Clawed Frogs and Surinam Toads: Pipidae
With their flat bodies and their wide and fishlike heads, the clawed frogs and Surinam toads are an odd-looking bunch. The head is flat in some species and shaped like a wedge—taller in the back and tapering down toward the front—in others. They have tiny eyes on the top of the head, but they do not have tongues. Their eardrums do not show on the sides of their heads, as they do in many other frogs. Another unusual feature is the line of stitchlike marks that run down each side of the body from the head to the rump. The marks are not actually stitches but allow the frog to feel the movements made by other animals in the water. This line of marks is known as a lateral (LAT-eh-rul) line system. While such a system is common in tadpoles and in fishes, it is unusual in adult frogs. The lateral line, which senses vibrations in the water, is helpful in finding prey.
Clawed frogs and Surinam toads have long hind legs with large, fully webbed feet. The webbing may match their foot color, or it may be a different color, such as the orange-yellow of the Müller's plantanna. The forelegs of clawed frogs and Surinam toads are much smaller than their hind limbs, and the thin toes on the front feet in most species do not have any webbing between them. The only members of this family with front toe webbing are the dwarf clawed frogs, which fall into the groups known as Hymenochirus and Pseudhymenochirus. All except two species, including the Surinam toad, have three claws on each foot. The claws are the second, third, and fourth toes. The outside two toes on each hind foot are clawless. A typical species in this family has a tan, greenish brown, or gray back, usually with dark spots or markings, and a lighter colored underside with dark markings. Depending on the species, they may either have bumpy or smooth skin. The adults of some species grow to 0.8 to 1.2 inches (2 to 3 centimeters) long from the tip of the snout to the end of the rump, but others can reach as much as 4.1 to 6.7 inches (10.4 to 17 centimeters) in length.
None of the 30 species in this family has vocal sacs. In most other species of frogs, the vocal sac looks like one or two bubbles located below the chin that blow up with air and deflate when the frog makes its call. The clawed frogs and Surinam toads do not even have vocal cords, which are the structures inside the throat that most animals, including humans and other mammals, use to make noises. Instead, these frogs have two disks in their throats that move back and forth to produce clicking sounds. Since these frogs do their clicking underwater, the sound travels through the water as a vibration, rather like an underwater ripple. Another frog can hear the clicks through a different disk that sits under its skin and on the side of its head. This disk picks up the vibration and passes it along to the inner ear, which is the part of the ear located inside the head, and the frog hears the click.