Friday, 26 May 2017

Diadema antillarum

Diadema antillarum, also known as the spiny urchin, was a key herbivorous species in the Florida Keys. It exerted massive downward pressure on algal species in coral reefs, rocky bottoms, and sandy banks. Numbers were so great, that at one point it was even suspected that D. antillarum feeding on hard bottoms removed more calcium carbonate than any other natural erosion process (Lessios, 2015, https://www.stri.si.edu/sites/publications/PDFs/2015_Lessios_annurev-marine-122414-033857.pdf). The massive abundance of the spiny urchin was extinguished seemingly overnight in a massive die off event that lasted from 1983 to 1984. The die off obliterated essentially every spiny urchin population in the Caribbean, with reef populations being reduced 93-100% only a week after the first signs of urchin death appearing. The dying is suspected to have been due to some water born disease, but we do not have enough information to determine for certain what exactly it was. What we do know for certain is that the removal of these top grazers resulted and a phase shift in coral reefs from coral dominated, to algal dominated systems. The effects are related to the drastic reduction of coral cover in the keys, as more macrology crowd out space, outcompeting the slower growing corals for light and space. D. antillarum on the other hand is very slowly recovering, but is still not even close to reigning in the algae dominance which is becoming more and more dominant all across the keys. This is mostly thought to be due to poor larval supply and recruitment, resulting in a very slow recovery of key populations (Miller et al, 2009, https://link.springer.com/article/10.1007/s00338-008-0458-4). A pair of Spiny Urchins were found during our dive on Sweat bank. This is encouraging because there are almost none found on the gulf side of the keys, and may mean the species is finally expanding out in this direction. However it is far too soon, and there is far too little data, to make any sort of assumption. The fate of D. antillarum, and in truth the fate of the entire structure of Florida key reefs, is far from certain. Continuing changes in climate and ocean acidification may continue to drastically alter the ecology of these reef systems. Only through careful observation, adequate protection and management strategy can we hopefully prevent any further disruption of these cherished environments.
 

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