Variability in nematocysts from a single individual of the sea anemone Actinodendron arboreum (Cnidaria: Anthozoa: Actiniaria)

Abstract

The taxonomic value of nematocyst size in sea anemones is still being assessed. We evaluate size distribution of nematocysts of one type in a single individual anemone. Length of unfired nematocysts was measured along the column, tentacles, and actinopharynx of a preserved specimen of Actinodendron arboreum (Quoy & Gaimard, 1833). Mean, range, minimum, and maximum length of nematocysts vary along the column, those in the middle region being least variable. The length of nematocysts in mature (split) acrospheres is less variable than in immature (unsplit) acrospheres. There is significant variability between nematocysts in tentacles of the primary and quaternary cycles, and along a tentacle, the middle being least variable. Size distribution of actinopharynx nematocysts is complex. The results of this study suggest that assembling data on nematocysts from multiple individuals for taxonomic purposes should be used with an awareness that sampling site can be an important variable. Ideally, the position of tissue sampled should be documented, an attempt should be made to be consistent in sampling from the same position in individuals being compared, and the variability of nematocyst length at each sampled site should be assessed. Inferences can also be made on ontogeny from these data; we conclude that an actinodendrid tentacle grows from the base and at the tips of its branches.