Johnson, D. S. (1951)
A study of the physiology and ecology of certain Cladocera.
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Comparatively little work has been done on the ecology of Cladocera except in regard to planktonic varieties, and special subjects such as cyclomorphosis. This thesis is an attempt to ascertain the factors governing the occurence and distribution of Cladocera, by means of a special, mainly laboratory, study of certain factors combined with a general field-survey. Temperature, the factors influencing parthenogenetic reproduct ion. and food-supply were specially studiedIt was not possible, except for a few varietie, to correlate resistance to high temperatures with occurence and distribution. Resistance to low temperatures was correlated with seasonal but not with spatial occurence. The effects of temperature, crowding, amount of food, and light on parthenogenetic reproduction were investigated. Increased temperatures increase frequency of moulting, and shorten the duration of the pre-adult phase. Egg-number may or may not be reduced, the behaviour being probably associated with the seasonal occurenece of the species.The other factors all have marked effects on egg-number, varying some-what as between species. They have no effect on frequency of moulting or logevity. Food and temperature effects show no inter-action. A general survey was made of the distribution of Cladocera in lakes and pools in Scotland, England, and Denmark. Notes were made on individual species. The opportunity was taken of clearing up some taxonomic difficulties. Within habitats Cladocera often show marked localization. Factors responsible for this include food-supply; presence and nature ofvegetaion ; water-movement; and type of bottom. It is more difficult to correlate Cladoceran distribution with the general nature of the habitat. Most,but not all, Cladocera appear to be indifferent to trophicity and pH. Larger habitats have more varied faunas, and also somewhat acid habitats. General stability of the habitat is probably important.Parasites are often fatal but are tooo rare to be important. Epibiotes are usually harmless. Many species prey on Cladocera, but only fish and tadpoles certainly control their numbers. Competition is probably very important in determining distribution, but related species often occur together. Apaper on Daphnia systematics ( in Ms. form ) is included with this thesis.
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Institution: University of London, Bedford College (United Kingdom).