The Fishes

60-90% of deep-sea fish exploit bioluminescence in some way. A survey of stocks around Bermuda showed that 97 % of fishes living at 500 to 1000m exhibit some form of bioluminescence (Beebe 1937). This is higher than in any other area of the ocean despite the Bermuda’s reputation for storm systems, which undoubtedly must have some effect on water clarity. Regardless, abundance of bioluminescent species is high here and helps convey how important light production is to the deep-sea way of life.

Being complex organisms, bioluminescence has evolved in the fishes to clear a multitude of life’s hurdles. Finding food, avoiding becoming food and attracting the right mate at the right time are all particularly tricky tasks to carry out in an environment set in perpetual darkness.

All but wavelengths of light between 470 to 490nm are lost with increasing depth (Turner et al. 2009); reds, greens and yellows are filtered out leaving just the narrow band of blue light. This explains the dominance of blue bioluminescent point sources with wavelengths ranging from 450 to 510nm (Turner et al. 2009). As a result many fish species cannot see red light at all but instead have a very accute ability to distinguish between ambient natural light and that produced by the photophores of their prey species.

There is remarkably little variation in colouration not only between species but between entire phyla when viewing the deep-sea. Reds, browns and blacks dominate as they best absorb the light wavelengths experienced at these depths; both ambient light and bioluminescent searchlights (Johnsen 2005). Fish in the deep-sea not only need to avoid being seen by predator’s searchlights but must blend into the ambient light filtering vertically down from the surface. Simply put they must hide their silhouettes by matching the background lighting. It seems a little counter productive to create light in order to not be seen but this is a well documented phenomenon known as counter-illumination. Even predatory species that spend large amounts of time luring prey or searching them out with bright bioluminescence utilize counter-illuminescent photophores.

Figure 6.1. A member of Myctophidae, the Lanternfish (innovate-vision.com)

Myctophids (Figure 3.1) are distributed widely throughout the world’s oceans with around 240 species described and some 600 million tonnes of biomass, despite this relatively little is known about them (Turner et al. 2009). In recent years they have become a very important family; largely abundant around the world and occupying the mesopelagic zone, ranging from waters poorly illuminated by daylight through to dark waters only broken by bioluminescent point light. This makes them incredibly interesting animals to examine bioluminescence in as they exploit the system for all three of the main applications. Their undersides are covered with photophores to hide their silhouettes, they also (as the common name suggests) have lanterns at their anterior end to illuminate potential prey and can further employ these large light sources to attract a mate.

Figure 6.2. Porichthys notatus, the midshipman fish (public.gettysburg.edu)

The Midshipman fish (Figure 6.2) occurs all along the western coast of North America from Baja, California up to Alaska. Rather spectacularly this species is thought to acquire exogenous luciferins in order to utilize bioluminescence as it can cross react with Vargula hilgendorfii; an ostracod species (Cormier et al. 1967, Warner & Case 1980). Other species have been recorded as obtaining their luciferins exogenously from sympatric ostracod species as well, encompassing them into their photophores to allow for the chemiluminescent reaction to occur. It is thought that without these ostracod luciferins Porichthys notatus would be unable to bioluminesce (Warner & Case 1980).

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