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Rhodeus sericeus (Pallas 1776)

Figure 44. Bitterling, TU 8889, 71.2 millimeter SL, from Westchester County, N.Y.

Figure 44. Bitterling, TU 8889, 71.2 millimeter SL, from Westchester County, N.Y.


       The Bitterling (fig. 44) is a small species, rarely exceeding 7 cm SL (Smith and others, 2004). Maximum size is about 11 cm TL (Page and Burr, 1991). The species is deep-bodied and moderately compressed, with the back arched anterior to the dorsal fin. The scales are moderately large, with 32-45 lateral scale rows. The lateral line is absent or incomplete, with only 0-10 pored scales. The head is relatively small and the mouth is small, oblique, and lacks barbels. The upper jaw is slightly longer than the lower one. The pharyngeal teeth are hooked and in a single row (0,5-5,0; fig. 45). Gill rakers on the first arch range from 9-16. The dorsal fin has with ii-iii (7-11) rays and originates slightly anterior to the anal fin. The anal fin has ii-iv (6-11) rays. The caudal fin is moderately forked, with roughly equally sized lobes. The back is gray-green to gray-brown, and the sides and belly are silvery. The sides may include some pink. The fins are pale orange, except in breeding males when the dorsal and anal fins become bright red. A gray-green metallic stripe runs along the mid-line of the body from just under the dorsal fin to the caudal peduncle. Meristics for Bitterling are given in appendix B.

Figure 45. Pharyngeal teeth of Bitterling, TU 8889, 71.2 millimeter SL, from Westchester County, N.Y.

Figure 45. Pharyngeal teeth of Bitterling, TU 8889,
71.2 millimeter SL, from Westchester County, N.Y.

       Breeding males are brilliantly colored (Smith and others, 2004). They turn iridescent along the sides and the stripe along the mid-line of the body turns bright blue (Smith, 1985). In addition, the dorsal and anal fins, breast, and belly become orange to blood-red. According to Smith and others (2004), the dorsal fin of mature males is almost black, with a red triangle at its tip, and the anal fin is red with a dark border. They also noted that the upper part of the iris is red in sexually mature males and remains pigmented the remainder of life. During the spawning season, mature males also develop two patches of white nuptial tubercles on either side of the head (Wheeler, 1978; illustrated in Schmidt and McGurk, 1982). The color of females remains unchanged during the breeding season: gray-green dorsally, silver laterally, and yellow ventrally, and without red pigment around the iris (Smith and others, 2004). Breeding females develop a long, tube-shaped ovipositor anterior to the anal fin. The ovipositor is extended during spawning, but is retracted into the body at other times (Smith and others, 2004). Length of the ovipositor is variable; it is longest shortly before or after spawning when the structure may exceed the body length of the female (Seeley 1886; Berg 1964; Smith and others, 2004). Outside the spawning season females retain an ovipositor. Although much reduced in size, the organ remains obvious and readily distinguishes females from males (Smith and others, 2004).

Similar Species

       The Bitterling can be distinguished from most native North American cyprinids as well as all other non-native cyprinids by a combination of characteristics, including large scales (32-45 lateral scale rows), a deep body, and short lateral line (0-10 pored scales).


       The taxonomy of Bitterling remains problematic (Smith and others, 2004). Banarescu (1964) recognized three subspecies: Rhodeus sericeus amarus ranges from the Seine River of France east through central and eastern Europe to the Ural Mountains (but not Spain, Italy, England, or Scandinavia); Rhodeus sericeus sericeus from the Amur River Basin region; and Rhodeus sericeus sinensis from the Yangtze River Basin. Some researchers consider the western populations a separate species, R. amarus (Smith and others, 2004). However, Holcik and Jedlicka (1994) documented clinal gradation in characteristics used to separate the geographically isolated subspecies and recommended ending the use of subspecies designations. They noted that characteristics used to separate the eastern and western forms were size and temperature dependent.

       Only a few hybrids are reported for the Bitterling, and all are from crosses with Asian cyprinids (Schwartz, 1972, 1981).


       Sexual maturity is reached in the second to fourth year (Nikolsky, 1954; Zhul’kov and Nikiforov, 1988). In Europe and nearby regions, spawning occurs between April and August, typically with a peak in May (Smith and others, 2004). According to Reichard and others (2004), spawning is determined by photoperiod and water temperature. Seeley (1886) noted that spawning occurred from May to August in the Seine River (France), and during April in Austria. Wheeler (1969) reported spawning in April and June in northwestern Europe. Zhul’kov and Nikiforov (1988) reported spawning at temperatures from 12-24 °C in Russia.

       Reproduction of Bitterling is unusual, involving a symbiotic relationship with various species of freshwater mussels from the families Unionidae and Margaritiferidae (Smith and others, 2004). During the spawning season, the brightly colored males defend territories around mussels. Gravid females use their elongate ovipositor to deposit their eggs into the mantle cavity of the mussel. Male Bitterling then fertilize the eggs inside the mussel by shedding their sperm over the inhalant aperture of the mussel so that water entering the mussel carries the sperm to the eggs (Smith, 1985; Smith and others, 2004). Sterba (1973) reported that females condition the mussels to the stimulus by repeatedly nudging them with their mouth, thus ensuring against closing of the valves during the insertion of the ovipositor. The same female may use a number of mussels, and she deposits only one or two yellow, oval eggs into each (Seeley 1886; Axlerod and Schultz, 1955). The ovipositor is only present during the spawning season, after which it gradually shortens until reduced to a papilla (Seeley 1886; Sterba, 1973). Smith (1985) reported that the ovipositor of one female shrank to one-third of its maximum length within 3 days after spawning. Breder (1933) demonstrated that the Bitterling could use freshwater mussels native to the U.S. for reproduction (for example, Elliptio complanata and Pyganodon [formerly Anadonta] cataracta). Contrary to what was previously thought, there is evidence indicating the Bitterling is selective about which mussel species are used for spawning (Smith and others, 2004).

       Fecundity of the Bitterling is low compared to cyprinids that broadcast their eggs. A fecundity of only 31-53 eggs per female was reported in the population inhabiting the Bronx River, New York (Schmidt and McGurk, 1982). Zhul’kov and Nikiforov (1988) reported a fecundity of 203-408 eggs per female in a Russian river. Aldridge (1999) studied an introduced population of Bitterling in Britain, and documented the large size of the eggs (2.6 X 1.7 mm). The elliptical shape of the eggs (versus the more common spherical shape) is reportedly an adaptation to increase transport of respiratory and excretory products across the membrane (Aldridge, 1999). It has been hypothesized that female Bitterling discriminate among egg hosts by inspecting flow velocity and oxygen content of water emerging from the exhalant siphon of the mussels (Smith and others, 2001; Mills and Reynolds, 2002). The stages of development from the egg to the free-swimming larvae are described and illustrated in Aldridge (1999).

       The larvae stay inside the mussel for 3-4 weeks, until the egg sac is absorbed and they have reached a length of about 11 mm (Aldridge, 1999). Larval drift of Bitterling in the Danube Basin peaks in late June (Jurajda, 1998; Reichard and others, 2002). Jurajda (1998) reported that the larvae drifted at night near the surface of the water. The larval period of the precocial Bitterling is shorter in duration than many other cyprinids (Reichard and Jurajda, 1999). Phosphorus pollution (500 g/L and higher) has been shown to increase the rate of premature expulsion of larvae from the mussels, leading to increases in larval mortality (Reynolds and Guillaume, 1998).


       The Bitterling is a relatively short-lived species, generally surviving only about 5 years (Nikolsky, 1954). Maximum size is 11cm TL (Page and Burr, 1991), but it usually only grows to 6-7 cm in Europe (Wheeler, 1969). A total of 138 individuals ranging from 38-66 mm SL (mode = 48 mm SL) were taken from the Bronx River, New York, by Schmidt and McGurk (1982).

       The Bitterling inhabits slow-flowing or still waters, such as ponds, lakes, marshes, muddy and sandy pools, and river backwaters (Wheeler, 1969; Page and Burr, 1991). Because the Bitterling depends on freshwater mussels to reproduce, its range is restricted (Smith and others, 2004). Przybylski and Zieba (2000) reported that in Poland the preferred habitats were characterized by low flow and covered by vascular plants.

       The species is omnivorous, feeding on both invertebrates (especially in the juvenile stage) and plant matter. Seeley (1886) reported diatoms and algal fragments from the long, coiled intestines of the Bitterling. Wheeler (1969) reported that European Bitterling fed on phytoplankton, crustaceans, insect larvae, tubificid worms, and vegetable detritus. Five specimens examined by Schmidt and McGurk (1982) from the Bronx River, New York, contained diatoms and detritus. Przybylski (1996) documented that the diet of small (22-36 mm) individuals primarily consisted of chironomids, whereas larger fish (50-75 mm) fed on detritus and plant matter. Feeding occurred primarily during the day (Przybylski, 1996). Koutrakis and others (2003) documented that the diet of a population of Bitterling in Greece consisted of algae, insects, crustaceans, and organic and inorganic matter.

       The Bitterling derives its name from its apparently noxious taste (to both humans and fishes), and thus was not recommended as a bait fish by Seeley (1886). Wheeler (1969) suggested its main appeal was as an aquarium fish, and hypothesized that populations found in Britain were due to aquarium releases.

Native Distribution

       The Bitterling has a disjunct distribution that has lead to some taxonomic confusion. The species occurs in Central and Eastern Europe from the Seine River of France to the Ural Mountains (but not in Spain, Italy, England, or Scandinavia). Pallas described the species in that region as Rhodeus sericeus. Bitterling also naturally occurs in the Amur River Basin of eastern Asia, where it was described as Rhodeus amarus by Bloch (now Rhodeus sericeus amarus). Additionally, a subspecies of Bitterling is found in the Yangtze River Basin (Rhodeus sericeus sinensis). There is a wide geographical gap in the northern part of Asia separating the ranges of the European subspecies from those of eastern Asia.

U.S. Introductions

       The only known introductions of Bitterling into the U.S. were in the state of New York. The species was introduced into the Sawmill and Bronx rivers sometime before 1925 (Dence, 1925; Myers, 1925). No Bitterling have been collected in the Sawmill River since 1951, and that population is assumed to be extirpated (Schmidt and others, 1981). In the early 1980s, the Bronx River population was estimated to number only about 900 individuals and inhabit 1-2 km of the river (Schmidt and McGurk, 1982). Although native mussels (needed for reproduction) still occur in the Bronx River, the population of Bitterling appears to be declining (R. Schmidt, personal commun., 2005; J. Rachlin, personal commun., 2005).

Rhodeus sericeus
Plate 10. Distribution of Bitterling in the United States. - click to enlarge

Plate 10. Distribution of Bitterling in the United States. See Methods for details regarding
data used to create maps, definition of “reproducing” and shading of HUCs and states.

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