recently a clip of one of the most mysterious animals on Earth enchanted everyone on the internet a clip of a black sea devil swimming in bright shallow water this angler fish normally lives in an environment where the sun has never touched and yet this jet black alien creature looked like it was reaching towards the light it's a completely unprecedented image and one that sparked the imagination of many who saw it why was this deep sea beast venturing into our world the black sea devil also known as a humpback angler fish typically inhabits depths between 200 and 2500 m and like all deep sea angler fish it looks profoundly eerie their unsettling features seeming like a reflection of the inky abyss in which they live and this is normally where I would say "No they're not that weird they're just misunderstood." But this time they actually are that weird there are aspects of them that you've likely heard about before like the fact that their jaws are highly flexible allowing them to open wide enough to consume prey almost as large as themselves or that their stomachs are also expandable to accommodate these massive meals or most likely you've heard about their most iconic feature the bioluminescent lure but how the lure glows is not simple and not obvious and it's not even the angler fish that's glowing but a bioluminescent bacteria that they cultivate from their environment and helping in their hunting strategy is another incredible adaptation some species have skin pigment that absorbs almost 100% of the light that hits it rendering them essentially invisible in dark water it's similar to vanta black on top of this the females and males are extremely sexually dimorphic like the most different any two animals of the same species could be the females are the teethy nightmarish goblins we're all familiar with and the males are basically sentient free swimming balls and I say that as a joke but also not really because they are small balls sack-shaped little fish who eventually latch onto a female and fuse there forever effectively becoming a parasitic partner providing sperm in exchange for nutrients for the rest of eternity literally why though and also how how do two organisms come together and share a circulatory system without their immune systems completely freaking out all of these weird adaptations obviously have something to do with their incredibly deep and extreme environment but to me it's not immediately obvious how or why these things have come about how is their skin one of the blackest natural materials on Earth and how do they find or cultivate glowing bacteria why does becoming balls help your species survive the deep sea and why did that one specimen come up to the surface the black sea devil is just one of about 300 known angler fish species the anglers make up the group loia formies most of which share a peculiar mode of feeding that involves an appendage that acts as a luring apparatus we might be the most familiar with the ones that look like the black sea devil thanks to Finding Nemo or Planet Earth type films but there is so much variety there are five main suborders there's the goosefish suborder which includes monkfish which is the freaky and delicious guy you might sometimes see at the fishmonger there's the group that contains frogfish of which there are many different kinds like the hairy frogfish or the warty frogfish there's the sea toads and coffin fishes the batfish like the red lipped batfish and finally there are the deep sea angler fishes or pelagic angler fishes scientists believe that the ancestors of deep sea anglers were shallow water fish who invaded the deep sea during a period of extreme global warming and major ecological changes and in these depths some very strange things evolved the deep sea anglers are the most specialized and most diverse of all of the anglers there are about 170 known species which makes them by far the most species richch vertebrate taxon within the ocean midnight zone deep sea angler fish inhabit the zone of the ocean where sunlight does not penetrate this zone typically starts around 1,000 m below the surface and extends to the sea floor which can reach depths of over 4,000 m in some regions this is not your average everyday darkness this is advanced darkness the deepest living species of anglerfish ever recorded is Giganticus gargantua a type of whip-nose angler which was found 45,00 m below the surface the body of Giganticus gargantua is about 30 to 50 cm which is small compared to the length of its super long alysium the fishing lure which can extend out to 1 m or more this makes it one of the most strikingly long and specialized lures among angler fish species but not all angler fish are even this big some look like they might be in certain videos like this one of the fanf fin angler fish but it's a little deceiving females reach a maximum size of only 20 cm or like 8 in the same is true for the black sea devil from the viral video it surprised me how small it is adult female black sea devils typically grow to a length of around 20 to 25 cm or like 8 to 10 in this one seems like it was more like 3 or 4 in at most this is the specimen seen swimming to the sun now in a jar as you might have guessed it didn't make it one theory as to what happened is that the fish was sick or injured unable to maintain its normal depth or it may have been fleeing a predator or got swept up in an unusual current or if we want to be anthropomorphistic optimists it knew it was dying and wanted to see the sun for the first and only time but whatever the reason for its swim to the surface or its death it's provided scientists with an incredible opportunity to study this rarest of deep sea creatures one thing you'll notice is that the alysium of the black sea devil is relatively short compared to some other deep sea angler fish species the alysium is a modified dorsal fin spine which extends from their heads like a fishing rod at the tip of the alysium is the esca or the lure and there's something special about the lures of the deep sea anglers most of them are bioluminescent while the lures of the other loiaform anglers are not the esque of the deep sea anglers are therefore a lot more complex at the heart of the lure is the bioluminescent bacteria which produce light through a chemical reaction these bacteria are typically part of the genus interero vibbrio the bacteria contain a lightemitting molecule called luciferin and an enzyme called luciferase when the luciferin reacts with oxygen it produces light but how did the bacteria get in there in the first place the process begins when free-living bioluminescent bacteria in the ocean are attracted to chemical signals released by the angler fish's esca once the bacteria are near they enter the esca through specialized pores or duct inside the angler fish provides a nutrient-rich environment with a stable supply of oxygen and other compounds that the bacteria need to thrive in return the bacteria produce light which the angler fish uses to lure prey in the dark depths of the ocean it's an incredible symbiotic relationship this type of glowing sounds kind of simple but the thing is an angler fish does not necessarily want its esca to glow constantly and in all directions it needs to direct and control the glow to maximize its effect as a lure and also to not shine too much light on its menacing waiting open mouth so although anglerfish can't directly turn the bacteria on or off they might influence factors like oxygen supply to the bacteria indirectly affecting light production on top of this they can direct the light through what is called a light guide this is a specialized structure that directs and controls the emission of light produced by the bacteria it functions similarly to a fiber optic cable or a lens channeling the light outward in a focused manner to maximize its visibility to prey the esca also contains reflective tissues that help focus and direct the light outward these reflective structures are often composed of guinine crystals which are highly effective at bouncing and scattering light and beneath that some anglerfish species have additional adaptations like pigments that prevent light from leaking backwards towards the angler fish's body but still some ambient light will inevitably hit the angller's body and if the prey sees the trick that's happening they might not fall for it in shallower water many animals use transparency or mirrored surfaces for camouflage since these work well in diffuse solar illumination but in the deep where there is zero sunlight these strategies don't work so some deep sea anglers use what is essentially an invisibility cloak in the form of ultra black skin that absorbs nearly 100% of the light that hits it this allows them to vanish entirely into the surrounding darkness one example of this is the jet black football fish aka the Pacific football fish these deep sea anglers live up to 1,000 m deep it has a jet black football-shaped flat body others are anglers in the dreamer genus like my favorite the bulbous dreamer found at depths of around 2500 m this dreamer was found by the Mterrey Bay Aquarium Research Institute 781 m deep in Mterrey Canyon it's so black that you can hardly make out any of its features it just looks like a silhouette a 2020 study found that their skin reflects just 0.5% of the light that shines on it and this super black coloration comes from structures within its cells called melanosomes which are organels that contain the same lightabsorbing pigment as human skin melanin the outer layer of an angler fish's skin contains a seamless thin sheet of densely packed large melanosomes and to achieve ultra blackness there are no gaps at all between pigmented cells like there are in other darkly colored fish melanin is naturally excellent at absorbing a broad spectrum of light from UV to visible the melanoomes are also irregularly arranged which causes incoming light to bounce between them multiple times much like how light bounces around inside the carbon nano tubes in vanta black with each bounce more light gets absorbed by the melanin and with this black skin and the sophisticated lure the anglerfish can employ a relatively simple hunting strategy sit motionless and lure prey to mouth area once the prey is close snatch prey with disproportionately enormous mouths filled with sharp needlelike teeth and by not pursuing prey or swimming around looking for it the angler fish saves energy and the lure gives them their best chance at success for this reason female angler fish generally have low muscle mass especially compared to other predatory fish they simply don't need much for their type of hunting plus muscle mass takes calories to maintain and in the deep sea it is a constant battle for calories food is extremely scarce so deep sea anglers have evolved to have incredibly slow metabolisms female angler fish may only need to eat once every several weeks or even months so when they eat they make it count their stomachs are highly distensible and expand easily allowing them to consume meals weighing more than themselves one black sea devil weighing 8.8 g was found to have three snipe eels totaling 12.3 g in her stomach but these big girthy stomachs crazy needle teeth and dangling glowing lures are only the norm for half of the deep sea anglers the female half the other half of their population has an entirely different story a story of what it's like to not have testicles but to be testicles quick side note here to watch this video with no ads head over to Nebula it's also where you can watch extended interviews with the scientists you see on this channel and where you can get tons of original exclusive content the life of a male angler fish begins like any other hatching from its egg in a massive free floating veil that drifts through the water column a gelatinous nursery that harbors hundreds of thousands of eggs we don't know much about the deep sea angller's eggs but the related monk fish have egg veils that are over 60 ft long the baby anglers start as larae eventually growing into small fully developed fish the males of many deep sea angler species only grow to about 1 to 3 cm less than an inch this is way smaller than the females who can be up to 60 times longer and 500,000 times heavier in fact the males look so different from the females that for a long time scientists didn't even recognize male angler fish as the same type of fish because besides being tiny they also don't have the trademark alissium or esca this is because they don't need to lure they don't even really need to hunt male anglers of some species stop eating altogether once they're fully grown in fact they couldn't even if they wanted to their digestive system degenerates making them incapable of feeding independently this provides some extra pressure for the next part of its mission the singular mission of its life to find a female and become her balls okay that isn't actually scientifically accurate but it's not far off before we get to how and why he becomes balls let's first address another fascinating question in the pitch black vast ocean how does this 2 cm little dude find her the process begins with the female her ESA is doing more than just luring prey it also lures mates not with light but with pherommones when she's ready to mate she sends out chemical signals these pherommones drift through the deep sea and act as a homing beacon for males the males have extremely well-developed olfactory organs that can detect traces of these pherommones from great distances he also has surprisingly well-developed muscles to be able to reach her and large eyes optimized to detect faint traces of light from her esca as he gets closer once he spots her he prepares for the most important moment of his life he swims closer and bites into her and releases an enzyme that digests her flesh and also fuses the skin of his mouth to her body they fuse so completely that even their circulatory systems merge over time the male atrophies and ceases to exist as an individual he loses his eyes digestive organs brain function and everything else not related to reproduction he becomes an appendage essentially a pair of gonads his body releases sperm to the female in response to her hormones triggering egg release for the rest of both of their lives so is he balls kind of in the sense that he's become an organ that just provides sperm but maybe not so much balls in the sense that the female now has her own pair and is self-fertilizing her eggs and also making them less like the balls we're used to is the fact that a single female deep sea angler fish can have up to eight males attached to her at once maybe even more and perhaps the more nagging question I have is does he still exist or is becoming balls the ultimate self-sacrifice for the benefit of his lineage or is he the ultimate freeloader getting all of his life-giving nutrients from a more capable woman does he still have a consciousness is his consciousness uploaded to hers okay I am joking about some of these questions but when two individuals become one it's hard not to wonder about some existential ideas this whole strange arrangement is called sexual parasetism and it's reallying weird and if you're wondering why this happens though me too the answer to this is actually pretty simple sexual parasetism ensures that when the female is ready to spawn she has a mate immediately available in the deep it's exceedingly hard to find a member of the opposite sex so rather than have to do it over and over again they just do it the one time so this sort of leads to a different question if sexual parasetism is so effective for deep sea anglers why don't other deep sea fish do it one reason might be that deep sea anglers had an evolutionary head start with strong sexual dimmorphism evolving earlier for other reasons it's feasible to imagine a scenario where this behavior began by males sort of just congregating around a female sticking with her for his entire life maybe he would latch on for efficiency sake like a raora from there it's not so hard to imagine him deciding to just latch on and easily get nutrients forever however it's still a pretty extreme thing to do in sexual parasetism the male gives up everything vision independence organs and even a brain other deep sea fish may have evolved different but successful strategies that didn't need to take the parasitic route and there's one more thing that might be the key thing that makes it so unique and also is my final remaining question about it all most animals could not tolerate an entire organism latching onto their bodies like this humans can't even easily accept an organ donation without a massive immune rejection response how can an animal accept an entire additional animal without its immune system rejecting it at the heart of an immune response are major hystocompatibility complex or MHC antigens they are proteins found at the surface of cells and alert the body when there are foreign invaders present like viruses bacteria or even transplanted organs this is part of an animal's adaptive immunity the part of an immune system that learns and remembers specific pathogens so it can fight them more effectively the next time they show up it's like the immune system's smart branch built for long-term defense and precision attacks so in 2020 scientists decided to try to understand the MHC antigens of angler fish to see how they could tolerate such extreme tissue donation and what they found was that deep sea anglers just didn't have these antigens so the scientists moved on to investigate other parts of their adaptive immunity like killer tea cells these are the cells that kill the infected cells after they send out their alert and in deep sea angler fish their function was almost completely lost the scientists then looked towards the angler fish's antibodies produced by B cells in normal adaptive immunity and you guessed it in some species of deep sea angler fish these were missing altogether these anglers seem to start out with the genes for these systems but then they get turned off so their adaptive immune system is essentially gone or nonfunctional which is pretty mind-blowing since scientists have long believed that having an adaptive immune system was a defining part of being a vertebrate and isn't foregoing an immune system a death sentence in humans losing immune functions like this would be considered fatal immuno deficiency the angler fish have to have some way to defend against infection scientists believe they likely have enhanced innate immune system function which relies on non-specific defense like inflammation fever and general white blood cells innate immune systems are sort of the basic less specialized systems that exist essentially in all animals for things like worms this is sufficient but for a vertebrate to nearly exclusively rely on it is extremely unusual and if scientists can figure out exactly how it works in anglerfish it could hold answers for human medicine we often struggle to find the balance in immune system interventions that leave the immune system strong enough to handle microbes but weak or smart enough to not attack its own tissue imagine if organ transplants were easily tolerated without the need for dangerous immune suppressants or if people with autoimmune diseases like lupus could have those harmful self-attacking immune responses stopped or what if anglerfish could hold answers to infection resilience without overreaction like the COVID 19 cytoine storms that have killed millions of people understanding the strangest organisms on Earth could very well provide us with solutions to problems that we never considered what looks like a devil from the depths of Hades could actually be a beacon of hope for humanity and regardless of its value for the medical world and regardless of what it can do for us these deep sea little freaks have inherent value there's value in studying these creatures just for the sake of knowing how cool and weird our world really is that's the goal of this channel to share everything I learn about the natural world from the impressive and beautiful to the creepy and kind of gross all of it's important fused ball sack fish Komodo dragon gastric pellets or lion prides taking down a zebra and eating it however lately I've had to be extra careful with how I make these videos because even if we love the science of animals gross stuff and all corporate advertisers do not and therefore my videos are at constant risk of demonetization when I recently showed Lions Hunting that video got demonetized which is devastating to the 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