- River otters range in size from about 87 to 153 cm (34-60 in.) and 3 to 14 kg (7-31 lb.).
- The largest river otter is the North American river otter, ranging from 100 to 153 cm (39-60 in.).
- The smallest river otter is the marine otter, averaging 87 to 115 cm (34-45 in.).
- The giant otter averages 145 to 180 cm (57-71 in.), with a maximum recorded length of 240 cm (94 in.). Male giant otters average 26 to 34 kg (57-75 lb.), while females average 22 to 26 kg (49-57 lb.).
- The Asian small-clawed otter is the smallest of all otters, measuring 65 to 94 cm (26-37 in.) and weighing just 1 to 5 kg (2.2-11 lb.). The other two clawless otters are similar in size to river otters.
- Alaska sea otters are slightly larger than California sea otters.
- Male Alaska sea otters measure up to 148 cm (58 in.) and average about 27 to 39 kg (60-85 lb.). Large males have been known to reach a weight of 45 kg (100 lb.).
- Female Alaska sea otters measure up to 140 cm (55 in.) and average 16 to 27 kg (35-60 lb.). Large females may reach 33 kg (72 lb.).
- Male California sea otters average 122 cm (4 ft.) and 29 kg (64 lb.).
- Female California sea otters average 20 kg (44 lb.).
- River otters typically have slender, streamlined, serpentine bodies. Clawless and giant otters have similarly shaped bodies.
River otters are slender with streamlined, serpentine bodies.
- In comparison to other otter species, sea otters are somewhat stockier with larger ribcages.
- All otters have very flexible bodies. This flexibility allows them to groom almost every inch of their fur.
- In general, otters exhibit various shades of brown (light cinnamon to dark brown), with a darker dorsal (back) surface and much lighter ventral (underside) surface. In some species, the color boundaries are sharp and distinct; in others, they are less clearly defined.
- The hairy-nosed otter is very dark brown with a creamy white throat.
- Giant and spot-necked otters have creamy white splotches on their throats and chests, sometimes blending to form a "bib".
- All otters have four relatively short legs. They enable otters to swim, walk, groom, and manipulate prey. Paws have bare sole pads on the undersides, with the exception of the sea otter, which has no sole pads on its hind feet.
- River otters have webbed digits and strong, nonretractile claws on all four feet.
River otters have webbed digits and strong, nonretractile claws on four feet.
- Clawless otters have partial webbing on their feet. Congo and Asian otters have small, blunt, peg-like claws. Cape clawless otters have similar claws on the three middle toes of their hind feet; claws are absent on the other toes.
Asian otters have partial webbing and small, blunt, peg-like claws.
- Giant otters have large, fleshy feet with thick webbing that extends to the tips of the digits. Claws are well developed.
- Sea otters have small dexterous forefeet with retractile claws. They use their forefeet for grooming, finding food, and eating, but not for swimming. The sea otters' flipper like hind feet are large, broad, and webbed. The outer digits of the hind feet are the longest.
- The tails of river otters and clawless otters are long, and about one-third of total body length. They are thick at the base, muscular, flexible, and taper to a point.
- The giant otter's tail is similar to those of other freshwater otters. However instead of tapering to a point it becomes lance-shaped - flattened top-to-bottom, with a noticeable flange on each side.
- A freshwater otter uses its tail to propel itself when swimming at high speed, to steer when swimming slowly, and to help balance when standing upright on its hind legs.
- Compared to other otters, sea otters have shorter and less muscular tails, with no marked tapering. While floating on its back, a sea otter can scull with its tail to turn.
- Like most other mustelids, freshwater otters have subcutaneous anal scent glands at the base of the tail. Sea otters lack scent glands.
- Otters have flattened heads and short, thick necks. A sea otter has a blunter muzzle than those of other otters.
- Eyes are located toward the front of the head.
- Ears are very small, and either rounded or pointed. The valve-like structure of the ears enables them to close when in the water.
- The nose pads of otters vary considerably with the species.
- The nose pads of most otters are flat and bare with some variation on a spade or anvil shape.
- Most river otters have black nose pads. The smooth otter and clawless otters have nose pads that are dusky or pinkish in color.
- The nose pads of hairy-nosed and giant otters are partially or completely covered with hair.
- A sea otter's nose pad is large, bare, black, and diamond-shaped. An adult female's nose pad often bears pink scars from wounds inflicted during mating, when the male grasps her nose with his teeth.
- Like the ears, an otter's nostrils close when under water.
- Teeth vary with diets.
- An otter is equipped with vibrissae (whiskers) on its muzzle. Vibrissae are sensitive to touch and to underwater vibrations. Otters use their vibrissae to detect the movements of prey.
- The fur of all otters is fine, dense, and velvety.
- Like other mammals, otters have two types of fur: long, stout guard hairs, and a more dense arrangement of short, fine underhairs.
- Fur length varies considerably with the species.
- Most otters have guard hairs that average about 12 to 17 mm (0.47-0.67 in.) in length. The underhairs average 7 to 9 mm (0.28-0.35 in.).
- The marine otter has the second longest fur of all otters, with guard hairs measuring 20 mm (0.79 in.) and underhairs measuring 12 mm (0.47 in.).
- With 8 mm (0.32 in.) guard hairs and underhairs measuring just 4 to 5 mm (0.16-0.2 in.), giant otters have the shortest fur of all otters.
- Sea otter fur.
- Sea otters have the longest fur of all otters, but length varies greatly with location on the body. Guard hairs and underhairs range from 8.2 to 26.9 mm (0.32-1.06 in.) and 4.6 to 15.8 mm (0.18-0.62 in.) in length, respectively. The longest hairs are on the back, stomach, and sides.
- Sea otters have the densest fur of any mammal. Hair density varies dramatically with location on the body, ranging from about 26,000 to 165,000 hairs per square centimeter (170,000-1,062,000 per square inch). The highest density occurs on the forearms, sides, and rump; the lowest density is on the chest, legs, and feet. A single large male may have 800 million hair fibers covering its body.
- One guard hair may have from 12 to 108 underhairs bundled with it, depending on the location.