Edinburgh Napier University

  The spionid polychaete Pygospio elegans Claparede (1863) is a small, tube-building opportunist. On the intertidal sandflat Drum Sands, Firth of Forth, Scotland, this species is numerically dominant and forms areas of increased density or 'patches'.

Grid surveys, together with mapping and spatial autocorrelation analysis, revealed that these patches were areas of statistically significant higher numbers of P. elegans
compared with surrounding areas where this species was present in very low numbers. These patches, 1-1.5m2, could be seen as areas of smooth, raised sediment within an
otherwise wave-rippled sandflat. The majority of the other macrobenthic invertebrate species also exhibited small- or meso-scale patchiness, but none of these patches were
spatially coincident with those of P. elegans.

Although life history characteristics and disturbance have previously been postulated as being responsible for the generation of spionid patches, these have never been
explicitly tested. The P. elegans population on Drum Sands was studied with respect to its population structure and reproductive biology and its response to macroalgal mat
establishment and sediment disturbance. The possible role of these in the formation of small-scale patches of P. elegans are discussed.

The P. elegans population on Drum Sands displayed reproductive activity for the majority of the year although intense larval recruitment was confined to two acute
periods, April/May and November/December. P. elegans reproduced exclusively via planktotrophic larvae: no evidence of asexual proliferation or benthic larval production
was found. This life history provides a large larval availability for patch formation.

The role of macroalgal mat establishment in structuring the spatial distribution of P. elegans was investigated by a controlled, weed-implantation experiment and a
comparative survey. Implanted Enteromorpha prolifera and naturally establishing Vaucheria subsimplex caused underlying sediments to have increased silt/clay
fractions, increased organic and water contents and increased sorting coefficients andmedium grain size. The sediment below these macroalgal mats also became
significantly more reduced. While the communities under E. prolifera mats became dominated by C. capitata, those under V. subsimplex mats were dominated by P. elegans.

The effect of sediment disturbance on the faunal communities of Drum Sands was investigated by studying the initial colonisation of defaunated sediments. P. elegans
numerically dominated the early stages of succession only during periods of high larval availability, C. capitata dominated at other times. Furthermore, P. elegans larval recruitment and adult immigration to defaunated sediments within P. elegans patches were higher compared with non-patch defaunated sediments.

The micro-scale spatial distribution of P. elegans within small-scale patches was examined. P. elegans was found to be non-randomly distributed throughout the year, patches formed were commonly less than 3cm square. Correlation analyses implied that these micro-scale patches may have been generated and maintained by adult-juvenile
interactions and/or sediment heterogeneity.

Small-scale P. elegans patches were found to be distinct ecological areas when compared with surrounding sediments. Sediment properties and invertebrate
community structure of P. elegans patches were significantly different to those of nonpatch areas. These findings emphasise the ecological importance of patches of tubebuilding spionid polychaetes in allowing certain species to occur in habitats where they would otherwise be unable to survive.