A comparative study of populations of Ectopleura crocea and Ectopleura ralphi (Hydrozoa, Tubulariidae) from the Southwestern Atlantic Ocean

Abstract

Ectopleura crocea (L. Agassiz, 1862) and Ectopleura ralphi (Bale, 1884) are two of the nominal tubulariid species recorded for the Southwestern Atlantic Ocean (SWAO), presumably with wide but disjunct geographical ranges and similar morphologies. Our goal is to bring together data from morphology, histology, morphometry, cnidome, and molecules (COI and ITS1+5.8S) to assess the taxonomic identity of two populations of these nominal species in the SWAO. We have observed no significant difference or distributional patterns between the so-called Brazilian E. ralphi and Argentine E. crocea for both morphological and molecular data. Therefore, SWAO populations of Ectopleura belong to the same species. In a broader view, it is difficult to find decisive character distinguishing E. crocea from E. ralphi, and both species have indeed recently been synonymized, with the binomen E. crocea having nomenclatural priority. Geographically broader genetic analysis should be carried out in order to test the validity of this synonymy because taxonomical procedures such as studying type specimens and documenting broad phenotypic variability have not yet been conducted.

Areas of endemism in the Southwestern Atlantic Ocean based on the distribution of benthic hydroids (Cnidaria: Hydrozoa)

“…This potential barrier might hinder larval transportation/advection (or survival) between regions, restraining colonization. Another putative barrier is the La Plata River, though this freshwater input does not seem to be a real barrier in more recent times, including different populations of hydroids (e.g., Imazu et al 2014). Last but not least, a potential barrier would be the availability of hard substrate at the southern Brazilian coast, mainly between 30-34°S, which is a region characterized by the lack of rocky shores.…”

Deep-Sea Anemones (Cnidaria: Anthozoa: Actiniaria) from the South Atlantic

“…Until now, however, these species had not been recorded north of Rio de La Plata, a proposed biogeographic transition zone due to the collision of the warm Brazilian Current and the colder, mostly sub-Antarctic, Malvinas Current creating the subtropical Atlantic convergence (30° S-36° S) and the South Atlantic Central Water (SACW), which flows northward beneath the Brazilian Current (Acha et al, 2004). Although the Rio de la Plata acts as a strong barrier for shallow water sea anemones (Zamponi et al, 1998), some species can cross this barrier (e.g., Antholoba achates Drayton in Dana, 1846) as well as other cnidarians (e.g., hydroids, Imazu et al, 2014). Antholoba achates is even more widespread in South America than A. lacteus and H. pectinata, being found from Peru to Cabo Frio in Brazil (Riemann-Zürneck, 1986), a distribution that corresponds to the temperate South America realm of Spalding et al (2007) for shallow water species (0-200 m).…”