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systematic sampling design to investigate the density variability of the bivalve Chione
stutchburyi in Ohiwa Harbour, New Zealand. The 5-15m areas of increased densities
found using two-dimensional spatial autocorrelation analysis were suggested to have
been due to an environmental gradient. The spionid polychaete Marenzelleria viridis
was found to form patches of 0.04m 2-9m2 in the Southern Baltic by Zettler and Bick
(1996) which were attributed to result from changes in sediment structure. Gage and
Coghill (1977) found that many invertebrate species in Scottish sea-lochs were
patchily distributed. For example, the bivalve Abra alba formed patches of 3.5m and
the polychaete Diplocirrus glaucus formed patches around 2-3.5m in length. They
suggested that these patches probably resulted from environmental heterogeneity such
as that created by patches of seaweed debris.
However, the most visually discernible small- to meso-scale patches formed by
marine macro-invertebrates are probably the dense arrays of tube-beds formed by
certain polychaete species. The tube-beds of several infaunal polychaete species have
been studied such as Lanice conchilega (Carey, 1982, 1987; Ragnarsson, 1996),
Owenia fusiformis (Fager, 1964) and Clymenella torquata (Sanders et al., 1962).
However, it is within the family Spionidae that many of the tube-bed forming species
of polychaetes are found, such as Spiophanes cf. wigleyi (Featherstone and Risk,
1977), Marenzelleria viridis (Zettler and Bick, 1996), Polydora ciliata (Daro and
Polk, 1973; Noji, 1994) and Pygospio elegans (Dupont, 1975; Morgan, 1997). The
features which make spionids particularly capable of mass development and patch
formation together with the ecological significance of such patches have been
reviewed by Noji and Noji (1991).
The sizes of such patches have been found to vary greatly between taxa, for example,
one of the large C. torquata tube-beds noted by Sanders et al. (1962) in Barnstable
Harbour, Massachusetts, was estimated to cover 150,000m2, while those formed by M.
viridis varied between 0.04-9m 2 (Zettler and Bick, 1996). Often, these patches can
readily be seen as large mounds or plateaus of smooth, raised sediment with numerous
tube-heads protruding from the sediment surface. Within such patches, where worm
densities can be very high (e.g., 200,000 P. elegans/m2; Morgan, 1997), sediment
stabilisation may occur due to the hydrodynamic effects of the tubes acting as
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