Cod and haddock are teleost fish.  Teleost fishes have some common features, including a bony skeleton and a swim bladder.  The swim bladder gives the fish neutral buoyancy, so that it can maintain its position in the water column without swimming.  Another common feature is the operculum, or gill covering, which serves to protect the gills and aids in the movement of water over the gills for respiration.   This diverse group of teleost fishes includes between 19,000 and 20,000 different species.  96% of all living fish are teleosts.

Cod and haddock belong to the order Gadiformes.  This commercially important order includes the following fish:  burbot (Lota lota), cod (Gadus morhua), pollack (Pollachius pollachius), whiting (Merlangius merlangus), and haddock (Melanogrammus aeglefinus).  All are marine fish, except for the burbot, which lives in freshwater environments. 

There are 25 different species in the cod family alone.  Some are very similar, differing only in geographic distribution or small anatomical variances.  Others differ greatly in biology, geographic habitats or behavior.  All information contained in this section will focus on the Atlantic cod, Gadus morhua and the haddock, Melanogrammus aeglefinus.  The categories to the left contain biological information pertinent to the cod and haddock.   All information is relevant to both the cod (Gadus morhua) and the haddock (Melanogrammus aeglefinus), unless otherwise noted.