Antionette Smith, age 12, of Winston Salem, North Carolina, for her question:

Which are the different fish species?

Ichthyologists have listed more than 20,600 different fishes and a fair description of all the species would fill a fair sized library. The fishy multitude is sorted and separated into large, small and smaller groups    so that each type has a genus name and his own personal species name. This helps us to see similarities and relationships in the various groups. However, it is very unfortunate that the fish experts of the world cannot agree on the same sorting system.

The fishes have had more than 400 million years to branch out in various shapes and sizes. Many of the more primitive types died out as more advanced ones arrived and survived in the fresh and salt waters of the modern world. Ichthyologists have identified and sorted about 20,600 of these existing species into three mayor classes, which some call superclasses.

The experts are not agreed on how these major classes should be subdivided into smaller groups of mire closely related species. However, most systems included 40 to 45 orders and some include a few catagories called subclasses and suborders. This is confusing to students interested in the fish world. Let's hope the world wide ichthyologists finally agree on a single system of classification.

Three major classes or superclasses separate the well known bony fishes from the sharks and the jawless lampreys. Class Agnatha, the jawless ones, includes about 235 species of lamprey and hogfish types. Class Chondrichthyes, the gristle bone fishes, includes about 600 sharks, rays and chimera types. The great majority belong in Class Osteichthyes    the bony fishes. The 20,000 or so species in this popular group are separated in 35 or so smaller orders.

One of everybody's favorites is Order Clupeiformes, which is crowded with silvery herrings and stuffed with sardines. Some systems include the rich salmon and trout in this order. Others group them with an assortment of pikes and minnows in Order Salmoniformes. Another system groups some of the minnows with the catfish, the carp and goldfish, the electric eel and the voracious little piranha.

The largest and most famous group is Order Perciformes, the spiny rayed fishes. It includes the perch and 8,000 to 10,000 typical fishy type fishes of fresh and waters. Some systems include in this group the mackerel and the snapper, the bass and the whopping tuna and the bad tempered barracuda.

In most cases, an order is large and subdivided into several families of more closely related fishes. A family is subdivided into genera, and the members of each genus are cousins. Each cousin is an individual species. The genus and species names often vary. This, sad to say, is because ichthyologists are not agreed on how their 20,600 or so subjects should be sorted, classed and named.