Daphnia, commonly known as water fleas, are small freshwater crustaceans found globally in various aquatic environments, including ponds, lakes, and other standing freshwater bodies. Although they are referred to as “water fleas” due to their jerky swimming movements, Daphnia are not insects but belong to the class Branchiopoda. They play a significant role within aquatic food webs, serving as primary consumers and a food source for many other aquatic creatures.
The Primary Diet of Daphnia
Daphnia are primarily filter feeders, consuming microscopic suspended particles in their aquatic habitats. Their diet predominantly consists of phytoplankton, which includes various types of algae, along with bacteria and detritus, which is decomposing organic matter.
Among the phytoplankton they consume, green algae are a preferred food source, with species such as Scenedesmus and Chlamydomonas being readily ingested. While they can consume certain types of cyanobacteria, these are a poor food source and may be avoided due to their tough cell walls or potential toxicity. Bacteria also form a substantial part of their diet, and Daphnia are efficient at collecting these microorganisms. Bacteria can contribute significantly to their nutrition, though the quality can vary.
Detritus supplements their diet, especially when concentrations of live algae or bacteria are low. Some Daphnia species, like Daphnia magna, can even switch to a “browsing” behavior, stirring up sediment to access detritus when suspended food is scarce.
How Daphnia Filter Feed
Daphnia employ a specialized mechanism to acquire their food from the water column. Daphnia utilize several pairs of flattened, leaf-like thoracic appendages. These appendages beat rhythmically to create a continuous water current that flows through a filtering chamber located within their carapace. This current draws in water along with any suspended particles.
As water passes through this chamber, microscopic food particles, typically (1 to 50 micrometers) are captured by specialized hair-like structures called setae. These setae act as a filter, retaining the particles while allowing water to pass through. The collected food particles are then transferred to a food groove and moved towards the mouth for ingestion. This efficient filtering process allows Daphnia to consume a wide range of small organisms and organic matter.
Factors Influencing Their Diet
Several factors influence the feeding behavior and dietary intake of Daphnia in their natural habitats. Water temperature directly affects their metabolic rate and feeding efficiency. The ideal temperature for feeding in species like Daphnia magna is 24°C, with feeding rates decreasing outside this range. Water quality parameters, including pH, also play a role, with a pH range between 6.5 and 9.5.
The concentration and size of available food particles are also significant determinants. Daphnia have limits on the size of particles they can efficiently filter, generally consuming particles between 1 and 50 micrometers. While they can ingest particles up to 70 micrometers, larger particles may be rejected. If food concentration falls below a certain level, their feeding rate becomes proportional to the food availability; however, above a certain concentration, their feeding rate can become constant as their filtering rate may decrease. High concentrations of suspended non-food material can interfere with their ability to take up edible particles.
Different Daphnia species or life stages may exhibit slight variations in their dietary preferences or feeding efficiencies. For instance, some larger species may be able to utilize different food sources or feeding strategies when preferred suspended food is scarce. The overall availability of nutrients in the water, which impacts the growth of their primary food sources like algae, also indirectly influences their diet.