An integrative systematic approach to species diversity and distribution in the genus <i>Mesophyllum</i> (Corallinales, Rhodophyta) in Atlantic and Mediterranean Europe

<div><p>For the first time, a comprehensive assessment of <i>Mesophyllum</i> species diversity and their distribution in Atlantic Europe and the Mediterranean Sea is presented based on molecular (COI-5P, <i>psb</i>A) and morphological data. The distribution ranges were redefined for the four species collected in this study: <i>M. alternans, M. expansum, M. macroblastum</i> and <i>M. sphaericum. Mesophyllum sphaericum,</i> which was previously known only from a single maerl bed in Galicia (NW Spain), is reported from the Mediterranean Sea. The known range of <i>M. expansum</i> (Mediterranean and Macaronesia) was extended to the Atlantic Iberian Peninsula. The occurrence of <i>M. alternans</i> was confirmed along the Atlantic French coast south to Algarve (southern Portugal). <i>Mesophyllum lichenoides</i> was only recorded from the Atlantic, whereas <i>M. macroblastum</i> appears to be restricted to the Mediterranean Sea. A positive correlation was observed between maximum Sea Surface Temperature (SSTmax) and the depth at which <i>M. expansum</i> was collected, suggesting that this species may compensate for higher SST by growing in deeper habitats where the temperature is lower. The latter indicates that geographic shifts in the distribution of coastal species as a result of global warming can possibly be mitigated by changes in the depth profile at which these species occur. <i>Mesophyllum expansum</i>, an important builder of Mediterranean coralligenous habitats, may be a good target species to assess its response to climate change.</p>