100 Water 



Amphibians are similarly able to utilize both air and water environ- 

 ments. Many species can leave the water to forage on land, and, if 

 their pools dry up, they can migrate to other bodies of water. Such 

 animals are not limited by land barriers as most fish are. Yet am- 

 phibians must re-enter the water or visit damp places at intervals to 

 keep their skins moist, and most species need water for reproduction. 



Organisms living in swamps and pools must be able to deal with 

 periods of drought when they occur. Physiological adaptations mak- 

 ing this possible have been evolved sometimes without any accom- 

 panying special morphological structures. Certain species of fish in 

 India can live in wet grass for as long as 60 hours, and other animals 

 burrow in the mud where they remain in a dormant condition during 

 the dry period. Water mites have been shown to be able to live 

 under debris for intervals up to 6 months after their pools have dried. 

 The African lung fish, on the other hand, constructs a special mud 

 cocoon and curls up inside for the duration of the dry spell. The 

 fish secretes about its body an impervious sheath which prevents 

 the loss of water from its tissues, and is thus enabled to survive drought 

 for more than two years. Other specialized structures for tiding over 

 dry periods are the spores and seeds of water plants, and the "resistant" 

 eggs and cysts of various aquatic animals ranging from the Protozoa 

 to the Crustacea. 



Another adaptation for meeting the water problem in temporary 

 pools is that of speeding up development and taking quick advantage 

 of water when it is available. In certain animals living in this type 

 of habitat the larval stage is accelerated; in others parthenogenesis 

 has been adopted. If a single female cladoceran such as the water 

 flea, Daphnia, finds itself in a temporary pool, it does not have to hunt 

 around for a male. It can go ahead and reproduce rapidly by par- 

 thenogenesis. The young animals mature and continue reproducing 

 parthenogenetically until in a relatively short period a large population 

 has built up. Later, however, if the pool tends to dry up, the adverse 

 conditions cause the appearance of males in the population. The 

 females now produce a different type of egg, one which requires 

 fertilization. Only one of these "winter" eggs is formed by each 

 female and the brood pouch is modified into an ephippium (Fig. 

 2-11). After the death and disintegration of the female, the ephip- 

 pium remains as a resistant egg case within which the egg can survive 

 prolonged periods of drought and even freezing. In this condition 

 the egg may be blown into another pond or carried in dried mud by 

 animals to another region. When water is again supplied to it, the 

 egg hatches out and is able to start the cycle over again. 



