Identifying
fishes

Appearance

The simplest way to identify fishes is by their physical shape and appearance. Everyone can easily recognize a shark because of its distinctive shape. Different species have different profiles when viewed from the side. Many also differ when viewed from the top or front. Some are slim and elongated, others fat and rounded, while others have characteristic fins, tails, bumps or curves.

Note the common external parts of a fish and observe how these parts differ across species. When you see a fish, look out for the body profile, size, colour & markings, the number of fins and fin shape, location and size of mouth and eyes, and the shape of the tail. Even if you do not recognize it, taking note of these characteristics will help you to identify it later out of the water when you are thumbing through fish reference books.

Note that the young (juvenile) of some fish species may look different from the adults (e.g. batfish and sweetlips). Some fishes may also mimic other species (bluestriped fangtooth blenny mimicking the cleaner wrasse).

Habitat

Note the place and environment that the fish is in. Some fishes prefer to live on the seabed, reef flat or reef wall (e.g. scorpionfish, goatfish, lizardfish, flounders, gobies). Others like to live in holes or crevices (jawfish, soldierfish, cardinalfish, eels) while others prefer to swim openly (sharks, jacks, trevallies, snappers, damsels etc.).

Behaviour

Some fishes like to sit motionless on coral (hawkfish, lizardfish, blennies). Some prefer to congregate in schools (snappers, fusiliers, bumphead parrotfish). Others move or swim in a peculiar fashion, like wrasses 'flapping' their pectoral fins like a bird and gobies 'walking' on their pectoral fins. Others sleep in the day and are active mainly at night (soldierfish & cardinalfish).

Markings and body patterns

Many of the colourful reef fishes are adorned with pretty patterns and markings on the body. Their common names usually reflect the kind of marking found on them. For example, the 'six-bar regal angelfish' has 6 vertical bars on its body. The 'chevron barracuda' has visible chevron markings on its body. The 'saddled butterflyfish' or 'banded wrasse' has saddles and bands. Some of these markings act as camouflage to break the outline of the fish, while others like the false eye spot are used to confuse predators.

Tail shapes

The tails of fishes also come in a variety of shapes. Fusiliers, jacks, soldierfishes and cardinal fishes typically have forked tails, while wrasses and parrotfishes typically have truncate or rounded tails.

Mouth shapes

The mouths of fishes differ in their location on the head, and in the size of their upper and lower lips. The mouth can be right at the front of the head or at the bottom of the head. They can be elongated or protruded, making the fish appear to have a long nose. Lips can be thin or thick, sometimes one lip might be bigger than the other, or one lip may protrude more than the other. Groupers and rock cods are easily identified by their superior mouths, where the lower lip or jaw protrudes. Sharks have inferior mouths, while parrotfishes have terminal mouths. Needlefish have elongated mouths, while goatfishes have barbels under their mouths.

Fins

Although most fishes have the usual set of fins at the top (dorsal fin), side (pectoral fin) and bottom (pelvic and anal fins), the size, shape and number of fins may differ. Groupers, damsels, wrasses, surgeonfish and angelfish have one continuous dorsal fin, while barracuda, goatfish, gobies, soldierfish and cardinalfish have 2 dorsal fins.

Sometimes, it is hard to tell apart fishes because at first glance they look very similar. In such cases, look out for distinguishing features that may be present in one species and absent in the other, such as barbels (whiskers), spines and fin arrangements. Shown below are some of these distinguishing features and how to tell apart similar-looking fish species.

Angelfish and butterflyfish
Angelfish have a clearly visible spine on the gill cover which is absent in butterflyfish. In general, butterflyfish are smaller and many species are predominantly yellow in colour.

Squirrelfish and soldierfish
Both are predominantly red, have large eyes and large scales. Like the angelfish, squirrelfish have a visible spine on the gill cover which is absent in soldierfish. Many squirrelfish species also have horizontal stripes running across the length of the body.

Bannerfish and moorish idols
Bannerfish have yellow tails while moorish idols have black tails. In addition, moorish idols have a predominantly black face and a tubular white nose with a yellowish-orange patch. The character 'Gill' in the movie 'Nemo' is a moorish idol.

Gobies and blennies
Both can be small, making identification difficult. Gobies have 2 distinct dorsal fins while blennies have 1 continuous dorsal fin. Many blennies also appear to have 'eyelashes' or small feelers near the eyes and faces that resemble grasshoppers. The pelvic fins on gobies are joined at the base forming a little cup for support, while the pelvic fins of blennies are separate and located clearly behind their pectoral fins. Prawn gobies are easier to identify as they live with a prawn as a housemate, the goby standing guard at the entrance to the hole while the prawn cleans out their living quarters.

Surgeonfish have a scalpel-like spine at the base of the tail. When folded, the spine may not always be clearly visible and sometimes appear as a fine horizontal line. The character 'Dory' in the movie 'Nemo' is a palette surgeonfish.

Porcupinefish and pufferfish
Both can inflate their bodies by sucking in water but the porcupinefish has many small spines all over its body while the pufferfish has a smooth body. When the spines are folded flat against the body, they look like little uneven bumps on the body of the porcupinefish.

Napolean wrasse and bumphead parrotfish
Both of these large fishes have pronounced bumps (or humps) on their heads especially in mature adults. The bump on the Napolean (or Humphead wrasse) appears more on the forehead while the bump on the parrotfish appears more at the top of the head. The Napolean also has a longer protruding jaw with subtle striped patterns on its face while the parrotfish has a mouth shaped like a beak, shorter face and a light stripe running down its nose.