I've been super interested in deep sea fish since I first watched the Blue Planet episode on The Deep as a child. My father wandered downstairs in the middle of the night to fall asleep to the TV noise, and sometimes I was allowed to come with him and watch the nature documentaries with him (if I promised I wouldn't tell Mom). I begged him over and over to watch the same deep sea episode because the creatures down there are truly 'not of this world'.....even though they are probably the most contained population of ancient creatures we have on the planet. A lot of people don't like deep sea fish because they are objectively terrifying to look at with their huge mouths and lamp-like eyes and overall unnerving design. Lucky for us, they are so so far underwater that it's truly impossible for them to ever getcha unless you are in a submarine going to visit them. I love them because, more often than not, we don't know a lot about the creatures or why they behave the way they do! We keep discovering new things! How fascinating!
Bioluminescence!!!
A huge part of why I love deep sea fish is because of how prominent bioluminescene is down there! Bioluminescence is a light-producing chemical reaction which happens when a specific enzyme reacts with a substrate inside of an organism. We see bioluminescence on the surface in lightning bugs/fireflies and some fungi, but it is extremely common once you hit the deep sea (up to 76%!). I believe it's the most common form of communication, and we know relatively little about what it's trying to communicate; the video below does a great job of articulating the different survival tactics bioluminescence helps with!
The Monterey Bay Aquarium Research Institute (MBARI) does some truly amazing research on bioluminesence. As shown in the video, most bioluminescence is blue-green, and some fish have red bioluminescence, which isn't a wavelength of light visible to most fish. And then there's that gossamer worm with golden light... and we have no clue why it does that! It's those kind of questions which make the deep sea fascinating to me.
Recently, we discovered that many of the examples of bioluminescence we know of don't capture the whole picture. In the little clip above, it shows researchers find out for the first time that this dragonfish doesn't only emit light from its clearly visible photophores, but it rather sparkles all the way down its body! We had never seen that until 2016!!
My Favorites
My favorite deep sea fish are the gulper eel, the gigantocypris ostracod, and the barreleye fish! (pictured below in that order)
I've loved the gulper eel for a very long time; I've even bleach painted a shirt of it so I can share my love for it in the wild! Its most obvious feature is its crazy jaw which take up a quarter of its body and unhinges to grab prey in it much larger than itself. They have comically small eyes, significant olfactory glands (which we think it uses for mating), and a little pink/red bioluminescent blub at the end of its tail. But do not fear! This is a smaller eel (around 3ft long on average), and a trip to the surface usually rips its mouth :( I think this one has been my forever favorite because of how truly absurd its design is. Every part of this eel embodies the deep sea: its wild mouth as an adaptation to the difficulties of deep-sea hunting, its bioluminescent lure (we don't really know why it exists since its so far away from the mouth), and the trade-off of other senses in favor of....? I love mysteries, and eels, especially this one, remains one!
My love for the gigantocypris ostracod is entirely due to that The Deep episode. The scene in the documentary shows a squid expelling a bioluminescent fluid in order to trick the hunting ostracod, and it floats aimlessly looking around with its big eyes and propelled with its mustache-looking fins. It's extremely large for ostracods, but that only means its around 1.3 inches. I just think its cute!
The barreleye fish is a recent favorite! Since it was discovered in 1939, researchers have wondered how its green, upward-facing eyes are at all helpful for hunting the fish it spots swimming above. Especially since we've only seen it a total of nine times, its hard to see it hunt naturally. Awesomely, we found in 2009 that it can rotate its eyes to look horizontal!
Further Reading:
Amazing PLOS Findings about the prevalence of bioluminescent fish
Recent article estimating about how many deep sea fish likely exhibit bioluminescence
The MBARI 2009 discovery of the barreleye's eyes moving!