Factors Affecting the Distribution of Tylos punctatus (Isopoda, Oniscoidea) on Beaches in Southern California and Northern Mexico

Populations of the isopod Tylos punctatus were studied at five widely separated beaches between Los Angeles, California, and Punta Banda, Baja California (Mexico). Population densities at these sites were 10 3-10 5 animals per meter of beach frontage; smaller populations were not located, although more than 30 intermediate sites were sampled. Cross-beach distribution appears to be related to sand moisture content rather than sand coarseness. Longshore distribution on a given beach is patchy, but the causative factors for this were not determined. Studies of the isopods' population dynamics on two beaches indicated that the populations were increasing, with doubling times of 2-12 years. If these are minimum values, it is hypothesized that the discontinuous distribution of the animals on this coast can represent a balance between catastrophic beach erosion and the natural ability of the populations to regenerate and disperse. Tylos punctatus Holmes & Gay is a dominant member of the fauna on a number of marine sand beaches in southern California and northern Baja California (Mexico). On this coast, it is the only species of its genus that has been collected, although a somewhat different form, possibly an undescribed species or subspecies, is present in the Gulf of California (Dr. Milton Miller, personal communication). Other species, such as T. niveus Budde-Lund and T. latreillei Audouin (a Mediterranean form), are definitely known from the Caribbean area, where T. punctatus does not occur. Schultz (1970) discussed the various species that have been recorded from North and South America. Where it occurs, T. punctatus can build up enormous populations. Hamner, Smyth, and Mulford (1969) reported concentrations of up to 64,000 individuals per meter of beach frontage (a I-meter-wide strip of beach from the upper beach margin to the water line) at Estero de Punta Banda, near Ensenada, Baja California. I have measured nearly 90,000jm beach frontage there, and more than 30,000jm beach frontage at some southern California beaches (Hayes 1969). Since the animals are detritovores, concentrations such as these will have decided effects on the local patterns of beach energy flow, which are generally based on detrital kelp. Hayes (1974) investigated this problem. The isopods are nocturnal, emerging from the sand at night after high tide to feed on kelp, primarily Macrocystis pyrifera (L.) Agardh, which washes up on the lower beach face. They rebury about dawn, near the high-tide mark, and are not seen during the day. During the winter (October-April), the animals appear to hibernate and do not emerge to feed. The present paper is concerned with the distribution pattern of T. punctatus on various scales of measurement in that portion of its range which has been well sampled. An attempt will be made to relate the rather peculiar distribution of this species to environmental factors and to its probable evolution. GEOGRAPHICAL DISTRIBUTION I Manuscript accepted 3 November 1976. 2 University of Georgia, Geology Department, Athens, Georgia 30602. Tylos species tend to be warm-temperate to tropical in their distribution throughout the