Are corals plants or animals? Well, they are a bit of both. A few hundred years ago scientists used to think they were plants, because they are sessile (fixed in one place) and require photosynthesis to survive, but we now know that they are animals made of little animal organisms called polyps.
Coral structures that we see underwater are actually hundreds or millions genetically identical polyps living in compact colonies.
Coral polyps are evolutionary cousins of jellyfish. Think of them as tiny upside-down jellyfish. Of course they differ from jellyfish, which drift with the currents; instead these polyps are fixed to one spot and build calcium-carbonate skeletons that shelter them.
What also makes corals different from most animals is that they live in a symbiotic relationship with tiny algae that live inside the skin of the polyps. The algae get safety and in return provide the coral with photosynthetic energy.
Coral reefs are the rainforests of the sea. While they only extend over one per cent of the ocean, they host over 25 per cent of all known marine life. These ecosystems are also of vast economic importance.
In the case of Tobago, research done by the World Resource Institute in 2006 valued the contribution of reefs to Tobago's economy between US$100-130 million annually. Tobago's fringing coral reef systems are of critical importance for Tobagonian livelihoods. They attract tourist dollars, provide food and income through coastal fisheries and protect shorelines from storms, which prevents erosion of beaches and coastal properties.
Think of a coral reef, and you probably imagine a blaze of colour. However, coral is in fact white! Remember the dead bits of coral you have picked up from beaches? Coral polyps are actually translucent and it's the micro-algae that give corals their colour.
When something happens to upset the balance between the coral-algal symbioses, corals react by ejecting the algae. For instance, if you have high temperatures, it messes up the symbiotic relationship. Algae go into overdrive production and produce a lot of oxygen radicals. Oxygen radicals are poisonous to coral polyps, so the polyps kick out their now-toxic tenants.
When this happens the corals turn white, in an event called "coral bleaching." When they are bleached they are starving. If warm waters persist for too long, first corals experience partial mortality, but eventually they will all die.
Corals bleach for many reasons. Like when sedimentation blocks sunlight, and algae can't photosynthesise, and become a burden on the polyp. Or when salinity levels change, or it gets too cold.
In the last two decades climate change has made air and sea temperatures warmer, and in extreme events it causes entire coral reef systems to bleach, and thus are called mass bleaching events. Some of the most severe mass-bleaching event in the Caribbean occurred in 2005, and again in 2010. Tobago reefs were not spared.
At the Gulf and Caribbean Fisheries Institute (GCFI) conference in Barbados, a few weeks ago, I caught up with Trinidadian biologist Salome Buglass, a student at the University of British Columbia, who has done research for her master's thesis on the effects of coral bleaching on Tobago's reefs, after the 2010 event. Buglass focused her research on Buccoo Reef, which is a long-ignored Marine Protected Area (MPA) that is adjacent to urbanised land and that has a water treatment plant that spills raw sewage onto the reef, and the more remote Culloden and Speyside reefs. She wanted to know how the main reef-building corals, which are essential to the existence of a reef structure, were recovering since the last bleaching event. She did this by looking at the juvenile coral population.
Worryingly, she found very low juvenile numbers, especially of the important reef-building species.
Buglass says: "Conditions for juveniles are very harsh. I only found five juveniles per square metres, in comparison to the 15-17 juveniles per square metre on a healthy reef. To make matters worse, most juveniles are not of the reef-building species. The important large growing corals do not appear to be successfully reproducing."
Anthropogenic climate change is often cited as the main reason for the degradation of coral reefs, but Buglass points out that so many things negatively affect the resilience of Tobago's reefs.
"Corals need nutrient- and sediment-poor waters. They need light and they need temperatures between 25-29 degrees C.
"We have greatly altered the ecosystem, making the sea warmer, more nutrient- and sediment-rich and we are taking away the herbivores, like parrotfish, that keep the reefs clean from algae. For a while algae-grazing urchin replaced the role of herbivore fish, as urchin predators also became fished out.
"With no predators to check their growth, the spiny sea urchin population exploded. The urchin population became so dense that disease could easily spread through the population. This happened in 1983 and it resulted in a mass die off of spiny urchins, and increase in algal growth on reefs.
"Overall since the 1980s Caribbean coral reefs, like the ones in Tobago, have lost a lot of coral cover.
"We need to focus on stopping anthropological threats. Climate change is one of those, but locally we have little control over global greenhouse gas emissions. What we can control is overfishing, sediment runoff and excess nutrients entering the marine ecosystem. The Caribbean must focus on reducing local impacts. One of the best ways to do that is through the establishment of functional marine protected areas."