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Classification of Fishes

There are several ways of classifying fish. One of these is on the basis of its habitat (where they live). Based on this, we have three types of fish.

(a) Fresh water fish: These are fishes, which live in water with little or no salt at all. They are found in rivers and streams, ponds and in enclosed water bodies or lakes; examples are the tilapia, mud fish/cat fish, electric fish, tiger fish, etc.

(b) Marine water fish: These are fishes from water with high salt content like Oceans, seas, etc. The fishes are more salty and tasty than those from fresh water habitat; examples are king fish, mackerel, sardine, butterfly fish, sword fish.

Figure 1: Glyphisoaon saxatilis (Sergeant-major)

(c) Brackish water fish: These are fish that live in a habitat with varying degree of Salinity (salt). They are found in lagoons and estuaries. The fishes in this habitat can adapt themselves to both fresh water and marine habitats. Examples are the bongafish, saw fish, catfish, which adapt to fresh water habitation and the mudskipper which adapt to a marine habitat.

Figure 2 (a) Elops lacerta (ten pounder) a marine fish species
Figure 2 (b) Eutropius niloticus (catfish) a freshwater fish species
Figure 2 (c) Periophthalmus Spp (mudskipper)

(d) Morphology: Fish can also be grouped into two main types on the basis of there form and structure (Morphology); those are bony and cartilaginous fish.

(i) Bony fish: These are fishes with bony skeleton e.g. sardine, tilapia, tiger fish, etc.

(ii) Cartilaginous fish: These are fishes whose skeleton is made up of cartilage (a firm elastic) tissue throughout their life e.g stockfish, hammerhead, shark, etc.

Differences Between Bony and Cartilaginous Fish
Figure 3. External structure of a bony fish (tilapia)
Figure 4. External Structure of Cartilaginous Fish (shark)