9780521861724htl 1..2

Schwinghamer (1981 ) found a biomass trough at exactly the same body size as the

species body-size trough (between 0.5 and 1mm equivalent spherical diameter)

and argued that this size defines the upper limit of the interstitial meiofauna and

is the region of a shift from interstitial to burrowing lifestyles, with a class of

intermediate-sized animals capable of neither. This hypothesis, involving the

importance of habitat architecture (Holling,1992), is not only intuitively appealing

but is also open to empirical test (Steadet al., 2005). However, the only experimental

test on soft-bottom marine benthos is that of Leaperet al.(2001)whofoundno

support for it. Furthermore, this explanation does not bear critical examination

based on comparative studies, since the bimodal species body-size distribution is

apparent in thixotropic muddy sediments where the interstitial mode is not pos-

sible (Warwick,1984), and also in sediment-free algal habitats where such sedi-

mentary constraints do not exist (Gee & Warwick,1994 ). Further, in freshwater

sediments, evidence suggests that the meiofauna/ macrofauna dichotomy does not

exist. Strayer (1986) found that, in the sediments of Mirror Lake, New Hampshire,

USA, both the biomass spectrum and the species size distribution were unimodal,

in marked contrast to the marine situation, and this unimodal pattern of taxon

richness in body-size classes has also been found in freshwater streams (Steadet al.,

2005)(Fig.11.3). Physical sedimentary constraints are clearly the same in the

freshwater and marine situations, but the reproductive characteristics of the

species involved are very different. Most shallow-water temperate marine macro-

benthos have planktonic larvae, whereasmany species in the freshwater benthos

are the larval stages of flying insects. Theconservative nature of the species body-

size spectrum in marine habitats therefore adds credence to the suggestion that it

results from evolutionary adaptationsto the spatial and temporal structure

of the marine environment, which will affect the regional species pool, rather

than ecological constraints imposed by the physical nature of particular habitats.

The meiobenthos are considered to be the first metazoans to evolve in the

Middle Precambrian, with the Plathelminthes (including Gnathostomulida) and

Northumberland mud Carmarthen Bay sand Algoa Bay sand

Number of species

0

4

8

12

16

20

24

2 6 10 14 18 22 26 30 2 6 10 14 18 22 26 30 2 6 10 14 18 22 26 30

X2 geometric weight classes

Figure 11.2Benthic species-size distributions in subtidal soft-sediment assemblages off NW

England (Northumberland), South Wales (Carmarthen Bay) and South Africa (Algoa Bay)

showing the conservative bimodal pattern (after Warwick, 1984 ).

214 R.M. WARWICK