Sharks and Batoids (rays) show great diversity in size. The largest shark is the Whale shark (Rhincodon typus) at 13+m. Among the smallest is the Midwater shark (Squaliolus latidicaudus) and the Pygmy ribbontail catshark (Eridacnis radcliffei) which measures in at 22 -25cm. The largest batoid is the Manta ray (Manta birostris) with a disc width of over 6.7m and a weight of more than 2 tons.
B. Body Shape:
Sharks typically have a fusiform body (rounded and tapering at both ends). This type of body shape reduces drag and minimises swimming effort. The Great white shark is a good example of this altough it has more of a spindle shaped body. Batoids are flattened, with a ventral (underneath) mouth and gill openings.
Sharks and rays usually have different colours on the ventral and the dorsal (top) side. Countershading is when the dorsal side is darker than the ventral side. This darker colouration on top makes it more difficult to be seen from above while the lighter colour underneath blends in with the lighter surface of the sea when viewed from below.
Some coastal sharks and batoids have other markings on them like stripes which dissapears with age. In contrast the whale shark keep its spots throughout its life.
Fins are composed of cartilaginous(cartilage) rods (for support) covered by a thin layer of flesh and skin. There are five types of fins:(a) Paired pectoral fins lift the shark as it swims as it swims, (b) paired pelvic fins stabilize the shark, (c) One or two dorsal fins, with or without spikes, to stabilize the shark, (d) A single anal fin (if present) provides extra stability and (e) The caudal fin which propels the shark.
Batoids show various amounts of fin fusion and reduction especially the greatly expanded pectoral fins which is used for propulsion.
Eyes are lateral(side) on the shark and dorsal on the batoids. Some sharks may have an eyelidlike membrane, called a nictating membrane, to protect its eyes while it is feeding. In general deep water sharks have larger eyes than shallow water sharks. In clear water a shark can see up to 15 m.
Sharks and batoids have ventral external nostrils which may have appendages, called barbers, near the nostril eg. houndsharks.
The sharks mouth is usually on the underside with the execption of th whale shark, Megamouth shark(Megachasma pelagios), frilled sharks and some carpet sharks.Teeth are modified, enlarged scales which, in the case of the sharks) are continually being replaced, when it falls out, by teeth pushing out from the jaw.
Sharks have five to seven pairs of lateral gill slits and batoids have five pairs of ventral gill slits. Sharks also have a spiracle which brings water into the gill chamber but this is absent in active, fast swimming sharks. Spiracles are remnants of rudimentary gill slits.
Sharks and batoids have scales also called dermal denticles(“skin teeth”). This scales does not grow as the shark grows, the shark only grows more scales. These scales create whirlpools or vortices which helps the shark to swim more efficiently. In some countries shark skin is also used for sandingpaper.