460 Fishes, Biodiversity of Skates and Rays The batoid skates and rays are 535 species of mostly benthic (bottom-living), mostly marine forms In skates and rays, the pectoral fins are fused to the sides of the head and the five gill slits are under the head Skates are most diverse in deep water and at high latitudes, whereas stingrays are most diverse in tropical, inshore waters Some batoids live much or all of their lives in fresh water Largetooth Sawfish frequently swim up rivers in Central and South America Two stingray families contain entirely freshwater species – the river stingrays of South America and several species in the large stingray family Dasyatidae The latter inhabit African, Southeast Asian, and New Guinea rivers Skates and rays feed mostly on benthic invertebrates, except for the huge (up to 6-m wide) manta rays, which capture small crustaceans and fishes in the water column Torpedo rays stun prey with powerful electrical discharges (50 V and 50 A ¼ kW output) Batoids reproduce by either laying eggs (skates) or bearing live young (rays) Embryonic skates develop inside ‘‘mermaid purse’’ egg cases for as much as 15 weeks (Figure 5) Skates in some locales are actually increasing in number because of overexploitation of competing bony fishes, such as cod in the North Atlantic However, the giant Barndoor Skate of the northwest Atlantic and its relative, the ironically named Common Skate of the northeast Atlantic, are caught incidental to bottom trawling for bony fishes; they have been seriously depleted and may face extinction Largetooth Sawfish in Nicaraguan lakes have been drastically overfished Chimaeras Class Chondrichthyes Subclass Holocephali Order Chimaeriformes: families, 46 species, chimaeras Chimaeras, also known as rat- or rabbitfishes, share a cartilaginous skeleton and other features with elasmobranchs They differ by having (1) the upper jaw permanently attached to the braincase, (2) continually growing tooth plates in the jaws instead of replaceable teeth, (3) a single gill flap instead of five or more gill slits, and (4) no scales Chimaeras swim by flapping their pectoral fins (something sharks never do) and by undulating their bodies (Figure 6) All chimaeras are egg-layers, the egg being protected by a horny shell Adult chimaeras range in size from 60 to 200 cm Chimaeras are cool-water, marine fishes that live at shallow to moderate depths between 80 and 2600 m, where they usually swim just above the bottom Chimaeras eat predominantly hard-bodied benthic invertebrates, which they crush with their tooth plates Surprisingly little is known about their general biology and natural history Bony Fishes Modern bony fishes, often referred to as Osteichthyes (literally ‘‘bony fishes’’), consist of seven major taxonomic orders Many living members belong to groups that were much more diverse during Paleozoic and Mesozoic eras Several of these primitive but modern fishes are classified as ‘‘bony’’ even though they have cartilaginous skeletons Their skeletal condition is actually an advanced, specialized trait; their immediate ancestors were bony Figure A mermaid purse (egg case) of a Big Skate, Raja binoculata, from the North Pacific Each case holds a single embryo, which can be seen because part of the egg shell has been removed Some skates are overfished because they are born at a size that is already too large to escape through the mesh of drag nets Figure A Pacific Ratfish, Hydrolagus colliei, showing the beak-like mouth, flexible pectoral fin, and lack of scales that set chimaeras apart from sharks and rays The large, reflective eye is characteristic of fishes that live in deep, dark waters