Sponge Spicules in Sediments: A Proxy for Past and Present Sponge Fauna Assessment in Ha Long Bay (Vietnam)

The study of cryptic sponges can be challenging, as they are difficult to detect in the environment without the use of destructive methods. The permanence of mineral skeletal components of Porifera in sediments is therefore a tool that can provide support for a more comprehensive study of sponge fauna. In this context, sediments collected from four karst lakes in Ha Long Bay (Vietnam), plus two surrounding sea areas, were processed to extract spicules belonging to sponges with siliceous skeletons. From the morphology of the spicules, it was possible to trace the taxa of the sponges present in the area at that time or earlier. It was found that each lake had a sponge fauna composition characterized by different families and genera. Moreover, sponge taxa identified in the surrounding sea areas differed significantly from the ones inside of the neighboring lakes. In addition, it was possible to identify the presence of four sponge genera not yet recorded in the South China Sea area. Thus, the study of spicules trapped in sediments proved to be a useful tool for a comprehensive study of sponge fauna.