Narrow valve gaping in the invasive mussel Limnoperna securis: implications for competition with the indigenous mussel Mytilus galloprovincialis in NW Spain

The black pygmy mussel Limnoperna securis (Lamarck 1819) is endemic to the brackish waters of New Zealand and Australia, but over the past decade, it has successfully invaded the inner Galician Rias of NW Spain. There is growing concern that L. securis will expand its range to the outer zones of the Rias, where it would pose a threat to the intensive raft culture of the indigenous mussel Mytilus galloprovincialis (Lamarck 1819). In this paper, we compare the valve-opening behaviour of the two mytilids under simulated raft conditions, i.e. full-strength seawater (35 g l(-1)) and a low current flow regime (2-5 cm s(-1)). Modes of valve-opening amplitudes that were most frequently observed in both species were in the range of 60-90 %, indicating a tendency towards full valve openness. Both species displayed circadian periodicity (tau = 24 h): maximal gaping was generally observed during periods of darkness and minimum gaping during daylight hours. The only prominent difference in behaviour between the two species was related to the degree of valve opening. The maximum recorded gape angle was 8.2A degrees (SE = 0.9) for L. securis versus 14.8A degrees (SE = 1.4) for M. galloprovincialis. This difference may place L. securis at a competitive disadvantage on substrates, where the two species coexist, such as over rocky shores or potentially mussel culture ropes.