ABSTRACT
The Longosomatidae, a poorly known polychaete family, includes only 23 recognized species; in this study, based on morphometric and taxonomic analyses, we describe a new species with three morphotypes: Heterospio variabilis from the Gulf of California, Mexico. The specimens examined exhibit large morphological variations but were clearly separated from close species due to a unique combination of morphological characters: chaetiger 9 as the first elongated chaetiger, four to eight branchial pairs; chaetae from chaetiger 10 forming rings in two rows, posterior row with thin and robust capillaries, anterior row with subuluncini, aristate spines, acicular spines and thick acicular spines. With the discriminant analysis, carried out on 11 morphometric characters, the presence of three morphological groups were recognized (Wilks' lambda= 0.093, p = 0.0001). However, the variables selected to discriminate the specimens (partial Wilks' lambda > 0.57) were correlated to their size: number of branchiae, body width, prostomium width, rate length CH9/CH1-CH8, length CH1-CH8 and length CH9 (r > 0.5). So, we concluded that they belong to a single species with three morphotypes: morpho A with eight branchial pairs, morpho B with 5-6-7 pairs and morpho C with 4 pairs. No correlations between the distribution of the distinct morphotypes along the eastern gulf shelf and the environmental conditions where they settle were detected.