are able to coexist. Warwick (1989 ) went on to suggest that it had been ‘necessary’
for larger animals to evolve a planktonic larva to avoid competition with and
predation by the permanent meiobenthos, which constitute a highly efficient
consumer unit. Whilst it is difficult to invoke the strength of present-day
interactions between the meiobenthos and macrobenthic larvae as evidence
for selection pressure in the past, there is strong present-day evidence that the
meiobenthos can significantly depress the densities of newly settled macro-
benthic larvae. This is mainly by predation but also by competition and disturb-
ance (Bell & Coull,1980; Watzin,1983, 1986). Meiobenthic predators can take
rather large prey relative to their own body size, even larger than themselves
(Bilio,1967; Straarup,1970; Watzin, 1985 ), including the young of traditionally
macrobenthic taxa (Fig.11.4). Eggs produced by macrobenthic species, and
consequently the larvae hatching from them, fall exactly in the same size
range as the adult meiobenthos (Fig.11.5), and settle out of the plankton to
the bottom at a size corresponding to the position of the trough in the bimodal
species body-size distribution (Figs.11.5 & 11.6). It had already been suggested
that the planktotrophic larval phase in macrobenthos is principally a migration
for feeding and safety (with dispersal as an inevitable secondary by-product),
and that ‘Life-history theories for marine animals cannot ignore a strong histor-
ical component stretching back to the origin of the metazoa’ (Strathmann,1985 ).All-animal body-size distributions in integral benthic assemblages
The above discussion concerns the diversity of metazoans divided among adult
body-size categories. Of course, natural assemblages also comprise the smaller
developmental stages of these species that are potentially interacting with the
adults. Warwick, Dashfield and Somerfield (2006 ) explicitly addressed the issue
of all-animal size distributions by determining the degree to which infaunalTable 11.2Size related biological traits that switch abruptly at about 45g dry
weight in temperate shallow water marine benthos (from Warwick, 1984 ).Smaller than 45mg Larger than 45mg
Development Direct benthic Planktonic
Dispersal As adults Planktonic larvae
Generation time Less than one year More than one year
Reproduction Semelparous Iteroparous (usually)
Feeding Discriminate use of particles Indiscriminate use of particles
Resource partitioning Particle selection (size,
shape, quality)Spatial segregation and particle
size selection
Growth Reach asymptotic adult size Continue growth throughout life
Mobility Motile Sedentary or motile216 R.M. WARWICK