There are over 28,000 species of fish in waters around the world. Almost 27,000 of these are bony fish – the fish commonly associated with the group. Sharks, skates and rays, or cartilaginous fishes, comprise over 900 species, and jawless fish such as hagfish and lampreys have about 100 species. So, how can you be expected to tell them all apart? Who has that kind of brain capacity available anymore? Well, honestly, no one. But there are some general patterns you can use to learn the bigger family groups in your chosen ecosystem.
Fish anatomy generally mirrors their behaviour and functional needs in addition to showing evolutionary similarities of closely related species. So by starting to take notice of different aspects of a fish, you can begin to correctly predict many things about its life and ancestry. You’ll start to look at your dives in an entirely different and more scientific way.
Before we begin, let’s cover some technical vocabulary. On a fish, just like on a person or other animal, there are many areas of its body. Anterior refers to the front of the fish, near the mouth. If you use a boat as a metaphor, anterior would be the bow of the ship. Posterior by comparison, is the stern of the ship, or the tail end of the fish. The ventral side is towards the belly, like the keel of a boat. The dorsal is the opposite – in the back, towards the spine. This is where the mast or crow’s nest would be. The laterals are the sides of the fish, like the port and starboard sides of a boat. The anal or caudal parts are, unsurprisingly, near the anus of the fish and toward the tail or the rudder of the ship, respectively. And the pelvic area is anterior to that, where you would expect the pelvic bone to be on any other vertebrate. Likewise, the pectoral area is slightly posterior from the very front of the fish, just posterior of the mouth and gills, similar to where you would expect pectorals on a human.
Of course, fish can take shapes of all sorts, but there are some molds that most fish fit into. The first thing I look at to ID a fish is the whole picture, the general shape the fish body takes and go on to narrow it down from there. For beginners, it’s probably best to start with the body shape without fins and add those in later. Body profile is most generally helpful, but taking note of how depressed or compressed the whole body is has equally importance. Depressed and compressed body shapes are sometimes erroneously interchanged. Depressed organisms are those that are flattened from the ventral (stomach) and dorsal (back) sides. So, imagine a steamroller running over the dorsal side of a fish to flatten it, like a halibut or a stingray. They tend to live on the bottom of the sea. Compressed organisms are those that are flattened from the laterals. Many reef fish are laterally compressed: butterflyfish, surgeonfish, mola mola, etc.
Oval shaped fish might be the most distinctive. They are generally strongly compressed and have a very unmistakable round shape. Surgeon and unicorn fish have almost perfect oval shapes, whereas butterfly and angel fish have more of a disk shape to their profiles. Batfish, beneath all those fins, also have a disk to oval body shape. They are good at short bursts of speed and can weave between algae/coral with agility.
Elongate body shapes have emerged many times over evolutionary history and are thus found in a large variety of fish from jawless fish to eels and even pipefish. The extent to which a fish is elongated can be a great clue. Goatfish are mildly elongated while eels are extremely so. The more elongated the fish, the more sedentary the fish’s lifestyle tends to be. Swimming generally takes much more energy for an elongated fish than a fusiform one.
Fusiform fish as a general rule are strong swimmers and can frequently swim the lengths of the ocean. Think of tuna, sharks, or marlin. They are shaped like a torpedo which makes them streamlined and powerful swimmers. Stout, robust fish like groupers and sweetlips can swim well, but tend not to do so as much. They hang out on reef shelves and under overhangs when not hunting.
This still leaves plenty of “odd shaped” fish like filefish, lion fish, pipefish, etc. But these fish are so uniquely weird, that you’re just going to have to recognise them individually. Start looking at the body shapes some of your favourite fish make and compare them to others. You’ll quickly be able to recognise patterns between families.
Ok, so once you’ve identified the shape, you should have a pretty good idea of which family it fits into and what type of life it leads. From there you can look at fin shape and presence/absence of fins and spines. There are 6 types of fins fish species may have (see top diagram). Paired pectoral and pelvic fins, an anal fin, one or two dorsal fins, adipose fins, and a caudal fin. All of these fins can be adapted for different functions, lending to the fish’s lifestyle. Frog fish pectoral fins have been adapted for walking more than swimming. Eel dorsal and anal fins span the length of the elongated body to aide in the ungulation movements. Caudal fins have a large variety of shapes and sizes, even between species of a genus, so they can be very useful when distinguishing species. Fusiliers always have forked caudal fins while parrotfish have an almost infinite variety in caudal shapes. Many fish have dorsal spines used as protection, or to help with some other nifty behaviour, but many fish have no spines. Surgeon and unicorn fish have “scalpel blades” at the base of their caudal peduncle (the narrow part right before the tail) used in protection or territory disputes. Some fish even have spines on their gill covers.
Next on your ID journey, move on to eye placement and size. These can tell you so much about a fish including it is a predator with the eyes facing forward, or more of a prey species with the eyes firmly placed on the sides to make sneaking up on it difficult. If the eyes are huge, it is probably a nocturnal or deep water species that needs every bit of light it can direct towards its retina. Smaller eyes generally signify a shallow water and diurnal species, or one that doesn’t rely on sight as its main source of sensory information.
From there, you can notice the mouth positioning. This will signify how the fish feeds. If it lives near the surface and feeds on insects or other small things above it, the mouth will be pointed upwards. Predators will have mouths straight forward because they eat what is directly in front of them, and bottom feeders will have months facing down. Some ambush predators that technically feed on the bottom have upward facing mouths because they will eat an unsuspecting fish that travel above them. Fish that pluck coral polyps from their skeletons will have tube-like snouts. Parrotfish, who eat the algae off of hard structures, have huge, strong teeth that have fused into a beak.
Schooling fish tend to have prominent lateral lines. These are extra sensitive organs that span the length of the fish’s sides. They can feel minute pressure changes and allow an individual fish to know what changes in direction other animals are making. The can feel when the rest of the school changes direction or if something suspicious is coming their way. Some species (usually schooling species) have more visible lateral lines than others.
Now, after all of that, only then will colour start to potentially matter. Colour, while easily picked up by human eyes in the shallows, can be a deceptive marker to use. Many fish of the same species will have different colours or markings and many fish of vastly different evolutionary lines will have the same colour, so it is not very reliable. Try to start paying attention to all these other factors first and see how much you can learn about the fish you’ve previously overlooked. Start with a few common families in your area and slowly add more. Fish ID doesn’t have to be intimidating or unmanageable. You got this!
‘Fish ID for Beginners’ was written by Roya