CHAPTER TWO
Body size and suspension feeding
STUART HUMPHRIES
Department of Animal and Plant Sciences, University of SheffieldIntroduction
Suspension-feeding animals are ubiquitous in aquatic ecosystems, and all major
taxa have members for whom suspension feeding is the main foraging mode.
Suspension feeders are often the chief primary consumers in aquatic systems but,
because of the diverse nature of the particles they collect, they also contribute
significantly via their effects as both secondary consumers and detritivores (Gili &
Coma,1998 ;Jørgensen,1966 ). In removing particulate food from the surrounding
water, suspension-feeding organisms act as mediators of energy flux between the
photiczoneandthedeepsea,betweencontinentalwatersandthebenthiczone,and
between local systems in freshwaters (Gili & Coma,1998; Wildish & Kristmanson,
1997;Wotton,1994). Their role in energy transfer means that they are key
components of aquatic ecosystems, representing important pathways for energy
flow, and are crucial determinants of the productivity of aquatic environments.
Suspension feeders are characterized by the possession of an organ used to
capture suspended particles from the water (feeding structure). The feeding struc-
tures utilized by suspension feeders are highly variable, and include appendages
bearing hairs, mucus or silk nets, gill rakers and baleen plates, lophophores,
tentacles, and ciliated and flagellated cells. Within a feeding structure, indivi-
dualcollecting elementsare the first point of contact for food particles. Transport
of particles (particle flux) to the feeding structure is achieved by the flow of water,
provided either by active pumping or by external flow. Suspension feeders are
generally categorized as belonging to one of two major groups, based on the
extent to which ambient (external) water flow is utilized for feeding (LaBarbera,
1984 ; Vogel, 1994 ; Wildish & Kristmanson,1997).Passive suspension feedersrely
entirely on ambient flow to deliver food particles to their feeding structures,
whileactive suspension feedersuse energy to regulate water flow over or through
their feeding structures using a biological pump (ciliary or muscular). However,
further intermediate categorization is possible, with facultative-active suspen-
sion feeders able to switch between passive and active feeding, depending on
ambient flow conditions. Combined passive-active suspension feeders (Wildish
& Kristmanson, 1997 ) utilize both passive and active methods at the same time
and to varying extents, while deposit-suspension feeders switch between
Body Size: The Structure and Function of Aquatic Ecosystems, eds. Alan G. Hildrew, David G. Raffaelli and Ronni
Edmonds-Brown. Published by Cambridge University Press.#British Ecological Society 2007.