Circulation of water as a main factor influencing the development of helmets in Daphnia cucullata Sars

Abstract

1. The problems concerning the development of helmets in Daphnia cucullata SARS were studied. The height of the head was expressed in per cent of the total length of Daphnia. The relative length of head was used as a measure characterising the size of helmet 2. In relatively deep small ponds with surface area less than 1 hectar the relative height of head in adult specimens reaches its maximum in autumn i.e. at the beginning of circulation. 3. The relative height of the head in neonata instar at 20°C is different in different populations. In the clones from 3 ponds with surface area more than 15 hectare the relative height of the head in neonata instar at 20° C is greater than in 4 clones from small ponds the surface of which is less than 1 hectar. 4. Populations possessing under natural conditions a low helmet were cultivated in the laboratory; their adult descendants did not develop the helmet at all. In the descendants of populations with high helmets a reduction of helmet could be observed after 5–10 generations cultivated in vessels in laboratory. 5. In the descendants of specimens which under laboratory conditions ceased to develop helmets in adult instars this ability is renewed when a water circulation is produced in the cultivation vessels by means of air bubbles. In this way we obtained higher helmets under laboratory conditions than could be ever found in populations under natural conditions. 6. According to our observations the function of the helmet can be interpreted as an adaptation to resist more easily the water current. 7. The role of fishes as selective factor is discussed.