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Ventilatory Reactions of the Lugworm Arenicola marina (L.) to Ambient Water Oxygenation Changes: A Possible Mechanism

The ventilation of the lugworm was studied at different controlled PO₂ values in the ambient water at 20 C. When ventilation in normoxia is taken as a reference value, hyperoxia (300 and 500 Torr) depressed ventilation, mild hypoxia increased ventilation slightly, severe hypoxia depressed ventilation, and extreme hypoxia arrested ventilation. At each controlled PO₂, 30-s exposures to oxygen-saturated water (PO₂ ca. 700 Torr) were performed-the "oxygen tests"-and the subsequent ventilation were measured. Oxygen tests led to no change of ventilation in hyperoxia, a fall of ventilation in normoxia and mild hypoxia, and an increase of ventilation in severe and extreme hypoxia. This ventilatory behavior is best explained by assuming, as a base ventilation, the low ventilation observed in hyperoxia that corresponds to the spontaneous activity of a pacemaker located in the nerve cord. The decrease of ambient PO₂ toward normoxia and mild hypoxia activates a chemoreflex ventilatory drive, and ventilation increases. With stronger hypoxia, ventilation is inhibited by the activity of special chemoreceptors located near the tip of the tail, which discharge maximally in extreme hypoxia and anoxia. The functional significance of this unique ventilatory behavior relates to the way of life of the lugworm. Within its gallery in the sand of the intertidal zone, it is cyclically exposed twice a day to normoxia at high tide and to complete anoxia at low tide.