Gammarus
Information Sheet: Gammarus
Gammarus are small crustaceans from the order Amphipoda, which means “feet all around.” Nicknames “scuds,” Gammarus have a unique habit of swimming on their sides and are well known for their acrobatic behavior. Fossil records indicate that crustaceans such as Gammarus have existed since the early Paleozoic period.
Preferred Environment
Gammarus are one of the few types of freshwater crustaceans that have adapted to marine coastal and cave environments that have a mix of saltwater and freshwater. They are bottom dwellers, usually living on the floors of lakes, rivers, and steams that are well oxygenated, clean, and cool in temperature. Gammarus can reach densities of more than 1000 individuals per square meter when food is abundant and there is protection from predators. They are sensitive to bright light and have specialized receptors that help them to detect even slight differences in light intensities.
Gammarus are know to tolerate water pH ranging from 6-10.
Many scientists categorize them as cold water stenotherms, meaning that they require a narrow range of cold temperatures in order to survive (10-15°C, with temperatures of 20-24°C being tolerable, and temperatures above 34°C causing death).
Feeding Habits
Gammarus have a well-developed chemical sensory system that enables them to identify potential food and mates by scent. Juveniles normally feed on algae and bacteria that are attached to aquatic plants. Adults and juveniles alike consume dead organic matter, which provides protein.
Reproduction
Mating occurs soon after the female sheds her shell (molts), when she is vulnerable and needs a male’s protection. During this time, she exposes her eggs for fertilization and waits for her new shell to harden. She then holds her larvae in a birthing chamber called the marsupium. Normally, females produce only one brood during their 1-to 1 ½ -year lifetime. The type of Gammarus most common to the United States needs short days and long nights to reproduce. Therefore, reproduction usually takes place during the winter.
Ecological Relationships
Gammarus are sensitive to changes in their environment, and changes such as acidification of the water can threaten their survival. Because Gammarus are a main source of food for larger freshwater organisms, observations of the effects of pollution on Gammarus can indicate how pollution affects freshwater environments as a whole.
Daphnia
Information Sheet: Daphnia
Daphnia are small aquatic animals. Sometimes called water fleas, they are about 1 mm long. They move by making rapid strokes with their two feathery antennae. Their clear outer shells make it easy to observe their internal structures, including the beating heart. The heartbeat of a Daphnia varies with the temperature of the water.
Preferred Environment
Daphnia thrive in an aquatic environment with a temperature around 20º C. Higher temperatures are usually fatal, but lower temperatures do not affect them adversely. Daphnia eggs can survive up to eight thaws and freezes; this thaw/freeze cycle stimulates hatching. The optimum temperature for Daphnia magna is 18-22º C.
In terms of water chemistry, Daphnia prefer a pH range of 7.9-8.3. Although they have been know to tolerate a pH as low as 4.5 and as high as 10.1.
Daphnia orient themselves by focusing on light that is coming from one direction; they have difficulty swimming when they are exposed to equal amounts of light coming toward them from all directions.
Feeding Habits
Daphnia feed on a variety of organisms such as bacteria, algae, and protists (microscopic, single-celled organisms). Daphnia eat by moving their legs to produce small water currents that sweep the organisms into their mouths. Their feeding rate depends on food concentration, food quality, water temperature, light intensity, oxygen concentration, pH levels, and crowding. Large concentrations of Daphnia can process immense amounts of algae at a time. When food is scarce, Daphnia may out compete other small organisms, causing those organisms to die.
Reproduction
Daphnia produce eggs that develop without fertilization. An adult female may carry 10 to 15 eggs, which develop and hatch in a special brood pouch under its shell. These eggs hatch as female offspring. Adverse conditions, such as excessive crowding, increased water temperature, decreased food supply, or environmental change such as the drying up of a pond, trigger the production of males and females that reproduce sexually. These sexual Daphnia produce eggs in the fall that survive through winter and hatch the next spring.
Ecological Relationships
Daphnia are a principal food staple for fish and thus are an important link in the food chain. Fish stomachs may contain 95 percent Daphnia by volume. Daphnia also help maintain water quality by efficiently cleaning up cyanobacteria blooms in lakes. These blooms are caused by an overabundance of inorganic nutrients such as phosphates. Phosphates are found in a variety of household cleaning products and they can leach into waterways through runoff. Because Daphnia can reduce the amount of algae and cyanobacteria by half in a small amount of time, they are important to lake restoration programs.
Daphnia are small aquatic animals. Sometimes called water fleas, they are about 1 mm long. They move by making rapid strokes with their two feathery antennae. Their clear outer shells make it easy to observe their internal structures, including the beating heart. The heartbeat of a Daphnia varies with the temperature of the water.
Preferred Environment
Daphnia thrive in an aquatic environment with a temperature around 20º C. Higher temperatures are usually fatal, but lower temperatures do not affect them adversely. Daphnia eggs can survive up to eight thaws and freezes; this thaw/freeze cycle stimulates hatching. The optimum temperature for Daphnia magna is 18-22º C.
In terms of water chemistry, Daphnia prefer a pH range of 7.9-8.3. Although they have been know to tolerate a pH as low as 4.5 and as high as 10.1.
Daphnia orient themselves by focusing on light that is coming from one direction; they have difficulty swimming when they are exposed to equal amounts of light coming toward them from all directions.
Feeding Habits
Daphnia feed on a variety of organisms such as bacteria, algae, and protists (microscopic, single-celled organisms). Daphnia eat by moving their legs to produce small water currents that sweep the organisms into their mouths. Their feeding rate depends on food concentration, food quality, water temperature, light intensity, oxygen concentration, pH levels, and crowding. Large concentrations of Daphnia can process immense amounts of algae at a time. When food is scarce, Daphnia may out compete other small organisms, causing those organisms to die.
Reproduction
Daphnia produce eggs that develop without fertilization. An adult female may carry 10 to 15 eggs, which develop and hatch in a special brood pouch under its shell. These eggs hatch as female offspring. Adverse conditions, such as excessive crowding, increased water temperature, decreased food supply, or environmental change such as the drying up of a pond, trigger the production of males and females that reproduce sexually. These sexual Daphnia produce eggs in the fall that survive through winter and hatch the next spring.
Ecological Relationships
Daphnia are a principal food staple for fish and thus are an important link in the food chain. Fish stomachs may contain 95 percent Daphnia by volume. Daphnia also help maintain water quality by efficiently cleaning up cyanobacteria blooms in lakes. These blooms are caused by an overabundance of inorganic nutrients such as phosphates. Phosphates are found in a variety of household cleaning products and they can leach into waterways through runoff. Because Daphnia can reduce the amount of algae and cyanobacteria by half in a small amount of time, they are important to lake restoration programs.