The functional morphology of feeding in three species of maldanid polychaetes

The functional morphology of deposit feeding relative to proboscideal morphology of the maldanid polychaetes Axiothella rubrocincta (Johnson, 1901), Clymenella californica Blake & Kudenov, 1974, and Praxillella affinis pacifica (Berkeley, 1929) from central California is presented. The proboscides of these species are externally and internally similar. They consist of a cuticulatized buccal mass, a bucco-pharyngeal junction and ciliated pharynx. A functional unit of the feeding process is the palpode because its secretions enable the proboscis to be effectively everted. These worms exploit die physical properties of the substratum by changing its dilatancy (and thixotropy) while feeding. The rapidly emerging buccal mass increases the dilatancy of the sediment and provides a penetration anchor. The buccal mass partially deflates and loosens the substratum just prior to the forceful extrusion of the bucco-pharyngeal junction. Sediment about the latter region becomes dilatant. The proboscis then shortens and widens, causing the substratum about the pharynx to become thixotropic while it becomes dilatant about the region of the proboscis proximal to the pharynx. The worms push their proboscides into the loosened sediment while pharyngeal ciliary currents and the undulating bucco-pharyngeal margin draw food into the pharynx. Pharyngeal cilia and the buccal mass function in particle selection.
The maldanids and arenicolids examined to date share similar and basic proboscideal morphologies and activities. It is suggested that the feeding mechanism described for the maldanids, A. rubrocincta, C. californica and P. affinis pacifica , will be found in some members of both families.