The effect of Sargassum on settlement of Diadema antillarum larvae around Saba
Before its mass-mortality in the 1980’s, Diadema antillarum was the most important benthic herbivore on Caribbean reefs. Since then, reefs have experienced an increase in turf/macroalgae abundance. Despite the approximate 40 years that have past, natural recovery is slow and variable. Therefore efforts are taken to boost D. antillarum recovery artificially. Several bottlenecks for recovery have been found, but in the last decade a new potential problem has risen: massive influxes of floating Sargassum species. Studies have shown that algae or its biofilm can induce sea urchin settlement to a certain extent, but no in vivo experiments have been conducted for D. antillarum. Here we investigated if these Sargassum floats interfere with the recruitment of D. antillarum by measuring the abundance of D. antillarum settlers in submerged and floating, rinsed and unrinsed Sargassum units and comparing values with a positive control. While the amount of data collected with the low sample sizes did not suffice to answer the research question in any statistically significant manner, hypothesis could be formulated.
D. antillarum certainly is attracted to sargassum patches, whether as post-settler only or also as settler is however still debatable. Cues by conspecifics could possibly play a role in increased settlement in Sargassum, blurring the effect of Sargassum solely. A large variation in D. antillarum abundance was found for the unrinsed submerged treatment, which was possibly the result of migration from nearby existing ecosystems with older D. antillarum into the experimental units, or it was an artefact of the low sample size. To confidently distinguish signal from noise in a similar study, some alterations to the research design need to be made and drifting sargassum patches need to be sampled to get a sense of D. antillarum settler abundance on naturally occurring Sargassum.