Particle-capture mechanisms in suspension-feeding invertebrates

A large number of suspension-feeding animals (e.g. bivalves, polychaetes, ascidians, bryozoans, crustaceans, sponges, echinoderms, cnidarians) have specialized in grazing on not only the 2 to 200 µm phytoplankton but frequently also the 0.5 to 2 µm free-living bacteria in the aquatic environment, or they have specialized in capturing larger prey, e.g. zooplankton organisms. Here we give an overview of the different types of particle-capture mechanisms in order to illustrate the many different solutions to the common problem of nourishing on a dilute suspension of microscopic food particles.. Despite the many differences in morphology and living conditions, particle-capture mechanisms may be divided into 2 main types. One type of mechanism (i) is some form of filtering or sieving (e.g. through mucus nets, stiff cilia, filter setae), which is found in both passive suspension feeders, that rely on external currents to bring suspended particles to the filter, and in active suspension feeders that themselves produce a feeding flow by a variety of pump systems. Here the inventiveness of nature does not lie in the capture mechanism but in the type of pump system and filter pore-size. The other type of mechanism (ii) involves some paddle-like flow manipulating system (e.g. cilia, cirri, tentacles, hair-bearing appendages) that acts to redirect an approaching suspended particle, often along with a surrounding ‘fluid parcel’, to a strategic location for arrest or further transport. Examples include (i) sieving (e.g. by microvilli in sponge choanocytes, mucus nets in polychaetes, acidians, salps a.o., filter setae in crustateseans, “ciliary sieving” by stiff laterofrontal cilia in bryozoans and phoronids), (ii) “cirri trapping” in mussels and other bivalves with eu-laterofrontal cirri, ciliary “catch-up” in bivalve and gastropod veliger larvae, some polychaetes, entroprocts, and cycliophores. These capture mechanisms may involve contact with a particle, and possibly mechanoreception or chemoreception, or may include redirection of particles by the interaction of multiple currents (e.g. in scallops and other bivalves without eu-laterofrontal cirri). Based on the review, we discuss the current physical and biological understanding of the capture process and suggest a number of specific problems related to particle capture, which may be solved using advanced theoretical, computational and experimental techniques.