The most striking phenomenon in the physiology of Artemia salina L. is the diapause phase under the form of a cyst in the gastrula stage. Thus for example, can dried cysts resist to extreme stress conditions, without affection of their hatchability after hydration. They withstand complete dehydration exception made for some traces of residual water (Whitaker, 1940); they resist to a temperature of 100°C for more than an hour (Hinton, 1954) , and can be kept at the absolute zero temperature for an indefinite period (Skoultchi and Morowitz 1964). Eventhough hydrated cysts are less tolerant of these "extravagant" conditions, they nevertheless are only slightly sensitive to anoxia for periods up to 5 months (Dutrieu, 1966). The system inducing diapause is the dehydration by osmotic withdrawal of water (increased salinity of the environment) or by simple dehydration (e.g. after being washed ashore) . After rehydration of the dried cysts the metabolism and further development can be started again. With regard to respiratory metabolism, the diapause situation is characterized by the absence of activity or by an unmeasurable activity (Muramatsu, 1960). During the first 3 hours after the onset of cyst's development (hydration), the respiration increases fast (0-1.2 µl O2/mg WD.h) , for a short time it decreases around the 6th hour to 1.0 µl O2 and increases furthermore to 1.5 µl O2 after 13 hours. The respiration remains constant until the emergence (18-24 hours) with which the oxygen consumption increases again to 1.7 µl O2. A third plateau is reached after an increase up to 2 µl O2 during the hatching (28-38 hours) (Emerson, 1967) . Hereby Artemia follows quite accurately the 2/3 rule which states that with increasing weight the respiration rather increases with the surface (2/3 of the weight) than with the weight. Adult male Artemia , however, acclimated in 35 promille seawater, do not seem to follow this rule (Gilchrist, 1959) . If, however, they are reared in 140 promille seawater, the rule holds. This phenomenon could be explained by Gilchrist, by demonstrating that the surface of the second antennae (which in the males are transformed into large flat claspers) increase faster in size with increasing body size, in animals acclimated in 35 promille than in those acclimated in 140 promille. Due to this fact, the oxygen consumption at the first salinity, increases faster with increasing body size, than is current according to the 2/3 rule. The influence of the osmotic pressure upon the respiration of developing embryos was studied by Clegg (1964) .The oxygen consumption decreased with increasing concentration of the medium (NaCl of 0.25-2 molarity). The respiratory quotient (RQ = ratio of the volume CO2 released to the volume of O2 taken up) during the development of the cysts, remains a source of discussion . From the RO values one all too easily concludes which fuel was used for the development. Thus Muramatsu (1960) , who found a rather constant RQ-value (0.90- 0.93) from 1 to 13 hours after the onset of hydration, and who concluded that the fuel had to be glycogen. During emergence, the RQ is lower, namely 0.71 (Dutrieu, 1960; Emerson, 1963) and in this case lipids would be burned up. Even though some biochemical approaches exist, such as those by Ewing (1969) and Clegg (1964) who found that the initial fuel had to be trehalose, it appears to us, that more in depth research is required to ascertain these findings. The RQ-values, 1 for carbohydrate metabolism and 0.7 for lipid metabolism, are indeed only valid when these substances are used exclusively as respiratory substrate. In growing tissues for example in which sugar is converted into protein, the RQ exceeds 1 (to 1.2 in the fattening of pigs and geese) , when reserve materials are being used (e.g. conversion of lipids into proteins) , the RQ decreases (to 0.4 at the awakening of hibernating animals). Since at the embryonic development of Artemia we are dealing with growing organisms, it seems to us that pure carbohydrate is not probably used as fuel. We consequently consider this fact in conjunction with the complex conversions of carbohydrates during the first development, as a problem to be solved by far reaching biochemical research. The influence of temperature and salinity on the respiration of nauplii was determined by Engel and Angelovic (1968). They found a distinct higher oxygen consumption at low salinity (2-5 promille) and by extrapolation of the curves they determined a respiratory metabolism equal to zero between 5°C and 8°C. These are indeed the lowest temperature limits for Great Salt Lake Artemia these authors used. Dutrieu (1960) also gave some respiration values for nauplii. The influence of acclimation of adults to various salinities was researched by Kuenen (1939) and Gilchrist (1956). At increasing salinities, the abdomen becomes longer and more narrow, the caudal furcae becomes smaller, and less setae occur. The females lay less eggs and the beat of the thoracopods is slower. As far as respiration is concerned, the 2/3 rule is followed. No apparent difference in respiration is to be noted between animals acclimated in seawater of 35 promille and those acclimated in 140 promille. Because at higher temperatures more energy is consumed for osmoregulation, in order to safeguard the hypotonic level of the blood, Gilchrist believes that energy is being saved in other areas such as growth and reproduction. Although a decrease of the dissolved oxygen at 100% saturation accompanies an increasing salinity, little research has been done on this point, which could be of primal importance with regard to the physiological reactions of Artemia to its environment. It is indeed known that extra haemoglobin is synthesized as a result of low oxygen contents (Gilchrist, 1954). Although far from being complete, some respiration measurements were carried out in these conditions by Gilchrist (1956, 1959) and Dutrieu (1960). Dutrieu (1960) furthermore combines the formation of cysts at low oxygen levels with the occurrence of haemoglobin. Sorgeloos (1975) extended this theory to an "oxygen controlled reproduction theory" which would offer an ecological explanation for the alternating reproduction mode of Artemia in nature. Also on this subject, detailed research is necessary. Going through the various publications concerning respiration, it strikes us that an attempt was never made, neither to determine the standard and active respiration, nor the entire activity field. Finally, the very important study of the respiratory pigments will be dealt with elsewhere. |
