Gorgonocephalus eucnemis (Echinodermata: Ophiuroidea) and Bursal Ventilation

The basket star Gorgonocephalus eucnemis is an aerobic organism highly dependent on dissolved oxygen in surrounding waters. Previous observations on the anatomy of Gorgonocephalus state that five pairs of ossicles (the radial shields and genital plates) regulate the position of the roof of the body disc and are responsible for flushing seawater into and out of the bursae, though this seems never to have been empirically tested. In the current study, rates of bursal ventilation were investigated in response to an increase in the availability of food and, separately, exposure to hypoxic levels of dissolved oxygen. When fed with suspended krill particles, basket stars increased rates of bursal ventilation, ranging from 13% to 155%, resulting in a similar increase in volume of water moved in and out of bursae. This rate remained elevated for an average of 25 minutes after active feeding ended. Bursal ventilation rates also increased significantly (~60% average increase) when basket stars were exposed to hypoxic conditions (dissolved oxygen ≤ 3.5 mg O₂ L^-1 = 2.45 mL O₂ L^-1). Some specimens exhibited a loss of coordination in hypoxic conditions. All specimens recovered and resumed a normal rate of bursal ventilation when returned to normoxic conditions. Measurements show that dissolved oxygen levels decreased from outside to inside bursae and suggest that dissolved oxygen is absorbed in bursae during bursal ventilations. Increasing rates of bursal ventilation may help meet the increased oxygen demands when feeding and may help animals endure some exposures to hypoxia.