Global warming and ultraviolet radiation (UVR) affect marine organisms worldwide. However, we still lack knowledge about the consequences of these environmental changes to range-restricted macroalgae species, such asGelidium canariense, a habitat-forming endemic of the Canary Islands. The aim of this study is to provide useful data for analyzing the conservation status of this endemic species. To do this, (1) we compared distributional data from 1987 with the data collected in 2008 and 2019 along the northern coast of Tenerife; (2) we assessed the variation in macroalgae assemblages dominated byG. canarienseover the last decade on this same cost; and (3) we explored the correlation of these results with the actual trends of sea surface temperature (SST), air temperature at 2 m above the sea surface (T2m), UVR, and downward solar radiation (DSR). The results showed significant changes in the distribution ofG. canariensein the last 30 years at Tenerife. More than 90% of its populations have declined. The endemic macroalgae have been replaced by the globally distributed speciesPterocladiella capillaceaat nearly all the sites studied. The cover ofG. canarienseandGelidium arbusculadecreased significantly with increasing SST and T2m. Additionally,G. canarienseis negatively related to the rise in UVR. These results highlight the vulnerability of both species ofGelidiumto global warming and UVR, but especially ofG. canariense. The drastic decline of this species reveals the need to upgrade its conservation status and implement protection measures to reduce the damage that predicted global change scenario may have to their relict populations.

Is a canopy-forming endemic macroalga from the Canary Islands. Vegetative and reproductive characteristics of all phases in its life cycle are examined. Key macromorphological features are its blackish color and the corymbose-to-subfastigiate branching pattern that differentiates

Understanding and forecasting the effects of climate changes on vulnerable species are leading concerns for ecologists and conservation biologists. Herbaria are invaluable for use in long-term data series, and one of the few available methods for quantifying biodiversity changes over large periods of time. Gelidium canariense is an endemic and habitat-forming macroalga of the Canary Islands that coexists with two other habitat-forming Gelidiales: G. arbuscula and Pterocladiella capillacea. This study assesses long-term changes in thallus size and reproductive effort of all specimens deposited in the Herbarium of Universidad de La Laguna of these three Gelidiales species. Also assessed were the effects of seawater temperature and increased incident light on net primary production (NPP), and the effects of extreme desiccation conditions on the relative water content and NPP of the three Gelidiales species. The length of the thallus of the endemic species G. canariense was halved during the past 40 years. The shortening of the thallus coincided with a significant decrease in the number of reproductive structures in both Gelidium species. These morphological changes coincide with a significant increase of the sea surface temperature, air temperature above sea surface and ultraviolet radiation in the studied area. The experiments have revealed the deleterious effects of extreme desiccation and extreme irradiance on all three species. Hence, these results suggest that air temperature and irradiance are related with these morphological changes over time in the habitat-forming Gelidium species and that are most likely compromising the survival of their populations which are already declining.

Gelidium canariense and Gelidium arbusculum coexist in the upper sublittoral zone in the Canary Islands and are the only Gelidiales registered as vulnerable species. Spore germination and the formation of rhizoids are vital steps for the successful growth of new plants. We investigated the initial germination stages of tetraspores and the growth of the primary rhizoid in these two vulnerable species. Both Gelidiales exhibited Gelidium-type germination. However, liberated tetraspores of G. canariense and G. arbusculum germinated earlier than those of other species of the Gelidiales. The growth curves of primary rhizoids were similar in both species.