Pedicellariae of the crown-of-thorns sea star Acanthaster planci are not an effective defence against fouling

Abstract

This study focused on the speculative role of the pedicellariae of the sea star Acanthaster planci in fouling control, as quantitative measurements showed that this species is free of any macro-fouling organisms. The morphology and distribution of its pedicellariae were measured to determine if larvae or propagules of fouling organisms could settle between pedicellariae without being in their physical range. The elementary and straight pedicellariae of A. planci had a mean length of 0.7 mm and a mean distance of 2.6 mm. The total number of pedicellariae was proportional to the estimated surface area of A. planci. To determine how pedicellariae respond to tactile stimulation, pedicellariae were stimulated by touching either inner, outer or basal sites of pedicellariae with a hypodermic needle. Pedicellariae closed rapidly on touch and closed for significantly longer when touched at their inner sites (8.9 s) than outer (6.7 s) and basal (7.9 s) sites. Settling larvae were simulated by dropping silica beads (size: 50.2, 181.5, 255.7 and 510.7 µm, density: 2.5 g ml-1) and zirconium/silica beads (size: 191.2 and 507.6 µm, density: 3.7 g ml-1) over the pedicellariae. The percentage of responding pedicellariae increased proportionally with increasing size of the silica beads. However, the percentage also increased when zirconium/silica beads of similar size but higher density were used, demonstrating that the mass, not size, of the beads was the main driving factor for the closure of pedicellariae. Pedicellariae were also stimulated by placing larvae of the bryozoan Bugula neritina (250 µm) and fragments of the alga Chrysocystis fragilis (150-1000 µm) over the pedicellariae. However, the response of the pedicellariae to the larvae of B. neritina was consistently low and none of the pedicellariae responded to the fragments of C. fragilis. These results demonstrate that for A. planci pedicellariae offer little or no defence against fouling.