Defensive

In order to keep up with advances in predatory detection methods, pelagic fauna have evolved many unique evasive adaptations. Most accounts of bioluminescence in the deep-sea are used as defence mechanism (Herring 2002). The favoured technique is simply not to be seen at all. Many species, from cephalopods such as the Cockatoo squid (Figure 1) through to predatory Lanternfish, have incredibly sophisticated photophore systems lining ventral surfaces to help blend in to the ambient background light (Johnsen et al. 2004). The Cockatoo squid, mid-water decapod shrimps and many other species exploiting this technique, can adjust their photophores to be shining directly down at all times (Herring 2007).

Figure 9.1. Bioluminescent dinoflagellae Noctiluca scintillans varias (tomclarkblog.blogspot.com)

Failing to blend in leaves organisms with a fight or flight situation. Due to the relative scarcity of food in the pelagic twilight, being built for speed is rather inefficient and undesirable, until one meets a predator. Some organisms do simply flee utilising strong swimming muscles, however many others make use of their bioluminescent properties. The ability to flash or strobe can work to disorientate or stun a predator. Smaller organisms, such as dinoflagellates (Figure 9.1), let off bright flashes giving away not only their location but the location of the shrimp (or whatever be predating on them) to larger predators (Herring 2007).

Figure 9.2. The defensive secretion of bioluminescence by Heterocarpus sibogae (Digital Taiwan)

Similarly, secretions by decapods, ostracods and other crustacea from specialized labral glands can deter an attack by a predator (Herring 1985), as the bioluminescent discharge could decorate the animal making them a beacon to attacks from bigger fishes or cephalopods (Figure 9.2). Such secretions offer a secondary defence by way of stunning or confusing the attacker. Being caught in a bioluminescent cloud can momentarily act as a decoy for the prey organism to flee under the cover of, or in the case of weaker swimming organisms simply evade the snapping jaws and hope the light show did enough to dazzle and send the predator swimming in the wrong direction.

Vampyroteuthis infernalis, the soul member of its genus not only exploits bioluminescence to catch prey but by the same method uses it to avoid becoming prey. The species uses a bright lure, a photophore on the end of one of its eight arms to attract prey. If a larger animal comes along it is hoped they deal minimal damage by simply attacking the lure arm; a sacrificial limb if you will (Robison et al. 2003).

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