Withdrawal behaviour of the red sea pen pennatula rubra (Cnidaria: Pennatulacea)

Aggregations of sea pens are important soft-bottom communities providing a three-dimensional complexity from which several associated species can benefit. The red sea pen Pennatula rubra is one of the Mediterranean coastal field-forming sea pens able to establish dense aggregations on the sandy/muddy bottoms of the infra-and circumlittoral zones. This species was first described at the end of the 17th century, but since then little information has been published about its biology, ecology and biogeography. Even less is known about its behaviour, its reactions after disturbance and its possible escape strategies. Several species of pennatulaceans can withdraw partially or completely into the sediment, usually in a fast (i.e. a few seconds) process of polyp closure and expulsion of part of the water contained within the colony. The present study reports and discusses the withdrawal behaviour of P. rubra after disturbance. This behaviour has never been documented before in this species. It proved to be a slow process requiring between 210 and 340 seconds (3 min 30 sec to 5 min 40 sec) for the complete withdrawal. Moreover, a soft bioluminescence was observed in two undisturbed colonies in the study area, while two other colonies were found to be out of the sediment, inflating themselves with seawater and getting carried by currents as a sort of dispersal behaviour.