Could the spiny grazer be the saviour of the Caribbean coral reefs?
Ah, the Caribbean! White, sandy beaches, warm waters, pirates and rum… As a final year zoology student, I got an amazing chance to do my dissertation in Honduras on the black long spined sea urchins, which, despite their lack of popularity among the public due to the terrible pain they can cause if you accidentally step on one, are one of the most important species on the Caribbean reefs. The Diadema antillarum is a herbivorous grazer of algae on coral reefs. The importance of this urchin as a keystone species was highlighted when it experienced a mass mortality event in the early 1980’s due to a waterborne pathogen that wiped out up to 99% of the populations in some localities. And as of yet, there has been very negligible recovery. This, along with other factors such as storm damage, coral bleaching and El Niño events, has helped pave the way for phase shifts from coral dominated reefs towards algal domination – pre-1982 surveys reported algal coverage on Caribbean reefs between 0 to 10%, with a mean of 6.6%, while post mortality surveys are generally reporting an algal cover up to four times that high.
While it is unlikely that losing the long spined sea urchins is the only reason we are seeing these phase shifts happening in the Caribbean, they are possibly one contributing factor that we can do something about. Studies undergoing in Honduras by Operation Wallacea are comparing two distinct populations of these urchins; one in the waters surrounding the bay island Utila, and one in the Banco Capiro reef, located 8km north from the coast town of Tela. The two reefs are only approximately 60km apart, but Utila has a low density of urchins and a coral and algal coverage resembling that of an average Caribbean reef on its way to algal dominance, while Banco Capiro boasts one of the highest post-mortality densities of urchins in the Caribbean, as well as an unusually high scleratinian coral coverage. Because of the close proximity of the two sites, it is a prime opportunity to try and tease out the reasons of recovery in one site over the other. One of the reasons is thought to be the level of habitat complexity. Higher heterogeneity of habitats on coral reefs is usually credited to the diversity and dominance of hard corals. When these hermatypic corals grow, they deposit a calcium carbonate skeleton, varying in complexity between species; some grow to form plate like structures and others build fragile but complex horn-like structures. The more different types of hard corals you have, the higher the heterogeneity of the habitat, which leads to more different kinds of microhabitats that can again support more species and individuals, including a higher number of urchins. But here comes the twist: in order to get a higher diversity of corals, one must have free substrate for the corals to settle on, and this is often a result of sufficient numbers of herbivores like D. antillarum which are removing young algal growths that are taking up the space. So, which came first? It’s a question much like “Which was first, the chicken or the egg?”, and the truth is that we don’t know. However, knowing more about one key species on the reefs will contribute towards finding the answer to the ultimate question.
This leads me to our study this summer, which concentrated on the predator avoidance response (PAR) of the urchins. For a benthic invertebrate with no eyes or ears or nose, how does it know when a predator is around? The answer: photopigments on their skin, that initiate a signalling pathway in response to a change in light intensity (i.e. the shadow of a predatory fish) that leads to muscle contraction and results in a fast “wiggle” of their long, barbed, defensive spines that are laden with toxins! One hypothesis suggests that urchins are more prone to predation in the flatter, less complex environments resulting from the phase shifts, and this could further be complicated by the predicted increases in sea water temperature under climatic change. We wanted to know if the environment in which the urchins were in had any impact on the intensity of the PAR, so we designed an experiment to test the potential effects on PAR of different temperatures, different reef materials and different reef complexity levels in a laboratory setting, using urchins from the healthier Banco Capiro population. Using video recordings we quantified the percentage of long spine movement in response to a “predator” a.k.a. a wooden board placed over the tanks, and we are currently in the process of analysing the results. Hopefully, our results will help reef conservationists make informed decisions and design effective management plans to ensure the healthy future of Caribbean coral reefs.
However, my summer in Honduras was not all about urchins. I had the pleasure to go and explore the mangrove forests, to snorkel in two different locations multiple times, and to learn more about the other projects going on in the Tela Marine Research Centre where I was based. For more on the studies of the invading lionfish, or the cool Cleaning Station projects, you can see my blog from Honduras.