Matt Clarke concludes his series of fish identification for experts by explaining why morphology is studied and how you can use it to identify your fish.
What is morphology?
Morphology is the study of form and structure. It's a very diverse subject and can look at anything from the type of teeth on the pharyngeal bone to the distance from the tip of the nose to the end of the tail.
Since it's so broad, it can also be immensely complicated, but thankfully fishkeepers really only need to know the basics of taking measurements to get by.
Combined with a knowledge of meristics (see PFK, April 2004) and access to the right information, you should be able to improve your chances of identifying a range of rare fishes.
Why is it studied?
Closely related fish species differ in the presence of various features, their structure and size in relation to other body parts. Taxonomists need to detail these features, and their relative sizes, when they describe a new species, and when they attempt to reconstruct the evolutionary family tree (or phylogeny) of a group.
Those fish that are derived from the same common ancestor and share a common feature (called a synapomorphy) are considered more closely related to each other than other ancestral species which lack the feature. As a result, they'll be grouped together in the phylogeny, and in a broad sense, might form a genus or species complex.
Are bones and muscles studied?
Yes. The study of bones is called osteology and the study of muscles is called myology. Both areas are very important for certain groups of fish.
Work on identifying and describing cichlids, for example, often involves painstaking work on the bones and teeth of the mouth.
It's not feasible for a fishkeeper to study these as the subject is too complex, particularly on the muscular side of things, but you'll often see these referenced in descriptions and classifications.
Osteology and myology involve lots of serious work. I did a research degree in the fish section of The Natural History Museum studying the osteology and myology of the pelvic-girdle region of pseudochromid reef fishes, with the aim of investigating how the fishes were related to each other.
This involved nearly a year of dissections and microscopic examinations of pickled fishes, and special transparent specimens which had been stained to make their bones and cartilage visible.
Which aspects of morphology can be studied by a fishkeeper?
Measurements of the form and features of a fish are within the grasp of most fish experts.
These are usually made on dead fishes, so you'll have a tough job trying to get them from fish in your tank, although you may get fairly close using a series of photographs of the same specimen.
A dead fish in alcohol is the ideal thing to use. Most scientists use special dial calipers, or sometimes needle dividers, to make measurements, although you can use a ruler for an approximation.
There are lots of different measurements that are widely used in all groups of fish, but some, such as the lengths of barbels and the like, are specific to certain orders. The main measurements used are on the picture of the fish opposite.
How can I make it easier to ID a rare fish?
Ask your dealer where the fish came from. If you can pinpoint the origin of the fish, you should be able to rule out some similar-looking species.
Bear in mind, though, that only the larger stores import their own fish directly, and most of the smaller shops will buy them from wholesalers who might know little more than their country of origin.
The other snag is that if you're looking at fish exported from say, Brazil, the area that a single airport covers can be an enormous geographic area of several thousand miles across! In this case, the country of origin may not narrow it down much.
The fish I have identified differs from the norm. What should I call the fish?
If the fish looks atypical, then you should put cf. or sp. aff. in the name you pin to the fish (ie Crenicichla sp. aff. regani or Neochromis cf. rufocaudalis).
These latin abbreviations, which stand for conferre and species affinis, basically state that the fish looks very much like this one, but may, or may not be, identical. Contrary to popular belief in the aquarium world "cf" does not mean colour form.
Many species have lots of intraspecific variation, so members of the same species can give different morphological measurements and have different meristics. As a result, just as things like fin rays will be shown as a range (say 12-15 dorsal fin rays), you'll often see measurements given in the same way (ie head length 5.1 to 6.2 of total length).
Why is it important to define the geographic race of a species?
An example of the importance of this has cropped up very recently. For donkey's years, all of the slightly different forms of Frontosa have been lumped into a single species - C. frontosa.
However, recently a second species has been described - C. gibberosa (Takahashi and Nakaya, 2003), although some see this as controversial.
This species, which includes the Zaire blue and other southern forms, has also been kept and bred by fishkeepers, some of whom have spotted subtle differences in its behaviour and reproduction.
Sadly, for those who chose not to define the specific race they were talking or writing about, all of the information has been essentially lost through mixing it up with that of C. frontosa.
If the supplier tells you the name of the race, take a note of it and pass it on if you provide information, or fry, to other fishkeepers. In this way, you'll preserve any knowledge of the species if it is ever split.
Common measurements
A-H Total length (TL)
The maximum distance from the nose to the tip of the tail. This includes long pointy noses, as in gars and halfbeaks, and long trailing tail filaments, such as those on many catfishes.
A-G Standard length (SL)
The distance from the tip of the nose to the end of the vertebral column, not including the tail. Since you can't usually see the end of the vertebral column, you'd normally measure up to the flexure point of the caudal peduncle. However, this doesn't always line up with where the scales stop and the caudal rays start.
BD Body depth (BD)
The distance from the highest part of the dorsal surface to the ventral surface in a straight, vertical line.
A-D Head length (HL)
The distance from the tip of the nose to the posterior-most part of the opercular membrane, including any fleshy extension or flap.
HD Head depth (HD)
Sometimes called height of head at occiput, this measurement goes from the top of the head (occiput) to the bottom of the head or breast.
B-C Eye diameter (ED)
The distance between the margins of the eyeball and the cornea.
A-B Pre-orbital (PreO)
Widely used on cichlid studies, this is a bone at the front of the head.
C-D Post-orbital (PostO)
The region behind the eye to the rear of the operculum.
A-E Predorsal length
A straight measurement from the tip of the snout to the first ray of the dorsal.
F-G Postdorsal length
A straight line measurement from the base of the posterior of the dorsal to the flexure line of the caudal peduncle or end of vertebral column.
More information
If you're trying to identify a fish from a scientific paper and get stuck on the terminology, try the glossary at FishBase.org which includes the vast majority of the common terms. It also includes measurements and meristics on a huge range of species.
This article (the final part of a three part series) was published in the May 2004 issue of Practical Fishkeeping magazine.