The Behavior and Life History of a Sand‐Beach Isopod, Tylos Punctatus

Tylos punctatus is an abundant oniscoid isopod on sheltered sandy beaches in southern California and in Baja California. It is strictly nocturnal, being buried in the sand or under wrack by day. During the winter it does not emerge onto the surface, remaining buried in the sand high up on the beach, the adults up to a meter deep and the juveniles shallower. From March to late October or November, however, it emerges at night to feed on stranded seaweeds and dead animals. When the high tide is early in the evening, T. punctatus emerges onto the surface only after the tide has begun to ebb from over the sand in which it is buried. Tylos moves seawards at first, but this strict seaward tendency diminishes the further down the beach it has walked. It apparently navigates by beach slope. Later in the night, having fed, Tylos moves back up the beach to bury itself at the previous night's high—water mark. T. punctatus is extremely abundant on El Estero Beach. Its reproductive rate is very low compared with that of other oniscoid isopods. Females breed once a year in summer. They incubate with that of other oniscoid isopods. Females breed once a year in summer. They incubate the embryos in their brood pouches in the sand above the high—tide line for about 9 weeks and produce an average of 13.6 young each. Larger females tend to produce more young. Very few if any females breed before they are 3 years old and most, if not all, die soon after. Some males, however, live for at least 4 years. T. punctatus only grows during a 5—month period in the summer. There is no significant difference between mean sizes or growth rates of males and females. Growth rate declines with age.