Minireview: Ciliary filter feeding and bio-fluid mechanics— present understanding and unsolved problems

The energy cost for various ciliary filter feeders shows that useful pump work constitutes 0.3–1.1% of the total metabolic expenditure. The ‘water processing potential’ (liters of water pumped per milliliter O2 consumed by the animal) is a useful tool for characterizing filter feeding and adaptation to the environment. The six types of ciliarycapture mechanisms (collar sieving, cirral trapping, ciliary sieving, ciliary downstream collecting, ciliary upstream collecting, mucus-net sieving) are reviewed as well as the aerosol/hydrosol filtration theory. A brief overview of fluid mechanical principles and tools for studying ciliary functions is given. Filter feeding (or suspension feeding) is widely represented among invertebrate taxa, and cilia-mediated feeding is particularly common in the marine environment. Ciliary filter feeders remove suspended food particles, mainly phytoplankton (2–200 mm), but in many cases also free-living bacteria (0.2–2 mm), from the water (Jørgensen 1966, 1975; Wotton 1994; Silverman et al. 1995, 1997; Riisgård and Larsen 1995, 2001; Gili and Coma 1998). These animals have adapted to life in a nutritionally dilute environment, and they show great variations in particle capture mechanisms and pump design (Foster-Smith 1976a,b, 1978; Jørgensen 1983, 1990; Jørgensen et al. 1984; LaBarbera 1984, 1990; Shimeta and Jumars 1991; Vogel 1994; Riisgård and Larsen 1995, 2001; Silverman et al. 1996). Because the animals have evolved to solve common basic problems, a great number of similarities exist between taxonomically distant species. Recent investigations of particle-capture mechanisms and properties of pumps include widely different groups such as sponges, bryozoans, polychaetes, bivalves, ascidians, and cephalochordates. Comparative studies have aimed to give more precise description of the particle-capture mechanisms and to characterize the pumps. Further, the studies have focused on assessment of energetic costs of filter feeding and adaptation to the environment. An analysis of bio–fluid mechanical aspects of ciliary filter feeders at the level of the organism may begin with an examination of the ‘‘overall effects’’ (pump characteristic, energetics, particle retention efficiency, and adaptation to the 1 Corresponding author ([email protected]).