Daphnia (Daphnia)

Daphnia are small, planktonic crustaceans, typically 0.2 to 5 millimeters in length. They belong to the order Cladocera and are commonly known as water fleas. Daphnia inhabit a variety of freshwater environments, ranging from lakes and ponds to temporary pools and ditches.

Description

Daphnia possess a translucent carapace that encloses most of their body, allowing internal organs to be visible under magnification. They have a single compound eye located on the head, and two pairs of antennae. The second antennae are large and branched, used for locomotion through rapid, jerky movements, hence the name "water flea". Daphnia lack true legs, but have phyllopods, specialized thoracic appendages used for filtering food particles from the water.

Reproduction

Daphnia exhibit cyclical parthenogenesis, a reproductive strategy that alternates between asexual and sexual reproduction depending on environmental conditions. Under favorable conditions, females reproduce asexually through parthenogenesis, producing diploid eggs that develop into genetically identical female offspring. This allows for rapid population growth. However, under stressful conditions, such as overcrowding, food scarcity, or changes in temperature, sexual reproduction is triggered. Females produce haploid eggs that require fertilization by males, which also appear under these conditions. The fertilized eggs develop into highly resistant resting eggs called ephippia, which are encased in a hardened carapace section. These ephippia can withstand harsh conditions and hatch when conditions improve.

Ecology

Daphnia are important primary consumers in aquatic ecosystems, feeding on algae, bacteria, and other organic matter. They are in turn a crucial food source for fish, amphibians, and aquatic insects. Their grazing activity can significantly impact phytoplankton populations and water clarity. Daphnia are also used as model organisms in ecotoxicology studies due to their sensitivity to pollutants and ease of culture. Their life cycle, reproduction, and physiological responses are well-characterized, making them valuable for assessing the impact of environmental stressors.

Classification

The genus Daphnia is diverse, with numerous species and subspecies exhibiting variations in morphology, habitat preference, and physiological characteristics. Classification and identification of Daphnia species can be challenging due to phenotypic plasticity and hybridization. Molecular methods, such as DNA sequencing, are increasingly used to clarify taxonomic relationships within the genus.