Bitterling (Rhodeus sericeus)

Synonyms: Cyprinus sericeus, Rhodeus amarus, Rhodeus sericeus amarus

Common Names: European bitterling, Amur bitterling

Description:

Bitterling (Rhodeus sericeus) is a small freshwater fish in the minnow family, native to Europe and Asia [1]. It is best known for an unusual habit: it lays its eggs inside living freshwater mussels [1][4]. In North America it has been found only in a couple of New York streams, where it was likely released from aquariums, and its impact so far appears minimal ? but it is tracked and prohibited as a precaution, and MISIN watches for it so any new release can be caught early [1][3].

Identification:

The bitterling is a small, deep-bodied minnow, usually about 2 to 4 inches (6?11 cm) long [1][2]. Its back is gray-green, its sides and belly are silvery white, and a shiny blue-green streak runs along the rear half of the body; the fins are pale orange with gray shading [2]. During the spring and summer breeding season, males become much more colorful, and females grow a long egg-laying tube (an ovipositor) that trails from the belly ? a giveaway when present [1][4].

Because it is a small underwater fish, the bitterling is most likely to be noticed when caught in a net or seen in clear, slow water near mussel beds, especially when breeding males are brightly colored in late spring and summer [1][4]. It is easy to mistake for native minnows and shiners, so a clear photo and an expert look are important [1].

At-a-glance cues:

1. Small, deep-bodied minnow about 2?4 inches (6?11 cm) long [1][2]
2. Silvery sides with a shiny blue-green streak along the rear half of the body [2]
3. Pale orange fins with gray shading [2]
4. In breeding season, colorful males and females with a long trailing egg tube (ovipositor) [1][4]

Habitat:

The bitterling lives in slow or still fresh water ? ponds, lakes, marshes, and the quiet backwaters and muddy or sandy pools of rivers [1]. Its one hard requirement is live freshwater mussels: the fish cannot breed without them, so it is tied to waters where native mussels are present [1][4].

The fish is native to Europe and Asia, from the Seine River in France east to Asia Minor and northern China [1]. In North America it has only ever been found in New York ? in the Sawmill River (where it disappeared by the mid-1900s) and the Bronx River, where a small population has persisted but appears to be declining [1]. It is not established in Michigan [2]. The bitterling eats small insect larvae and plant material [2].

Reproduction:

The bitterling reaches new waters mainly through people releasing aquarium fish ? that is the likely source of the New York populations [1]. It is a popular aquarium fish, so the real risk is someone dumping an unwanted tank into a pond or stream [1]. It cannot spread to a new water body and breed there unless native freshwater mussels are already present [1][4].

Its breeding is remarkable. In spring and summer the female grows a long egg-laying tube and uses it to place her eggs inside a living mussel, through the opening the mussel uses to expel water; the male then releases sperm into the current the mussel draws in, fertilizing the eggs inside the shell [1][4]. The young develop inside the mussel for about a month and then swim out as tiny larvae about 10 mm long [1]. Because the fish depends entirely on mussels to reproduce, protecting native mussels and never releasing aquarium fish are the keys to keeping it from establishing (see Management) [1][4].

Impact and Damage:

The bitterling is not a danger to people or pets. In North America its impact so far appears minimal: after roughly a century in the Bronx River, the population stayed small and is now thought to be declining, and biologists have judged it unlikely to cause appreciable harm to northeastern habitats [1]. It is best described honestly as a watch-list species rather than a proven damaging invader in the United States [1].

The concern that keeps it on watch lists is its dependence on freshwater mussels. North America's native mussels are already among its most imperiled animals, and a fish that places its eggs inside them could add stress to mussel gills and breathing, while the fish might also compete with small native fish for food [1][4]. These are potential risks rather than documented damage in the United States, which is why the species is prohibited as a precaution even though its measured impact has been low [1][3].

Similar species:

The bitterling is easily confused with small native minnows, and an expert look is usually needed to be sure.

Golden shiner (Notemigonus crysoleucas) is a common native minnow with a similar deep, silvery body, but it lacks the bitterling's blue-green rear-body streak and orange-tinted fins, and it does not breed in mussels.

Common rudd (Scardinius erythrophthalmus) is another introduced European minnow, but it grows much larger and has bright reddish fins, unlike the small, pale-orange-finned bitterling.

Native shiners and minnows (family Cyprinidae) include many small silvery fish that resemble the bitterling; the bitterling's blue-green side streak, orange-gray fins, and (in season) the female's long egg tube help set it apart.

Monitoring and rapid response:

Signs to watch for:

1. Small, deep-bodied silvery minnows with a blue-green streak on the rear body and pale orange fins, in slow or still fresh water near mussel beds [1][2]
2. Brightly colored males, or females with a long trailing egg tube, during the spring?summer breeding season [1][4]
3. Unfamiliar minnows turning up where aquarium fish may have been dumped [1]

What you can do:

1. Never release aquarium fish, plants, or tank water into any pond, lake, or stream ? rehome or surrender unwanted aquarium fish instead, since aquarium release is how this fish reached the wild [1].
2. Don't move fish, water, or bait between water bodies, and Clean, Drain, and Dry boats and gear between waters to avoid carrying this and other aquatic invaders [1].
3. If you catch or see a fish you think is a bitterling, photograph it, note the location, and report it (below) rather than releasing it elsewhere [1].
4. Natural area crews and lake associations can include this fish in early-detection surveys where it might be released and coordinate with the state aquatic invasive species program [1].

When to call a professional:

1. A suspected new population ? especially in a water body not known to have it ? contact your state aquatic invasive species program or a regional fisheries office for confirmation and guidance [1].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. U.S. Geological Survey, Nonindigenous Aquatic Species Database. Rhodeus sericeus (Pallas, 1776) ? Bitterling (Species Profile). nas.er.usgs.gov
2. Michigan Department of Natural Resources, Invasive Species Program. Bitterling (Rhodeus sericeus). michigan.gov/invasives
3. U.S. Fish & Wildlife Service. Bitterling (Rhodeus sericeus) Ecological Risk Screening Summary. fws.gov
4. Smith C, Reichard M, Jurajda P, Przybylski M (2004). "The reproductive ecology of the European bitterling (Rhodeus sericeus)." Journal of Zoology 262(2):107?124. semanticscholar.org

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