Distribution and range expansion of the invasive soft coral Chromonephthea (Octocorallia; Malacalcyonacea; Nephtheidae) in the southwest Atlantic

Abstract The introduction of nonnative octocorals (Octocorallia) is becoming increasingly more common around the world. In Brazil alone, we know of nine nonnative or cryptogenic species of octocorals representing eight different families. The nephtheid Chromonephthea braziliensis (a pseudoindigenous species) was probably first introduced in Brazil in 1994 and has since appeared in three distinct regions on the northeast and southwest coast. The aims of this study were to review and describe its current distribution, verify the taxonomic composition of populations, and identify probable vectors and pathways of introduction. In 2024, we assessed seven of fourteen known populations along the entire extent of the known species distribution. Population sizes were estimated, species interactions noted, and specimens collected. Colony morphology and color were assessed by examining sclerites, and DNA sequences were obtained for the mitochondrial mtMutS and nuclear 28S rDNA barcode markers. Genetic and morphological analyses confirmed the presence of two different species of Chromonephthea in Brazil, suggesting multiple introductions through fouling on hulls of oil and gas platforms. These species can be most easily distinguished by the color of their polyp sclerites, which are yellow in C. braziliensis and red in the other, as yet unidentified species. Both negative (contact necrosis) and positive (basibiont) species interactions were recorded.