Alcyonacea are sessile invertebrates, which can significantly shape the boundary layer in coral reefs and rocky habitats. Ecological aspects in this taxon have been well studied in the Caribbean, Mediterranean, and Indo-Pacific. With few recent exceptions, studies in the Eastern Pacific focused on taxonomy. We present a quantitative assessment of Alcyonacea communities from the southern Tropical Eastern Pacific, based on video transects in the Marine Reserve El Pelado. Seventeen species from the Plexauridae (8), Gorgoniidea (8), and Clavularidae (1) were identified, comprising 6963 colonies dominated by Muricea (86.7%), particularly M. plantaginea (48.6%). The overwhelming dominance of M. plantaginea was the most striking and previously unreported community trait, which contributed to a moderate Shannon entropy (n = 31, H mean 1.40, SD 0.22), equitability (n = 31, HE mean 0.16, SD 0.4), and species diversity expressed as effective number of species (n = 31, mean 4.16, SD 0.87). Few common species overprinted a more variable and subtle community pattern among rarer species, suggested in agglomerative hierarchical cluster analyses. Four species (M. plantaginea, M. purpurea, M. fruticosa and Leptogorgia alba) had the strongest influence on site groupings in the correspondence analysis between a principal component analysis of a Hellinger-transformed Alcyonacea species matrix and substrate categories, with filamentous turf algae and crustose coralline algae being the main determinants of site differentiation. Muricea plantaginea’s qualities of a keystone species, and the eurytopic and stenoecious distribution traits among some species are discussed. The invasive Carijoa riisei was confirmed as biological thereat to other Alcyonacea, and possible physiological distribution limitations are indicated.