Use of cnidae in taxonomy: implications from a study of Acrozoanthus australiae (Hexacorallia, Zoanthidea)

Abstract

Cnidae have been used in hexacoral taxonomy since 1900 but their value at species-level remains controversial. Data given in descriptions have generally been cnida types, their distribution, and capsule sizes presented as ranges (maxima and minima). More recently, measurements of cnida capsules, treated as populations, have been employed. Sampling cnidae extracted from zooids ranging from buds to full-grown polyps, we have analysed 28 300 random measurements of capsule length and width in 133 samples taken from two colonies of the zoanthid Acrozoanthus australiae, one from Queensland, Australia, and one from Indonesia. Using image analysis we measured large holotrichs, microbasic b-mastigophores and p-mastigophores, basitrichs being absent. Length has a lower coefficient of variation than width. Up to 12% of sample distributions were non-normal, depending on the test employed, but there was no consistent pattern of departure from normality. Sample sizes of 50 are regarded as adequate for the calculation of means and variances but need to be larger (contrary to current practice) for an accurate estimation of range. Moreover, some ranges were asymmetrical with respect to the sample mean. Statistical methods for comparing ranges are lacking, so sample size, mean and a measure of variance must always be included. Capsule sizes often differed significantly along the length of tentacles and filaments; they also showed significant differences between polyps within a colony, sometimes clearly correlated with polyp size. It is suggested (based on size frequency distributions that are right skewed or bimodal) that new buds may contain residual cnidae derived from the parent polyp as well as others that have differentiated in situ. Though length appears to be the most useful capsule descriptor, bivariate (width: length) scattergrams provide a concise summary of capsule size, also displaying cnida type and source. We make several recommendations for future practice.