The Seasonal Life History of Daphnia in an Arctic Lake

Edmondson, W

1. Daphnia pulex var. tenebrosa Sars 1907 was a prominent member of the zooplankton of Imikpuk, a shallow lake near Point Barrow, Alaska, sampled quantitatively in 1951 and 1952. It is an abundant and widespread arctic species, but is occasionally found at lower latitudes. 2. Changes in the density and the size composition of the population are described (Fig. 1). Adults died out during the winter, and the population overwintered as ephippial eggs. Animals hatched from ephippial eggs just as the ice was melting around the edge of the lake early in July, 1952, and in the course of three weeks grew to maturity. Apparently sporadic mortality caused decreases in the population, which was replenished by continued hatching of ephippia. One brood of non-ephippial eggs was produced which gave rise to a small generation that apparently produced ephippial eggs in September. Their development was followed in detail (Figs. 1 and 2). None of these animals produced non-ephippial eggs. This generation formed only a small part of the total population. 3. Most of the exephippial animals produced ephippial eggs as their first brood. The possibility exists that some of the ephippia produced in 1952 hatched before the lake froze in September, making an increase in population. Thus there is one principal generation each year with exephippial animals producing ephippia. There is a subsidiary generation of non-ephippial eggs by some of the exephippial animals. Further, possible hatching of ephippia during the year in which they were produced makes a second subsidiary generation. Ephippial eggs are more important than non-ephippial eggs in reproducing the species. The population size achieved in any year is highly dependent on the factors affecting survival and hatching of ephippia. 4. The duration of the embryonic period of the non-ephippial eggs was between 10 and 12 days. The water temperature varied from 7.0░ to 10.5░ during development. 5. An analysis of the probable instar composition of the population was made on the basis of length-frequency data (Fig. 3), and the growth rate determined (Figs. 4 and 6). 6. The length-weight relation was determined (Fig. 8), and from it the mass of the population computed (Fig. 4). 7. No males occurred. The ephippial eggs evidently do not require fertilization. 8. A discussion is given of some of the published work on Arctic Daphnia populations and related laboratory work.

The Seasonal Life History of Daphnia in an Arctic Lake

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