Drawing by Robert Savannah, USFWS
Currently, there is a certain amount of confusion regarding classification (taxonomy) in the soft-shell family Trionychidae. If you look at the latest version of Pritchard's Encyclopedia of Turtles (1979), you will find a total of eight different genera in two sub-families. If you look at Iverson's A Revised Checklist with Distribution Maps of the Turtles of the World (1992), you will see 14 different genera in the same two sub-families. Not only are there more genera, the genus name of North American soft-shells has "changed" from Trionyx to Apalone (Amyda has also been used). I will use Apalone here although I must admit I'm not used to it myself. I will confine myself in this article to the North American or New World soft-shells, although a lot of the habits and characterizations apply to many soft-shelled turtles around the world.
There are soft-shell turtles in North America, Asia and Africa. In North America, Iverson recognizes three species and 9 subspecies: two in Apalone mutica; seven in Apalone spinifera; and no subspecies in Apalone ferox. The Florida soft-shell, Apalone ferox, resides in South Carolina, Alabama, Georgia and of course Florida. The smooth soft-shell turtle, Apalone mutica, can be found throughout the entire central USA. The spiny soft-shell turtle, Apalone spinifera, is distributed throughout most of the USA and northern Mexico with the exception of the northwestern US.
I apologize ahead of time for the lengthy physical description, however, soft-shell turtles are very different from "the rest" of the turtles and tortoises that we are accustomed to, and I believe it is necessary. Archie Carr states that soft-shell turtles have existed in their present form since the middle Cretaceous period, 135-65 million years ago. It would have started its split from the other turtles even earlier.
The soft shell (bony part) is substantially different in a soft-shelled turtle from that of a "normal" turtle. The peripheral bones are gone or reduced in number, allowing the leathery edges of the shell to "flap". The plastron is cartilaginous or incompletely ossified. The turtles are covered by a leathery skin, not scutes. The shell is low in profile and round in outline (adults generally have a more elongated shape). The feet are highly webbed and three-clawed (hence the tri in Trionyx). The head and neck are withdrawn into the shell with vertical bends as with other Cryptodiroidea. The neck is quite long and the head narrow with a double-barreled proboscis for a nose. The long nose is excellent for snorkeling and "sniffing" amongst cracks and crevices for food.
The Florida soft-shell is the largest of the New World soft-shell turtles (which are all believed to have originated in the Old World) and has the most Old World characteristics such as: relatively large size, tolerance for brackish water and longitudinally wrinkled carapace. The young Florida soft-shell is olive-yellowish in color with large gray spots, yellow and orange markings on the head and a yellowish border around the carapace, (perhaps the most beautiful of the North American "softies"). These juvenile markings are mostly lost with age. Adults are brown-gray sometimes showing traces of the juvenile markings. The plastron of the juveniles is a beautiful slate-gray.
The spiny soft-shell has a brownish/olive background color to its carapace with many variations of darker markings and ocelli on the Eastern specimens and white spots on the Western specimens, dependent upon subspecies. The spiny soft-shell gets its name from the cone like projections on the leading edge of its carapace.
The smooth soft-shell gets its name because to a large degree it lacks the spiny protuberances of its relative. Smooth soft-shells have faint markings as juveniles on a tan carapace with light colored markings on the side of the head. Adults often develop a darker mottled brown color on the carapace. Plastrons of smooth soft-shells are very light or white in color with visible callosities.
Soft-shell turtles are believed to breathe anally and pharyngeally as well as "normally". This means that there is direct oxygen transfer through highly vascular papillae in the turtles throat and anus. Continuous movement of the hyoid bone (in the throat) pumps water over the vascular tissues somewhat like the action of gills in fish. This appears externally in the continual swallowing movements observed in soft shells while submerged.