Detecting the shellfish killer Heterocapsa circularisquama (Dinophyceae) by measuring bivalve valve activity with a Hall element sensor

Abstract

We developed a system for the early detection of noxious dinoflagellate Heterocapsa circularisquama blooms by utilizing the response of Akoya pearl oysters ( Pinctada fucata) to H. circularisquama cells. The system is based on valve movement activity in bivalves. The movements were measured by means of a magnet on one valve to a Hall element sensor on the other valve connected by thin cables to a recording device. In filtered sea water, valve movement activity of pearl oysters as displayed on the output screen of the detection system showed sporadic thorn-like spikes, indicating the shell closing for a short time, with a flat waveform indicating that the shell was open. The frequency of these spikes increased as the number of H. circularisquama cells in the filtered seawater increased. However, the frequency of the spikes almost immediately decreased when the water was changed to filtered seawater with no H. circularisquama. These results indicate that this system using the Hall element sensor can detect low densities of H. circularisquama cells in real time without needing complicated apparatus.