9780521861724htl 1..2

feeders. Evidence from vertebrates eating ‘small-prey’ (Peters, 1983 ) suggests a

relationship of the formWprey¼0.0018Wpred1.18, whereWis mass in kg, with an

exponent not significantly different from unity. For invertebrates, Warren and

Lawton ( 1987 ) describe a positive relationship between mean prey length and

predator length (L, in m) equivalent to the formLprey¼5.83 10 ^4 Lpred0.984.

Wilson (1973) givesLprey¼7.83 10 ^5 Lpred^2 – 6.61 10 ^5 Lpredþ4.4 10 ^5 , for

the maximum plastic bead diameter ingested by the copepodAcartia tonsa

(Fig. 2.3). The latter data are also fit well by a power function with an exponent

of close to unity (Lprey¼5.52 10 ^5 Lpred1.0397; Humphries, unpublished), but are

valid only for one species with a small size range.

The general predator–prey size relationship is also characterized by increas-

ing variance in prey size as predator size increases (Peters, 1983 ; Warren &

Lawton,1987; Cohenet al., 1993). Mechanisms for this pattern in ‘traditional’

predators are thought to relate to the ability to switch to small prey when larger

prey is unavailable, which becomes less feasible as predator size decreases.

However, when considering suspension feeders a physical basis for the pattern

can be proposed. If particles with a linear dimension greater than the filter-gap

size are utilized (sieving) then the maximum prey size will be determined by the

dimensions of the filter apparatus or mouth, and all particles larger than this

critical size will be retained. Thus, if sieving is utilized by a particular species, a

bimodal prey-size distribution is predicted (Silvester, 1983 ; Brownet al., 2005)

and maximum prey size will increase isometrically with predator size.

0.065

0.055

0.045

0.035

0.025

0.015

0.005

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

Animal length (mm)

Bead diameter (mm)

maximum

minimum

Figure 2.3The relationship between prey size and body size in the copepodAcartia tonsacapturing
plastic beads. Maximum ingested bead size varies asLprey¼05.52
10 ^5 Lpred1.0397, while minimum bead size is constant across the body-size range studied.
Redrawn from Wilson (1973 ).

BODY SIZE AND SUSPENSION FEEDING 25