The CO2/pH ventilatory drive in fish

That ventilation in fish is driven by O-2 has long been accepted. The O-2 v entilatory drive reflects the much lower capacitance of water for O-2 than for CO2, and is mediated by O-2 receptors that are distributed throughout t he gill arches and that monitor both internal and external O-2 levels. In r ecent years, however, evidence has amassed in support of the existence of a ventilatory drive in fish that is keyed to CO2 and/or pH. While ventilator y responses to CO2/pH may be mediated in part by the O-2 drive through CO2/ pH-induced changes in blood O-2 status, CO2/pH also appear to stimulate ven tilation directly. The receptors involved in this pathway are as yet unknow n, but the experimental evidence available to date supports the involvement of branchial CO2-sensitive chemoreceptors with an external orientation. In ternally-oriented CO2-sensitive chemoreceptors may also be involved, althou gh evidence on this point remains equivocal. In the present paper, the evid ence for a CO2/pH-keyed ventilatory drive in fish will be reviewed. (C) 200 1 Elsevier Science Inc. All rights reserved.