Massive accumulations of pelagic species of Sargassum have generated recent social, economic and ecological problems along Caribbean shores. In the Mexican Caribbean, these events have prompted the study of diverse biological and ecological aspects of these macroalgae. However, studies on their associated biota, including Hydrozoa, remain scarce. This research provides important species observations in an area where data is lacking. The occurrence and percent cover of hydroids on Sargassum thalli collected on the beach at Puerto Morelos, Quintana Roo, Mexico from April 2018 to March 2019 was studied. Three pelagic species and morphotypes of Sargassum from this area were analyzed: Sargassum fluitans III, S. natans I and S. natans VIII, as well as a benthic species, S. polyceratium var. ovatum. A total of 14 taxa of hydroids, belonging to the superorders “Anthoathecata” and Leptothecata, were identified. In our study, more hydroid taxa were observed on axes of the different species of Sargassum than on leaves or aerocysts. In general, the greatest species richness of hydroids was observed from February to April. Results show that live hydrozoans attached to pelagic Sargassum are transported into the area. This should be considered in future management measures that address the recurring coastal abundance of Sargassum and its associated biota in the Caribbean region.

Here, we provide the first list of shallow-water hydroids for Alacranes Reef, Mexico, comprising 49 species in 33 genera. The most speciose superorder was Leptothecata (36 species), with three particularly diverse families: Clytiidae, Sertulariidae, and Plumulariidae. Three species were recorded for the first time in the Gulf of Mexico (GoM), and 13 for the south-central GoM. The majority of fauna was substrate generalist with specific distributions: 38 hydroid species occurred on natural reefs, 18 on artificial reefs, 9 on drifting Sargassum spp., and 8 on the seagrass Thalassia testudinum K.D. Koenig. Our results significantly increase knowledge of the hydroids from Mexican waters. This is an important step toward detecting non-native species in the near future, as well as for improving and designing new conservation programs in the area.

The exoskeleton is an important source of characters for the taxonomy of Hydroidolina. It originates as epidermal secretions and, among other functions, protects the coenosarc of the polypoid stage. However, comparative studies on the exoskeletal tissue origin, development, chemical, and structural characteristics, as well as its evolution and homology, are few and fragmented. This study compares the structure and composition of the exoskeleton and underlying coenosarc in members of “Anthoathecata” and some Leptothecata, but does so mainly in bougainvilliid polyps histological analyses. We also studied the development of the exoskeleton under experimental conditions. We identified three types of glandular epidermal cells related to the origin of the exoskeleton and the secretion of its polysaccharides component. The exoskeleton of the species studied is either bilayered (perisarc and exosarc, especially in bougainvilliids) or corneous (perisarc). The exoskeleton varies in chemical composition, structural rigidity, thickness, extension, and coverage in the different regions of the colony. In bilayered exoskeletons, the exosarc is produced first and appears to be a key step in the formation of the rigid exoskeleton. The exoskeleton contains anchoring structures such as desmocytes and “perisarc extensions.”

Acanthophora spicifera, a red alga considered an alien species, was discovered for the first time on the Pacific coast of Mexico in 2006 from a locality inside La Paz Bay, Gulf of California. Since then, more records have shown its presence, 17 localities having been added up to 2015. A two-year field study (2020-2022) visiting 31 sites along the coast of La Paz Bay, complemented with data from literature and citizen science, resulted in a database of 709 entries that spans the data from 2004 to 2023. These data showed a distribution that goes from Punta Coyote, close to Boca Grande, the northern entrance to the Bay to Playa Tecolote in the south, more than 100 km of coastline, including Espiritu Santo Archipelago, an area considered a natural reserve since 2007. Anthropogenic activity and environmental variables did not present statistical differences that explain A. spicifera spreading. It represents a naturalised alien species without evidence of a negative impact. Still, it soon could acquire the status of invasive species together with its epibionts Bryozoa and Hydrozoa detected in this study.