Pharmalogical induction of larval settlement in the New Zealand mussel Perna canaliculus
The endemic New Zealand green-lipped mussel (Perna canaliculus) is a commercially important species. Although this mussel is extensively cultivated in New Zealand, very little is known about its early larval behaviour. After a brief planktonic stage, the young mussels settle onto specific substrates before undergoing metamorphosis into their adult form. However, the timing of this settlement, and preference for substrata is poorly understood. Chemical stimuli of biological origin that modulate neuronal signalling are thought to mediate this process. Exposure of marine invertebrate larvae to neurotransmitters, their precursors or similar molecules that have the ability to induce larval settlement, have been used in numerous studies to give important insights into the biochemical mechanisms underlying settlement behaviour.
In the present study, 16 pharmacologically active compounds were assessed for their effectiveness at inducing larval settlement in P. canaliculus. In the laboratory, mussel larvae were exposed to the chemicals for 48 hours. Among the chemicals tested, potassium chloride, acetylcholine, atropine, epinephrine, L-DOPA, hydrogen peroxide and cyclic adenosine monophosphate induced larvae to settle with minimal acute toxic effects. Exposure of larvae to potassium metabisulphite, sodium metabisulphite, ascorbic acid, caffeine, L-Tryptophan, L-Phenylalanine, and L-Tyrosine induced high levels of larval settlement but were acutely toxic to the mussels. Exposure of larvae to gamma aminobutyric acid had inhibitive effects on the settlement process. These results suggest that such compounds, many of which are present endogenously, are naturally involved in the biochemical signalling pathways of larval settlement. An exploratory meta-analysis was performed on larval settlement responses to various inductive compounds among nine marine invertebrate species, clustering the organisms by similarity of response. This investigation may have identified a novel approach to species clustering; illuminating currently concealed relationships based on the biochemical mechanisms involved in larval settlement behaviours. There also may be considerable applications for using these compounds in aquaculture to provide routine, inexpensive, and effective techniques for inducing synchronous settlement of hatchery-reared larvae.
A major concern for the green-lipped mussel aquaculture industry in New Zealand is the limited supply of larvae or juvenile mussel spat for cultivation. To stock farms, the primary source of juvenile mussels comes from ‘beach cast spat’, attached to drifting macroalgae washed up on the foreshore. Considerable effort also is made to catch planktonic larvae on ‘spat catching ropes' suspended in the ocean. In the present study, weakly surface-bound acetylcholine was tested as a spat attractant to determine if the compound has commercial applications in areas other than larval settlement. Results showed no apparent capacity for acetylcholine to modulate chemotaxis in P. canaliculus. The effect of substrate surface charge on the ability of spat to attach showed that mussels have a significant preference for positively charged surfaces. Charged surfaces in the marine environment may modulate the colonisations of particular substrata, and may have commercial applications in promoting or inhibiting biofouling. Another problem suffered in the mussel aquaculture industry is poor retention of mussels on growing ropes. Acetylcholine showed no ability to enhance spat retention in a practical capacity, and neither did alterations of surface charge.