Lurking in the Deep

As mankind’s technology advances, it is now possible to explore uncharted environments, previously unreachable and altogether unfathomable. As a species, we have now discovered all manner of weird and wonderful deep-sea animals and we continue to do so to this very day.

Along with the weird and wonderful comes many strange and frightening species, such as the goblin shark (Mitsukurina owstoni) or the giant Isopod (Bathynomus). Perhaps the most famous of them all is the anglerfish of the order Lophiliformes.

An anglerfish captured on camera. Image source.

Imagine you are travelling down through the sea, getting deeper and deeper. Above you are the surface waters, illuminated by sunlight. This is the photic zone and hosts a great diversity of marine life.

As you pass the photic zone, only 200 metres down, the light begins to fade and the number of species you encounter drops. At a depth of 2000 metres you’re well into the aphotic zone and approaching abyssal conditions. It is here where you would come face-to-face with the deep-sea anglerfish.

There are over 300 species of anglerfish, most of which live in the depths of the Atlantic and Antarctic oceans. Most anglerfish are less than a foot in length, but some have been known to grow larger. Due to the isolated nature of the anglerfish’s habitat, these animals are notoriously difficult to study.

As such, some aspects of the anglerfish’s ecology are unknown and will remain so for some time. One such example is the mystery of its lifespan. It is widely regarded that, like other teleost fish, the anglerfish lifecycle comprises of a larval stage followed by metamorphosis into an adulthood form. Larvae are released from eggs on a sheet of gelatinous material which floats to surface waters; anglerfish larvae spend their time feeding on plankton until they are ready to return to the depths.

Survival of the Fittest

Due to the extreme conditions posed on deep-sea species, effective adaptions are essential for survival and reproduction. Species living at great depths have to be able to withstand very high pressures, cold temperatures, and a complete lack of sunlight. The deep-sea anglerfish has an array of unique and interesting adaptations to cope with these unhospitable conditions.

Perhaps the most infamous of these adaptations is the glowing bulb protruding from its head. This adaptation is brought to life by bioluminescent bacteria and is only found on females. Bioluminescence as an adaptation is featured frequently in deep-sea creatures. Other animals that make use of bioluminescence include the firefly squid (Watasenia scintillans) that uses its bioluminescence to conceal its shape and confuse predators. There is also the Carribbean Ostracod that uses bioluminescent vomit to attract mates.

Anglerfish use their bioluminescent ‘lures’ to attract prey. This method of prey capture allows individuals to conserve energy: a trait that is highly desirable in the food-depauperate deep sea.

The glowing lure pairs well with their globular-shaped bodies which allow them to float with ease along the seabed as opposed to sustained periods of swimming which would consume a lot of energy.

Like the majority of deep-sea predators, anglerfish are opportunistic in nature, and are capable of eating a large range of prey items. This is facilitated by an expandable jaw, huge teeth, and an elastic stomach, further aiding them in digesting prey of varying sizes.

The diet of female anglerfish consists mainly of small fish, shrimp, and small squid. However, there is reason to believe that anglerfish take advantage of the food influx from surface waters, the main food source of the deep sea. This marine debris constitutes a huge diversity of dead and decaying organisms, from plankton to seabirds. Despite this, the anglerfish diet largely depends on what food is available. The expandable jaw allows individuals to feed on prey twice their size, however meals this large rarely reach the seafloor.

Battle of the Sexes

One of the most interesting aspects of anglerfish ecology is the relationship between males and females. When scientists first discovered anglerfish in the 19th century, they found the typical specimen that you might imagine: a set of razor-sharp teeth and glowing lure at the ready.

They soon realised that the specimens found were all female; nobody had any idea what the males looked like. A century later, an Icelandic scientist, Bjarni Saemundsson, found a specimen with two smaller fish attached to her belly and assumed that they were her offspring. A few years later the dissection of one of these smaller specimens revealed the existence and nature of the long-lost male anglerfish.

Further research revealed that the males exhibit ‘parasitic’ behaviour, essentially becoming an extension of the female. During mating, the male bites into the female’s underside and hangs on to her until both individuals are fused together. The circulatory system becomes intertwined. The male receives life support from the female in the form of nutrients and, in return, becomes her living sperm bank, facilitating multiple spawning.

Whilst some species adopt a monogamic reproductive strategy, some species of anglerfish females can host a multitude of male partners – as many as eight in some species.

What the Future Holds

Current constraints on technology as well as the associated expenses required for deep-sea studies, limits our ability to study anglerfish in their natural environment. However, as the awareness of our oceans grows, more and more will take part in uncovering the ocean’s mysteries. The anglerfish is just one of many deep-sea animals waiting to be brought to light and as such, represents an exciting opportunity for discovery for all aspiring marine explorers.

Written by Taylor Harris

Edited by Lucas King