In the second part of this series on fish identification for experts, Matt Clarke explains how meristics can be used to tell similar species apart.
What are meristics?
Meristics are countable features that are useful to scientists and fishkeepers in identifying their fish. Different species of fish have varying numbers of fins, scales, fin rays, barbels and other features that can be used to help tell them apart.
These characteristics can be compared to those listed in books, descriptions and in dichotomous keys to help you work out which species you are looking at. Meristics are often combined with details on anatomy and measurements of certain features called morphometrics.
What are the basic rules?
At their most basic, meristics are as simple as counting fins, fin rays or scales. However, there are lots of different features to count and there are slightly different ways of counting the same things, and expressing them in a way so other people can understand what you're going on about.
The first rule with meristic stuff is that the number is always whole (a discrete variable). Just as most people don't usually have 2.25 arms or 9.2 fingers, most fish have whole numbers of fins, rays, scales and other anatomical features, not parts of them.
Why is counting features so difficult?
Some people interpret things differently, so most experts follow the same basic rules on what constitutes a particular type of fin ray and how scales are counted.
Some specialists on certain groups of fishes do their meristic analyses in slightly different ways.
If you're reading a description of a new species or a review of a family, you'll often see in the methodology section of the paper details on whose technique they are following.
For cichlids, for example, most people use Barel's recommendations, which were written for those working on Lake Victoria haplochromine cichlids - a notoriously difficult group to work on.
Barel's paper is a useful guide to cichlidophiles serious about fish ID and has been fairly widely used by those working on other groups of fish, too.
How do you actually do the counting?
Meristic studies are usually undertaken by taxonomists working on dead fish that have been pickled in alcohol.
Counting the number of scales along the flank of a live Tinfoil barb or, perish the thought, a Kuhli loach, is a rather more tricky proposition.
If you are very patient and the fish you're looking at is fairly large and slow moving, you might be able to get reasonable results by watching the fish in the tank, but it might be easier to take some photographs instead.
If you do this, it's wise to use only pictures of the same individual - some numbers might differ between members of the same species.
Some fish are too small to study, unless you are really skillful and have access to a microscope. In the lab, most scientists working on typical aquarium-sized fish use a dissecting microscope and manipulate the fins with a pointed needle so they can count accurately.
If the fish is small, you'll have your work cut out, or you'll have to wait until a fish dies and you can use the corpse.
How do I count fin rays?
Fin rays fall into three main types: spinous (spines), soft and hard rays, and each is counted separately. This means that you need to be able to tell the difference between the two, and fin rays can differ a bit in their overall structure.
Soft rays are thin, flimsy and usually branched at the top end. The group includes branched rays, caudal fin rays (apart from the principal rays at the top and bottom of the tail).
Hard rays are made from a group of soft rays, are rigid and sometimes pointed. Hard rays are seen in the fins of perciform fishes, sisorid catfishes and in the principal rays of the caudal fin.
Spinous rays are harder than hard rays and are made of bony tissue, so they're stronger and often sharper. These aren't usually covered with skin tissue and often have serrated edges, such as in many doradid catfishes.
All of the rays in all of the fins are counted, including the tiny ones on the inside of the pelvic fins, which isn't easy on a live fish...
I'm trying to identify a South American cichlid, but the description contains loads of incomprehensible codes like D XII,10-12. What's that all about?
D XII,10-12 is a fin formula for the dorsal fin. To save repetition, ichthyologists use this special shorthand language for describing the fins.
The first letter stands for the fin, so D here stands for dorsal. Other letters, such as A for anal and P for pelvic, are also used. Roman numerals, such as XII, refer to the number of spinous rays in the fin, while the numerals, 10-12 in this example, describe the number of soft segmented rays.
The range covered, 10-12 here, shows the typical counts seen across the species, something called intraspecific variation. Importantly, not all members of the same species are identical!
Sometimes you might see lower-case Roman numerals used in the same formula alongside Arabic numerals, for example, P ii, 5. These differentiate between branched and unbranched rays.
The lower-case Roman numerals refer to the unbranched rays and the Arabic numerals refer to the branched rays. The rays of the caudal fin or tail are counted by adding up the number of branched rays and adding two, one for the upper and lower principal rays.
Sometimes the count is split into two bunches of rays, the dorsal group on the upper half and the ventral group on the lower half. So principal caudal rays 6 + 7 states that there are 13 rays, 6 on the dorsal portion and 7 on the ventral portion.
Some fish, like perch or polypterids, may have more than one dorsal fin, while others have a dorsal which is divided.
If the fin is divided, a slash shows the position of the split - DX/I, 8-10 shows a dorsal fin that's split into two after the tenth spine.
If there are several dorsal fins, each gets a separate number and formula.
How do I count the scales?
Fortunately, you won't have to count all of the scales, only those in specific places!
The most common scale count looks at the number of scales along the lateral line - the sensory line on the flank of the fish.
This normally looks at the number of scales on the lateral line which have a pore in them. The scales on the caudal peduncle (wrist of the tail) after the lateral line don't get included in the count.
But not all fish have a lateral line, and even if present, it is often only partially complete. In these cases, you simply count along an imaginary line.
Other common scale counts include counting the scales below the lateral line to the base of the pelvics; counting the scales from the insertion of the first dorsal fin to the scale above the first lateral line scale; recording the number of pre-dorsal scales and the number of scales around the narrowest part of the caudal peduncle - here (the circumpeduncular count) you'd also indicate how many lie above and below the lateral line.
Where do I go from here?
Once you've recorded the meristics on the fish you are trying to identify, you'll need to find a review of the genus, a dichotomous key, or a description of a similar sounding species so you can narrow down your search.
The meristic (and morphometric) data on a huge number of fish species can be found in FishBase - a global database managed by taxonomists around the world. This is a fantastic resource when you know how to use it to its full potential. It's found at www.fishbase.org
Further reading
Barel et al. (1977) - An introduction to the taxonomy and morphology of the haplochromine Cichlidae from Lake Victoria. A manual to Greenwood's revision papers. Neth. J. Zool., 1977 27(4), 333-389
This article (the second in a three part series) was first published in the April 2004 issue of Practical Fishkeeping magazine.