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Human adolescence is one of the great challenges en route to adulthood. If you are like me, you broke out in pimples, did the brace-face thing, maybe had glasses, grew hair (so much hair!) where it didn’t belong, and were probably picked on by your peers.

Now let’s compare that to growing up as a fish. Sure, braces, body hair, and acne don’t have fishy parallels. But then there isn't really a human equivalent for exaggerated body filaments, having eyes on the wrong side of your head, or the dreaded external anus. You may have thought the opposite sex saw right through you growing up, but transparent fish larvae take this literally. And while kids will bully each other around from time to time, life in the open ocean is more likely to resort to cannibalism. Life as a young fish in the ocean is a lot tougher than high school. 
 

Alectis ciliaris: Fin filaments are pretty common among adult fish, but larval fish really flaunt them. The posterchild for fin filaments is probably the threadfin jack, whose young are sometimes encountered in harbors and over shallow reefs (f/11, 1/125s, ISO 100)

Most marine fish species go through a life phase where they drift around at the surface of the ocean, termed pelagic larval dispersal phase. This gets them away from the abundance of predators found on a reef and puts them in an environment that is packed with appropriately sized food particles. However, while the reef has structure and holes, caves, or loose sand to hide in, the open ocean is a relative and vast desert. Therefore, pelagic larvae have to look very unusual if they are to survive in a completely different habitat from their parents. 

While the early life of nearshore fishes might be a little intense, deep-sea fishes take things to a whole new level. That’s why this second part in this black-water series will focus primarily on fish larvae from deepwater species to explore the challenges facing a developing fish, and the amazing adaptations they have evolved in response.
 

Carapidae: Filaments like this can make for a confusing photo. If you want the photo to make visual sense, make sure the eye is in tack-sharp focus. Usually, once the viewer recognizes the eyeball, they make a connection and the rest of the animal takes shape (f/8, 1/160s, ISO 200)

When pearlfish (family Carapidae)—aka assfish—grow up, home will be in the anus of sea cucumbers, cushion stars, or bivalves. And if that isn’t unusual enough, Carapids are one of the only fish known to shrink as they grow.

The animal pictured above is around three feet long, of which more than two-thirds of its body is a tail-fin filament and that elongated dorsal ray springing from the top of its head. These ornaments are meant to make it look like a stinging siphonophore. Before the fish can hope to fit in its adult host, these extra filaments have to disappear. That means that pearlfish actually shrink as they grow up.
 

Cetomimidae: Be sure to “look small” on a black-water dive—the total length of this Cetomimid was probably 5mm. Bring a macro lens with a short focal length (60mm is the lens of choice), and be ready to go super macro if you can. Flip diopters are a great idea for these dives (f/8, 1/100s, ISO 100)

This photo is of an incredibly rare Cetomimid, or whalefish, whose future is uncertain. I only found out after taking the photo that there is a bounty for a live one.

For decades, researchers kept bringing up fish and placing them into three very different-looking families: One consisted only of juveniles called “tapetails,” another family with only males with degraded digestive tracts, and a family of only huge, ugly females. It wasn’t until 2009 when someone discovered that these three fishes were all the same species. In fact, if this little fish grows up to be a male, its entire digestive tract will transform into something of a modified liver, and it will have to survive for the rest of its life off of the food it managed to eat as a larva. 
 

Malacosteinae: Shooting clear fish with highly reflective eyes like these presents a huge challenge. Set your ISO low to make use of the highest dynamic range your camera will manage, and take as many photos as you can. I can’t count how many dragonfish photos I have that just don’t make the cut (f/8, 1/160s, ISO 100)
 

Sure, rarities like the Cetomimid are fascinating, but do they have an external anus like a juvenile dragonfish? (For the super-nerds out there, it’s in the subfamily Malacosteinae.) Well, this creature will grow to be long and eel-like, and it will reabsorb its anal feature in due time, but let’s talk about the rear appendage for a moment.

First off, it is often longer than the fish—that’s to grab every last nutrient from its meals. The faster it grows, the less time it spends as a swimming tasty morsel. But you might have also noticed its trailing gut is “butt-dazzled” with photophores, or light-producing organs. They produce a light with a reddish hue. Most fishes have eyes that can detect the light wavelengths which penetrate water best (blue to ultraviolet, typically between 46–490nm). In addition to producing red light, dragonfish have astaxanthin in their visual pigments, so they can see light wavelengths well into the red spectrum. This gives dragonfish a whole spectrum of wavelengths to use in interspecies communication and super-secret prey detection—all completely undetectable by other deep-sea inhabitants!
 

Brotulataenia nielseni: These animals are hiding behind nothing. Spend your time looking at every piece of flotsam with suspicion. You never know what you are going to find! (f/10, 1/160s, ISO 200)
 

Brotulataenia nielseni is one of those animals that doesn’t have a common name. The young have taken fin filaments to a whole new level. The filaments give the small fish a lot of extra surface area, meaning that the drag from the filaments keeps it near the surface. That way the young fish doesn’t have to spend precious energy staying buoyant, so it can focus on a more important task: growing up into a big, ugly, brown deep-dwelling eel that most of us will never see. The filaments play another role as well: mimicry. Some claim that it is meant to look like a venomous siphonophore, others think it resembles a bird’s feather.
 

Lophiodes fimbriatus: Being set up ahead of time is the only reason I came away with this shot. Know where you want your strobes and focus lights, and set your camera’s aperture, shutter speed, and ISO as soon as you hit the water (f/6.2, 1/160s, ISO 100)
 

When the filaments extend from the skin and not the fins, we call them cirri, and this young anglerfish is covered in them. Lophiodes fimbriatus has only ever been observed a scant handful of times in Japan and Indonesia, so imagine my surprise when this thing showed up across the ocean for the first time in Hawaii! The cirri make it look like a fried egg jellyfish, which are also incredibly rare in Hawaii. What caused this little fish to take a Hawaiian vacation is beyond us. Encounters like this are rare and often fleeting, so you have to be ready to focus and fire.
 

Lamprogrammus: For a creature that grows into a large, brown nondescript fish, larval cusk eels are a quiltwork of fantastic adaptations (f/10, 1/160s, ISO 200)
 

And finally, we come to Lamprogrammus brunwicki—the fish that was so incredible I nearly lost the boat following it! By the time I looked up, the boat was but a faint glow in the distance. And that’s why we tie divers to the boat, because five miles home is a long way to swim. No matter how enthralling your subject, always be aware of your surroundings!
 

Author’s Acknowledgement: These identifications would not have been possible without the tireless help of professionals such as Dave Johnson, Bruce Mundy, and Jeffrey Leis, among many, many others. Thank you for all of your help.
 

About the Author: Jeff Milisen is an underwater photojournalist with a degree in biology from the University of Hawaii Manoa. Jeff’s art has always reflected his curiosity for the natural world, as has his penchant for exotic and remote environments. He has spent two field seasons working with NOAA’s marine debris team in the northwestern Hawaiian Islands and, more recently, he spent six months living and diving from a sailboat between 10 and 70 miles offshore of the Big Island of Hawaii. His work is frequently seen in popular publications, and he has been awarded honors in numerous international competitions including the Ocean Art photo contest, Our World Underwater, Beneath the Sea and most recently, Nature’s Best Ocean Views photo contest sponsored by the Smithsonian Institute. See more of his work at iphotograph.fish, and follow him on Instagram and Twitter @JeffMilisen.