The number of parthenogenetic eggs and embryos carried in the brood pouch of Daphnia varies greatly. In some natural populations of the pond-dwelling species females carry many eggs,* in other populations they have few. We have found in nature a female of D. magna Straus carrying 95 eggs, and de Kerhervé (1926) had one in culture with 105 eggs, whilst, on the other hand, in some natural populations of the same species no more than one, two or three eggs are borne. Two potent causal factors influencing egg number are the quantity of food and the aeration of the water : up to a certain maximum level of feeding, the number of eggs produced is proportional to the quantity of food eaten (Ingle, Wood & Banta, 1937; Fox, Hardcastle & Dresel, 1949), and below a certain degree of aeration of the water the egg number falls off proportionately to the amount of dissolved oxygen (Fox, Gilchrist & Phear, 1951). In addition, fewer eggs are produced at a high temperature (MacArthur & Baillie, 1929; Fox & Phear, 1953). Other factors influencing the egg number are the nature of the food (Lefèvre, 1942) and the age of the mother (Anderson, Lumer & Zupancic, 1937).
Daphnia swims by oar-like movements of its pair of antennae. Swimming is necessary both for progression and to keep the body from sinking. The eggs are heavier than water, since isolated eggs sink in water; therefore the specific gravity of an animal with many eggs may be greater than that of one with few eggs in the brood pouch. This might be expected to necessitate swimming more vigorously in order to keep up in the water, and the rate of antennal movement would then increase with the number of eggs. Another factor intervening is the dorsal and posterior position of the brood pouch containing the eggs, which must change the centre of gravity of the female and perhaps necessitate a modified mode of swimming. With all this in view, we set out to count the rate of antennal movement of parthenogenetic females of D. magna carrying different numbers of eggs.
The time to 0·1 sec. for ten antennal strokes was taken with a stop-watch. This was done 10 times for each animal and the mean reckoned, from which the rate of antennal movement, in beats per minute, was calculated. Each animal was studied singly in tap water at 20° C., with standard illumination. Subsequently, the number of eggs or young was counted, after removal from the brood pouch if numerous. Of 103 individuals studied, sixty-five bore eggs and thirty-eight had empty brood pouches.
The results are shown in Fig. 1, where each point gives the relation between the mean rate of antennal movement and the egg number for one animal. It is clear that the rate of antennal beat increases with the number of eggs carried in the brood pouch. It is also evident that the relation between the two variables is not a linear one; indeed it could not have been such, or the rate of beat for 100 eggs would have been impossibly fast.
The curve of this equation has been drawn in Fig. 1.
The mean rate of antennal movement of each of the thirty-eight animals carrying no eggs in the brood pouch is also entered in Fig. 1. It is clear that the values have a wide scatter. For these animals the mean rate of antennal movement to be expected from the regression equation is of the order of 75 beats per minute, with a standard error of 4·6. Less than one half of the thirty-eight values fall within the upper 95 % confidence limits for the regression equation, and it must therefore be presumed that many of the animals carrying no eggs belong to a different statistical population from those with eggs.
Counts were made, too, of the rate of antennal strokes of eighteen females with ephippia. These data are also entered in Fig. 1. It is seen that the rate is midway between the fastest and slowest rates of parthenogenetic egg carriers.
It was necessary next to find if a possible greater specific gravity of animals with many eggs does make them sink more rapidly. This was tested by timing the fall, through a certain distance in water, of dead D. magna carrying varying numbers of eggs. It was done in a 11. glass measuring cylinder containing tap water at 20° C., within a glass museum jar of water acting as a thermostat. Rubber bands marked two levels 30 cm. apart in the cylinder, with 10 cm. of water above the upper band. Each individual Daphnia was killed by momentary immersion in water at 50° C. and then pipetted gently into the cylinder. Thirty-three individuals with and without eggs were tested but no significant difference was found between the rates of fall with many, few or no eggs.* This uniform rate of fall must mean that friction with the water has a greater effect on sinking rate than small differences in specific gravity due perhaps to different numbers of eggs. In animals killed by heat the antennae were spread out†, and, with their setae, they must have acted as parachutes. In life the antennae are, of course, not always spread out, but only during part of each swimming stroke. Between strokes heavier animals may sink faster.
A study was also made of a possible influence of food contained in the gut on the sinking rate of D. magna, as different amounts of gut contents might have accounted for some of the scatter of points in Fig. 1. Eight individuals were kept for 2 days without food in several changes of tap water so that their guts were more or less emptied. The rate of fall of these animals through 30 cm. of water, after killing them as before, was compared with that of eight freshly caught individuals with full guts. But the gut contents showed no effect on rate of fall. Eyden (1923) had tested this by keeping four individual D. pulex for 8 days, feeding them in the day-time and starving them at night. She narcotized them every morning and evening, and timed their rate of fall through 20 cm. of water. She found that they fell quicker in the evening.
It was suggested to us by Dr Barbara M. Walshe that since egg number in Daphnia depends upon feeding, individuals with many eggs might move their antennae more quickly merely because they were well fed, just as a horse is ‘fresh’, or fit for hunting, after being given extra corn. To test this in the absence of eggs we had recourse to males of D. magna. They were fed for 5 days on Chlorella, one lot receiving 10 times as much food as the other. Counts of the antennal rates of fifteen individuals of each lot showed no significant difference in the means, so this hypothesis was abandoned.
We conclude that the rate of swimming movements in Daphnia increases with the number of eggs or embryos in the brood pouch. This is not a direct result of abundant food, which might have resulted in greater swimming vigour. Although eggs may augment the specific gravity of Daphnia, yet dead animals with many eggs do not sink faster than those with few eggs or none. The faster rate of swimming movements of those carrying more eggs may, however, arise from the position of the brood pouch, which is like a knapsack, changing the animal’s centre of gravity in proportion to the number of eggs it contains. The mode of swimming of Daphnia has been described by Scourfield (1900). Between each antennal stroke and the next one the animal sinks with its tail-spine leading. The antennae beat backwards, dorsally to the line from head to tail-spine, thus making the head incline forwards. A knapsack of eggs must prevent some of the forward tilt. Therefore, to swim at the same water-level as an eggless female, one carrying eggs should make more frequent antennal strokes.
Individuals of Daphnia carrying many eggs are, on the whole, bigger than those with few eggs. This is to be expected, since abundant food increases both body size and egg number (Ingle, et al. 1937). We measured the length of 114 egg-carrying individuals of D. magna with an eyepiece micrometer, taking the distance from the crown of the head to the base of the tail-spine : the biggest measured 4·9 mm. and the smallest 2·3 mm. In each of these individuals we counted the number of eggs in the brood pouch, which varied from 1 to 58. The correlation coefficient between length and egg number was found to be +0·55 (P<0·001).* Thus the rate of antennal movement, which increases with the number of eggs carried, necessarily increases also with the size of the female. Could it be, then, that the correlation found between rate of antennal movement and number of eggs is really one between rate of antennal movement and size of female? This is not so, for the following reason. We made measurements of the lengths of thirty-three of those females carrying no eggs whose antennal rates are shown in Fig. 1 : the extreme values were 4·2 and 2·3 mm. There was no connexion here between size of animal and rate of antennal movement, for the correlation coefficient was only 0·06. Thus antennal rate is proportional to size of clutch, but merely incidentally to size of animal.
The rate of antennal swimming movements of Daphnia is proportional to the number of eggs or young carried in the brood pouch. An increased rate of antennal movement is not a direct result of the better nutrition which produces more eggs. The increased swimming rate may be necessary to counteract a change in centre of gravity caused by the posterior position of the brood pouch containing eggs.
We wish to thank Dr R. J. Whitney for statistical help.
In this paper ‘eggs’ in the brood pouch means ‘eggs or embryos’.
Eyden (1923) concluded that narcotized D. pulex sinks more slowly after the young leave the brood pouch, but she studied only one individual!
Narcosis with urethane was tried, but was abandoned because the position of the antennae was not uniform, as it is in animals killed by heat.
Big animals with numerous eggs did not have smaller eggs ; measurements were made to test this possibility, but no evidence was found in its favour.