Northern Redbelly X Finescale Dace - Chrosomus eos x chrosomus neogaeus
The northern redbelly x finescale dace hybrid (Phoxinus eos x P. neogaeus) is a Montana Fish Species of Special Concern, Class C (Hunter 1997). It was placed on the species of concern list due to its rarity and unusual form of genetic reproduction (Holton and Johnson 1996). Montana appears to be the only state that designates special status for this hybrid fish (AFS website 2003).
The hybrid is usually larger than the northern redbelly dace and has a larger mouth. The finescale dace has never been recorded for this state. The hybrid persists due to a unique strategy. Typically, hybrid females breed with redbelly dace males, but the male's genetic material is not incorporated during egg development and is not passed on to the next generation. The offspring are all female and clones of the mother (that is, they are genetically identical to the mother). Unisexuality is not common among vertebrates but has been found in amphibians and reptiles as well as in fishes.
The midside stripe is less distinct than on the northern redbelly dace (see record for northern redbelly dace), and its upper stripe is faint and usually broken.
There is no easy field technique to differentiate the dace hybrid from the northern redbelly dace (Holton and Johnson 1996). According to William Gould (Emeritus Professor of Fisheries, Montana State University) the hybrid dace can be differentiated from the northern redbelly dace by pharyngeal teeth counts. Northern redbelly dace have pharyngeal tooth counts of 0, 5- 5, 0 and finescale dace has counts of 2, 5-4, 2. Hybrid Phoxinus spp. have intermediate counts such as 1, 5-5, 1 or 1, 5-4, 1 or 1, 5-4, 0. New (1962) gives a detailed description of morphometric differences between the species (AFS website 2003).
Summary of Observations Submitted for Montana
Number of Observations:
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(Records associated with a range of dates are excluded from time charts)
Northern redbelly dace prefer quiet waters from beaver ponds, bogs and clear streams (Scott and Crossman 1973, Holton and Johnson 1996). The finescale dace likes similar habitat but is also found in larger lakes (AFS website 2003).
Schlosser et al. (1998) studied responses to environmental stresses on the hybrid, finescale and northern redbelly dace. He found that the hybrid had a greater physiological tolerance and survived longer in oxygen stressed environments than the other two species (AFS website 2003).
Two years of experiments on New England populations indicate that the hybrid dace utilize a unique reproductive strategy called gynogenesis (Dawley et al. 1987). The hybrid dace are female clones with identical eggs. In gynogenesis, sperm from the male of a sexually reproducing related species is needed to stimulate egg development, even though the genetic material is not incorporated into the offspring. Entire populations can have the same genes.
In Montana, preliminary studies indicate that at least two genetically distinct clones occur in the Pine Butte Fen (Allendorf 1991a). Studies by Goddard et al. (1998) have shown that some female hybrid dace clones reproduce by cloning while other individuals make haploid eggs that can be fertilized by northern redbelly dace to produce diploid northern redbelly dace offspring. Phoxinus spp. spawn in the spring and early summer (Scott and Crossman 1973) (AFS website 2003).
Further inventory is needed to better define Phoxinus spp. distribution in Montana. Due to difficulties of field differentiation it is likely that some waters thought to contain only northern redbelly dace may also have the hybrid. Phoxinus spp. are not extremely common in Montana. Dr. Robert Bramblett (personal communications) has conducted surveys on 43 prairie streams of the Missouri and Yellowstone rivers and identified Phoxinus spp. at just three sites, one of which contained the hybrid. Few prairie streams in Montana have the clear pool-type habitat preferred by Phoxinus spp. Due to the limited distribution and knowledge of this species it is important to reduce impacts to their known habitat, such as is described for Big Coulee Creek, Montana (Palmer 1994) (AFS website 2003).
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- American Fisheries Society (AFS), Montana Chapter Website.
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