Cartilaginous Fish: Groups, Examples and vs Bony Fish

Cartilaginous fish are jawed fish whose skeletons are made mainly of cartilage instead of bone: the sharks, rays, skates, sawfish and chimaeras, grouped in the class Chondrichthyes (Wikipedia). Features they share:

• Skeleton of cartilage rather than bone tissue (Wikipedia).
• Five to seven pairs of gills, each opening separately to the outside, instead of one bony gill cover (Wikipedia; Saltmarsh Guide).
• Tooth-like scales called dermal denticles or placoid scales (Wikipedia).
• No swim bladder. Sharks and rays keep buoyancy with oil stored in the liver, which also serves as an energy reserve (Wikipedia, Elasmobranchii).
• Replaceable teeth in several rows in sharks and rays (Wikipedia).
• Internal fertilisation, with males using claspers on the pelvic fins; many species give birth to live young (Wikipedia).
• Electrical sense: small pores around the snout, the ampullae of Lorenzini, detect the electric fields of prey (Saltmarsh Guide).

Cartilaginous fish fall into three familiar groups:

• Sharks: more than 500 species in 13 orders (Wikipedia). Examples: the whale shark, the largest cartilaginous fish at over 10 m (Wikipedia); the bonnethead, up to 4 ft, with a shovel-shaped head; the Atlantic sharpnose, up to 3.5 ft (Saltmarsh Guide).
• Rays, skates and sawfish: well over 600 species in 26 families, the most species-rich group (Wikipedia). Example: the Atlantic stingray, a flat disc up to 2 ft wide found in shallow estuaries from New Jersey to the Gulf of Mexico (Saltmarsh Guide).
• Chimaeras, also called ghost sharks, ratfish, spookfish or rabbit fish: over 50 species, mostly in deep water, some below 2,000 m (Wikipedia).

Formally, sharks and rays together form the subclass Elasmobranchii, and chimaeras form the subclass Holocephali (Wikipedia). Five quick examples: whale shark, bonnethead, Atlantic sharpnose shark, Atlantic stingray and a chimaera such as a ratfish.

Bony fish, such as trout, bass and goldfish, have skeletons made mainly of bone tissue (Wikipedia). The practical differences you can see or check:

• Gill openings: bony fish have one gill cover (operculum) on each side; sharks show five to seven separate slits, and rays have them on the underside (Saltmarsh Guide; Wikipedia, Batoidea).
• Buoyancy: most bony fish have a swim bladder; sharks and rays have none and rely on an oily liver, and many must keep moving to avoid sinking (Wikipedia; Saltmarsh Guide).
• Tail: sharks usually have a heterocercal tail, with the backbone running into a longer upper lobe; most modern bony fish have a homocercal tail that looks symmetrical from outside (Wikipedia, Fish fin).
• Teeth: sharks and rays replace teeth in rows throughout life (Wikipedia).

The odd one out is the chimaera: it has a single gill opening covered by a flap of skin, and six permanent, continuously growing tooth plates instead of replaceable teeth (Wikipedia, Chimaera).

Body shape is not enough, because some rays look like sharks and some sharks are flattened. Wikipedia gives the defining features of rays: a flattened body, enlarged pectoral fins that are joined to the head, and gill slits on the underside. A shark's gill slits are on the sides of the head.

So for a photo ID, try to get a shot that shows where the gill slits are. If the slits are on the side, it is a shark; if they are hidden underneath and the pectoral fins run forward to the head, it is a ray. Count the slits too: five to seven is normal for cartilaginous fish (Wikipedia).

Some chimaeras carry a venomous spine on the front edge of the dorsal fin (Wikipedia), and many sharks and rays can bite or sting. Do not handle an unknown cartilaginous fish with bare hands. If you are stung or bitten and the pain, swelling or bleeding is severe, get medical care; in the US, Poison Control (1-800-222-1222) can advise on venomous stings.

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