than small taxa, paralleling the scenario in marine environments but contrast-
ing with Finlay ( 2002 ) andFinlay and Esteban’s (this volume) proposed model.
Dispersal in marine systems
In contrast to many of their freshwater counterparts, marine invertebrates are
often benthic as adults and disperse as planktonic larvae. Three main types of
developmental strategy have been described in marine taxa: (i) non-planktonic
development, in which species are either viviparous, eggs and young develop
demersally, or eggs are maternally brooded; (ii) lecithotrophy, in which a rela-
tively large egg is produced and the yolk provisions the developing larva for at
least part of its planktonic existence; and (iii) planktotrophy, in which the larvae
derive their sustenance from exogenous sources, usually by predating other
plankton. The division of development along such simple grounds, however,
represents a considerable over-generalization (see Poulin, von Boletsky & Feral,
2001 ). For example, most planktotrophs develop feeding structures during their
planktonic phase and therefore commence their development with maternal
provisioning (Miner, McEdward & McEdward,2005) whilst others which are
primarily lecithotrophic may become facultatively planktotrophic (Emlet,
McEdward & Strathmann,1987).
The relative frequency of different developmental modes differs both across
clades of marine organisms, and across biogeographic regions within clades.
Opisthobranch molluscs from the northeast Pacific are primarily plankto-
trophic (Goddard, 2004 ), whilst the relative proportions of lecithotrophs and
brooders are higher in other groups such as echinoderms and prosobranchs in
the same region. Groups such as peracarid crustaceans, meanwhile, are almost
exclusively brooded, whilst most decapods have planktonic phases. The most
profound patterns affecting dispersal mode are biogeographic trends in devel-
opmental pattern. Thorson’s rule (Mileikovsky,1971) describes a trend towards
decreased incidence of pelagic and planktotrophic development (towards direct
demersal development) with increasing latitude (Collin, 2003 ). These trends
mean that dominant modes and extents of dispersal may vary significantly
with latitude. Most marine phyla exhibit a spectrum of modes of development,
indicating that evolution of traits such as lecithotrophy and direct development
have occurred separately in most of these groups (e.g. Jeffery & Emlet, 2003 ).
It is difficult to justify the dichotomy of dispersal into active and passive
modes in the marine environment. Whilst there are active components to
planktonic existence, for example migrations within the water column, and
small-scale dispersal by viviparous or brooding species can be highly effective
(Kelaher,2005), the majority of large-scale dispersal by both planktonic (both
lecithotrophic and planktotrophic) and non-planktonic organisms is primarily
passive. The duration (and therefore, potentially, scale) of pelagic transport may
be influenced by a variety of both passive and active factors ranging from
BODY SIZE, DISPERSAL AND RANGE SIZE IN AQUATIC INVERTEBRATES 191