The Ha Long Bay marine ecosystem. An unprecedented opportunity for evolutionary studies on marine taxa.

TheHaLongBay(Vietnam)isashallowarealocatedinthenorthernpart of the Tonkin Gulf, in the South China Sea. It includes more than 3000 islands of variable sizes. A continuous action of karstic processes, initiating around 280 million of years ago, shaped the formation of extraordinary caves and shallow salt-water lakes. These impressive structures recall the marine lakes found in Indonesia and Palau, creating one of the most important UNESCO’s World Heritage Sites. The protected area covers a surface of 434 km2 and comprises 775 islands, forming a triangle with the Dau Go Island (Driftwood Island) to the west, the Ba Ham Lake (Three Shelter Lake) to the south, and the Cong Tay Island to the east. During the Pleistocene, multiple sea level rise episodes that span for 10,000 – 15,000 years contributed to the geographic isolation of marine taxa. Along the coasts of Ha Long Bay islands, several coral species formed very shallow reefs that host peculiar sponge assemblages adapted to low depth and murky waters. In the marine lakes instead, corals can be quite rare while sponges are common. Their diversity though, is only known for a few lakes. Extreme variations in environmental conditions occur yearly in the lakes, due to heavy monsoon rains that cause stratification of the water column and drastic alterations in the communities. The aim of this study is to review the sponge diversity inside marine lakes and along the coastal areas of this peculiar region, comparing data collected more than fifteen years ago with recent data. During three expeditions between 2003 and 2004 we recorded 63 demosponges, of which 46 were living in and outside marine lakes, 17 were characteristic outside the lakes, and 23 were exclusive inside the lakes. After 15 years, a survey in August 2018 was performed to combine morphological data with DNA barcoding, to reveal and enhance the value of the biodiversity of the area. The diversity of sponges displayed drastic recession, showing the decline of more than half of the species typically present. In lieu, the exploration of a new lake (with very low salinity and high- water temperature) unveiled a seemingly new sponge species able to endure these conditions. Our findings suggest the urgency to develop measures of protection for these extraordinary but endangered environments, where biodiversity is still poorly explored. Indeed, these characteristic systems subjected to such intense geophysical isolating mechanisms represent optimal natural laboratories for the study of evolution and speciation of marine biota developing from segregated peripheral populations.