Early Development of Pseudacris clarkii

Abstract

THE spotted swamp tree frog, Pseudacris clarkii (Baird), ranging from eastern Texas to southern Kansas, has been treated generally as a subspecies of the swamp tree frog, P. nigrita, and is so listed in the fifth edition (1943) of Stejneger and Barbour's Checklist. Smith (1947) found breeding populations of P. n. triseriata and P. clarkii in the same ponds, but without mixing, in areas of overlap in Texas. He inferred that clarkii is a distinct species, and has discussed the differences. Youngstrom and Smith (1936) described the tadpoles of P. n. triseriata, as Green (1938) has done for P. brachyphona. Descriptions of eggs and tadpoles of some other forms of Pseudacris are presented by the Wrights (1929, 1931, 1942). We collected eggs and adults of P. clarkii near Hackney, Cowley County, Kansas, April 26, 1948, in a temporary rain pool. The egg masses were conspicuous and abundant, attached to dead weed stems as shown in Wright's Handbook (1942: 110). By following the development of the more recently laid eggs we obtained an approximate chronology of the embryonic and early larval stages, up to the attainment of definitive tadpole characters. Comparison with numbered stages of Rana pipiens (Shumway, 1942; Rugh, 1948) shows that development of P. clarkii up to the time of hatching goes at approximately the same rate, but the change to an active tadpole is then completed more rapidly. For example, the gills had become internal and the operculum was complete at about 41/2 days from the presumed time of fertilization in clarkii instead of 71/2 days (pipiens) at 25? C. The growth of the tadpole, however, is much more rapid in Rana, for a considerably larger size is reached before metamorphosis. Another point of interest is the failure of the external gills in P. clarkii to become slender and functional. They are covered by the operculum while still in a rudimentary form (Fig. 16). This abbreviation of a step in larval development may be correlated with the reduced size of Pseudacris embryos, which are perhaps small enough to permit adequate respiration through the body surface alone. The changing features of early stages may now be summarized: 24-36 hours (Figs. 1 and 2). Gastrula and yolk-plug. Average vertical diameter of 16 gastrulae 1.28 mm., extremes 1.0-1.66 mm. (measured in life by ocular micrometer). Color pale brown, not heavily pigmented, which would be an advantage in experimental embryology, although the small size is a handicap in manipulation. Wright (1942) gives citrine drab and ivory yellow for the animal and vegetal poles, respectively, of the eggs.