MEPS 154:223-239 (1997)  -  doi:10.3354/meps154223

Filter-feeding in fifteen marine ectoprocts (Bryozoa): particle capture and water pumping

Riisgård HU, Manríquez P

The particle capture mechanism in ectoprocts was described, and the pumping rates in 15 species of marine ectoprocts with divergent lophophore morphometry were quantified in order to comprehend and characterize the lophophore as a filter-pump. Further, the effects of algal concentration and temperature on clearance were studied. The most characteristic feature of particle capture, apparent from video recordings, was that when the path of a particle was altered from downwards, towards the mouth, to outwards, between the tentacles, the particle was stopped by a tentacle. In most species (but never in Crisia eburnea which lacks frontal cilia) some of the trapped particles were seen to move along the tentacle surface towards the mouth. But more frequently, another downward transport mechanism was involved. As a result of the action of tentacle flicking restrained particles were propelled back into the central lophophore current to be carried further downwards, perhaps to be restrained by a tentacle again. These observations, supplemented with theoretical calculations, support the assumption that a mechanical laterofrontal-filter is at work which filters the water while the central current, created by the special lophophore pump-design, together with the flicking action of the tentacles, cleans the filter and transport the particles towards the mouth. Also, the measured particle retention efficiency, expressed as simultaneous clearance of particles of different sizes offered as a mixture of flagellates, supports the assumption of the presence of a mechanical laterofrontal-filter in the ectoprocts. The video recordings of particle trajectories revealed that there is a velocity profile at the lophophore entrance, the highest velocities being found in the central part of the lophophore. Thus, the mean velocity through the central area of the feeding core and the velocity through the outer area was used to estimate the pumping rates of ectoprocts. The pumping rate was found to vary between the 15 species, from 0.14 ml h-1 zooid-1 in C. eburnea to 7.5 ml h-1 in Flustrellidra hispida. The pumping rates (Q, ml h-1 zooid-1; 20°C) of all examined species as a function of the total lophophore tentacle length (NL, cm) was expressed by the equation: Q = 3.390NL - 0.704. A linear relationship between tentacle length specific pumping rate and total tentacle length indicates that the ciliary pump in small lophophores such as that of C. eburnea is relatively weak compared to large lophophores as found for example in F. hispida. The maximum zooidal clearance rates (F) of C. hyalina at 10, 15 and 20°C, measured after an initial stimulating period, was 0.12, 0.16 and 0.17 ml min-1 zooid-1, respectively. The ratio F/Q showed that about 40% of the water pumped through the lophophore entrance may subsequently pass through the laterofrontal filter.

Pumping rates · Clearance · Lophophore morphometry · Filter-pump · Particle trajectories · Tentacle flicking · Laterofrontal-filter · Retention efficiency · Effects of algal concentration and temperature

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