Ecological effects of the introduced fanworm, Sabella spallanzanii (Gmelin, 1791), on hard substratum epifaunal assemblages

Brief description

Sabella spallanzanii is an introduced sabellid ploychaete that is thought to be native to the Mediterranean and east Atlantic coast. Since its first identification in Port Phillip Bay in 1992 it has spread rapidly and occurs in most parts of the bay. This thesis examines the ecological impact of Sabella spallanzanii aggregations on hard substrate epifaunal assemblages in south eastern Australia. Specifically the effects of Sabella on: a) recruitment of sessile invertebrates; b) development of epifaunal assemblages; c) distribution of larvae and other planktonic organisms and d) growth and survival of sessile animals are examined. These effects are evaluated in relation to spatial and temporal changes in Sabella canopies resulting from changes in populations and patterns of fan deployment. Studies were carried out between 1995 and 1998 at four sites within Port Phillip Bay, Victoria (Geelong Marina, Breakwater Pier, Victoria Dock and St Kilda Marina) and at Outer Harbour, within the Gulf of St Vincent, South Australia.

The effects of Sabella on recruitment of sessile taxa were complex, but a number of generalisations could be made. Recruitment of some barnacles, bryozoan, spirorbid and ascidian taxa to large experimentally cleared (pilings) areas was higher than on uncleared areas. On the scale of individual 20 by 20 cm settlement plates, where the effects of physical structure was controlled using fanworm mimics, most of the effects were caused by the presence of physical structure on the plate. Positive and negative responses of some taxa were observed but responses were not consistent across experiments. Recruitment onto tubes increased the abundance of several taxa on plates with fanworms, but not all taxa recruited onto tubes. Results were not consistent within taxonomic groups or between sites.

Longer term experiments revealed fewer changes to the structure of the understorey assemblage. The apparent disappearance of canopy effects with time could be caused by processes associated with assemblage development such as overgrowth and senescence of early recruits. Alternatively it could simply be caused by differences in the composition of assemblages between experiments.

Multivariate analyses showed that spatial variation in recruitment on the scale of 10's of metres was important in determining the composition of the assemblage in both early and later stages.

The planktonic abundances of larval solitary ascidian, spionid polychaetes and pooled larval taxa adjacent to the substratum were significantly reduced by the presence of a fanworm canopy, suggesting that larval flux to these areas may be lower. Little difference in larval recruitment between cleared and uncleared areas suggests that settlement rates may be enhanced or post-settlement mortality rates might be lower beneath canopies.

Growth of the arborescent bryozoan Bugula stolonifera and an encrusting bryozoan Watersipora subtorquata were affected by the presence of a canopy, but these effects were not consistent between experiments. This variability suggests that canopies may act in different ways at different times.

The overall impact of Sabella spallanzanii on other species at the scales examined in this thesis can not be explained by its modification of the habitat. The use of tubes as settlement sites, and the predominance of small scale responses to physical structure support this idea. Any impact of Sabella spallanzanii must also be evaluated in the light of observations that the canopy was not constant though time, due to population crashes that occurred at a number of times and places.

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