Utilization of Algae by
Daphnia as Influenced by Cell
Senescence and UV Irradiation
R. G. STROSS, Associate Professor of Zoology
F. M. UNGER, NSF Fellow
J. C. JONES, Research Assistant
J. M. VAIL,   Research Assistant
Department of Zoology
University of Maryland
College Park, Maryland
INTRODUCTION
Suspensions of algae, bacteria and detritus are ingested by many groups of
animals. Some of the Crustacea are equipped with special appendages which are
capable of removing dilute suspensions of particles from large volumes of water.
The thoracic limbs of Daphnia act in this manner and mav collect with equal efficiency particles ranging in size from 0.9 to 18,000 jr3 (1). Within this size
range, the rate of ingestion may be in direct proportion to the volume of particles
collected by the appendages when the density of particles in suspension is at or
below a certain density (1,2). A constant volume is ingested when the concentration in the medium exceeds the critical density which, according to the above
workers, is determined by size and digestibility of the particles.
Qualities other than density and size of the particles have been shown to
regulate utilization. Nutritional deficiencies or indigestibility has been shown by
several investigators to inhibit the growth of the animal population, thereby
hindering control of the algal suspension (3,4,5). Ryther (6) showed thatfewer
cells of algae, (Chlorella, Scenedesmus, Navicala) were removed from suspension
by Daphnia when the cells were taken from senescent, or stationary phase cultures.
The possibility of artifact, resulting from larger volumes of the senescent cells
and cell concentrations above the critical density, has been raised (2) and discounted (1). In this study we measure the utilization of senescent cells of algae
and evaluate the effect of senescent and UV-irradiated algae on the intrinsic rates
of increase of Daphnia populations.
METHODS AND MATERIALS
Cultures of Chlamydomonas reinhardi Dangeard I. U. strains No. 89 and 90
were grown axenically on a 1. 5 per cent agar gel of a standard inorganic nutrient
medium (7). The cultures were incubated in 125-ml cotton stoppered flasks at
temperatures of 21.0 ± 1.0 C. The flasks were under constant light from "cool
white" fluorescent lamps at an ambient intensity of approximately 500 ft-candles.
Prior to testing, the cultures were flooded with 5.0 ml of a nonsterile medium
used for culturing the assay animal,  and allowed to stand in the dark overnight.
The test animal, Daphnia pulex De Geer, was purchased from Connecticut
Valley Biological Supply House, Southampton, Massachusetts, in 1960. Stocks
were maintained in the laboratory in a synthetic pond medium (Table I) on a
diet of   Chlamydomonas, cells.    Stock and test cultures were maintained in 50 ml
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