Ecological Determinants of Spionid Distributions Within Dense Patches of Deposit-Feeding Polychaete Axiothella rubrocincta

Abstract

Total spionid polychaete density (composed of eight species) on a California sandflat was significantly lower within dense, upper-intertidal patches of the tube-dwelling polychaete Axiothella rubrocincta (Maldanidae). Interspecific competition for a limited supply of organic-mineral aggregates (OMA) between A. rubrocincta and the spionid guild was implicated to account for the spionid distribution pattern at these patches. 'Within Patch' sediments contained significantly less OMA than sediments on the 'Outer Edges' of patches, at equivalent tidal heights (2 "/D vs. 25 O/O). By contrast, at a lower intertidal A. rubrocincta patch, where OMA abundance (42 O/O) was significantly greater, spionids were equally dense inside and outside. OMA were found to be an important selected food of deposit-feeding adult A. rubrocincta and Pseudopolydora paucibranchiata (Spionidae). In addition, OMA are incorporated into the spionid tubes (found in all six species checked). The sediment reworking, feeding activity of A. rubrocincta depletes OMA abundance in the sediment, both in the laboratory and in nature. OMA depletion was experimentally shown to lower adult P. paucibranchiata feeding and tube-building rates because of a strong OMA 'preference'. Furthermore, adult P. paucibranchiata had higher mortality and lower larval recruitment when in sympatry with A. rubrocincta. Pseudopolydora kempi (Spionidae) larval recruitment was significantly lower in cultures containing A. rubrocincta. Predatory flatfish were attracted by and attacked feeding P. paucibranchiata. These attacks were unsuccessful when the spionid food (OMA) supply was abundant as spionids were able to withdraw rapidly into their tubes. When food abundance was low, a condit~on found within upper-intertidal A. rubrocincta patches, spionids extended farther out of their tubes to increase their OMA search area. This behavior is likely to increase the probability of a successful attack by a predatory fish.