Abstract
This is the third of a series of papers examining the taxonomic relevance of some statistical treatments of measurements of cnidae from sea anemones (Actiniaria). Some cnida lengths from fresh tissue samples (column ectoderm, tentacles or acontia) from Nematostella vectensis, Haliplanella lineata, Sagartia elegans, Metridium senile, Cereus pedunculatus, Sagartia troglodytes, Anthopleura thallia, Urticina eques and Sagartiogeton lacerates were measured. Five measures of statistical dispersion (sample standard deviation, coefficient of variation, observed sample range, standard range, and 99% probable maximum value of the standard range) were calculated, and their interrelations and potential applications were appraised. It has long been the convention to use the largest and smallest cnida sizes (observed sample range) from tissue samples in attempts to establish differences between actiniarian taxa. However, such data do not reflect the true extremes of a population range. In the present study, the 99% probable maximum value of the standard range for a standard abundance of 1,000 gave the greatest and, therefore, the most cautious estimate of a population range of cnida sizes for a species. This maximum standard range is the only measure of dispersion of cnida sizes that may be used validly to demonstrate that anemone specimens are of different species, and then only if there is no overlap between the extreme cnida sizes being compared. However, partial or complete overlaps of cnida size extremes do not necessarily indicate that specimens are conspecific; other taxonomic characters must also be considered. Coefficients of variation may provide valuable clues as to the homogeneity or heterogeneity of samples of cnida measurements. This paper should be read in conjunction with the first two in this series, which address the taxonomic relevance of differences between mean cnida sizes (Williams, 1996, Sci. Mar., 60: 339-351; 1998, Sci. Mar., 62: 361-372).
Highlights
The fundamental characteristic of members of the phylum Cnidaria is their possession of stinging cells called cnidae, of which there are many types (Mariscal, 1974)
It has already been shown that differences demonstrated, by classic inferential statistical tests, between mean lengths of samples of single cnida types cannot be used in isolation to distinguish actiniarian species (Williams, 1996, 1998)
1) Examination of five measures of dispersion of cnida lengths indicates that cnida size expressed as extreme values of observed ranges is not a reliable taxonomic character
Summary
The fundamental characteristic of members of the phylum Cnidaria is their possession of stinging cells called cnidae, of which there are many types (Mariscal, 1974). The cnidome may be valuable as a taxonomic character in some groups of Cnidaria, but the value of additional information on cnida sizes, conventionally included in species descriptions of sea anemones (Actiniaria) since it was first suggested by Carlgren (1900), is controversial (Fautin, 1988). MEASURES OF DISPERSION OF ACTINIARIAN CNIDA SIZES 49 of a single type tend to be normal (Gaussian). It was recommended that mean cnida sizes should not be used in isolation from other taxonomic characters to differentiate anemone species, because statistically significant differences may occur between replicate samples from the same specimen, and between samples from different specimens of the same species. No predictable relations between the sizes of conspecific anemones and the mean lengths of their cnidae were established, either for sexually or asexually reproducing species
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