loosejaws have tear-drop shaped bioluminescent photophores (light-producing organs) under their eyes. They produce light of two different colours; red from the upper photophore and blue-green light from the one below (hence I would have called them “Traffic light” fish but you get the gist). If you live in a dark, watery world, being bioluminescent (creating your own light) can be quite handy. Light production can have lots of useful functions; from blending in with lighter surface waters to avoid predators to attracting potential mates. Stoplight Loosejaws, however, use their luminance powers to become invisible hunters.
Sunlight is made up of a spectrum of colours of different wavelengths. Only the short wavelengths; the blue/green end of the spectrum penetrate past the surface waters – hence we have the “deep blue sea” and you won’t bleed red if you cut yourself in the deep. Most marine bioluminescent creatures produce blue/green light and deep sea fish have evolved only to see these colours. Clearly there’s a gap in the evolutionary niche for animals which are not restricted to the usual light forms – enter the Stoplight Loosejaws. The fish produce red light which acts as their own, private signal. Their red light communications with other Stoplight Loosejaws are the ultimate Enigma code as they cannot be detected by species which are blind to red light. Similarly, the red light beacon illuminates redcoloured crustacean prey which would otherwise appear black to the animals which can only see blue light. So Stoplight Loosejaws can have private communications and locate prey in the murky deep thanks to a unique light ability… sneaky! Stoplight Loosejaws seem to rely on a specific diet to fuel their unique bioluminescent abilities. They eat a large volume of copepods; small crustaceans which are a major component of zooplankton. Copepods contain a derivative of chlorophyll (the green pigment molecule which is necessary for photosynthesis) which the fish need to produce their red light. So a copepod diet seems logical when it comes to light production but not when you think about the “Loosejaw” part of their name.
SCIENCE SPIN Issue 61 Page 25
Relative to their size (around 30cm), Malacosteus have one of the widest gapes of all fish species – their loose lower jaws can measure up to a quarter of the fish’s total length. They also have no base to their lower jaws but rather an open gap between the two rows of teeth which allows them to open and close their jaws very rapidly without much resistance from the water. All of these characteristics; large gape, rapid-moving jaw and lots of sharp teeth – appear to be good adaptations for hunting large prey, not the small copepods which make up the majority of their diet. With their red-light stealth-hunting tactics and oversized jaws, yet having primary reliance on unexciting copepod prey, Stoplight Loosejaws may seem to have gone for the “overkill” side of evolutionary adaptation. While they might have to rely on the “bread and butter” of snacking on copepods, they are still ready to make use of fleeting opportunities for large meals which are often missed by other predators. If you’re in the deep sea, diversity and flexibility are key – and if you’re weird and wonderful enough to use a communication and hunting systems which are undetectable by most other creatures you can go far. Sive Finlay, a Zoology graduate, is currently working as a postgraduate scholar with the Macroecology and Macroevolution group at TCD.