Seasonal changes in the population structure of Echinoderes coulli (Kinorhyncha)

Abstract

The population ecology of Echinoderes coulli (Kinorhyncha) in a South Carolina, U.S.A., Spartina alterniflora salt marsh was studied at two nearby intertidal sites. Seasonal cycles in population density, spatial dispersion, age structure, size distribution, sex-ratio and gonad condition were determined. Abundance peaked in midsummer and again during fall months at both sites. Density declined during February and abundance minima were recorded during spring. Overall, density ranged from 0·4 to 72·0 × 10 cm −2 . Based on ANOVA, significant (at the 0·05 level) seasonal variations in adult/juvenile ratio and frequency of occurrence of mature gonads in both sexes were demonstrated. Juveniles were fewest in the population during the late fall, winter and spring. Except for males in March, sexually mature individuals were present the entire year with maxima in spring and early fall. The mean length of adults fluctuated seasonally, the minimum occurred in February. Reproduction and the frequency of juveniles were greatest from March to September when an intrinsic rate of natural increase of 0·018 day −1 was achieved. Two distinct female morphotypes, one with long and one with short lateral terminal spines, are present in E. coulli . Two-way ANOVA revealed that the two morphotypes reacted similarly to seasons as no difference across seasons or interaction with seasons in the relative frequency of occurrence of each morphotype or the occurrence of eggs in each morphotype could be identified.