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Ecosystem engineers in the sea: Revealing mechanisms of colonization

Pyura stolonifera

Pyura stolonifera is the most conspicuous ascidian on rocky bottoms and walls along the South African coast.

Coastal environments are among the ecosystems most heavily affected by biological invasions worldwide. Ascidians (Chordata: Urochordata) are a group of sessile, filter feeding marine invertebrates that include both important ecosystem engineers and aggressively invasive species.

Ascidians have a planktonic larval stage with limited dispersal capabilities. However, larvae can be caught in ballast pumps and survive in transit to other harbours, or individuals or colonies attached to floating structures such as drift algae or loose debris can be pumped in. In addition, adults can also be transported as fouling on the hulls and sea chests of ships and recreational vessels, and can release their larvae in the locations where these ships stop. Once they arrive at a new coastal environment several factors may influence the way in which the larvae settle (settlement behaviour). In a recent paper in the journal Marine Ecology Progress Series C·I·B post doctoral researcher Dr Marc Rius and co-authors investigated the influence of larval settlement behaviour on the distribution and abundance of adult ascidians. Species that aggregate often owe this characteristic to larval behaviour, and in this paper the authors investigated whether this predisposes ascidians to becoming invasive, by increasing their capacity to maintain their populations.

The paper explored the interactive effects of larval phototaxis (movement in response to light), geotaxis (larval response to gravity) and conspecific adult extracts (tunic extracts) on settlement rates of a representative suite of six species of ascidians that form aggregations in the field. In addition, the adult abundance and habitat characteristics in the field was studied and compared to the settlement patterns. Species investigated included 4 introduced species with global distributions (Ciona intestinalis, Ascidiella aspersa, Styela plicata, Microcosmus squamiger) and two native species (Pyura herdmani and P. stolonifera).

The authors found both strong geotactic and phototactic preferences of the larvae, which in most species correlated well with adult distributions in the field. Adult extracts inhibited settlement of half of the studied species and failed to influence settlement on the other half, arguing against conspecific attraction being a cause of aggregation and an explanation of the propensity of ascidians to become invasive. Results therefore favoured the view that the aggregated distribution of the solitary ascidians reflects responses towards abiotic factors rather than biotic factors.

Dr Marc Rius is a post doctoral fellow with the C·I·B at the University of Cape Town and can be contacted at marcriusvil@gmail.com.

Access the paper (abstract):

Rius. M., Branch, G.M., Griffiths, C.L., & Turon, X. (2010). Larval settlement behaviour in six gregarious ascidians in relation to adult distribution. Marine Ecology Progress, 418, 151163.