Frequently asked questions on fish identification - Part one


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Matt Clarke begins a new series for expert fishkeepers, explaining how you can improve your fish identification skills.

Surely fish identification is as simple as comparing a fish to a picture in a book?

Although this is the easiest way to identify a fish, it does have some obvious drawbacks. The differences between some species are sometimes so subtle that they might not even show up on a photograph.

There is often considerable variation within a species (called intraspecific variation) which can make a fish look unlike other members of its own species. Sub-species and geographic races can also exist, and there is a possibility that the fish pictured might not be a typical example of the species, which could lead you to believe you have a different fish.

And with over 25,000 fish species, it's equally likely that the fish may have been incorrectly identified - even if it appears in a book or magazine. Few of the popular books used for identifying aquarium fishes are 100% accurate...

How do experts identify fish?

The scientists involved in fish identification are called taxonomists or ichthyological systematists. They are concerned with not only identifying fishes, but also investigating how closely related they are to each other to produce both a classification and an evolutionary family tree (called a phylogeny) as well as a name. The branch of taxonomy that deals exclusively with the identification, characterisation and naming of species is called alpha taxonomy.

When a new fish species is named, the taxonomist produces a description that outlines in detail all of the characters it possesses that make it distinguishable from other fishes, especially other members of its genus.

These differences, or taxonomic characters, are often very subtle such as a slight difference in the number of scales in the lateral line or a couple of extra rays in the dorsal fin.

Collectively, they can be used to identify a fish by comparing the features present to those in the description.

The fish used to make the description are called type specimens and are lodged permanently in a natural history museum for other scientists to study.

Clearly it isn't practical for a taxonomist to sit down in the museum with his pickled fish and compare it to a stack of descriptions, so to make life easier, from time to time, taxonomists produce special identification tools called keys.

How do keys work?

Keys are documents (and more recently, computer programs) that contain a logical series of steps to help identify a fish based on its characters, such as the number of fin rays, the number of scales or the length of the dorsal fin in relation to the body. The most widely used type of key is called the dichotomous key.

Keys sound rather more complicated than they really are, but the best way to think of them is like a game of 'Guess Who'. To start the game you'd think of a person.

For the sake of argument, let's choose Kylie Minogue. Your opponent now has to ask a series of questions that give a 'Yes' or 'No' answer that can be used to identify, or key out, the person: Is it a woman? Is she most famous for singing? Is she tall?

Did she appear in a poor quality Australian soap opera? Does she have a sister who is also a singer, but with darker hair, who also appeared in a similarly poor quality Australian soap opera?

It's much the same with fish, only you start with an unidentified fish and gradually go through each step, eliminating those that don't have the right features until you finally end up with the name of the species. To double-check, you backtrack through the key and find the original description, just to make sure you've not made any errors.

Are keys difficult to use?

As keys aren't designed as tools for fishkeepers, some of them aren't going to be very practical or easy to use. Taxonomists rarely work on live fish or photographs - they spend most of their time looking at dead specimens that have been preserved in alcohol.

Preserved fish have the obvious advantage that they can be picked up, turned over, dissected, measured and looked at more closely under a microscope. Clearly it is not an easy task to count the number of dorsal fin rays on a 3cm/11/3" tetra as it whizzes around your aquarium.

However, a knowledge of fish anatomy and the ability to use keys and understand the more complex fish descriptions can be useful for the expert in making a proper identification.

Can you give me an example?

Here's a simple example to the three Asian cichlid species of the genus Etroplus by Jarayam. It's not hard to follow as there are only three fish. If you meet a number you simply jump to that step in the key.

1. Body with one to three dark blotches along sides. Dorsal fin with 17-20 spines. Etroplus maculatus

Body with transverse bands.2

2. Dorsal fin with 18 or 19 spines. Anal fin with 12 or 13 spines and 11 or 12 rays. Etroplus suratensis

Dorsal fin with 21 or 22 spines. Anal fin with 14-16 spines and 6 or 7 rays. Etroplus canarensis

What sort of taxonomic characters are used for fish identification?

It depends entirely on which species you are looking at. In some cases, a step in a key might be as simple as comparing the number of rays in the dorsal fin, but at other times you could be asked to do something much more complicated, such as identify the type of dentition on the pharyngeal bone - something common when identifying certain cichlids to species level.

Typical characters used in a key might be body proportions, colours, or what are known as meristic features. These traits are essentially those things that can be counted, such as the number of rays in the anal fin, scales along the lateral line or even vertebrae in the spine.

Meristic stuff isn't usually excessively complicated, but the terminology can be mind-boggling. At the most basic level, it's concerned with the presence or absence of a feature (such as whether an adipose fin is present or not) and goes on to look at more detailed things like the number of soft branched rays in comparison to hard spinous rays.

Basic fish anatomy

Ventral surface

The underside of the fish is called the ventral surface. Fins, muscles, bones and things on the bottom half of the fish are hypaxial.

Dorsal surface

The top surface of the fish is called the dorsal surface. Stuff on the top half of the fish is called epaxial.

Anterior and posterior

The head end is called the anterior. The tail end is called the posterior.

Median fins

These fins sit on the median axis of the fish. On a 'normal' fish like a Red tailed black shark, this includes the dorsal fin (on the back), the caudal fin (tail), and the anal fin at the lower back end of the fish. Some fish have more than one dorsal fin, and others have a little fleshy fin at the posterior end of the dorsal surface called the adipose. This is common in tetras, members of the salmon family and in many (but not all) catfishes. In some fish, such as the Swordtail, the anal fin has evolved to become a sexual organ called the gonopodium.

Paired fins

The pectoral fins (which sit behind the gill covers in most fishes) and the pelvic fins (also called ventral fins) are paired, so there's one on each side of the body. Not all fishes have pectoral fins (they're missing in lampreys and hagfishes, for example), and they can be greatly enlarged in some species such as hatchet fishes or gurnards. Similarly, the pelvics can also undergo modification. In the balitorine loaches, such as the Chinese plec, they form a sucker-like disc to hold the fish in place in fast flowing water.


On most bony fishes like goldfish, cichlids or tangs, there's a single opening on each side of the head. The bony flap covering this is called the operculum, or gill cover. It's virtually always in front of the pectoral fins, apart from in the ogcocephalids, also called Batfishes, where it sits behind them.

Not all fishes have a bony operculum - it's missing in hagfishes and lampreys, and some fish have more than one slit (there are 5-14 on a hagfish, 5 on rays, and 5 on most species of shark).

This article (the first of a three part series) was first published in the March 2004 issue of Practical Fishkeeping.