Morphological characterization of anterior axial bifurcation in Holothuria (Halodeima) grisea and other puzzling occurrences in Holothuroidea

The present study explores rare but taxonomically and geographically broad cases of anterior body axial bifurcation in adult sea cucumbers (Echinodermata: Holothuroidea). The case of Holothuria (Halodeima) grisea, a common intertidal species along the Brazilian coast, was studied externally and internally. Individuals of Apostichopus californicus in the United States of America, H. (Mertensiothuria) leucospilota in La Réunion (France), H. (Panningothuria) forskali in Spain, and Parastichopus tremulus in Norway, were also reported to display similar anterior body axial bifurcation. In each of them, the anterior body presented two mouths with their respective crown of tentacles. In vivo observations of H. (Halodeima) grisea showed that the two tentacle crowns and mouths were independently functional. Micro-computed tomography (μCT) revealed that the anterior region of the body (up to the pharynx) was duplicated. Internally, two aquapharyngeal bulbs were found, each with their own calcareous ring (one with two plates missing), madreporite, and Polian vesicles. The body presented a normal set of five longitudinal muscle bands, some of which bifurcated orally to support more than one calcareous ring plate. This is the first report of such abnormalities in Holothuroidea and hypotheses that may explain such phenomenon are discussed.