Life History of an Aquatic Gastrotrich

Abstract

The gastrotrichs employed in this study were collected from a pond in Vermilion County, Illinois and established in cuture in 0.1 percent malted milk solution according to the mass-culture method described by Brunson (loc. cit.). It was found that L. squammata could reproduce in isolation culture only in the presence of living protozoa. During this study, Petalomonas sp. was the accompanying protozoan. The temperature maintained was 22 degrees C. In view of the cumulative but reversible effects noted by Lansing (1947), animal A, whose progeny were to be studied, was selected as follows: an immature animal was selected from the clone and isolated in a depression slide. After oviposition, the animal was removed and the egg allowed to hatch in the depression. When the resulting animal had laid its first egg, the animal was again discarded and the egg retained. The animal resulting from this egg was designated animal A and its life history and that of its progeny studied. The succeeding animals were designated using the system employed by Packard (1936) in which the offspring of animal A are Al, A2, A3, and A4, the offspring of Al are All, A12, etc. Boerner Micro-Test Slides having ten depressions were fitted with glass covers for use as culture dishes. Animal A was introduced into the first depression of a Micro-Slide. After each oviposition, the animal was transferred to a new depression. Following its last and fourth oviposition, the animal was transferred to a fresh depression and observed until its death. The same procedure was followed for each animal studied. The Micro-Slides were examined at eight-hour intervals using a stereoscopic dissecting microscope. Those depressions in which hatching, oviposition, or death appeared imminent were reexamined after four hours, or in some instances, after a shorter interval. In those depressions in which oviposition had already occurred, the embryonic stage was determined and an approximate time of oviposition assigned based upon the timetable of the embryology which had previously been determined (Sacks, 1955). The age of an animal is calculated from the time of hatching, and time of death is considered as the time of cessation of ventral cilia beat.