That’s an excellent and important question! Behavioral endpoints in Daphnia biotests provide significant insights into the toxicological impacts of contaminants, especially with the increasing diversity of emerging pollutants. Here are some contaminants that I believe should be prioritized for testing:

1. Pharmaceuticals:

• Antibiotics: These are increasingly being detected in aquatic environments and can disrupt aquatic organisms by affecting gut microbiota or causing direct toxic effects.
• Antidepressants (e.g., fluoxetine): Behavioral changes in aquatic organisms have been observed in response to these compounds, which can alter normal behavior patterns like movement, feeding, and predator avoidance.
• Hormonal drugs (e.g., birth control): These can impact reproductive and behavioral endpoints, such as mating behaviors and predator responses.

2. Microplastics and Nanoplastics:

• Microplastics (and nano-scale variants) are widely present in aquatic ecosystems and can disrupt the behavior of aquatic organisms, including swimming patterns and feeding behavior. The chemical additives that are often associated with plastics (e.g., flame retardants, plasticizers) also pose additional risks.

3. Pesticides and Herbicides:

• Neonicotinoids: These have been shown to affect sensory behavior and movement in aquatic organisms, including Daphnia. The rising use of these chemicals means they should be prioritized for behavioral assays.
• Glyphosate-based herbicides: These can interfere with the behavior of aquatic organisms, especially through changes in movement and feeding, affecting the ecosystem balance.

4. Heavy Metals (e.g., mercury, cadmium, lead):

• These contaminants are persistent in the environment and can significantly alter behavioral patterns such as swimming, feeding, and predator-prey interactions. Daphnia have been used widely in studies of metal toxicity, but newer, more complex exposures are increasingly relevant.

5. Industrial Chemicals (e.g., Bisphenol A, phthalates):

• Endocrine-disrupting chemicals like Bisphenol A (BPA) and phthalates can interfere with development, reproduction, and behavior in aquatic organisms. Behavioral endpoints such as changes in movement or social behavior would help assess their impact.

6. Flame Retardants:

• These chemicals are often present in the aquatic environment and can cause behavioral disruption in aquatic species. Given their widespread use, assessing their impact on Daphnia behavior is important.

7. Per- and Polyfluoroalkyl Substances (PFAS):

• These substances, often referred to as "forever chemicals," are resistant to degradation and have been detected in water systems. They can affect various biological functions, including behavior, movement, and reproduction.

8. Endocrine Disrupting Chemicals (EDCs):

• This broad category includes various industrial chemicals, pesticides, and even some pharmaceuticals, which can impact hormone signaling and result in altered behavioral patterns in aquatic organisms.

Why Behavioral Endpoints Matter:

Behavioral endpoints are essential because they provide real-time, observable effects that might occur before more severe physical or reproductive effects. For instance:

• Movement patterns: Changes in activity or swimming patterns can indicate toxicity before other symptoms like mortality or growth inhibition appear.
• Feeding behavior: Alterations in feeding or prey-capture efficiency can signal sub-lethal effects on the organism, which could lead to ecosystem-level consequences.

In summary, focusing on emerging pollutants like pharmaceuticals, microplastics, industrial chemicals, and endocrine disruptors is crucial. The behavior of organisms, particularly Daphnia, can serve as an early warning system for ecosystem health, especially given the growing diversity of contaminants.