TRANSITIONAL ECOLOGICAL REQUIREMENTS FOR EARLY JUVENILES OF 2 SYMPATRIC STICHAEID FISHES, CEBIDICHTHYS-VIOLACEUS AND XIPHISTER-MUCOSUS

The distribution patterns, diets, and substratum (refuge) requirements of early juveniles of two sympatric stichaeid fishes Cebidichthys vio laceus and Xiphister mucosus, were investigated in a rocky intertidal habitat at Diablo Canyon, California. Monthly investigations were cond ucted at low tide for four consecutive months, to assess ontogenetic d ifferences in distribution, diet, and refuge requirements within and b etween the two species. Distinct differences in vertical zonation were exhibited by both stichaeids throughout the study. Interspecific zona tion patterns were similar to those recorded for adults of both specie s. Diet analyses showed that early juveniles of both stichaeids were z ooplanktivorous, differing markedly from the primarily herbivorous die ts of adults. Changes in diet were largely due to the selection of lar ger prey taxa as both fishes, and their mouth size, grew over the stud y period. Predation by both fishes on water-column planktors (calanoid copepods, zoea and polychaete larvae) was greatest following initial intertidal settlement and habitat establishment by early juvenile fish es. Greater dependence on substrate-oriented and/or benthic prey (harp acticoid copepods, gammarid amphipods and mysid shrimp) was exhibited by both fishes as they grew in size. Affinities for sand, gravel, and pebbles during monthly field surveys were similar for both species thr oughout the study. As the fishes grew, their substratum preferences ch anged in relation to the substrata which provided the best refuge. Res ults from laboratory experiments indicated that young stichaeids selec t very specific substrata based on fish age (size) and substratum suit ability (i.e. adequate refuge). Similarities in diet and substratum pr eferences, and changes in those preferences over time, appear to be th e result of morphological similarities (body size and shape and mouth gape) for both species at a given age.