determining rates of spread. No attempt has been made explicitly to model
spread in passively dispersing freshwater taxa, but future studies should focus
on this, and on including the influence of dispersal vectors (cf. animal dispersal
of seeds), fecundity and propagule resistance into rates of spread. Hard data on
these parameters are lacking in freshwater taxa, and at present our empirical
understanding of the comparative dispersal biology of passive dispersers in
freshwaters is cursory, limiting our ability to explore the possible influence of
body size.
Dispersal in marine systems
There have been few studies of body size and range size in marine systems
(Gaston, 2003 ) and fewer still that consider invertebrates and their modes of
development and dispersal. For marine invertebrates, passive dispersal via
planktotrophic larvae represents the most effective means of achieving
long-distance transport. However, the probability of successful recruitment to
adulthood from such meroplanktonic or teleplanic lifestages is low, and
broadcast spawners adopting such a strategy must compensate by producing
larger numbers of dispersing propagules. For brooding and viviparous species
such as Amphipoda, active movement, whilst effective at small spatial scales,
is likely to contribute little to their large-scale dispersal (Costaet al., 2004;
Kelaher, 2005 ), with rafting and other passive means of dispersal dominating
(Thiel & Gutow,2005). We might draw comparisons, therefore, between all
marine invertebrates (given that passive dispersal dominates in this realm)
and passively dispersing freshwater taxa, with the caveat that size spectra in
these two groups overlap little, if at all, with the marine organisms represent-
ing orders of magnitude greater adult body sizes than their freshwater
counterparts.
Given that selection for egg sizes in planktotrophic taxa tends to produce
intermediate sizes of egg across species (Levitan,2000), and that there is no
overall observed relationship between egg size and adult body size (Eckert,
1999 ), we might expect no relationship to result between range size and body
size in passively dispersing marine taxa. Larger propagules should develop
faster and have a shorter pelagic larval duration and, thus, we might expect
an egg size-range size relationship to exist amongst planktotrophic taxa.
However, larger species are generally longer lived and more fecund (Llodra,
2002 ), and therefore generate more dispersing individuals. Goodwinet al.
(2005) demonstrate that in the case of marine egg-laying teleosts, geographic
range indeed scales positively with body size, and Reaka (1980) reported a
similar relationship for Stomatopods (most species initially brood their eggs,
which then hatch and the larvae disperse planktonically). These trends are
probably related to adult fecundity rather than to the size of the dispersing
particle.
BODY SIZE, DISPERSAL AND RANGE SIZE IN AQUATIC INVERTEBRATES 199