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Nocturnal coral reef bio-erosion: parrotfish scraping and sea urchin grazing at Unawatuna?

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K
Kenji Benthic•915 rep

How do nocturnal herbivorous reef grazers and boring bivalves break down calcium carbonate reef matrix into fine coral sand along the southern shelf?

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5 Answers from travellers

✓Accepted Answer

Bio-erosion of shallow coral reefs around Unawatuna and Rumassala is driven by biological scraping and chemical boring. At night, parrotfish (Scarus species) sleep inside secreted mucous cocoons in reef crevices, while nocturnal long-spined sea urchins (Diadema setosum) emerge from deep cracks to graze epilithic algae off live and dead coral skeletons. Their five-plated calcium chewing apparatus (Aristotle's lantern) scrapes away thin layers of dense aragonite coral matrix. Simultaneously, boring sponges (Cliona) and lithophagine bivalves dissolve internal limestone using acid secretions, converting up to 5kg of solid reef per square meter into fine white lagoon sand annually.

38
D
Dr Anouk Marine•1890 rep

Excessive nutrient runoff from coastal hotels triggers explosive turf algae blooms, which causes sea urchin populations to spike and accelerates destructive reef erosion.

22
K
Kenji Benthic•915 rep

Night divers must avoid resting hands on bare coral rock; Diadema urchin spines are needle-sharp, brittle, and inject painful protein toxins upon contact.

18
C
Capt Roshan Sea•1720 rep

Bio-eroded coral sand is swept landward by swell action, continuously nourishing the golden crescent beaches of Unawatuna Bay.

15
M
Marco Pelagic•910 rep

Predatory triggerfish (Balistidae) control urchin numbers by flipping them over to bite the unshielded mouth underside during late twilight hours.

12
V
Valerie Chelonian•920 rep

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