Shellfish are divided into molluscs like mussels, oysters, and scallops which have one or two shells, and cephalopods like squid and cuttlefish which have an internal shell. Crustaceans like crabs, shrimp, and lobster have segmented shells. Finfish are divided into whitefish like cod, haddock, and pollock, and oil-rich fish like mackerel, salmon, and trout. Whitefish are further divided into round and flat varieties. Finfish can also be classified by where they swim - demersal fish live near the seabed while pelagic fish swim in mid-waters or near the surface.

1. FISH FACTS - FISH
CLASSIFICATION
Nivoshan Mahendran 2011

2. Shellfish
Shellfish are broadly divided into two categories—
molluscs and crustaceans.
mullusc crustaceans

3. Molluscs can be divided
into three categories:
Uni-valve molluscs have one shell—periwinkles, whelks
and limpets. These molluscs can be found on the
seashore.
whelks limpet
periwinkles

4. Bi-valve molluscs
Bi-valve molluscs have two shells hinged at one end—
mussels, oysters and scallops
oysters scallops
mussels

5. Cephalopods
Cephalopods have no outer shell, just a single internal
one called a pen. Examples include squid and cuttlefish.
squid cuttlefish

6. Crustaceans
Crustaceans are mobile creatures with hard, segmented
shells and flexible joints. Examples include prawns,
shrimp, crab and lobster.
crab lobster
prawns shrimp

7. Finfish
Finfish are divided into whitefish and oil-rich fish.
whitefish oil-rich fish

8. Whitefish
Whitefish are sometimes referred to as "lean fish"
because all the oils are contained in the liver, which is
removed during gutting.
lean fish

9. Whitefish are further sub-
divided into:
Round Whitefish—Examples include cod, haddock,
hake and pollock.
cod haddock hake

10. Flat Whitefish
Flat Whitefish—Examples include plaice, lemon sole,
brill, turbot and black sole
plaice lemon sole brill
turbot black sole

11. Cartilaginous Fish
Cartilaginous Fish—Examples include ray, dogfish,
shark
ray dogfish

12. Oil-Rich fish
Oil-Rich fish are so called because the oils are
distributed throughout the flesh of the fish. Mackerel,
herring, salmon and trout are examples.

13. Finfish are occasionally classified in a different way—
according to where they swim in the sea.
Demersal fish are those which live on or near the
seabed. Round and flat whitefish fall into this category.
Pelagic fish swim in mid-waters or near the surface.
Examples are oil-rich fish such as mackerel, herring and
tuna.