Structure and function of the external gill filaments of embryonic skates (Raja erinacea).

We have investigated structure and function of the external gill filaments, which occur transiently in the embryonic little skate, Raja erinacea. Approximately 25-30 days after spawning (body mass 0.03-0.05 g) external gill filaments appear as an outgrowth from the caudal side of the gill arches. These filaments are thread-like, each containing one afferent and one efferent blood vessel, and by day 70-75 (body mass 0.4-0.5 g) they reach their maximum size at a length of about 1 cm and a blood vessel diameter of 70-80 microns. Subsequent resorption of the filaments is characterized by a decrease in both length and diameter of the blood vessel. By day 90-95 (body mass 0.9-1.0 g) the external gill filaments are completely resorbed and replaced by internal gills. Blood velocity, measured in these external filaments, increased with development from 0.1 mm.sec-1 to about 0.7 min.sec-1, and decreased again during resorption. Blood flow, calculated therefrom with blood vessel diameter, showed a similar maximum curve. A model analysis supports the hypothesis that in a full grown filament respiratory gas exchange is mainly perfusion-limited and can contribute significantly to the total oxygen uptake of the embryo. Analysis of the results indicates, however, that the gill filaments are not adequate as a gas exchange organ for later developmental stages.