Feeding and particle selection in the echiuran worm Bonellia viridis Rolando (Echiura: Bonelliidae)

Abstract

The feeding behaviour of Bonellia viridis Rolando is described. Small individual particles ( ⪕ 94 ± 25 μm ) are taken onto the proboscis by means of cilia. Individual particles of intermediate size (150±40 μm) are taken up by means of a combination of ciliary and muscular activity. The larger particles (230 ± 60 to 290 ± 60 μm) are picked up by muscular action. When presented with large accumulations of particles, the terminal lobes pass them onto the ventral surface of the proboscis by active muscular action. The ciliary currents on the ventral surface of the proboscis have been mapped. Particles are carried from the terminal lobes of the proboscis to the mouth via the stem gutter and boluses may be formed at the neck region. Transport along the proboscis may be ciliary for the smaller particles ( ⪕ 290 ± 60 μm ), ciliary and muscular for the intermediate sizes (230±60 to 480 ± 70 μm) or purely muscular for the largest particles (550 ±140 μm). Particles may be rejected at various levels along the proboscis, the site of rejection also being dependent on size. The density of ciliation in the various regions of the proboscis has been estimated and correlated with function. Ventrally ( i.e., on the uppper surface) cilia are densest at the fringe region, the terminal lobe gutters and the lateral margins of the proboscis. The dorsal surface of the proboscis is always extremely sparsely ciliated except for a short distance from the ‘leading’ edge which has a dense covering of locomotory cilia. When presented with a monolayer of clean sand grains of different sizes, most animals only pick up particles of 150±40 m and less. When presented with a choice of clean sand grains or sand enriched with various plant and animal extracts, the terminal lobes actively pick up enriched particles but not, to any appreciable extent, clean sand. The terminal lobes are not attracted to the enriched substratum from a distance. Once on Isochrysis-enriched sand they spend significantly more time on it. With two other plant-enriched substrata ( Phaeodactylum and Ulva) they again spend more time on them than on clean sand, but the difference was not statistically significant. The terminal lobes do not feed exclusively on enriched substrata but graze over all the available area.