Black Dragon-Fish: Hunters in The Dark
The Black Dragonfish
The Black Dragonfish (Malacosteus niger) is among the most fascinating creatures to be found (or more often, not found) on the planet today. Possessing adaptations that make them almost unique in the animal kingdom. M. niger are found in the deep sea, and tend to occupy the area below 700m, rarely migrating into shallower water. At this depth they face challenges that may seem insurmountable to us land based organisms without the use of some of the most advanced technology available. Most notably the lack of light (it is almost pitch dark at this depth) and the scarcity of food. The fine chisel of evolution however, has sculpted these fish into beings that are capable of thriving in such conditions, and in fact, they could not survive anywhere else. There are two aspects of the black dragonfish that are especially interesting, the size of its jaws compared to the rest of its body, and its ability to shine red lights from special pits (called photophores) from just below its eyes. These two factors combined lend it another name; the stoplight loose-jaw dragonfish.
When looking at M. niger, perhaps the first thing that strikes any viewer is the size of its jaws. In the picture above, it is clear to see that the jaws are huge in proportion to the rest of the animal, with the lower jaw being around a quarter of the total length (they grow to a maximum of 26.5cm). However they become even more impressive when considering the animals gape. The well named “loose jaw” can open its mouth a massive 120 degrees! The means that it can open its mouth so wide that it is almost a flat line. Us humans on the other hand can open our mouth to a measly 50 degrees.
While long jaws that can open extremely wide are an interesting adaptation, there are a great many organisms that can accomplish similar feats such as lions and hippos. Perhaps the most striking feature of the black dragonfishes mouth, is the lack of tissue it has. They have no skin between the mandibular rami, with the exception of the protractor hyoideus (‘ph’ in the diagram above). In less scientific terms this means that they have no skin on their lower jaw between the jaw bones. The protractor hyoideus in this case is a small “cord” running from the back of the throat to the centre of the lower jaw, linking the lower jaw muscles to the jaws themselves. This gives their head a rather skeletal appearance. Add on to this the ability to “throw back” their head using an unossified vertebrae in their ocipito vertebral hinge, which allows them to achieve the massive jaw gapes mentioned above, and swallow prey that can weigh as much as themselves.
Having long jaws makes perfect sense, as they allow them to capture much larger prey than otherwise. However, the purpose of the hinge and the lack of skin below the lower jaw takes a bit more thinking. Using computer modelling, scientists worked out that the lack of skin greatly reduced the water resistance faced when closing the jaws. In fact it was theorised through the use of these same computer models that if they did have skin between the rami of their lower jaws (as is seen in shorter jawed relatives) they would simply not be able to close the jaws fast enough to catch prey.
The Red Light District
Perhaps the most impressive feature of M. niger however, is its incredible bioluminescence. It (like the vast majority of deep sea organisms) is capable of bioluminescing, that is, capable of creating its own light in much the same manner as a firefly. However the light that M. niger produces is special, as it is one of only a very small group of animals on the planet that can produce red light. While almost all bioluminescent animals produce light that is blue-green, M. niger, along with a small number of its close relatives (and one species of beetle) can produce red light, so why?
In the picture above, the red photophore can be very clearly seen just below the eye. Its location implies that it is not just to lure prey close, or to signal to a mate, but to help the fish see in the near pitch darkness in which it lives. What is so special about the red light is that, to most of the animals on which this fish preys, red light is invisible. This is because red light gets absorbed by sea water very quickly, meaning it never reaches very deep. As such, the majority of deep sea organisms have never evolved the ability to see the colour red, and instead can only see light that is blue-green, the colour of most bioluminescence. What all this means is that the black dragonfish possess an undetectable light source, allowing it to see its food without the fish or copepod ever being aware of this lurking predator.
A mysterious world.
The adaptations of the M. niger arguably make it one of the most interesting fish on the planet, and it is quite unlike almost anything else on earth. What is so fascinating and exciting as a scientist, is the mystery that it represents. While computer simulations and theorisations have made great insight into the life of this and many other deep sea organisms, so much is still unknown, no-one has ever seen an individual feeding, and their ability to distribute themselves globally is still poorly understood. They are hardly alone, they are merely one of a veritable ark of deep sea creatures that are a mystery not just to the public but to the scientists who try to study them. It is this this that makes them such exciting and compelling animals.