All About Our Oceans’ Fish Species

Magnificent whales, comical seals and sleek sharks frequently steal the show when it comes to marine creatures; they are the species we most often hear about in the news and see on TV programmes and conservation campaigns. Fish are often forgotten in favour of these larger animals yet this group is actually incredibly diverse and intriguing. From fluorescent spots to neon pink stripes, ruby red spines to rainbow ombre scales, fish are undoubtedly our most colourful creatures. However, they also have some of the most ingenious adaptations, partly due to 70% of the planet being water, giving them multiple different habitats and temperatures to live in but also thanks to them living on Earth for the grand total of 500 million years, giving them plenty of time to evolve and develop many quirks. In fact, there are more fish than all other vertebrate species on the planet combined.

Technically, fish are a group of aquatic vertebrates, meaning they live in the water and have a backbone. However, most also have gills, scales and swim bladders whilst they reproduce by laying eggs. Common examples are pufferfish, angelfish, sharks, stingrays, eels and seahorses but there are also some more unusual names such as the blobfish, red-lipped batfish, vampire squid and the sarcastic fringehead. Whilst the total number of fish species is debated due to our lack of knowledge about large areas of the ocean, a common estimate believes there are almost 34,000 different species of fish in the world. The largest of these is the gentle dappled whale shark which can grow to 50 feet long and weigh several tonnes. In comparison, the goby, one of our smallest fish, is a mere half an inch. The ocean’s oldest fish include lampreys and the unfortunately named hagfish which have sucking mouths, showing that they stopped evolving before the development of biting jaws. Interestingly, you can tell the age of an individual fish much in the same way you can with trees; by counting the growth rings on its scales or ear bones. In the ear, the bone slightly changes colour depending on the season, with white highlighting summer and translucence depicting winter. On the scales, a pair of fine rings appear each year with the bigger in the summer as the fish grow faster from more nutrients.

The difference between fish and our iconic mammals, such as whales and dolphins, is that fish breathe oxygen in from the water whilst mammals need to come to the surface to breathe air. In order for fish to breathe, they take a gulp of water into their mouth which then passes over the feather like filaments which make up the gills. The gills are filled with blood and oxygen passes from the water into the bloodstream. This takes place via a process called diffusion as oxygen always wants to move from an area where it is in high concentration, in this case the water, to a place of lower concentration, the fish’s blood. The heart then pumps the oxygen around the body whilst waste carbon dioxide leaves the bloodstream in the gills.

For those wondering if fish sleep, the answer is kind of but not as we do. Some, such as tuna, have to swim constantly in order to be able to breathe whilst sharks act like dolphins by shutting down half their brain at a time, allowing one half to rest whilst the other is alert for danger. Other fish float still in one place whilst sleeping or maybe they hide amongst corals and bury themselves into the mud to hide from danger. The parrotfish has a clever but slightly disgusting way of having a snooze as they create a sticky mucus cocoon which they wrap around themselves like a sleeping bag. This not only keeps them warm but stops parasites attacking them as they doze.

Some of the most exciting adaptations have come from the need to protect themselves from predators however, with our fish species using multiple different methods of keeping themselves safe. Of course, there is simple camouflage which allows prey to hide from their predators. The brilliantly named leafy sea dragon, David Attenborough’s favourite fish, disguises itself to look much like the seaweed drifting in the ocean currents. This creature appears very similar to a seahorse but is decorated in plant-like protrusions which help it disguise itself into seaweed forests. Similarly, the stonefish has evolved to look like a harmless rock on the seafloor and even has algae growing on its skin. Despite their benign appearance, these are some of the deadliest creatures in the ocean with spines containing strong toxins which cause pain, swelling and tissue death. Luckily, they mostly use these as a defence mechanism and not an attack strategy! Whilst you may not realise it, sharks actually have a specific colour to help them fade into the ocean. Their dark upper sides mean that when looking down into the deep ocean it all appears a dark hole whilst their light under sides mean they blend into the sunlight for the fish below them. Others, such as the angelfish, have dark lines running through their eyes so other species can’t tell that it is in fact an eye, helping them appear less like a tasty living snack.

Then we have bioluminescence, an incredible adaptation which is most often seen in the dark deep sea. Bioluminescence can be caused in two ways; either the fish itself has self-luminous cells called photophores or they have bacteria living on the fish which are able to glow. For example, Hawaiian bobtail squid glow in the dark via bioluminescent bacteria living in their organs. By glowing in the dark, they are able to camouflage themselves against the moonlight shining down through the water whilst also eliminating their shadow, all of which enables them to escape predators. The angler fish has a large glowing lantern protruding from its head which is used to attract prey, much as a moth to a flame. 1,500 fish species are believed to use bioluminescence for attracting prey, scaring predators and even finding mates within the same species yet a new use has recently been discovered. The flashlight fish have a light organ beneath each eye where bioluminescent bacteria live. On the command of the fish, these tiny organisms can blink light which is used to help schools of flashlight fish swim together in harmony in dark water. Most fish give up schooling overnight and wait till morning to reassemble but this species uses flashing lights to co-ordinate their synchronised movements. Scientists have discovered that only 5% of the fish in the school need to use bioluminescence for this incredible event to be possible, which may help reduce their visibility to predators.

Other species have taken a more deadly approach to protecting themselves by using either venom or electric organs. Most venomous fish deliver their toxins via spines, such as stingrays and pufferfish. In fact, oceanic pufferfish contain a toxin which is 1,200 times more powerful than one of the most lethal poisons known to man, cyanide. This toxin is produced by bacteria that live in the fish’s gut and is stored in the skin and flesh, ready to inflict paralysis, breathing difficulty and loss of consciousness. The skates and rays which use electricity to zap their prey do so via tiny gel filled canals in their head which detect the weak electric fields produced by moving prey. They also have electricity producing organs which give their next meal a shock, stunning them and allowing them to take a quick bite. Finally, the peacock mantis shrimp has a rather quirky method of attacking potential food. In fact, this tiny unassuming critter has one of the fastest punches in the animal kingdom at a shocking 50mph! This is used to break open molluscs and crabs but has also been known to shatter aquarium tank glass.

Despite often being overlooked, our amazing fish species are facing just as many life-threatening problems as our more popular species. It is thought that 50% of all fish species in the ocean have been either fully exploited or over-exploited, leaving their populations on the brink of collapse. For the species which haven’t been overfished, they are still at risk of becoming unwanted by-catch or tangled in ghost fishing gear. The habitats of many fish are also rapidly declining as coral reefs bleach, seabeds are torn up by trawlers and climate change ramps up the heat in their home ranges. Let’s not forget the threat of plastic, oil and chemical pollution as well. So next time you’re in the supermarket looking for a fish supper or on the beach and spot some windblown rubbish, don’t forget our eclectic and colourful fish species which fill our oceans around the world.

By Neve McCracken-Heywood