No rhythm is seen, but an artificial light-dark cycle activates a vestigial rhythm. Both are entrained by light. Neither involve neurons. The cycle period is about 0.5 seconds. The evolved from a common ancestor.
Most species have a number of different biological clocks.
A master clock controlling the others by the nervous and endocrine systems. The animal's daily activity rhythm. The presence of food sources at certain times of the year. The turning on an off of genes in each separate clock. Dispersion to new feeding grounds.