Basic Features:

Pisces is a Class in the Subphylum vertebrata of the Phylum Chordate, so they have all of the characteristics of both of these groups in addition to the following:

Skin covered in scales
Ectothermic, cold blooded
Soft shelled eggs that must be laid in water
External fertilization
All members are fully aquatic
Limbs modified into fins
Gas exchange through gills

Kingdom - Animalia
Phylum - Chordata
Subphylum - Vertebrata
Class - Pisces

The group includes:

The status of fish groups, whether they are Classes, Orders, infraorders, subclasses etc. is something that is currently changing and somewhat confusing as to what they should be called. I have named some of the major groups without saying which level the group is (class, order, etc.) as doubtless the hierarchy will change whereas the names are relatively constant. The following list is representative but not complete.

Agnatha - jawless fish

Cyclostomata - no jaw so the mouth cannot close, retractable teeth - hagfish and lampreys

Chondrichthyes - cartilaginous fish, skeleton made of cartilage rather than bone, gill slits at the side of the head, urea retained in the blood for osmotic balance.

Elasmobranchii - no swim bladder, teeth in several series, heterocercal (vertically asymmetrical) tail - sharks and rays

Osteichthyes - bony fish

Sarcopterygii - fleshy finned fishes (ancestors of tetrapods) - Coelocanths, lungfishes
Actinopterygii - ray finned fishes, fins are skin supported by a number of bony spines or rays
- Teleostei - moveable lower and upper jaw, homcercal (symmetrical) tails - Most living fishes are members of this group
  - Anguilliformes, reduced fin number, dorsal, anal and caudal fin often fused to make a ribbon running much of the upper and lower length of the body - the true eels
  - Clupeiformes, include many important food fish, usually silvery, schooling, plankton-eating and pelagic- herrings, sardines, anchovies
  - Cypriniformes, mostly fresh-water - carp, goldfish, minnows
  - Siluriformes, prominent barbels around the mouth which resemble the whiskers of a cat - catfish
  - Protacanthopterygii
    - Salmoniformes, all members spawn in fresh water, though many spend much of their lives in the sea - salmon, trout, char, graylings
    - Esociformes, mainly ambush predators and highly adapted as such - pike
  - Lampridiomorpha
    - Lampriformes, oceanic pelagic feeders - oarfish, opah and ribbonfishes
  - Paracanthopterygii
    - Lophiiformes, a fleshy growth on the head of the fish is used a lure - anglerfish
    - Gadiformes - cod, burbot, hake
  - Acanthopterygii
    - Mugiliformes, identifiable as they have two dorsal fins and no lateral line, detritivores - mullet
    - Beloniformes, includes the flying fishwhich have greatly developed pectoral fins into wings for gliding
    - Tetraodontiformes, mostly found in the sea around coral reefs, many departures from the normal streamlined fish shape - file fish, puffer fish, box fish, oceanic sunfish
    - Pleuronectiformes, flatfish, asymmetrical fish, both eyes lie on one side of the head - halibut, flounders, plaice, sole
    - Scorpaeniformes, - lion fish, scorpionfish, stone fish
    - Perciformes 40% of all fish species - bass, cichlids, gobies, gouramis, mackerel, perch, tuna, bonito, whiting, wrasse

What do fish eat?

As in many animal groups, between them all fish eat a whole range of different foods. Some fish are specialists on just a few sorts while others are generalists. The specialists often have very highly adapted mouths and teeth to enable them to feed more efficiently on their particular types of food. Here are some examples amongst the variety of carnivores, omnivores and herbivores:

Sharks - mainly carnivorous on other fish, often have several series of teeth that move forwards to take the place of damaged or missing ones.

Plankton feeders - Plankton are tiny often microscopic, plants and animals of many different groups that live in the upper levels of waters, particularly seas. They are the basis of many marine food chains and many fish are specialized to sieve them out of the water. Such fish include anchovies, mackerel, sardines and the largest of all fish, the whale shark (a shark but not a whale). Many other fish feed on plankton when they are in the larval stage and many kinds of fish make up part of the plankton themselves while they are larvae.

Detritus and algae in mud and sand - fish such as mullet feed in this way, they have highly adapted digestive systems to separate out the organic food matter from the sediments.

Anything that fits in their mouth and is edible - some fish are not at all fussy about what they eat and end up being pretty successful as a result. Cod in particular fit into this category, they will eat detritus, shrimps, worms and other invertebrates, smaller fish and even birds if they are big enough and get the chance. Fish that do this tend to be demersal - feeding at the bottom the water.

Ambush predators - These fish feed on other fish or crustaceans such as shrimps, crabs, worms etc. They may lie on the bottom of the water as do flatfish such as flounders, sole, turbot and halibut. These fish are highly unusual in that they are asymmetrical with both eyes on the same side of the face and a twist to the mouth. The Anglerfish take ambush to another level by having a lure that has evolved from the first three spines of the anterior dorsal fin. They may lie in wait camouflaged against the sea bottom, or may float in the water column in the dark depths with a bioluminescent lure that attracts smaller fish or invertebrates that see it as prey, in the last instant the anglerfish opens its usually enormous mouth and swallows the would-be predator.

Shell crushers - many rays and skates have their teeth adapted to crush shelled organisms such as snails, oysters, mussels, clams, shrimps and crabs.

Cleaning stations - Some small reef fish, particularly wrasses and gobies make their living by cleaning larger fish or turtles. The larger fish come to the site of the cleaning station and often a number of the cleaners will remove parasites from the skin and even inside the mouth and the gills of the fish which refrain from making a meal of the cleaner fish which they might do in the normal course of things.

What eats fish?

Fish are an important part of many food chains and webs. Many terrestrial or partly terrestrial animals such as birds, bears, reptiles etc. also feed on fish so their food webs stretch beyond the water.

The biggest natural consumers of fish are other fish.

Most fish are not cared for by the parents, the eggs are fertilized externally and often left to drift in the water, particularly at sea, though may stick to rocks where in some cases the parents may waft water over them to ensure they survive and hatch. After hatching the care of even this small number of fish mainly stops and the young larval fish are on their own.

As fish start off so small, in large quantities (Atlantic cod lay around 5 million eggs in a single spawning for example) and are on their own, this means that they are eaten by all manner of other fish and invertebrates of all sizes with a huge mortality in the early larval stages particularly when they are part of the plankton.

FISH IN PAKISTAN

Fisheries contribute a significant amount of value to economy of Pakistan, and apart from that, it is one of the popular hobbies in several parts of the country. In Sindh and coastal areas of Baluchistan, fishing is widely regarded as a hobby and nearby dwellers practice their this hobby whenever they get some time for this activity.