In zoology, an annulus is an external circular ring. Annuli are commonly found in segmented animals such as earthworms and leeches. The bodies of these annelids are externally marked by annuli that are arranged in series with each other.
An annulus may also be an indication of growth in certain species, similar to dendrochronology. For example, in fish, it is a series of concentric rings (or annuli) formed in the scales of bony fish. In bivalve mollusks, annuli are concentric growth rings in their shells.
annulus: One of a series of concentric rings or bands of varying width and opacity which are formed in the scales of bony fish. Winter rings are often narrower and denser than summer rings. The number of annuli is indicative of the age of a fish.