current movements (Largier, 2003 ; Shanks, Largier & Brink, 2000 ; Siegelet al.,
2003 ) to larval responses to their own energetic status or settlement cues (e.g.
Marshall & Keough,2003a). Non-planktonic species, meanwhile, may display
both active dispersal by swimming or crawling, and passive dispersal, the latter
being by means of rafting on floating objects (see Grantham, Eckert & Shanks,
2003 ).
Given these considerations, it is possible to make some generalizations as to
how body size could relate to dispersal ability in marine invertebrates. Some
life-history models predict a dichotomy of egg sizes in planktonic marine inver-
tebrates, with eggs giving rise to planktotrophic larvae having small optimal
size and those giving rise to lecithotrophic larvae being optimally large (Vance,
1973 , reviewed in Roff,1992). However, empirical data do not support such a
bimodal distribution of egg sizes, and instead often demonstrate a right-skewed
or intermediate size distribution, even within wholly planktotrophic groups
(Levitan, 2000 ). It remains possible, however, to make the generalization that,
compared with lecithotrophic propagules (eggs and larvae), those of plankto-
trophic taxatendto be smaller, spend longer suspended in the water column,
and disperse further, especially in the case of specialized long-lived ‘teleplanic’
forms (Eckert, 1999 ; Largier,2003; Shankset al., 2000). Amongst planktotrophic
dispersers, there is evidence for a positive correlation between adult body size
and egg size within species (e.g. Marshall & Keough,2003b; Mineret al., 2005),
but little evidence for patterns between species, although a trade-off between
size and number of eggs as a result of energetic and physical constraints
might be expected. Second, there is a tendency within clades for small body
size to be correlated with either brooding or lecithotrophy, especially amongst
co-occurring species (Strathmann & Strathmann,1982).
A variety of hypothesized mechanisms exist to explain these relationships
between life-history traits and dispersal patterns (Sewell, 1994 ). Chia ( 1974 )
hypothesized that limitations upon energetic reserves for reproduction in
small-bodied taxa limit gamete production and hence favour viviparity, brood-
ing or lecithotrophy. Strathmann and Strathmann (1982) cite three hypotheses
to explain this trend: dispersal limitation, adult longevity and recruitment, and
allometries in scaling of fecundity and space available for brooding. It is also
possible that smaller, more specialized species may experience increased selec-
tion for retention of juveniles if their habitats are widely dispersed and the
probability of successful dispersal to new habitats is low. Such a case has
certainly been made for organisms inhabiting estuaries (Bilton, Paula &
Bishop,2002; Foggo, Frost & Attrill, 2003 ). Finally, it is probable that small-
bodied organisms face a double jeopardy in employing broadcast spawning:
quantities of sperm and eggs produced by small organisms may be inadequate
to ensure fertilization (especially in denser waters at higher latitudes) (Yund,
2000 ). Also, the probability of successful development of relatively few192 S. D. RUNDLEET AL.