DIADEMA - OUR PRICKLY FRIEND

I'm sure that many of you will already have noticed that the black spiney urchin, Diadema antillarum, is making a tentative appearance on our reefs. One of the best places to find them is around the Cargill salt pier and the chances are good that it won't be too long before you will have to watch your fins when you go shore diving. Scientist are really excited by the Diadema come back and we should be too because Diadema really is our prickly friend.

Recent immigrant or old hand?
Anyone not living on Bonaire before 1983 could be fooled into thinking that Diadema is a recent immigrant, maybe even an invasive species. But as Capt Don Stewart can confirm, Diadema has been around for a long, long time. In those days, divers concerned with the integrity of their feet, placed an absurdly high bounty of $0.25 per urchin on its prickly head. Divers had such an antipathy for this prickly grazer that they would routinely chop them up to provide a tasty snack for marauding fish. All this might lead you to assume that Diadema is an aggressive species prone to sneak attacks on invading divers. But it was not their speed or stealth which was causing the problem, simply their numbers. Several decades ago Diadema was not only present on our reefs, it was positively abundant.

Diadema the Echinoderm
Diadema is in fact a slow moving, bottom dwelling plant grazer of the echinoderm ("spiney skin") family. It lives on and around reefs, preferring places where it can find plenty of shelter. It is rather cosmopolitan in its food preferences and will eat just about any plant material it comes across. Diadema is also one of the most sociable invertebrates you will find on our reefs. During the daytime it loves to aggregate into dense prickly huddles. Most Diadema sport jet-black spines, but sometimes you will see them with white or with black and white striped spines. The colour variation seems to be determined by light level. In very bright light Diadema will routinely be black, lower the light level and it will tend to have whiter spines.

This little creature is capable of some truly stunning feats of survival. Did you know, for example, that it can live for up to six months without food. Or that their larvae can live in the plankton for up to 60 days. This is an absurdly long time by anyone's standards.

Diadema die off
Then, without any warning, between January 1983 and January 1984 populations of Diadema throughout the Caribbean mysteriously sickened and died. In less than a week they went from being perfectly healthy to developing "sticky" spines, shedding their spines and dying in hordes. The pathogen which caused this mass mortality event has still not been identified. It traveled with terrifying swiftness from an epicenter in Panama following the main circulation patterns to Florida, Bermuda, St Croix and finally and St Vincent and the Grenadines leaving destruction in its wake. Scientists speculate that it was originally carried through the Panama Canal by shipping and it seems to have arrived here in the Antilles in the same way.

The consequences …
The consequences for Caribbean coral reefs were dire. In places like Jamaica, with few grazing fish due to chronic overfishing, the reefs underwent what has been called a "phase shift". In other words healthy coral reefs rapidly turned into an algal wilderness. On Bonaire we were lucky that parrotfish moved into the niche vacated by Diadema, cropping the algae and keeping our reef from becoming overgrown.

In nearly 20 years there has been no significant recovery of Diadema … until now. And finally it seems to be making a tentative come back. It is certainly starting to reappear on Jamaican reefs and you won't have to look too far to find Diadema here either. But their come back is patchy and it's still not clear whether they are here to stay. With all the multiple pressures on our reefs you can understand why scientists are starting to get really excited about the Diadema come back.

Divers and snorkellers: I do believe now is the time to strike up a whole new relationship with our prickly friend, Diadema!