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

Abstract

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 Cr~sia 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 ' zooid' in C eburnea to 7.5 ml h'l in Flustrellidra hispida. The pumping rates (0, ml h ' zooid"; 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 m i n ' zooid", respectively. The ratio F/Q showed that about 4 0 % of the water pumped through the lophophore entrance may subsequently pass through the laterofrontal filter

Downstream collecting in ciliary suspension feeders: the catch-up principle

“…Ciliary sieving by means of stiff (laterofrontal) cilia is now well documented and appears to be restricted to larval and adult ectoproct bryozoans (Strathmann & McEdward 1986, Riisgård & Manríquez 1997, Nielsen & Riisgård 1998.…”

Feeding activity, retention efficiency, and effects of temperature and particle concentration on clearance rate in the marine bryozoan Electra crustulenta

“…However, in preliminary experiments in this study no behavioural effect was detected. Riisgård & Goldson (1997) and Riisgård & Manríquez (1997) have had similar results. Common to all of the experiments showing no effect was the use of an Elzone ® , whereas a Coulter ® particle counter has been used when an effect has been observed.…”

“…Pumping rate (Riisgård & Manríquez 1997), particle capture (Nielsen & Riisgård 1998) and clearance rate (e.g. Riisgård & Goldson 1997, Lisbjerg & Petersen 2000 have been studied applying techniques used in the studies of other benthic suspension-feeders (e.g.…”

“…Petersen & Riisgård 1992, Clausen & Riisgård 1996, Petersen et al 1999. Effects of temperature on bryozoan clearance rate have been studied by Menon (1974), Riisgård & Manríquez (1997), Lisbjerg & Petersen (2000) and particle concentration by Bullivant (1968), Best & Thorpe (1983, 1986, and Riisgård & Goldson (1997) ABSTRACT: Various factors influencing clearance rate were elucidated on the bryozoan Electra crustulenta (Pallas). Measurements of clearance rates were performed using the algae Rhodomonas sp.…”

“…Due to the open structure of the bryozoan filter, small particles are not efficiently retained. Distance between laterofrontal cilia varies from 3 to 4 µm in Crisia (Nielsen & Riisgård 1998) and experiments show that particles smaller than ca 5 µm in diameter are retained less efficiently than larger particles in the filtering apparatus of Electra crustulenta (Riisgård & Goldson 1997) and Celleporella hyalina (Riisgård & Manríquez 1997).…”