A Great Barrier Reef pearlfish has been filmed by the BBC living inside a sea cucumber's bottom. But it is not the only fish with a somewhat unusual home. Where are some of the other strange places fish live?In bottoms: Pearlfish
Inside another animals' bottom might seem like very worst place to be, but there are big advantages to the pearlfish's chosen home.
"The key thing for fish living on coral reefs is to find somewhere to hide and not get eaten," says Prof Martin Attrill, head of the marine institute at Plymouth University.
This "fascinating way of living", he says, is very specific to this group of fish; they can also live inside clams, starfish, or anything else "with a hole".
"Sea cucumbers are like bags - they make a good hiding place for pearlfish.
"They are essentially living in a burrow and they come out at night to feed from the colonic cavity. It is moist and safe in there, and quite often they live in there in pairs."
And there is no appreciable harm to the sea cucumber; pearlfish are commensal, which means they have a non-parasitic relationship with their hosts.
In trees: Mangrove killifish
This "amazing little fish" can survive up to 66 days out of water.
Kryptolebias marmoratus lives in the mangrove forests of the Western Atlantic and has the capacity to survive a long time out of water in trees and logs, "as long as it keeps moist", says Prof Attrill.
It is able to do this because of its specially adapted gills; it excretes nitrogen waste through its skin while out of the water and has been recorded as being able to survive up to 66 consecutive days through cutaneous respiration.
One fish can also repopulate an area, even if it ends up stranded on its own.
"They are hermaphrodites - they have [both] male and female organs... which is really unusual in vertebrates," says Prof Attrill.
"It is a great way to maintain a population and allows them to reproduce without a mate, enabling them to live alone in logs."
In hot water: Desert pupfish
In the summer, temperatures soar in California's Death Valley.
To survive this extreme environment - in salty pools that are heated up to 40C - the desert pupfish, or Cyprinodon macularius, has adapted over time.
"People think they survived the last ice age, and evolved to survive in a process of quite extreme natural selection," says Prof Attrill.
"In fact, only the best survive each summer and reproduce the following summer."
Despite its apparent resilience, the pupfish is endangered. It is threatened by habitat loss and by competition from other fish species.
Males live inside females: Anglerfish
In the deepest, darkest part of the ocean, it is pretty hard find a mate.
So the anglerfish has come up with a straightforward solution. When a male finds a female by following her scent, he latches on with his teeth and doesn't let go, fusing to the female.
"This fish is an extreme adaptation in terms of a sexually developed female. A parasitic male is carried by the female and eventually merged in to her body as a bump; all he is used for is his sperm," explains Prof Attrill.
"In the deep sea, it's dark, and hard to find another anglerfish, so the best way is to carry another fish around.
"This has developed so that the female carries around just a couple of testes to pass on the genes."
In the depths: Hadal snail fish
True creatures of the deep, the Hadal snail fish, Pseudoliparis amblystomopsi, was described as "surprisingly cute" when it was first discovered.
The "deepest ever" fish found alive, a 17-strong shoal were filmed at depths of 7.7km (4.8 miles) in the Japan Trench in the Pacific, and have to develop to cope with the pressure at that depth.
"The Hadal snail fish is flying the flag for an undiscovered area; they are swimming around in a place man has never set foot," says Prof Attrill.
The fish use vibration receptors on their snouts to navigate the ocean depths and to locate food.
The researchers who found them thought they would be motionless.
"We certainly thought, deep down, fish would be relatively inactive, saving energy as much as possible, and so on," Prof Monty Priede from the University of Aberdeen's Oceanlab told BBC News.
"But when you see the video, the fish are rushing around, feeding accurately, snapping at prey coming past."
In mouths: Jawfish
Many parents aim to protect their young. But "mouthbrooders" take this to a different level - keeping their young in, as the term suggests, their mouths.
Jawfish, are some of the very few saltwater fish to have embraced this mostly freshwater fish practice.
The female lays the eggs and the male takes them in his mouth.
"The bigger the mouth the better," says Prof Attrill. A male can have up to 400 eggs in his mouth at one time.
"The male can't feed (while he hosts the young), it's a case of males, get on with it," Prof Attrill explains.
"It's a great place to hide, and as the young get older they spend more time out of the mouth."
In deep sea vents: Hydrothermal vent eelpout fish
Hydrothermal vents can be up to 380C, and are home to one of the most extreme ecosystems on Earth.
The eelpout fish (Thermarces cerberus) is the one group that lives on the vents, and feeds on crustaceans. So how does it stay out of hot water?
"It is a long, thin eel-like fish, almost a knife shape, which is great for getting in and around the mussel clumps it feeds on," says Prof Attrill.
"It has the ideal body form to deal with that.
"What is very interesting is how they travel between vents - using whale bones etc on the sea floor to 'jump' between vents."
In shells: Cichlids
Different to true shellfish, these tiny fish live in shells in the Great African Lakes.
More than a thousand new species have evolved over the last 12,400 to 100,000 years, and live in shells as they "need to find really good homes to protect themselves".
Prof Attrill says: "If they don't have shells, they feel very nervous as they don't have anywhere to live."
In the dark: Blobfish
Heralded as the ugliest fish alive, the blobfish lives in the deep waters off Australia and New Zealand.
In the deep sea, swimming is hard work if you are a heavy protein-filled fish, so their tissue has become water-based.
"They are almost like jellyfish, and feel quite gloopy... and are charismatically ugly," says Prof Attrill.
But interestingly, blobfish show "nesting" behaviour.
"Pairs lay eggs on the sea floor and look after them together - it's really the first time we've seen parental care in deep sea fish," says Prof Attrill.
In anemones: Anemonefish
For most small fish, living within the grasp of the tentacles of predatory sea anemones would mean dicing with death on a daily basis.
But the anemonefish, also known as a clownfish, or Amphiprion percula, is unaffected by anemones stinging tentacles.
A substance in the mucous that covers their bodies prevents anemones' stinging cells from firing and killing the fish.
Clownfish parents clean an area of their host before laying eggs so their young are not harmed either.
The male then protects the young until they are ready to leave their parents.
Underground: Brazilian blind characid
Like most creatures that live underground, this incredibly rare fish is blind, and was only recently rediscovered in wells fed by underground springs in Brazil.
Stygichthys typhlops also lacks pigmentation, a common trait in organisms that live underground in the dark.
Biologists cannot be sure, but they suspect the fish may be a living relic that has survived deep under the ground while its relatives above became extinct.
Unknown home: Baby coelacanths and eels
Initially thought to be extinct only one baby coelacanth has been seen since the species was discovered in 1938.
"Coelacanths are probably hiding in rocky crevices and caves, but it really highlights how little we know," says Prof Attrill.
"We don't know where they go, or what they do. It's really more of a case that we've only recently rediscovered them."
But there is another species we know very little about - eels.
"The whole migratory cycle of eels is a mystery.
"Once they hit the open ocean we don't know what happens," says Prof Attrill.
"We only recently found out what basking sharks do - if something the size of a basking shark can elude us then it may be a while before we find out what eels do."
It is thought eels hatch in the Sargasso Sea, in the middle of the North Atlantic, where they feed on young plankton, and then migrate to Europe and North America, before returning later in life to spawn.
But very few have ever been caught en route.