Floristics and Biogeographical Affinity of Diatoms Attached to Sargassum fluitans (Børgesen) Børgesen and Sargassum natans (Linnaeus) Gaillon Arriving on Mexico’s Caribbean Coasts

The environmental contingency caused by the recent massive arrivals of pelagic sargasso (Sargassum natans and S. fluitans) on Mexico’s Caribbean coasts have given rise to several areas of scientific research. Our work proposed identifying the diatom flora adhered to the thalli of these two sargasso species collected on the coasts of Cancun, Isla Mujeres, and Puerto Morelos. We recorded 184 diatom taxa (all illustrated) from 68 genera. Taxa from the genera Mastogloia (37), Cocconeis (11), Nitzschia (10), Diploneis (8), and Amphora (9) represented 41% of the total, while 44 of these genera were represented by a single species. In the total floristic count, 41 taxa occurred exclusively on S. fluitans, 53 exclusively on S. natans (22 and 29%, respectively) and 90 (49%) were found on both. Species of Navicula were scarce, and Navicula barbara var. densestriata was here transferred to the genus Lyrella (Lyrella barbara var. densestriata (Foged) López-Fuerte & Siqueiros Beltrones comb. nov.). Overall, 17 (9%) of the identified taxa were new recordings for Mexico’s coasts. Supporting the hypothesis proposed, the 37 Mastogloia taxa suggested a tropical affinity, while the high species richness denoted that the surfaces of both sargasso species constituted favorable substrata for the growth of diatom assemblages.