what is the difference between circuli and annuli and what causes them to form? (referring to fish scales)

Circuli: Each growth ring is called a circulus (plural: circuli). Like a tree, the fish grows faster during the summer when the temperature is warmer and there is more food available, and the circuli are spaced farther apart. During the winter, growth slows down and the circuli are very close together.

Annuli: 1 : ring. 2 : a part, structure, or marking resembling a ring: such as. a : a line of cells around a fern sporangium that ruptures the sporangium by contracting. b : a growth ring (as on the scale of a fish) that is used in estimating age.

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.