Towards a standard measure of sea anemone size: assessing the accuracy and precision of morphological measures for cantilever-like animals

Abstract

Our capacity to detect and interrogate patterns in nature depends on the use of standard methods for measuring biological units. Consensus methods to quantify the size of individual animals and characteristics of biological communities are critical for comparisons across time and space. Nowhere is this more important than when dealing with organisms such as sea anemones that display high plasticity in body shape. Despite the need for accurate measures of anemone size for ecological comparisons, there is little consensus on the accuracy and precision of size inferences for these animals. We assessed several morphological parameters to determine which in-field measure accurately and reliably reflects the reference size of an anemone measured in the laboratory: (i) column height, (ii) column diameter, (iii) limbus diameter, (iv) pedal disc diameter, (v) pedal disc area or (vi) pedal disc perimeter. The results revealed large variability in the accuracy and precision amongst measures, which have implications for their suitability as a standard method for insitu measurements. In general, measures of diameter were preferable to those of height, area and perimeter; and those associated with attachment (i.e. the limbus and pedal disc diameter) performed the best. Overall, considering concurrence with measures obtained from two differing but useful reference states, pedal disc diameter was the most accurate parameter (mean percentage difference=0.6) with which to estimate the size of sea anemones in the field, and we thus recommend its use as an effective, non-destructive means of gaining insights into their behavioural and evolutionary ecology.