Observation on the Breeding and Development of the Viviparous Fish, Heterandria Formosa. by Elizabeth A
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Observation on the Breeding and Development of the viviparous fish, Heterandria formosa. By Elizabeth A. Fraser, D.Se., Reader, assisted, by Rachel M. Eenton, Curator of the Aquarium. Department of Zoology, University of London, University College. Witt Plates 26-29, and 13 Text-fignres. Breeding.—Heterandria formosa, one of the small- est viviparous fish, has been known for many years as a common species in tropical aquaria. It is a member of the family Poeciliidae (Garman, 1895-7; Began, 1913; Hubbs, 1924,1926), and a native of fresh-water streams from South Carolina to Florida. Although some records as to its breeding are available, no details of its development or mode of viviparity are known beyond the fact that the egg has little or no yolk and the embryo does not leave the follicle until ready for birth. We have now bred the fish successfully in the aquarium at Univer- sity College, London, for four and a haK years in order to obtain exact information of its habits and life-history. The fish have been kept in small rectangular tanks holding from 3 to 4 litres of water with a layer of coarse sand at the bottom, and in each were usually one adult female and two males. The tanks were placed in a specially heated room which varied in temperature from 20° to 26° 0. Heterandria are hardy and easy to keep; they can stand quite a large range of temperature, but seem to thrive best in water between 22° and 26° C. They are of a placid temperament and rarely fight, and have never been known to eat their young during the four and a half years that they have been under observation.1 Their food has consisted of Enchytraeid worms, finely minced, or small Daphnia, and the young were fed on newly hatched Ar- 1 A fairly large female since bought (1939) has been unusual and habitually swallows her young as soon as they are born. NO. 324 I i 480 ELIZABETH A. FRASER AND RACHEL M. EENTON temia salina. It was never found possible to induce these fish to eat anything but live food, although many of the usual dried fish foods on the market were tried at different i imes, but all without success. On the whole they were remarkably healthy and there was little disease; it was found, however, that if several males were kept in a tank together without a female, there was quite a heavy mortality among them. Copulation was never observed although the male follows the female unceasingly, and it is quite probable that it takes place at night. The birth of young has frequently been watched, the young being born tail first. Usually they are strong and active almost at once, and after resting on the sand for a few minutes they swim to the top to fill the air bladder. Occasion- ally a fish1 (usually a very young female) will give birth to one or two weaklings, which would appear to be born prematurely. These weak young have great difficulty in reaching the surface of the water, they lie on the sand and make repeated efforts to rise. Unless successful soon after birth they do not live long. The life of a female appears to vary considerably with different individuals, but lasts from two to three years. The males have a shorter span. Lebistes, the millions fish, and many other viviparous forms give birth to a large number of young at one time, with an interval of three or four weeks between each delivery. Heterandria, on the other hand, though much less prolific, breeds fairly continuously from February to September or October, with intervals between the births, but produces on an average only one to three at a time. The numbers tend to decrease towards the end of the summer, when a resting period sets in until the following February. Very occasionally an exceptional female will give numbers in excess of this, one individual having had ten at one time on more than one occasion. A female bought in November 1933, of unknown age, gave rise to fifty-one young before she died in June 1935, apparently of old age. Females, when at their prime, usually measure about 25 to 30 mm. from the tip of the snout to the end of the tail, whereas adult males do not usually exceed 17-5 mm. The largest females examined by Eegan (1913) measured 30 mm., whilst the males varied from 15 to 20 mm. DEVELOPMENT OF HETEEANDEIA 481 A remarkably large female adult, when acquired in April 1934, must have measured at least as much as 40 mm.; the exact measurement of her maximum length was not possible, for, when destroyed in May 1936, she was sick and dying and considerable shrinkage had taken place. During this period she gave birth to 170young, of which 150 were born in eight months; she showed signs of diminishing fertility only during the last few months of life, and after death her ovary was found to be a mass of de- generating tissue. Such fertility seems to be very exceptional. The newly born young measure on an average 7-5 mm. At about four weeks old the females can be distinguished by the appearance of a dark spot at the base of the anal fin, but the males cannot be recognized until the anal fin becomes modified into the long copulatory organ, a transformation which takes place quite suddenly at a later date, at an age of about eight weeks. Whilst the males are not capable of copulating before this change is complete, the females may receive the sperma- tozoa from an adult male before the dark spot appears and when they are still quite small. Some young which had been left in the parents' tank until the characteristic spot had become visible, were found to contain embryos a few weeks after removal. Fertilization in this case must have taken place through the male parent. By isolating the young shortly after birth, and separating the females as soon as the dark spot is perceptible, a number of virgin females can be secured. A few weeks later these may be mated with a male and some idea can thus be obtained of the time necessary for fertilization to take place. Accurate data are, however, difficult to obtain, for although the spermatozoa may be introduced almost im- mediately by the male, fertilization does not occur for some days owing to disintegration of ripe ova. Degeneration of the eggs appears to be quite common in viviparous fish, and has been observed in several forms, as well as in other Teleosts; in Heterandria it is especially marked in young females that have not been fertilized. The nearest approach one can make is that fertilization in isolated females seems to ensue in ap- proximately three weeks after the introduction of the males, for embryonic stages of a few cells have been observed about 482 ELIZABETH A. FBASEE AND RACHEL M. BENTON this time. In order to obtain more accurate information a young female was isolated in a separate tank. Fifty-one days later she was mated, and the first young were bor:: fifty-six days afterwards. Assuming fertilization does not take place for about three weeks, the gestation period would be about thirty-five days or about five weeks. This isolated female has had fifty-four young from January 1935 to May 1936, and she died the following July, having attained a length of 38 mm. Females in which the first young are developing measure about 15 to 17 mm., and at first only one or occasionally two are born at a time. As the mother increases in size the number of young also increases. It seems to be the rule for an actively breeding fish to have one to three at a delivery, though the young are not necessarily born on consecutive days, and intervals of one to two weeks may even occur; but this number may be greatly exceeded in exceptional cases. For example, the very large female measuring 40 mm. has brought forth no less than nine young between the hours of twelve and four in the afternoon. It must be borne in mind that these data are for fish bred exclusively in an aquarium under a necessarily artificial en- vironment, and how far they conform to natural conditions we have had no opportunity of ascertaining. Some years ago Seal (1911) published a few observations on the breeding habits of Heterandria formosa, and as far as they go these tally with ours. Recently Turner (1937a), in a paper on reproductive cycles in Poeeiliid fishes, contributed some further details for Heterandria. The numbers of young at a birth and the intervals between the broods corre- spond in general with the average recorded by us. His fish, however, begin to breed in December, and continue until the end of May or July. The resting period is therefore rather longer than in our aquarium, where it begins in October or November and lasts until near the end of January. Technique.—The fish were chloroformed and the ovary was then cut out, and either fixed entire or the larger embryos were removed and preserved separately. The fixation of later stages and isolated embryos presented no difficulty, the usual fixatives, such as Bouin and picro-nitro-osmic, giving satis- DEVELOPMENT OF HETEBANDEIA 483 factory results; but for the earlier stages where the zona is thick and firm, penetration was no easy matter, and moreover the fluid-filled vesicle readily collapses.