Resumen Se realizó un análisis biogeográfico y de diversidad a partir de la distribución geográfica de 118 especies de grandes macroalgas (Chlorophyta, Ochrophyta y Rhodophyta) presentes en el golfo de México y Caribe mexicano, con el propósito de identificar patrones de distribución que puedan resultar en posibles áreas de conservación y robustecer a las previamente establecidas. El análisis de parsimonia de endemismos (PAE) resultó en 9 áreas definidas por sinapomorfías y 4 más, definidas por autapomorfías. El análisis de trazos permitió identificar 2 trazos generalizados y 1 nodo biogeográfico ubicado en el arrecife Alacranes, resultado de la complejidad geológica del área de estudio. Dicho nodo corresponde a una de las áreas con mayor complejidad taxonómica para el golfo de México, además de albergar el mayor número de especies y junto con el sur de Tamaulipas presentan los valores más altos de complementariedad. Con base en la integración de los análisis anteriores, se propone como área a conservar al arrecife Alacranes, que además coincide con uno de los sitios prioritarios marinos considerado de extrema importancia para la conservación. Este trabajo constituye la primera aproximación biogeográfica en el estudio de las macroalgas marinas bentónicas en México y representa una primera etapa bajo el enfoque de la biogeografía evolutiva.

Species recognition in algae is often extremely difficult due to the paucity of morphological characters and high environmental plasticity. If environment is important to morphology, then related species growing in sympatry are good candidates to discover diagnostic characters. We studied algae collected in sympatric populations in tropical Atlantic Mexico. Hydropuntia cornea and H. usneoides have high morphological variation and have been difficult to diagnose morphologically. We used four molecular markers (RuBisCo spacer, cox2-3 spacer, rbcL and COI) to investigate the genetic relationship between samples that correspond to either H. cornea or H. usneoides; in addition, we determined if molecular-characterized groups were morphologically distinguishable. RuBisCo and cox2-3 spacers revealed low genetic variation but showed two genetic groups: groups I and II. rbcL and COI phylogenies also showed a separation into two groups, corresponding with cox2-3 spacer groups I and II. Group I matched sequences in GenBank of H. usneoides and group II with H. cornea. Populations were mixed for these genetic groups, with group I prominent in Quintana Roo populations. Morphological analysis of samples in both genetic clades showed that they are not distinguishable. As the difference between the two groups is only genetic, they are, consequently, cryptic species. While the extremes of morphology in allopatric populations may be distinguishable, we do not feel that in most cases these species can be recognized. Therefore, we propose that these two 'species' should be designated as a species complex (the Hydropuntia cornea/H. usneoides complex).

Red algae are the most conspicuous component in algal drifts that periodically arrive on the coasts of the Yucatan Peninsula. Given the presence of agars and carrageenans in their cell wall and the synthesis of secondary metabolites that act as antitumors or antioxidants, most of these species are highly valued in the international market. However, in this region of Mexico they are not used but represent a problem of waste from beaches. Our aim was to determine the composition of marine red algae of economic use in the algal drifts of the Yucatan Peninsula, providing brief descriptions, photographs that facilitate their identification and possible explanations for their causes and routes of origin. 13 samplings were carried out at 14 sites, collecting fresh and complete thalli from which the species of economic importance were described. 24 species distributed in six orders and seven families were identified. The most abundant families for both the number of species and the amount of biomass were Gracilariaceae (10 species), Rhodomelaceae (6 species) and Solieriaceae (3 species). The localities with the greatest diversity and biomass were those belonging to Campeche: Sabancuy (11 species), Punta Xen (14 species), Tortuga Bay (13 species) and Playa Bonita (13 species). From the present study we contributed six new records endemic to Campeche, including the first record of the genus Codiophyllum for the Western Atlantic Ocean. Our data on diversity, biomass, periodicity and growth rates of red algal drifts allow us to suggest that the Campeche and Yucatan drifts are a sustainable source of raw material.

The recognition of many species of Sargassum has been difficult because morphological characters show significant phenotypic variation depending on habitat, season, or even differences at the population level. To resolve some of these systematic issues along the Mexican Atlantic coast, molecular analyses of Sargassum species were carried out using DNA sequences of the COI-5P, ITS-2 and rbcL_S genetic markers. Of the 16 species recorded for the area, five species (S. acinarium, S. mathiesonii, S. pteropleuron, S. pusillum and S. ramifolium) were not found during the collections and their status remains undefined. Phylogenetic analyses of the three molecular markers showed that 10 species with their varieties (S. bermudense, S. buxifolium, S. cymosum, S. filipendula, S. fluitans, S. furcatum, S. hystrix, S. natans, S. polyceratium, and S. vulgare) were grouped into a polytomy, with low genetic diversity. If we consider these markers as suitable for proxy for species delimitation, these species should be synonymized under S. cymosum, the older name. Taxonomic independence was confirmed only for S. platycarpum and our data revealed a new morphologically and molecularly independent species within the genus, described here as Sargassum xochitlae.

Four species of Gracilaria (Gracilariaceae, Rhodophyta) with textorii-type spermatangial conceptacles (subgenus Textoriella Yamamoto) are recorded for the Gulf of Mexico and the Mexican Caribbean: Gracilaria blodgettii, G. cervicornis, G. mammillaris, and G. tikvahiae. The general distribution of the subgenus for Central America, both Pacifi c and Atlantic, displays a disjunct pattern explainable based on the geologic vicariant events that interrupted the connection between Pacifi c and Atlantic at the Isthmuses of Panama (closed 3.1-2.8 million years ago), and Tehuantepec (southern Mexico, closed 4-3.5 million years ago). Gracilaria cuneata/G. crispata, and G. mammillaris (G. hayi)/G. veleroae are 2 pairs of sibling species, or sister taxa, that diverged as a result of the fi nal emergence of the Isthmus, and of the same age as the Central American Isthmus itself.