Ecology

Skip to: Microhabitat, Associations, Ecosystem effects


Microhabitat

On the Heron Island Reef, D. savignyi was found from the reef flat to the reef crest, usually wedged in between rocks and in crevices. This not only prevents the urchin from being swept away by strong waves, but also offers it protection from predators. Other sources say that D. savignyi can occur on sand and coral rubble, at depths of up to 10m, and may also be found in the channels of mangroves, and in creeks and estuaries (Muthiga & McClanahan, 2007).

A D.savignyi urchin nestled between coral heads on the fore reef
of Heron Island Reef. 

Associations

D. savignyi, like most other Diadema species, is an omnivorous and detritivorous grazer and, and feeds mainly by scraping algal films off hard substratum (Randall, 1964). Gut analyses performed on D. savignyi have shown that the gut contents of this species contain not only algae (28%), but sea grass (20%),invertebrates (2%) and coral sediment (~50%) (McCalahan, 1988). Due to their diet, D. savignyi competes with other urchin species as well as herbivorous fish, especially during the day when these fish are most active (Kauffman, 1977). Because of their overlapping microhabitats, D. savignyi is also often in direct competition with other Diadema, especially D. setosum, for crevice space (McCalahan, 1988). D. savignyi is usually the competitively dominant species due to its reduced spine length to test size ratio, giving it a larger test size for the same crevice size. Because of this, D. savignyi tends to occur in smaller crevices while D. setosum is left to inhabit open spaces between coral.

The long spines of Diadema discourage most predators, however at least 15 species of fish, 2 species of large gastropods and the spiny lobster have been identified as predators of the genus (Randall et al, 1964). Most of these species have hard palettes to enable them to safely ingest the spiky urchin, while others feed on the tube feet or pedicellariae (Sakachita, 1992).

The oral surface of D. savignyi, showing the jaw used for
scraping and shearing the substrate during feeding. 

Ecosystem effects

Grazing by Diadema urchins plays an important ecological service on coral reefs. Consumption of algae by urchins reduces algal biomass, enhances the quality of algal turfs and thus increases productivity on the reef (Carpenter, 1981). If the reef becomes overrun by algae, corals are out competed by the algae, and the reef undergoes a ‘phase shift’ from coral dominated to algae dominated. This phase shift can be catastrophic to the reef and its inhabitants, because the loss in calcium carbonate due to biological or physical erosion is not replaced sufficiently by calcification by corals and coralline algae. This reduces the resilience of the reef, and ultimately may lead to the loss of the coral framework that is vital to marine organisms. 

While some grazing is beneficial to the reef to reduce algal growth, an over-abundance of urchins leads to a decrease in coral recruitment,which has an equally detrimental effect on the reef.  The abrasive grazing methods of urchins such as Diadema result in high levels of coral spat mortality, which reduces the formation of the calcium carbonate framework required for a healthy coral reef (Sammarco, 1980). Therefore, the optimal conditions for coral growth would be intermediate densities of urchins and herbivorous fish, because this way, the two opposing forces (competition with algae, and bioerosion from echinoid grazers) are in balance (Sammarco, 1980).

Scanning electron micrograph of newly settled Porites spat. Figure (a)
shows the specimens alive and intact, while in (b),the specimens are
dead and show evidence of sheared septa, most likely from echinoid
grazing.

Image appropriated from Sammarco, 1980.