Academic journal article The American Midland Naturalist

Potential for Bias in Using Hybrids between Common Carp (Cyprinus Carpio) and Goldfish (Carassius Auratus) in Endocrine Studies: A First Report of Hybrids in Lake Mead, Nevada, U.S.A

Academic journal article The American Midland Naturalist

Potential for Bias in Using Hybrids between Common Carp (Cyprinus Carpio) and Goldfish (Carassius Auratus) in Endocrine Studies: A First Report of Hybrids in Lake Mead, Nevada, U.S.A

Article excerpt


During a 2008 study to assess endocrine and reproductive health of common carp (Cyprinus carpio) in Lake Mead, Nevada (U.S.A.) we identified two fish, one male and one female, as hybrids with goldfish (Carassius auratus) based on morphology, lateral line scale count, and lack of anterior barbels. Gross examination of the female hybrid ovaries indicated presence of vitellogenic ovarian follicles; whereas histological evaluation of the male hybrid testes showed lobule-like structures with open lumens but without germ cells, suggesting it was sterile. Because common carp/goldfish hybrids are more susceptible to gonadal tumors and may have different endocrine profiles than common carp, researchers using common carp as a model for endocrine/reproductive studies should be aware of the possible presence of hybrids.


Common carp (Cyprinus carpio) belong to the carp and minnow family, Cyprinidae, and are native to Asia. During the period of 1880-1896, the U.S. Fish Commission distributed an estimated 2.4 million common carp across the country (Nico and Fuller, 1999). There are no records of introductions into the Colorado River but common carp were introduced to Nevada in 1881 and Arizona in 1885 and were present throughout the lower Colorado River before construction of the Hoover Dam in 1928 (Allen and Roden, 1978). Although common carp in Lake Mead are not important as a recreational species, they are relatively abundant, lending to their use as monitors of environmental health (Bevans et ai, 1997; Patino et ai, 2003; Goodbred et ai, 2007).

Goldfish ( Carassius auratus) are in the same family as common carp. They are native to eastern Asia where selective breeding of visible mutations like color, shape, and fin and eye structure have been extensively developed for use in aquaria (Hulata, 1995) . In the U.S. goldfish were initially brought from Europe during the 1600s and by midcentury were numerous in the Hudson River (Nico and Fuller, 1999). In Lake Mead they have been a popular live bait fish, which probably accounts for their presence in the lake (Minckley, 1973) where four goldfish were collected in 1975 (Allen and Roden, 1978).

Common carp have been routinely sampled for environmental studies in Lake Mead for almost 20 y. Since goldfish are also present in Lake Mead there is a possibility of hybridization between the two species. The objective of this study was to look for presence of hybrids using morphology and meristics under a larger endocrine and reproductive health study in Lake Mead, Nevada and Arizona.


We sampled 363 common carp in Mar. 2008 using seine and electrofishing techniques. Detailed methods can be found in Patino et al. (2012) and a brief summary follows. We electrofished in shallower water (<4 m) adjacent to shorelines where cover was present. Fish were collected and placed in a live well for processing on shore where we collected morphometric and meristic measurements, including condition factor, lateral line scale counts, and presence or absence of anterior barbels. Condition factor was calculated by; ? (TL) = W 105/L3 where W is total weight in grams and L is total length in millimeters. Methods for processing fish gonads followed those described by Patino et al. (2003). Gonads were dissected then the testes were preserved in 10% buffered formalin for histological analysis using standard procedures (Luna, 1992). Samples were embedded in paraffin blocks and sections cut at 6 µp? and stained with hematoxylin and eosin. Microscopical observations were made with a compound microscope and photographs taken with an Olympus digital camera (DP70; Tokyo, Japan). Criteria to differentiate hybrid common carp/goldfish from common carp or goldfish were based on Taylor and Mahon (1977), Hume et al. (1983), and Pulían and Smith (1987). These include lateral line scale counts (LLSC) of 37-40 for common carp, 28-31 for goldfish; the presence of both anterior and posterior barbels (two sets) in common carp, and the absence of barbels in goldfish. …

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