EMERSION OF THE AMPHIBIOUS CHILEAN CLINGFISH, SICYASES SANGUINEUS.

The amphibious clingfish Sicyases sanguineus attaches by means of its ventral sucker to vertical surfaces of large exposed rocks splashed by the cool and heavy surf of Chile and southern Perú. Although adult and halfgrown fish tolerate diluted seawater and can survive at least a few hours in warm and stagnant water, they seldom, if ever, occur in isolated tidal pools, bays, or estuaries. Factor analysis indicates that clingfish come out of the water more abundantly during periods of calm and often turn head-down. They avoid drying rocks outside the spray zone and emerge onto higher rocks as the water level rises. Adult fish come out of the water more abundantly in remote areas relatively undisturbed by civilization. Insolation apparently does not directly alter the abundance of clingfish, which act so as to minimize evaporative water loss and overheating. Terrestrial fish breathe air held in their gill cavities, probably through their gills. As they come out of the water, fish gulp air, then stop all opercular movements to seal their cavities, and often turn head-down. This positioning may facilitate airbreathing by easing the expulsion of spent gas upward through the watery gill cavities and by shading an area of delicate respiratory epithelium under the chin. The volume percentage oxygen in gas expelled into a viscous solution of resin in sea water decreased regularly for about 12 minutes in a young fish, about 30 minutes in a halfgrown fish, and about one hour in an adult before these fish renewed their branchial gas. Although a concomitant increase in percentage carbon dioxide indicated that the branchial gas contributed to respiration, rates of oxygen uptake calculated from modal volumes of expelled gas were only about 12-30% of the fish's total long-term rate in air as determined by other investigators. As in other airbreathing fishes, however, cutaneous respiration may supplement branchial respiration, which fills the total need only when the metabolic rate falls. Exposure of their head apparently stimulates clingfish to gulp, so that atmospheric air, rather than aquatic hypoxia, is their primary stimulation to airbreathing. Fish in stagnating water do not necessarily emerge and occasionally remain under water until they suffocate. Like other airbreathing fishes, clingfish appear insensitive to relatively large proportions of carbon dioxide in their branchial gas. And even with their opercles closed and their ventral mouths pressed against the substrate, clingfish in the field should easily eliminate excess carbon dioxide through their wet skin. Vargas and Concha (1957a) emphasized the ontogenesis of terrestriality in clingfish from erratic young to well-regulated adult: modification of gill surface to minimize clumping, control of aerial oxygen uptake, and greater composure on land. The present study indicates that exposure of an anteroventral respiratory membrane, assumption of a head-down position, improvement of gulping technique, control and slowing of ventilation, and greater efficiency of exchange between branchial gas and blood also contribute critically to the maturing fish's increasing independence of aquatic life.