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The eyclom orphosis of a natural population of Daphnia (D . gessneri H erbst, 1967) in Am ericana Reservoir (B razil) is investigated during one year. T he structures considered w ere helmet, head basis, carapace and spine of carapace, m easured in two ins ta rs : new born and last stage adult. T he relative length of the helm et and of the spine showed variation in time, w ith sum m er incre­ ment, while the head basis show ed a less expressive variation. T hree environm ental factors w ere considered: tem perature, turbulence and nutrition. T here are no linear correlation betw een the ratios he lm et/carapace, sp ine/carapace and the tem perature, but the la tte r seem s to have influenced above a limit (25°C). N utrition, estim ated indirectly through the egg production, seem s to have affected the proportions, but the cyclical variation of food is difficult to evidence in the environment. It is possible th a t the relative length of the mentioned structu res w as influenced by the jo in t action of the three factors. The eye size w as also considered, being sm aller in the low helm eted form. I N T R O D U C T I O N The eyclomorphosis in Cladocera, mainly in species of Daphnia, was investigated in detail in temperate regions. In tropical and subtropical ones, generally speaking, observations withouth mensuration data were made. The investigations, based on laboratory populations, natural popula­ tions, or both, have searched the causes and the adaptive value of eyclomorphosis. The environmental factors more intensely investigated as possible causes were: temperature (Ostwald, 1904; Wesenberg-Lund, 1926; Coker and Addlestone, 1938; Brooks, 1946; Jacobs, 1961a; Hazel­ wood, 1962, and Egloff, 1968), turbulence (Brooks, 1947; Hrbâcek, 1959, and Jacobs, 1961a), turbulence allied to light (Jacobs, 1962, and Hazel­ wood, 1966), and nutrition (Coker and Addlestone, 1938; Brooks, 1946, and Jacobs, 1961a). The adaptative value is still a very discussed theme. The hypotheses are concerned with density and viscosity of the water and the buoyancy of the organisms, defended by Ostwald and Wesenberg-Lund; with stabi­ lization and direction of the animal (Woltereck, 1909); with predation (Brooks, 1965; Green, 1967; Jacobs, 1967, and Dodson, 1974). It was even suggested that this variation of form has no adaptive meaning, although Jacobs (1961b) searched to prove the contrary. This paper deals with an attempt to verify the occurrence of cyclomorphosis in a species in tropical conditions and to test the factors known as important in the process in temperate zones. The lack of knowledge on the subject in tropical regions makes it exceedingly hard to extend the conclusions from researches in temperate regions to our latitudes. In this work an analysis is done of the monthly variation of the relative length of the head (divided in two parts) and of the spine of the carapace of Daphnia gessneri in two instars — newborn and adult — during one year (with a gap of two months). The factors to which the structure are related a r e : temperature, turbulence and nutrition. The results, however, are preliminary and intend to be a basis for further studies. I would like to express my deepest thanks to Dr. Claudio G. Froehlich, Dr. Antonio Carlos Azevedo and Miss Gisela Y Shimizu. MATERIAL AND METHODS Americana Reservoir is located in São Paulo State, Brazil, 22°44’20” S Latitude and 49°19’52” W Longitude. The samples were collected monthly at Station 6 (see Zago, 1976) from June, 1969 to June, 1970. Sampling was made with a net 85 ./¿mmeshed through vertical hauls. In the present work the attempt to utilize the allometric analysis introduced by Brooks (1946) failed, for k is not constant through the instars. In such case, two stages were chosen: newborn and last stage adult. From each sample 36 newborn and 50 egged parthenogenetic females were measured. Each specimen was measured in profile, at the micros­ cope with the help of a camera lucida. The eggs were counted and the averages of eggs per adult were computed. In fig. 1, the measured regions are delimited by parallel lines. The head is divided in two parts (according to Jacobs, 1961b): the helmet (E ) , delimited by the apex of the head and the anterior limit of eye pigmentation, and the basal part (B ), whose posterior delimitation is the tangent to the head limit. The carapace (C) is posteriorly limited by the line that passes by the end of the opening of the carapace, shown in the same figure. The spine (S) is measured from this line to the end. The length of the measured structures — helmet, basis of head and spine — was related to the carapace length — E/C , B /C and S /C . The average, the standard deviation and the confidence interval (9 5 % ) were calculated for each sample. The temperature values represent averages from superficial and deep water temperatures. The eye, from last instar adults, was measured according to Zaret (1972) at the microscope with a camera lucida. Twenty specimens were obtained from the samples of 2 5 /9 /6 9 and 28 /1 /70 . Fig. 1 — Outline of adult female of D aphnia gessneri, in profile, showing the measured regions. E — Helmet; B — Basal p a r t of head; C — Carapace; S — Spine of carapace. Fig. 2 — Outlines of high and low helmeted forms.

A preliminary investigation on the cyclomorphosis of Daphnia gessneri Herbst, 1967, in a Brazilian reservoir