First observation of mole-like burrowing behavior observed in a sea pen

Abstract

Sea pens are benthic marine organisms, which have important roles as three-dimensional structures and habitats on sandy and muddy seafloors. Although the behavior of a few species of sea pens has been sporadically reported since the seventeenth century, and despite their presence across a wide range of habitats, the behavior of most sea pen taxa has not been well studied or documented. In this study, via video images of an Echinoptilum sp. specimen in an aquarium (shown in this study at 128-times speed), we observed unreported aspects of behavior, namely mole-like movement within the seafloor sand, and bending of the colony when emerging from the sand. Such mole-like behavior may be advantageous to avoid dangerous and other unfavorable situations, such as predation and competition for space. Aspects of Echinoptilum sp.’s morphology, such as a digitiform colony shape, lack of an axis, and the presence of retractable autozooids and three-flanged sclerites may assist in this behavior. Finally, it appears that peristalsis and active movements are common features for anthozoans in sandy bottom environments, and have so far been reported from sea pens, zoantharians, and scleractinian corals.