upstream tidal pools, where they burrow into the mud at depths up to to overwinter. They can also bury themselves in mud if they are caught in a drying tidal pool between spring tides. Alternatively, they can travel short distances on land to get back to the sea. In the laboratory, mummichogs have yielded clear examples of free-running circadian rhythms, in both body colour and swimming activity. For the latter, clear rhythms were obtained in single individuals as well as in groups of 5 or 25 individuals. Evidence of free-running semi-lunar rhythms have also been obtained in mummichogs: in constant laboratory conditions, egg production peaked every 14.8 days for up to 5 months. Reproduction Spawning takes place from spring through fall. In the southernmost populations, up to eight spawnings are possible in a season. Spawning takes place most often at high tide and when the moon is new or full. Maximal spawning occurs when high spring tides coincide with night, though spawning during the day remains possible. During courtship, males may pursue females, and females may attract males by turning on their sides near the bottom and flicking their tails. A male and female may swim together for a while, after which the male crowds the female against a rock or a plant and clasps her: the male's larger dorsal and anal fins curve around the female's body. Fingerlike projections that develop on the male's scales behind and below the dorsal fin may help the male maintain contact with the female. The pair quivers vigorously and eggs and sperm are released. The eggs are pale yellow, about in diameter, and strongly adhesive. During a spawning event, a female can deposit up to 740 eggs in separate clutches of 10 to 300 eggs at a time. The eggs adhere to plants, algal mats, empty mussel shells, sand, or mud at sites that are reached by water only at high spring tides. Eggs therefore develop while exposed to moist air, and they hatch when the next high spring tides reach them. The eggs cannot hatch in air, nor can they do so in moving water; hatching is initiated by a lack of oxygen, something that can happen in the boundary layer of relatively still water surrounding the metabolically active egg at high tide, but not in air or in moving water. As opposed to their northern counterparts, the southern subspecies have eggs