MEPS 168:163-186 (1998)  -  doi:10.3354/meps168163

Tentacle structure and filter-feeding in Crisia eburnea and other cyclostomatous bryozoans, with a review of upstream-collecting mechanisms

Claus Nielsen1,*, Hans Ulrik Riisgård2

1Zoological Museum (University of Copenhagen), Universitetsparken 15, DK-2100 Copenhagen, Denmark
2Research Centre for Aquatic Biology (Institute of Biology, Odense University), Hindsholmvej 11, DK-5300 Kerteminde, Denmark

ABSTRACT: The upstream-collecting filter-feeding mechanisms occurring in many aquatic organisms are not adequately described. Tentacles of Crisia eburnea and other cyclostome bryozoans, with only lateral and laterofrontal ciliary bands, are among the least complicated upstream-collecting systems among metazoans. SEM and TEM revealed that the tentacles have 2 rows of very closely set lateral cilia and 1 row of stiff laterofrontal cilia on each side. The shape of the basal membrane and the longitudinal muscles indicate that the tentacles are specialized for flicking movements directed towards the centre of the tentacle crown. Video observations of C. eburnea feeding on Rhodomonas cells showed characteristic velocity gradients around the tentacle crown. Particles in the central current gain the highest velocity at the entrance to the tentacle crown from where the speed decreases to zero in front of the mouth. Usually the path of a particle deviates from the downward course to a more outwards directed course (between the tentacles), where they may be trapped by the filter formed by the stiff laterofrontal cilia; the tentacle then makes a flick that brings the particle into the central current and further down towards the mouth. A survey of the literature shows that a similar mechanical filter mechanism occurs also in gymnolaemate bryozoans and their cyphonautes larvae, but that the particle-collecting mechanism of larvae and adults of phoronids, brachiopods, pterobranchs, and enteropneusts is different. The differences in structure and function between the tentacles of ectoprocts and those of phoronids, brachiopods and pterobranchs support the idea that the 2 types of tentacle crowns are not homologous.

KEY WORDS: Ectoproct feeding · Ciliary filter · Sieving · Bio-fluid-mechanics · Video observations · Ultrastructure · Phylogeny

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