Biology of Branchinecta Mackini and Branchinecta Gigas (Crustacea: Anostraca)
LcshShrimps - Alberta
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AbstractThe biology of two species of fairy shrimp was investigated. Branchinecta mackini is the smaller species and is primarily a filter-feeder. Branchinecta gigas is the largest known anostracan and is carnivorous, feeding mainly upon diaptomid copepods and g. mackini. Both species were found in a very shallow, turbid, alkaline lake in east-central Alberta. The lake can be classified as a sulphate saline type. The shrimp hatched immediately after the spring thaw in early April. Their growth curves had a tri-phasic form. Growth was very rapid for the first 5 or 6 weeks. A 1 to 2 week period of reduced growth followed which may be associated with the maturation of the gonads. Thereafter, growth resumed at a rapid rate for 5 to 6 weeks and subsequently levelled-off by early July. The maximum mean body length of B. mackini was 27.1 mm at an age of 107 days. There was no significant difference between mean male and female body length. B. gigas males attained a maximum mean length of 56.6 mm at an age of 100 days; and the females, 66.4 mm at 91 days. 100% of B. gigas females were sexually mature at an age of 49 days, which was 28 days earlier than the corresponding development in B. mackini. Maximal egg production for B. mackini occurred about the 95th day, after which it declined; and for B. gigas, on the 84th day, after which no further observations were possible. Under experimental conditions, B. mackini produced one clutch of eggs every 3 to 4 days. Each female had a potential egg production of 1,200 eggs in a lifetime. Egg production by B. gigas was not assessed. The maximal clutch size for B. mackini was 332 eggs; for B. gigas, 670 eggs. B. mackini females slightly out-numbered males in all but the last sample. B. gigas males were more numerous than females in the final 4 weeks. Although the lake contained water until late August, no B. gigas were found after 100 days, and no B. mackini after 107 days (21 July). There is no evidence that chemical or physical factors were entirely responsible for the disappearance of the shrimp, although high temperatures may have played a part. Predation was not significant. There is an indication of senescence in both populations at the time of their disappearance. Their growth rates had nearly levelled-off and the rate of egg production was declining. The life span of B. mackini was 15 weeks; B. gigas, 14 weeks. The data suggest that B. gigas males may have a slightly longer life span than the females. The feeding behaviour of B. gigas was investigated. The observations support the hypothesis by Fryer (1966).
Bibliography: p. 69-73.