Driving through the midwaters of the deep sea feels a lot like staring at the starting credits to the Star Wars movies – without any words or music. The darkness is broken only by what scientists call marine snow, animals and debris that slowly fall from the upper ocean, and only very occasionally by larger animals swimming quickly in and out of view. 

As someone who has sat through many hours of these mid-water dives, I can tell you the monotonous scenery – hours of nothing but deep blue water – can be hypnotic, a strange way to feel when you're exploring hundreds to thousands of of meters deep. Considering how difficult it is to find animals in the vast expanses of these sparsely populated waters, it's amazing that wildlife filmmakers Kirsten and Joachim Jakobsen even spotted a small, globular anglerfish in the inky-black waters as they finished up a long dive off an island in the Azores.

It was unusual enough to find an anglerfish on a dive that the scientists changed course and followed this six-inch long fish for 25 minutes, capturing incredible video footage of the rare individual. While there are over 100 species of deep-sea anglerfishes, only a handful have been caught live on camera for scientists to observe. The anglerfish in this video was later identified as Caulophryne jordani, by Ted Pietsch, the University of Washington professor of fishery sciences emeritus who literally wrote the book on anglerfishes.

Following the anglerfish

This species is remarkably rare, and scientists have only been able to study it by using the mere 38 specimens that have been caught in nets and preserved for study. Unfortunately, these small, flabby-looking fishes are built for floating in the mid-waters and do not hold up well when they are dragged up to the surface – their fragile, paper-thin skin is often torn, and the long, whisker-like fin rays that can be seen in the video are often broken off and damaged. When scientists only have a few, beat-up specimens to look at, it's hard to learn much about the anatomy, biology, and ecology of a species.

Based on the few specimens available, scientists previously speculated this fish may fan its fin rays out to make a net-like array in the water, likely to help them sense and catch food that is hard to come by in the deep-sea. But they had never seen it in action to confirm the idea. The video footage shows the fish with long, filamentous fin rays spread gracefully through the water, supporting the scientists' hypothesis.

The video also revealed some unexpected aspects of the anglerfish’s biology. In the footage, light appears to glimmer off the fin rays. While anglerfishes are known to be bioluminescent — they produce light from a lure on their head that holds glowing bacteria — glowing organs, or photophores, have never been observed on its fins. Pietsch speculates the glow seen in the video could very well be never-before-described bioluminescence emitted from the fish’s fins (although further evidence will be required to be certain).

The parasitic male

But perhaps the most exciting part of this video footage is that the Jakobsens didn't just manage to film a graceful female with sprawling fins. They also captured a second anglerfish in the footage – a much smaller male parasitically attached to the female. Yes, you read that correctly – a parasitic male. In many anglerfish species, the females are much larger than the males. 

The small males do not have the prominent bioluminescent lure, but are instead equipped with large eyes and noses. They use these to seek out their lovely lady anglerfish counterparts in the dark waters. When they find her, they have no intention of missing a mating possibility. The male will bite on to the female to get a firm grasp. He will remain attached, eventually fusing completely to the female so he can obtain essential nutrients. Attached to her side, the male essentially serves as a convenient source of sperm whenever the female is ready to mate.

While this may seem ridiculous, it makes a lot of sense for fishes living in the dark expanses of the deep sea, where chances of bumping into a potential mate at the right time are slim. This strategy allows the males lucky enough to find a female a much better chance of actually mating in the future. Scientists have known of this strategy for some time, based on observations of preserved specimens, but this is the first ever recording of the parasitic mating behavior in a live anglerfish, making it, Pietsch explains, a “rare and important discovery.”

It may seem strange that scientists are so excited by this little fish, but that's because we simply don't know much about how these fishes – or most deep-sea creatures, for that matter – behave in their natural habitats. While specimens are invaluable in learning about certain aspects of fish biology, there are a lot of questions that can't be answered without seeing how an animal acts while alive in its environment. 

And even relatively basic observations of deep-sea animals have been hard to come by until recently. Though the deep sea is the largest habitat on Earth, its freezing temperatures and intense atmospheric pressure, which can be 100 times higher than it is at sea level, have made human exploration at these depths challenging. Only in the last 50 or so years have scientists been able to use robotic vehicles and manned submersibles, like the one used to capture this anglerfish footage, to watch the behavior of deep-sea animals.

Luckily, recent technological advancements are continuing to make it easier for scientists to expand our understanding of the ocean’s depths. Scientists can now program vehicles like autonomous underwater vehicles (AUVs) to explore parts of the ocean, or can manually control remotely operated vehicles (ROVs) in real time while watching high definition videos of the deep sea. Other vehicles, called submersibles, are capable of holding several people who drive and operate the deep-sea submarine while exploring the deep-water habitats.

Improvements to each of these vehicle types is making deep-sea exploration more accessible and allowing scientists to explore never-before-seen parts of our oceans. Recent discoveries of important ecosystems like hydrothermal vents, fissures in the ocean floor that spout geothermally heated water, and methane seeps, where cool methane gas bubbles into the surrounding area, are just a few of the exciting finds from our deep-sea explorations. 

You can even watch discoveries happen in real time, by tuning into watch the Okeanos Explorer and Nautilus explore new areas of the deep sea. This video footage is just one example of how our technological advancements and continued deep-sea exploration are instrumental in explaining the ecosystems and biology that, until now has existed in darkness.

Luckily for scientists (and those of us who are just amazed by the strange, and rather remarkable creatures living in the oceans' depths), we are certain to learn more as we continue exploring.