The bitterling is a small freshwater fish belonging to the carp family, Cyprinidae. These fish are known for their distinctive appearance and unusual reproductive strategy. The European bitterling (Rhodeus amarus or Rhodeus sericeus) is a well-known example of this group. Their unique biology makes them a subject of interest in aquatic ecosystems.
Physical Characteristics and Habitat
Bitterlings are small, typically reaching 6-7 centimeters in length, though some can grow up to 11.2 centimeters. Their bodies are narrow and laterally compressed, with a grey-green back and silvery-white sides and belly. A metallic streak often runs along their hind sides, and their fins are pale orange with gray shading. During breeding season, male bitterlings develop a brilliant orange belly and bright red fins. They have large, golden-red eyes and a slightly upturned mouth, with an incomplete lateral line extending only to about 5-6 scales.
These fish prefer slow-moving or still waters, such as marshes, lakes, ponds, and river backwaters. They thrive in environments with abundant aquatic vegetation and a substrate of sand or mud. Their geographical distribution spans across central and eastern Europe and northern Asia, from the Seine River in France eastward to northern China.
A Unique Reproductive Partnership
The bitterling’s reproductive strategy is an example of obligate symbiosis with freshwater mussels, primarily from the Unionidae family. During the spawning season, between April and June, female bitterlings develop a long, tubular ovipositor. This specialized organ allows the female to deposit her eggs into the gill cavity of a freshwater mussel, typically through its exhalant siphon. Females release one to six eggs per spawning event, and over 100 eggs can be found in host species.
Once eggs are inside the mussel, the male bitterling releases sperm near the mussel’s inhalant siphon. The mussel’s respiratory current draws the sperm inside, where fertilization occurs. The developing embryos remain protected within the mussel for about a month, benefiting from a safe, oxygenated environment. After hatching, young bitterlings, around 10 mm in length, leave the mussel through its exhalant siphon. While bitterlings gain protection for their offspring, mussels also benefit, as the fish are thought to rid the bivalves of parasites, establishing a symbiotic relationship.
Ecological Role and Conservation Status
Bitterlings serve as a food source for larger predatory fish and waterfowl. They consume algae, plant matter, and small invertebrates, including worms, crustaceans, and insect larvae. Their presence can also indicate good water quality, as they are sensitive to pollution.
Despite their abundance in much of their native range, bitterling populations face various threats, and some species are considered endangered. Habitat loss and degradation are concerns, often resulting from factors like dam construction, river straightening, and water pollution. A threat also arises from the decline of their host freshwater mussel populations, which are themselves impacted by water pollution and habitat changes. Introduced carnivorous fish can also pose a threat through predation.
Bitterlings as Aquarium Fish
Bitterlings can be kept as aquarium fish, and their unique breeding behavior makes them an interesting addition to a coldwater setup. They are peaceful and can be housed in community tanks. A tank size of at least 20 gallons is recommended for a small group, with larger tanks preferred for more individuals or mixed species.
Maintaining appropriate water conditions is important for their well-being. Bitterlings thrive in clean, well-oxygenated water with a pH level between 6.5 and 7.5, and a temperature range of 68-75 degrees Fahrenheit. The aquarium should include plenty of aquatic vegetation and hiding spots, along with a sandy or muddy substrate. For breeding observation, live freshwater mussels are necessary, though keeping mussels long-term can be challenging. Never release aquarium-kept bitterlings or mussels into the wild, as introduced species can harm native ecosystems.