Particle capture and processing mechanisms in Sabellaria alveolata (Polychaeta : Sabellariidae)

Sabellaria alveolata, a ciliary suspension feeder, is a reef-building polychaete widely distributed in European waters, and is an important element of the trophic web of the ecosystems they have colonised. The lack of data concerning the feeding mechanisms of polychaete sabellariids is an impediment to understanding their roles in these systems. In this study, we combine a descriptive approach based on SEM observations and histological mapping of mucocytes with video endoscopic recordings to provide a functional interpretation of particle capture and processing mechanisms. It is hypothesised that particle capture in S. alveolata is based on the beat of grouped cilia. The cilia types and distributions of tentacles are described, and both downstream and upstream collecting mechanisms are demonstrated in this species, Other feeding characteristics, such as the flicking of tentacles, are involved in particle capture. Feeding mechanisms suggest previously unsuspected similarities with lophophorids. The role of mucus in S. alveolata was characterised in the tentacles, grooves and buccal cavity, and the functional specialisation of organs in transport or rejection of particles was confirmed with video recordings. Tentacles and grooves collect and transport particles towards the mouth, and only viscous acid mucopolysaccharide-secreting mucocytes occurred on these organs, protecting particles from being removed with currents. The presence of mixed mucocytes on the cephalic floor, situated just posterior to the mouth, suggests that this area is involved in rejection of pseudofaeces. The implications of these results are discussed in terms of environmental adaptation and are compared to other families of ciliary suspension-feeding polychaetes.