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Kelps are among the most studied groups of macroalgae globally, as they are large foundation species that form some of the world's most productive and diverse marine habitats. Yet little is known about diversity and ecology of tropical kelps, which tend to inhabit the more difficult to survey mesophotic zone. Eisenia galapagensis is the first tropical kelp species to be recorded globally, and is an endemic and endangered species to the Galapagos. Here, we reveal the presence of previously unknown off-shore mesophotic kelp populations in the Galapagos, made up by a potentially new Eisenia species for the region, and present phylogenetics and morphological analysis to support our claim. We find that these novel populations form extensive kelp forest habitats with a kelp density of 2.6 ± 1.5 m.sup.-2 at depths of 48-67 m, where mean water temperatures are 3-5 â cooler than at the surface (18.1-19.9 â). Despite low levels of light and macronutrients (measured in August 2019), observations of juveniles and sori on blades suggests these kelp populations are successfully reproducing and recruiting. Our findings provide information about foundation species and complex marine communities previously unknown to managers of the Galápagos Marine Reserve. They also support the notion that this region is uniquely important for harboring brown macroalgal diversity. Our study highlights critical knowledge gaps on the distribution of primary foundation species in colder mesophotic zones, and how their invisibility from the surface prevents their inclusion in current conservation strategies., Author(s): Salome Buglass [sup.1] [sup.2] , Hiroshi Kawai [sup.3] , Takeaki Hanyuda [sup.4] , Euan Harvey [sup.5] , Simon Donner [sup.2] [sup.6] , Julio De la Rosa [sup.7] , Inti [...]