The deep-sea is a highly hostile environment. Temperatures in this region can fall as low as −1.8 °C and rarely surpass 3°C. In addition, both oxygen levels and nutrient concentration begin to diminish with depth, and pressure increases at a rate of one atmosphere every 10 metres. However, it is some deep-sea species’ ability to thrive in complete darkness that is most impressive.

The mesopelagic zone ranges from 200-1000m in depth. Conditions change quickly here; temperature decreases rapidly through a thermocline and light fades fast as one travels deeper in this region. The point at which no light can penetrate at all is called the aphotic zone. Consisting of the darkened bathypelagic (1000-4000m) and the vast black plains of the abyssopelagic (4000-6000m) beneath that; the pitch-black world of the deep-sea is home to specially adapted species, with well-developed strategies that allow them to succeed in this challenging environment.

In addition to the rapid changes in conditions between the mesopelagic and bathypelagic, species behaviour seems to vary significantly between the two zones also. The mesopelagic is home to highly mobile, active prey, possibly in response to the increased visibility and open waters present in this area. In contrast, fish in the bathypelagic would rather wait for their prey than expend precious energy hunting for it. They usually have slow metabolisms and unspecialized diets, meaning they will eat anything that they come across due to the relative scarcity of food in this region.

The rarity of interactions in this zone; whether that’s finding prey or meeting a potential mate, is a prominent selection pressure for deep sea species. Therefore, various adaptations to increase interaction rate have evolved in predatory bathypelagic species, the most impressive of which, is bioluminescence.

How does it work?

Bioluminescence is the emission of light by either fluorescence or chemiluminescence in an organism. Because light must first be absorbed by a substance for fluorescence to function, this mechanism is not exhibited in deep sea animals. Instead, they use chemiluminescence due to the lack of natural light.

Figure 1. Coelenterazine is the luciferin (light emitting molecule) found in many luminous aquatic species. – SOURCE

Chemiluminescence is a chemical reaction in which light is emitted when a product (luciferin) in an excited state returns to its relaxed ground state. During chemiluminescence, the luciferin reacts with oxygen in the presence of magnesium ions, ATP and luciferase enzyme. This reaction is 100% efficient; all the energy is converted to light energy without any production of heat. This light is therefore known as “cold light“. The only variation between bioluminescent species is the light emitting compound that is used as luciferin and its corresponding enzyme. Coelenterazine is the luciferin common in many aquatic species (Figure 1).

How it evolved?

The evolutionary development of many bioluminescent mechanisms remains a topic of discussion. Upon the beginnings of life on earth, the atmospheric makeup was very different to the one we breathe today. It had relatively little oxygen; however, oxygen was toxic to anaerobic organisms present during this era and had to be eliminated from their system. Hence the introduction of coelenterazine. Due to the strong antioxidative properties of coelenterazine, it is believed that the original function of coelenterazine was to detoxify toxic oxygen derivatives such as peroxides. So how did the role of coelenterazine change from the detoxification of reactive oxygen species to increasing interaction rate via light emission? It may have occurred when aquatic organisms began to colonise deeper and deeper in the ocean because the strength of oxidative stress decreases with depth. Upon colonisation of the bathypelagic zone, lack of interaction between organism may have become the greater selection pressure as opposed to reactive oxygen species, due to low oxygen levels at these depths. It would also provide an explanation for the low metabolic rate found in deep water animals, as it reduces the internal production of harmful oxygen derivatives.

What’s it used for?
Figure 2. A female humpback anglerfish (Melanocetus johnsonii) – SOURCE

The humpback anglerfish (Melanocetus johnsonii) is one of the most commonly known deep-sea species. Despite earning the nickname ‘common black devil’ for its terrifying appearance, this round bodied, sharp-toothed fish is built to survive in the darkness of the bathypelagic (Figure 2). The humpback anglerfish can expand both its mouth and stomach to the extent that it can consume prey twice the size of its body. It also has sharp, inward-facing teeth to prevent prey from escaping. This is crucial as food can be scarce in this region and the anglerfish may have to go a prolonged period without another meal.

However, the most impressive adaptation is this species use of bioluminescence. Melanocetus johnsonii possess a single light producing organ (photophore) on the end of its elongated dorsal spine. The photophore contains bioluminescent bacteria called photobacteria. The fish relies on a symbiotic relationship with the bacteria, gaining the necessary bioluminescent properties in exchange for nutrients and protection. The photophore emits a blue-green light and is waved back and forth on the highly manoeuvrable spine like a fishing lure to attract curious prey, hence the name ‘anglerfish’. The fish itself does not move, trying to remain as unseen as possible. The humpback anglerfish has specially adapted skin to help achieve this; it reflects the blue light that both it and other illuminated organisms produce. Also, the photophore can be concealed when necessary by a muscular flap. Upon luring its prey close enough, the anglerfish opens its mouth wide causing an in-pour of water, pulling the animal in and swallowing it whole.

What does it mean?
Figure 3. Once cracked, a bioluminescent reaction occurs within the glow stick which produces ‘cold light‘. – SOURCE

Chemiluminescence is an example of a functional shift over time. It shows how a change in selection pressures can result in the role change of a mechanism. The humpback anglerfish also demonstrates that extreme adaptations are required if one is to succeed in an extreme environment. Despite they’re gruesome appearance, they exhibit compromise by providing shelter and nutrients to photobacteria in exchange for the bioluminescence. This mutually beneficial relationship is what makes Melanocetus johnsonii such a successful ‘lie-in-wait’ predators. In addition, the bioluminescent reaction that occurs in the deep-sea can also be harnessed to help humans. Glow sticks light up via the same principle, ‘cracking’ the stick causes a chemical reaction that makes the atoms become excited and release photons (Figure 3). This is especially useful as it occurs without the production of heat, making them the perfect light for scuba-divers.

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