The terms Osteichthyes and Chondrichthyes may seem a trifle wonkish, as opposed to street talk like “bony fish,” “sharks” and “rays,” but you will encounter them from time to time and should at least be aware of them.
• Osteichthyes (os-tee-ik’-thee-eez, from the Greek for “bone” and “fish”) is the taxonomic class of bony fishes, those with hard, rigid skeletons based on calcium, phosphate and other minerals, smooth scales, covered gills and flexible fins. With more than 25,000 species, from tiny blennies to tunas, swordfish and other large fishy animals, bony fishes comprise the largest and most diverse class of vertebrate animals on the planet.
• Chondrichthyes (kon-drik’-thee-eez, from “cartilage” and “fish”) is the class of fishes with skeletons made completely of cartilage, a substance based on collagen, protein and other material and offering the advantage of being lighter and more flexible than bone (we humans have cartilage in our joints, cushioning our connecting bones). It encompasses sharks and rays in Subclass Elasmobranchii but also ratfish in Subclass Holocephali. Hagfishes and lamprey eels are cartilaginous fishes in a separate class, Class Agnatha.
• Elasmobranchii (ih-las’-muh-branks, from “flexible” and “lungs,” or in this case, “gills”) is the subclass including some 350 species of sharks and some 500 species of rays. As chondrichthyes fishes, they tend to have placoid (rough, somewhat pointy) scales, uncovered gills and rigid, fleshy fins.
• Teleost (tel’-ee-ost), bony fishes in the group Teleostomi. The word reflects the Greek term for “boned,” but in addition to bony skeletons, etc., it describes fishes usually with terminally positioned mouths – that is, right at the front of their heads (as opposed to the position of sharks’ and ray’s mouths on the underside of their heads). Telosts’ mouths are more flexible than those of elasmobranchs.
• Fish vs. Fishes: Semantically, and strictly formally, when you’re talking about multiple fish of the same species, you’re talking about “fish.” As in “a school of fish.” When you’re talking about more than one species, they’re “fishes.” But the distinction is mixed up a lot in common usage and I’ve given up worrying about it. On the other hand, the fishy stuff you buy at a seafood market is “fish,” no matter what kind it is.
If you particularly hate encountering taxonomic terminology, you might want to read The Specifics of Species to understand why it’s important.
PRINCIPAL SOURCES: Marine Biology, Peter Castro, Michael Huber; Oxford Dictionary of Zoology, Michael Allaby, Oxford University Press; Osteichthyes, Chondrichthyes, Wikipedia.com; Dictionary.com.