Essay: Daphnia sp. (crustacean)

Daphnia sp. is a small crustacean that has a wide distribution and high diversity with over 100 species of Daphnia identified by scientists (Ebert 2005). Daphnia sp. is more commonly known as the water flea and it is a transparent organism with a length of around a few millimeters and a rounded shape. It has a tubular digestive system that is clearly seen from the outside of the organism because of its transparent quality. The transparency of the organism makes physical changes within the organism easy to observe, but the small size means that a microscope is required to fully observe them. The head has two antennae protruding from it that are used to propel the Daphnia as it moves through the water. They also have another set of antennae located on their head called the antennules (Ebert 2005). To go along with the two pairs of antennae, Daphnia sp. also has multiple appendages on the ventral side that are contained within the exoskeleton or carapace. These appendages are used to move and filter food to the mouth of the organism for eventual digestion (Kloepfer, [ ]). Daphnia’s sensory abilities are mainly controlled by the compound eye located on the top of their head, which is able to detect variations in light intensity.
Species of Daphnia are spread around the world in every continent, including Antarctica (Center for Freshwater Biology 2013). Daphnia are known to live in many freshwater environments, from slight short-lived pond to larger lakes, but they often prefer areas with minimal water movement (Ebert 2005). The optimum water temperature for the adequate survival of Daphnia varies between species, but for many species, such as Daphnia magna, the tolerance is around 20??C to 25??C (Ocampo et al. 2012). In order for Daphnia to survive they require sufficient amounts of algae to be present in their environment, as this is one of their main food sources.
One of the most important abiotic factors in the habitat of Daphnia is the factor of light intensity, as it determines the distribution and abundance of Daphnia because it affects their food source, algae, and their predators, which often use light to hunt. Light intensity decreases throughout the freshwater environment at night and increases during the day. Seasons also affect light intensity; it increases during the summer months with increased sunlight, and decreases during the winter months with decreased sunlight. On a normal day, during the time of maximum sunlight, the light intensity hitting the surface of a lake that Daphnia are inhabiting is around 10,000-25,000 lux for an average of around 17,500 lux (Schlyter 2009). Horne and Goldman (1994), show that the light intensity in a relatively turbid lake at a depth of 2.0 m is around 25 percent of the light intensity at the surface. Therefore, this puts the maximum light intensity in the environment of Daphnia in a turbid lake at around 4,400 lux at 2.0 m. It decreases to around zero lux at lower depths of around 10-25 m. By nightfall the light intensity reduces significantly, approaching zero lux in all areas. While a light intensity of 4,400 lux is present in the environment of Daphnia, the maximum light intensity that Daphnia prefer to be living in is 1076 lux (Ocampo et al. 2012).
As mentioned before, Daphnia possess a compound eye on their head that is located just above the brain, or cerebral ganglion (Ebert 2005). This eye is able to detect light sources and the changes in its environment in regards to light (Kloepfer, [ ]). The movement of the eye is controlled by six muscles that work together to scan the Daphnia’s environment (Consi et al. 1987). Connected to the compound eye are many optic nerves which all go towards the brain of the Daphnia, where the information is processed (Frost 1974). Consi (1990) stated that when the eye moves and then detects variations in light, the body is signaled to move into a favoured alignment. This movement is carried out by the two larger antennae located on their head that act as paddles. This allows the organism to align itself into the exact position it needs to be in so as to carry out the activities that are required to survive. This response of changing position in relation to light sources is central to Daphnia’s survival and this is clearly seen in a common Daphnia behaviour, caused by varying light intensity, called vertical migration (Van Gool and Ringelberg 2003). Van Gool and Ringelberg discuss this behaviour by stating that during the night, Daphnia are found in the upper areas of their freshwater environment, but as the morning approaches and light intensity increases, they start to swim down to the bottom of the environment and into the benthic zone. This response to light intensity, that is initiated by the compound eye, takes place because it allows Daphnia to avoid predation from fish that rely on light to hunt during the day. This vertical migration shows how light intensity, and how they sense and react to it, is a very important factor in determining the survival of Daphnia in their environment.
The basis of our experiment is to investigate the behaviour of Daphnia in relation to the abiotic factor of light. Our main objective is to find out how Daphnia sp. responds to varying levels of light intensity. The following is our null hypothesis: in response to varying light intensities, Daphnia sp. will not have a preference for an environment of a certain light intensity. Our alternate hypothesis states: in response to varying light intensities, Daphnia sp. will have a preference for an environment of a certain light intensity. Our prediction is that the Daphnia will most often choose to move towards an environment of lowest light intensity as this fits in with the knowledge that they migrate away from light in their natural environment to avoid predation.