Population structure and morphological characteristics of the sea pansy Renilla amethystina in the Mexican Tropical Pacific

Renilla amethystina is a colonial cnidarian distributed throughout the Eastern Pacific. They are infaunal organisms found buried in soft bottoms, characterized by a rachis and a peduncle. This paper presents information on the population structure and ontogenetic development with morphological and color changes based on records obtained in Bahía Chamela, Jalisco, in the Mexican Tropical Pacific. Population density and size structure were determined at six sites within the bay between December 2013 and May 2017. The average density for the bay was 0.46 ± 0.20 col/m2, with the highest values in Villa Polynesia (1.13 col/m2) and El Embarcadero (1.08 col/m2). Distribution pattern of the colonies in the bay was aggregated. The abundance of small colonies was higher than that of adult colonies. The size structure shows a unimodal distribution based on maximum rachis length (12.2 ± 0.5 mm) and for the maximum length of the mid-dorsal tract (9.1 ± 0.3 mm). It was found that this species presents a negative allometric growth. The young colonies showed differences in their body shape and coloration pattern with the adults attributed to ontogenetic changes. Some albino colonies were recorded. It is suggested that efforts be directed toward ecological and taxonomic studies of sea pansies in order to recognize their ontogeny, diversity, and distribution patterns.