Facilitation effects of invasive and farmed bivalves on native populations of the sea slug Pleurobranchaea maculata

Invasive and native bivalves can facilitate higher trophic levels through habitat provision and food subsidies. In New Zealand, interest in the predatory sea slug Pleurobranchaea maculata increased when 10 dogs died after contact with beach-cast slugs on Narrow Neck Beach (Hauraki Gulf, Auckland) in August 2009. Investigations identified large populations of native P. maculata containing the deadly neurotoxin tetrodotoxin on nearshore beds of the invasive mussel Arcuatula (Musculista) senhousia. Subsequent studies revealed extensive populations of P. maculata beneath native Perna canaliculus mussel farms in Tasman Bay (Nelson, New Zealand). This study investigated whether P. maculata benefit from the trophic subsidy and/or habitat complexity provided by introduced and farmed mussels. Isotopic analysis suggested that P. maculata from Hauraki Gulf and Tasman Bay were most likely feeding on filter-feeding bivalves. Analysis of stomach contents using real-time PCR confirmed A. senhousia as a dietary source for P. maculata at one Hauraki Gulf site, and Perna canaliculus and Mytilus edulis (blue mussel) as dietary sources at Tasman Bay. Artificial habitat experiments in the Hauraki Gulf were confounded by a die-back of A. senhousia beds, but in their absence, P. maculata also disappeared. In Tasman Bay, P. maculata laid eggs on artificial mussel shell treatments beneath mussel farms, but no recruitment was recorded. Subsequent recruitment (ca. 2.7 P. maculata recruits per linear metre) was observed on overlying suspended mussel lines. Spatial and temporal changes in the availability of the food subsidy and habitat provided by native and invasive bivalves clearly have facilitative effects on P. maculata populations.