The cloaca is the only part that serves as an opening for the body like the digestive system, reproductive tract and urinary tract of most vertebrates. All amphibians, reptiles, birds, and a few mammals excrete both urine and faeces through this opening only.
In some invertebrates excretory organs with analogous functioning also known as the cloaca. Mating is also done through cloaca is some organism which is known as cloacal copulation or more commonly also called as cloacal kiss. Few organisms also have an accessory organ i.e. penis attached to them, which is used to transport sperm into the female cloaca.
Cloaca in Birds
Reproduction processes in birds are mainly done by using cloaca. And the use of cloaca occurs during a cloacal kiss in most of the birds, as during the matting process birds touch their cloacae together, in some species only seconds are enough to transfer the male gamete from the male to the female. Whereas some species of birds do not use cloaca for reproduction as they have phallus to perform the reproduction process. Example: in birds like ostriches, cassowaries, kiwi, geese, and some species of swans and ducks.
Cloaca in Fish
True cloaca in fish is only present in elasmobranchs like sharks and rays along with lobe-finned fishes. In some fishes like ray-finned fishes, the cloacal part remains in the adult to receive the urinary and reproductive ducts, whereas the opening of the anus is separate in them. In a few species of fishes, all three openings are separate. Examples of such fish are Chimaeras and teleosts.
Respiration With the Help of Cloaca
Turtles who are specialised in diving are highly dependent on cloacal respiration during the diving process. They do this process with the help of a pair of accessory air bladders which is connected to the cloaca. This turtle can absorb oxygen from the water very easily.
Even some fish along with crabs also take advantage of cloaca respiration during constant flow in water.
Functions of Cloaca
A. It acts as a waste elimination chamber for both urine and faeces. In animals in which cloaca is a common passage for both digestive and excretory systems, it acts as a single site for all kinds of excretion processes.
B. In few organisms it also serves the function of the vagina and in some, it works similarly to function as a penis. Female organisms having cloaca receive eggs through the cloaca and also give birth through it.
C. Mating in some species is also done by cloaca which is known as cloacal kiss, by which transfer of sperm takes place.
D. Few species of fish and turtle perform their respiration process by using cloaca.