Spatiotemporal Size-Class Distribution of Turbanella mustela (Gastrotricha: Macrodasyida) on a Northern California Beach and Its Effect on Tidal Suspension

摘要

The size-class distribution of the marine interstitial gastrotrich Turbanella mustela Wieser was analyzed at a high-energy beach in northern California. Five 100-#mu#m size classes, each corresponding to a particular sexual phase of the species, fluctuated in percentage abundance at both temporal and spatial scales. On average, the most abundant size classes over the 3-day period were the 100-199-#mu#m group (prereproductive juveniles) and the 200-299-#mu#m group (male phase). Significant differences were evident spatially, where aggregations at the vertical and horizontal level contributed to patchy size-class distributions. Members of the largest size class (postreproductive or male phase) were in low abundance, and juveniles and reproductive individuals made up the bulk of the population. The smallest size class (100-199#mu#m) was most aggregated in the top 5 cm of sand and differed significantly in percentage abundance from all other size classes at that depth. This size class is also the only size class to decrease significantly in percentage abundance on a vertical scale and increase in percentage abundance on a horizontal scale. Three hypotheses accounting for the observed size-class variations are entertained: sexual phase stratification, interspecific interactions, and intraspecific trophic relations. All three hypotheses are important for understanding the importance of these size-class aggregations and may lead to a better understanding of the factors that influence local spatial patterns in gastrotrichs. Size -class stratification may also function in the planktonic dispersal of individuals in both tidal and longshore directions, ultimately affecting the geographic distribution of the species.