Electrophysiological responses of the clam (Ruditapes decussatus) osphradium to amino acids and alarm cues.

Chemical sensing of the surrounding environment is crucial for many aspects of bivalve biology, such as food detection and predator avoidance. Aquatic organisms strongly depend on chemosensory systems; however, little is known about chemosensory systems in bivalves. To understand how the carpet shell clam (Ruditapes decussatus) senses its surrounding chemical environment, we used an electrophysiological technique - the electro-osphradiogram - to assess the sensitivity of the osphradium to different putative odorants (amino acids, bile acids) and odours (predator-released cues and signals from con- and heterospecific bivalves). The clam osphradium was sensitive to most proteinogenic L-amino acids, evoking negative, tonic, and concentration-dependent responses. However, acidic amino acids (L-glutamic and L-aspartic acid), L-arginine and bile acids (cholic, taurocholic and taurolithocholic acid) failed to evoke any response. Surprisingly, while cues from injured bivalves (con- and heterospecific) evoked strong responses, predator-released cues (green crab, Carcinus maenas) failed to elicit any response, whether fed or unfed. That predator-released cues failed to evoke an electrophysiological response in the clam osphradium may indicate that they use cues released by injured prey - alarm cues - to avoid predation and/or that predators are detected by different sensory modalities. Indeed, the behavioural assays, performed to understand how clams make use of such sensory inputs, revealed that the activity index decreased after exposure to water conditioned with injured conspecifics, suggesting the origin of such alarm cues. Further research is needed to identify the chemical nature of these cues. We suggest that the electro-osphradiogram will be a useful tool in this endeavour.