A Method For Prey Capture

The dark waters of the deep sea presents challenges to predators due to the low abundance of prey and the reduced ability to see any prey that is available (Young 1983). In order to combat these problems some organisms are able to use lures or visual aids in the form of ‘headlights’.

Lures:-

Figure 10. A deep-sea Anglerfish with a bioluminescent lure (Image from: Anemaw)

Anglerfish, belonging to the order Lophiiformes, are perhaps the most well studied species that uses a luminescent lure, however even the knowledge for this species is bases on observations of closely related shallow water species (Young 1983). A light organ, called an esca, is extended from the top of the fishes head and droops just above its jaws with the aim of attracting prey to the lure and then consuming it before it has time to bite. The esca is filled with bioluminescent bacteria and is thought to attract the prey by being a mimic of another food source, possibly faecal pellets (Young 1983). The esca has the ability for long periods of time (Haneda 1968), maximising the chance of a prey organism seeing the lure.

Deep-sea Erenna species, belonging to the order Siphonophorae, are another example of an organism using a bioluminescent lure. Their lure is in the form of bioluminescence which runs up and down their tentacles, attracting their prey to the area around their stinging cells (Haddock et al. 2010). Haddock et al. (2005) observed an Errena species that emitted a red fluorescent cloud around the lure which may suggest that this species preys on fish that have a have the ability to see long wavelengths (Douglas et al. 2002).

Video: Bioluminescence being used as a lure to attract prey

Headlights:-

To aid the search for food in the darkness of the deep, some animals have the ability to release beams of light illuminating their surroundings. The majority of organisms will use blue light in order to achieve this as it has the biggest range (Herring 2002). The photophores used to project this beam are often situated near the eyes, minimising the distance that the reflection has to travel to be received (Herring 2002).

Figure 11. The dragonfish Pachystomias microdon. (Image from: Widder)

There are some problems with this method linked to the scattering of light by the water and the suspended particulate matter. This scattering of light may lead to the prey being alerted of the predators presence before it has the chance to attack. (Young 1983).

There are three genera of dragonfish, Malacosteus, Aristostomias and Pachystomias, that have the ability to produce a beam of red light, as well an the usual blue beam, allowing them to see prey of a red colouration without the prey being aware that it is being watched, as the eyes of many marine organisms are not sensitive to this end of the spectrum (Douglas et al. 2000).

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