Body size changes during ontogeny and evolution
The flow regime that an organism experiences is critical to a number of physio-
logical processes (Palumbi,1986; Vogel, 1994 ; Rex, Montebon & Yap, 1995 ).
Given that the body size of suspension feeders determines the flow regime
around them, and considering the extent of growth in aquatic organisms,
ontological shifts between regimes are common. Physical limits to growth and
patterns of modularity (the way in which clones group together) are important
in many groups with indeterminate growth, where size of feeding surfaces is
traded against the risk of mechanical failure and respiratory costs of increased
tissue volume. Feeding itself is highly dependent on flow, and hence size of the
collecting elements within a feeding structure.
Although it has been commonly assumed that theReof collecting elements
(based on element diameter) is much less than unity, there is mounting evidence
that a large fraction of suspension feeders may operate in a 1Re100 regime
(intermediate-Re), where neither viscosity nor inertia truly dominates (Shimeta &
Jumars,1991 ;Table2.1 &Fig.2.1a ). The physics of fluid flow in this regime is
underdeveloped, and little experimental work has been carried out to investigate
the implications of this flow regime for the large number of suspension-feeding
animals whose collecting elements are thought to operate in this intermediate-Re
range. The few studies in this area have already shown some surprising shifts in
the way feeding appendages operate with a transition to intermediate-Re(Koehl,
1983 ; Cheer & Koehl,1987 ; Koehl,1993 , 1996 , 2000 , 2001 ).
It has been shown that hair-bearing appendages used for functions such as
suspension feeding, swimming, flying and olfaction, operate in fundamentally
different ways depending onRe(based on the gap between hairs: Koehl,1983;
Cheer & Koehl,1987 ;Koehl1993, 1996 , 2000, 2001). At very lowRethese append-
ages act as paddles, pushing water in front of them, whilst atRenear unity they
function more like leaky sieves with water passing easily between the hairs
(Fig. 2.1b ). The strong implications for suspension feeders are reinforced when
the relationship betweenReof the hair-gaps and body size is considered. Reynolds
number in this instance is related to body size via both appendage size and speed
of movement (proportional to limb length and body size). However, there is
currently a lack of evidence for ontological transitions within species, as com-
pared with between-species differences. This difference is probably a consequence
of ‘leakiness’ having a low sensitivity to changes in size at very lowRe(Koehl,1995 ).
Limits to maximum body size
The density of water is around 800 times greater than air, which (in addition to
reducing sinking rates and hence promoting the existence of suspension feed-
ing per se) provides substantial physical support not available to terrestrial
organisms. As a result, both maximum body size and appendage elaboration
in suspension feeders are somewhat less restricted by constraints related to
BODY SIZE AND SUSPENSION FEEDING 19