Diadema, otherwise known as spiny sea urchins are a valuable part of any coral reef ecosystem. Within the past decade or so massive amounts of coral bleaching as occurred due to increasing water temperatures and nutrient run-off into the bays here in Jamaica. These conditions favor the growth of macroalgae which smother the corals on the reef and eventually kill them and also prevent new larval corals from establishing themselves. Diadema are the power source behind the control of the macroalgae coverage. They are intense herbivores that are normally found in dense packs grazing the macroalgae along the reefs.
My project is to determine Diadema grazing preferences and consumption rates of vegetation.Previous research shows that Diadema prefer to graze on red turf algae, but I want to determine their preferences if red turf algae is not available for consumption. I currently have two set-ups in the wet lab of 3 tanks a piece (total of 6 tanks). Each tank has an equal wet-weight samples of Halimedia, Sargassum, and Thalassia for the Diadema to graze on. Each trial runs for a 48 with observations being taken ever couple of hours and then the remaining vegetation is taken out of each tank and re-weighed. My results so far show that after one trial the Diadema have a preferences for Sargassum, a brown algae, over the Halimedia and Thalassia. But, I have observed noticeable grazing marks on the Thalassia within the tanks. I have also noted that most of the grazing activity happens at night when there are no lights on in the wet lab. Tomorrow trials 2 and 3 will be finished with their 48 hour grazing period and the samples will re-weighed and I’m hoping to get at least 2 more trials done before the end of my experiment.
Along with the laboratory experiment I’m also currently working with other students, Dawn, who is working on determining natural Diadema grazing preferences out in the field and Brian who is doing scuba transects for Diadema abundances. Dawn and I have set-up 4 cages (4 exclusion and 4 inclusion) out in Discovery Bay on algae patches. Surveys of the algal coverage were taken before three Diadema were placed in each of the inclusion cages. So far Dawn’s observations show that the field Diadema prefer to graze on red turf algae and have been grazing pretty heavily within the cages.
The last part to my research will be attempting to do some Diadema dissections to determine what they have been grazing on out in the wild. Professor Peterson showed me how to do the dissections and the first couple were pretty rocky for me, but I’m willing to give it another shot. I would like to divide up what the Diadema are grazing on into categories: brown algae, green algae, red algae, and sea grass.The dissection information combined with the lab experiments and the field experiments will hopefully show that Diadema do prefer red turf algae as their main source for grazing, but also that other types of vegetation are consumed when red turf algae is not available.