From the Depths

A Creature of Nightmares Fishing With Light

Foxfire was rather spooky when we didn’t know what caused the strange glowing of the wood, but this week we’ll be looking at a group of bioluminescent animals that are much more frightening when you can see them entirely. Like a scene from a nightmare, these creatures have huge gaping mouths with razor-sharp teeth. Their appearance is almost otherworldly, and it should be considering that are found in the deep ocean where no light penetrates. These are the deep sea anglerfish.

Anglerfish are another example of aggressive mimicry like those detailed in Season 4. There are some species that live in shallower water and act as ambush predators, blending in with nearby rocks, seaweed, or other natural features. Many of these, as well as their deep sea counterparts, possess a lure composed of a fleshy organ called the esca at the end of a long stalk or filament. The stalk can be moved back and forth to simulate a small fish or other prey item moving. When the prey closes in on the lure, the anglerfish strikes, its large mouth making an excellent grabbing tool.

There are a few things that distinguish the deep sea anglers, however. This includes a large sexual size dimorphism for many species. Females are generally several orders of magnitude larger than males. For anglerfish that display this difference, the males usually do not possess a lure at all. Instead, once they reach maturity, they find a female and attach themselves by biting her. Then, their circulatory systems merge as most of the male’s organs are absorbed and lost, leaving only a set of gonads for producing sperm. Females can and do have multiple males attached to them.

Furthermore, most deep sea anglerfish (also known as ‘seadevils’) possess an esca that is bioluminescent. Unlike the fungi that emit foxfire, the anglerfish do not produce the chemical or protein required for the reaction. Instead, they have developed a symbiotic relationship with luminescent bacteria that live in the esca. The exact nature of the relationship is still poorly understood, including how they are acquired. The esca has pores that are always exposed to seawater, so there is the possibility that they are acquired in this way. However, each species of fish has its own species of symbiotic bacteria and some of those bacteria do not bioluminesce on their own (the fish produces some chemical also required for the process).

Regardless of exactly how these fish acquire their symbiotic partners and whether it is mutualistic or commensal (see Season 2 for more detail on symbiosis), the ability to make their esca glow creates a compelling lure in an otherwise completely lightless environment. For many of us, we are most familiar with these fish from a scene in the movie Finding Nemo.

Of course, in actual anglerfish only the esca will glow, not various dots along its dorsal ridge. Its eyes would also be much smaller (no need for them to be so big in a lightless environment), but if we want to get really technical, neither Dory nor Marlin would have been able to survive the pressures at that kind of depth anyway. However, the overall body shape and ‘fishing’ strategy depicted are fairly accurate. Even its size is not unheard of, as some deep sea anglerfish can reach over one meter in length, an absolute monster from the perspective of a smaller fish like those in the movie (a blue tang and a clownfish).

Anglerfish mouths really do span the entire length of their head (though some species have more vertical mouths instead). This gives them an incredibly jarring appearance for humans, like something that shouldn’t exist outside of some strange, nightmarish fantasy realm. The glowing esca only reinforces this atmosphere of strangeness. The fact that people cannot readily enter their world and they are difficult to find even by unmanned submersibles adds even more to their mysteriousness.

There’s still a lot we don’t know about deep sea organisms and this will be evident several more times this season, as many of the bioluminescent animals we will be looking at dwell in this habitat. However, despite their odd appearance, there is nothing alien or supernatural about anglerfish or their glow. As with any evolutionary development, they have found a way to exploit their particular environment to improve their survival ability.

I do hope you are finding Season 8 enjoyable so far. There are still many more different light-producing organisms to look at and talk about so stay tuned each week. As always, I am eager to hear any feedback you might have to offer regarding my writing or any future topics you would like to see me cover. If you like Nature Stories, please consider sharing it with your friends.