Mushroom corals overcome live burial through pulsed inflation
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Sedimentation represents a major stressor for scleractinian corals. Although many coral species exhibit the capacity of active sediment rejection (Stafford-Smith and Ormond 1992), only few are capable of freeing themselves after becoming completely buried. Fungiid corals appear to be an exception, as they can remove sediments through substantial polyp inflation (up to five times their normal size) in addition to mucus entanglement and ciliary action (Schuhmacher 1977). Using time-lapse photography (speeding up time 300×), we observed that this inflation occurs in rhythmic pulses (Fig. 1), allowing corals to completely exhume themselves after becoming covered in sand.
Specimens of Lobactis scutaria (Lamarck, 1801) and Herpolitha limax (Esper, 1797) from Heron Island (Great Barrier Reef) were placed in aquaria, covered with coarse sand (0.5–1 mm), and photographed every 10 s over a 10–20 h period (videos available as ESM and at: http://www.coraltimelapse.com/reefsite). Pulses occurred at intervals ranging from 10 to 20 min with each pulse characterized by (1) a gradual inflation phase of 1–3 min, (2) a rapid inflation phase (<10–15 s), and (3) a slower deflation phase of 3–5 min. Tentacles retracted during inflation, whereas they extended during deflation and waved from side-to-side in between pulses due to peristaltic waves of tissues on septal ridges. The extent of inflation in each pulse increased over time (15–25 cycles) until the coral was completely freed from sediment particles.