The majority of bioluminescence in the deep sea is used defensively (Herring 2002). Three of the main defensive methods are described bellow:-
Flashing, Secretions and Decoys:-
The ability to emit a sudden luminous flash or cloud is shared by a large proportion of the micronekton and planktonic species (Young 1983). Morin (1983) and Young (1983) have suggested that a bright flash may temporarily stun an attacking predator, encouraging them to pass by or giving the organism opportunity to flee.
The flash produced by dinoflagellates when under threat has been shown to increase the swimming speed of predating copepods (Buskey et al. 1983). This, in turn, results in the copepod passing by without having time to graze.
In larger animals a flash may be used to emphasise size in order to intimidate predators (Herring 2002). For example, when under attack or feeling threatened, the Black dragon fish Idiacanthus atlanticus will flash along its tail and fin, giving the impression of a very large size (Herring 2002).
Other organisms will use secretions in order to form a cloud either to appear threatening or to create a distraction while they disappear. For example, many copepods have luminous glands on their abdomen or feet, which when the organism is threatened, will secrete luminous material that can then be kicked away as the organism swims away, startling or misdirecting the predator (Hartline et al. 1999)
Vampyroteuthis infernalis is a deep-sea cephalopod, living at depths of over 600m (Robison et al 2003). This organism has two defence mechanisms which it can employ depending on the level of threat. A severe threat will stimulate the excretion of a viscous fluid from tips of all its arms, creating a luminous cloud around the organism. In less extreme circumstance a threat will stimulate a light organ in the tip of one arm causing it to glow. Studies have shown that this arm can be regenerated implying that it may be used as a sacrificial structure or decoy, attracting the predator to an insignificant part of the body in the hope it will leave the rest of the organism in tact (Robison et al 2003). Some brittle stars of and scale worms may purposely shed parts of their body as decoys e.g. arms or scales respectively (Herring 2002).
Burglar alarm effect:-
Burkenroad (1943) suggests that a luminous flash of a prey under attack will illuminate the predator which will in turn attract larger predators to potentially remove the initial predator before the luminescing prey in devoured.
As previously described, the deep-sea cephalopod Vampyroteuthis infernalis may produce luminescent secretion when under attack. These secretions are sticky and contact with a predator will results in it becoming ‘painted’ and highly visible to its own predators (Robison et al. 2003). The holothurian Enypniastes eximia also exhibits a similar strategy of defence (Robison 1992).
Camouflage in the deep sea in mainly done in the form of counterillumination, which is where bioluminescence is used to match the colour of the light filtering in from the surface above. In order to do this the intensity of the luminescence emitted must be under the organism’s control (Young 1983) and in the majority of organisms that use this method the photophores are required to be of ventral orientation (Herring 1977). This method of defence is useful for mesopalagic orgainisms whose silhouette is often visible to predators bellow, especially if they are not big enough to intimidate their predators or fast enough to escape them (Marshal 1979). Organisms that are known use this method include a few decapod crustaceans and cephalopods and many fish species (Herring 1977). The extent of their camouflage can range from breaking up their silhouette to making them near enough ‘invisible’.
An example of a group of organisms that becomes near enough invisible are transparent squid. Their only opaque organs are their eyes and ink sac so by having counterilluminating photophores beneath these features the squid is barely visible from bellow (Herring 1977).
Perhaps the best illustration of counterillumination is shown in the family Sternoptychinae which are the hatchetfish. Denton & Land (1971) describe the effectiveness of their attempts. Their silvered sides provide camouflage to any organism looking at it head on, while their ability to change the direction of their photophores depending on their orientation provides near perfect camouflage from bellow.
In all of these species, the effectiveness of the camouflage depends on how acute the vision of the observer is (Herring 2002).