Sea urchins

Sea urchins are a fascinating class of species from the Echinodermata or ‘spiny skinned’ animal group – they evolved 600 million years ago and today approximately 800 species exist worldwide, and around 60 species have been found in the shallow waters of the Western Indian Ocean. On our snorkel surveys, we have been recording the abundance of sea urchins and so far have found key species present to include the Needle Spined Urchin (Diadema setosum and D. savignyi) with long thin black spines up to 30m in length, Echinometra mathaei with short thick black spines, and Astropyga radiata with densely packed needles, vivid pink bordered with iridescent blue spots.

The Astropyga radiata - with distinctive pink and blue markings

Sea urchins have five teeth in their mouth on their underside and mainly feed on sea grasses or algae living inside dead coral; in times of high population numbers they can also attack living coral. Population outbreaks can occur on dead reef patches caused by an environmental disturbance, such as an El Nino event or over-fishing of predatory fish such as Triggerfish and Wrasse. This can lead to bio-erosion of large areas of limestone rocks and reefs, toppling erect coral structures, which reduces the topographic complexity of the reef. This removes certain niche habitats and can limit overall biodiversity of reef areas.
We are noticing through our surveys a trend common in many similar studies, that protected (non-fished) areas have higher abundance of predators such as Triggerfish, but they are rarer in unprotected reefs allowing sea urchins to exploit the area. It has been determined that the removal of top, invertebrate-eating, fish carnivores can have cascading effects on coral reef community structure and function (McClanahan and Shafir, 1990). It is also true that sea urchins themselves have an important role to play in the nutrient cycle and regeneration of reef habitats.



The Needle-Spined Urchin can bio-erode large areas of coral reefs



Sea urchins