VENTILATORY MECHANISMS AND THE EFFECT OF HYPOXIA AND TEMPERATURE ON THE EMBRYONIC LESSER SPOTTED DOGFISH

Small, intermediate and large-sized embryos of the dogfish Scyliorhinu s canicula utilize different ventilatory methods; small and intermedia te embryos rely on body movement alone to stir either the jelly or sea water in the capsule, large embryos use conventional pharyngeal pumpi ng to pump water through the case. The effects of environmental change s in O-2 tension (0.5-100% air saturation) and temperature (6-18 degre es C) upon ventilatory mechanisms in the developing embryo in situ wer e studied using non-invasive ultrasonography. Ail three embryo classes increased ventilation rate with rising temperature: for small embryos , y=2.02x+3.295 (P<0.01); for intermediate embryos, y=3.51x+0.395 (P<0 .01); and for large embryos, y=381x+9.39 (P<0.01); where y=ventilatory frequency (tail beats min(-1) or pump cycles min(-1)) and x=temperatu re (degrees C). Q(10) (6-16 degrees C)=5.0, 2.45, and 2.08 for small, intermediate and large embryos, respectively; corresponding Q(10) (5-1 8 degrees C) values were 2.09, 2.62, and 2.02. It is suggested that th e extreme response of small embryos to 6 degrees C is related to a dif ferent state of development in either chemoreceptors or muscle blocks. There was no significant change in ventilatory frequency induced by c hronic (2 h) hypoxia. Dogfish embryos are oxyconformers al 8 degrees C but oxyregulators at higher temperatures. Water flow through an eggca se occupied by a large embryo was studied also. Water enters the open eggcase of a large embryo, drawn in by the buccal/opercular pump of th e respiring embryo, via holes at the posterior end of the eggcase. Exp ired water exits holes at the anterior end of the eggcase. The mean re sidence time for water in the case is 50 s at 8 degrees C, giving a tr ansit velocity of 1.36 mm s(-1). (C) 1996 The Fisheries Society of the British Isles