9780521861724htl 1..2

For marine animals very little is known about the species numbers and body-

size distributions of guilds of species that co-occur in specific locations or hab-

itats. A much simpler exercise is to analyze these patterns in regional species

pools because comprehensive regional inventories are available for many taxa. In

Fig. 11.1the numbers of species of different sizes in various taxa that might

reasonably be defined as guilds, in what might reasonably be defined as regional

species pools, are plotted. The term ‘guild’ here is taken to mean a taxonomically

coherent group of animals with similar trophic roles. The taxa have been chosen

to cover the whole spectrum of marine metazoan body sizes from tardigrades to

toothed whales, and the body-length axis is divided into1.28 geometric size

classes. A remarkable feature of these plots is that each guild spans almost the

same range of sizes, from nine size classes (tardigrades, shrimps and prawns,

regular echinoids) to 14 (mysids), with a mean of 10.6. Thus, there seems to be a

rather fixed range of body sizes that a given body plan will allow, irrespective of

the nature of that body plan. If local assemblages were a random selection of

species from the regional pool, the resulting guilds would not be inconsistent

with the size ratio and guild size ‘rules’ noted above, and local competition or

partitioning of food resources need not be invoked to explain them.

Adult-body size distributions in integral benthic assemblages

The species numbers/adult body size relationship for marine benthic metazoan

assemblages is quite unlike that described for any terrestrial or freshwater

situation. Warwick (1984 ) found a highly conservative pattern for temperate

soft-bottom benthic assemblages, with two separate lognormal distributions cor-

responding to the size categories traditionally regarded as macrofauna and meio-

fauna (Fig.11.2 ). The meiofaunal mode occurs at a dry body mass 0.64mgandthe

macrofaunal mode at 3.2mg, with a trough between them at 45mg. Rather few

subsequent studies have attempted to count species numbers across the whole

Table 11.1Dyar’s Constants (increase in length between successive moults) for

seven species of free-living marine nematodes.

Species Constant Calculated from

Monhystera denticulata 1.28 Tietjen & Lee ( 1972 )

Chromadora macrolaimoides 1.41 Tietjen & Lee ( 1973 )

Steineria ericia 1.37 Lorenzen ( 1978 )

Steineria pilosa 1.34 Lorenzen ( 1978 )

Subsphaerolaimus litoralis 1.32 Lorenzen ( 1978 )

Sphaerolaimus gracilis 1.22 Lorenzen ( 1978 )

Parasphaerolaimus paradoxus 1.31 Lorenzen ( 1978 )

Mean 1.32

212 R.M. WARWICK