A circatidal rhythm is a rhythm or pattern of behavior that repeats approximately in concert with one tidal cycle. It is approximate because the cues that are used to determine the tide, such as salinity and changes in pressure and temperature, have been removed, usually in a laboratory. Such cues are called Zeitgebers. When Zeitgebers are removed things are said to be in constant conditions. Constant conditions car be constant high tide or constant low tide. If an animal in constant conditions performs a behavior at approximately the same time as an animal that is not in constant conditions and in its normal environment, the animal in constant conditions is said to have a circatidal rhythm and such a rhythm is endogenous (influenced by its internal environment or "memory" of its former natural environment). If an animal in constant conditions does not perform a behavior at approximately the same time as an animal that is not in constant conditions and in its normal environment, or the animal does not change its behavior at all, the animal in constant conditions does not have a circatidal rhythm and such a rhythm is exogenous (influenced by its external environment in the laboratory with no "memory" of its former natural environment).
Fiddler crabs are animals that are active during low tide because this is when they feed. At high tide they go into their burrows and are less active. If fiddler crabs are removed from their natural environment and are placed in constant high tide or constant low tide in a laboratory, they will be active and feed at approximately the same time that they would feed in their natural environment, and they would be less active at approximately the same time that they would be in their natural environment. This is a ciratidal rhythm and it is endogenous. Although fiddler crabs cannot keep this endogenous rhythm forever, they are better at it than most other marine animals.
- Akiyama, T., (1995). Circatidal swimming activity rhythm in a subtidal cumacean Dimorphostylis asiatica (Crustacea). Marine Biology 123: 251–255.