Walters Shoal's marine fauna and flora - The Walters Shoal's summit : an algal reef!
Among the members of the expedition, we were eight divers who would have the chance to look at the summit of the Walters Shoal ... What a privilege to be among the first to dive on this sea mont ! What a pleasure to glance at the earth, even underwater, in the far west of the Indian ocean! Despite much enthusiasm, I was worried about what we were going to discover on the bottom: a rocky bottom battered by the action of the swell and current? The canopy of an algal forest ? We were soon to discover a reef formed by seaweeds.
After jumping in the water and a descent into a deep blue, about fifteen meters above the bottom, we saw a dense concentration of pink spots punctuating the bottom. These spots are red algae that accumulate limestone in their cell walls. Because of their resemblance to coral they were named "corallines". Coralline algae are one of the most species-diverse groups of algae (~ 700 species). They are found in waters all around the world to depths exceeding 200 m when waters are particularly clear.
At the top of Walters Shoal, the Corallines constitute a dense network, forming three-dimensional structures that serve either as a substrate for the flora and fauna or as shelter for mobile animal species. In most tropical reefs, corallines grow between corals where they reinforce the reef by operating as a cement. On Walters Shoal we did not observe corals, and coralline algae were the main organism contributing to the formation of the reef. They grow on their top part, which faces towards the light and where are the cells which use the energy of the sun and the CO2 dissolved in seawater to form sugars and other organic molecules through photosynthesis. The lower layers of algae die but the calcareaous cell walls remain and form thick concretions. However, these calcareous structures are particularly brittle and when they are broken, they form a very white sand that accumulates in small grooves before descending the slopes of the mount and accumulating on its flanks.
Such algal reefs are therefore extremely fragile. They have already been recorded, in particular in the Mediterranean Sea, where they are commonly called coralligenous reefs and for which measures of protection are in the process of being established.