Adult Daphnia galeata mendotae (left) and Daphnia pulicaria, both with subitaneous eggs. The Daphnia pulicaria is about 3mm long.
Zooplankton (microscopic organisms suspended in water) are an important component of a diverse array of aquatic systems. Although zooplankton differ in form and function, one trait common to a variety of species is the ability to produce two types of eggs. During favorable conditions, females produce subitaneous eggs, which develop immediately. When environmental conditions begin to decline for any number of reasons, females can switch to producing diapausing (resting) eggs, which enter a period of dormancy before resuming development. These diapausing eggs sink to the bottom of the lake, pond, or river until they receive the appropriate cues to resume development.
Adult Daphnia galeata mendotae (left) and Daphnia pulicaria, both with
subitaneous eggs. The Daphnia pulicaria is about 3mm long.
The cues vary among species and are not well understood, but likely include changes in light and temperature. Historically it was thought that these eggs served to carry the population through short-term environmental catastrophes, such as the winter or the dry season. However, recent studies have indicated that some diapausing eggs are capable of remaining viable in the mud for over a century. This longevity increases the life span of these organisms from weeks or months to years or even decades. In addition to being extremely long-lived, diapausing eggs can accumulate in densities ranging from 1,000 to 1 million eggs per m2 of sediment. This storage of dormant stages forms an "egg bank" that is in many ways analogous to the seed banks of various terrestrial plant species. One consequence of storing dormant stages in either a terrestrial or aquatic system is that it creates a reservoir of both species and genetic diversity. This storage of biodiversity will affect the way in which a system responds to both natural and artificial changes.
In addition, the adults of many freshwater zooplankton cannot tolerate desiccation and therefore are not capable of actively dispersing overland from system to system. However, many of these specialized dormant stages resist drying, freezing, and gut passage through vertebrates, thus offering a series of passive dispersal options. Dormant propagules have been transported to new locations by ducks and other birds, wind and rain, raccoons, and insects. Humans have increased the rate of intercontinental dispersal when mud containing the diapausing eggs is introduced through ballast water exchanges or fish-stocking practices. These human-caused dispersal vectors are likely responsible for the introduction of at least two exotic zooplankters into Illinois waterways. The predacious Bythotrephes cederstroemi, which entered Lake Michigan in the mid- 1980s, has been implicated in changes in the Lake Michigan food web. More recently, the suspension feeding Daphnia lumholtzi has been found in the Illinois River and several Illinois reservoirs.
Adult Daphnia pilicaria with diapausing eggs. In Daphnia
the
female produces two diapausing eggs at a time, encased in a
saddle-shaped protectivecovering known as an ephippium. When
the female
molts, the ephippium is released and sinks to the water
bottom.
The role of diapausing egg banks is rarely considered in aquatic ecology. This oversight is unfortunate because they are often an integral part of the system, and the response of systems to perturbations will be related directly to the ability of an egg bank to store diversity. Current research being carried out by the Center for Aquatic Ecology focuses on the causes and consequences of these eggs in aquatic systems. Understanding the role of dormancy is essential in making long-term predictions about ecosystem responses to anthropogenic disturbances. Long-lived dormant eggs can affect the ecology and evolution of the populations that produce them. Therefore, to fully understand the dynamics of the planktonic stage we must consider its ties to the dormant stage as well.
Carla E. Cáceres, Center for Aquatic Ecology
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