Carassius

Carassius View in CoL View Figure

Carps of the genus Carassius are immediately distinguish- able from all other cyprinids in the region by having a long dorsal fin, a terminal mouth without barbels, and the last unbranched anal and dorsal rays strongly serrated. Crucian carp C. carassius is the only species native to the area. Three different species are known to be involved in the so-called C. auratus complex. These are (1) C. auratus , a widespread ornamental species that has become feral in many areas; (2) C. gibelio , an Asian species that may have been introduced with stocked fishes;(3) C. langsdorfii believed to be endemic to Japan and introduced as a pest with ornamental koi. An addi- tional unnamed lineage potentially originating from Central Asia was introduced mostly within countries of the former Soviet Union and may be found in West Asia in the future.

All three mitochondrial lineages of non-native Carassius in West Asia are diploid and reproduce normally. This means that haploid male and female gametes join and produce a diploid offspring. In C. auratus , this is the only mode of reproduction. In the other lineages, triploid, all-fe- male fish are common (but not found everywhere), and these reproduce by gynogenesis. Here, the sperm only trig- gers the triploid egg, and the offspring are clonal, triploid copies of the mother. If the sperm is included in the genome of the egg cell, then tetraploid individuals hatch. This might occasionally be the case in all populations with triploid fish. Triploid individuals are common, often even dominant, in many populations of the mitochondrial lineages called C. gibelio and C. langsdorfii . These lineages can switch between a recombinant and a clonal mode of reproduction.

The assignment of individual Carassius to C. auratus , C. gibelio, or C. langsdorfii by their mitochondrial DNA is not an accurate reflection of their species identities. Mitochondrial DNA lacks recombination, which is not useful to identify hybrids.In particular,triploid individuals frequently possess genomes from two (or all three) different “species”. There is strong evidence that C. auratus , C. gibelio , and C. langsdorfii form large and highly diverse hybrid swarms in their non-native range. As all three “species” could reproduce by diploid sexual reproduction, they appear to have hybridised extensively, and their genomes are largely intermingled. We are confident that future nuclear DNA studies will not support three distinct, reproductively isolated non-native species in West Asia or Europe. However, some local populations may be pure C. auratus , C. gibelio , or C. langsdorfii . Identi- fying non-native Carassius to the species level is impossible because of the lack of morphological and molecular charac- ters to identify pure species. We do not even know whether pure populations of non-native Carassius exist in West Asia, except for locally stocked C. auratus . We, therefore, refer to all populations as “ C. auratus complex”, pending future studies.

It should not be forgotten that Carassius hybridise with Cyprinus carpio . Their hybrids are fertile and may form populations independent of their parental species. Such hybrid populations are known from Europe but have yet to be discovered in West Asia. Carassius x Cyprinus hybrids can be recognised by their short barbels.

Further reading. Murakami et al. 2001 (polyploidy systems in Japan); Szczerbowski 2002 (biology); Hänfling et al. 2005 (genetics); Gao et al.2012 (diversity in Asia); Kalous et al. 2013 (mtDNA records of C. langsdorfii in Europe); Rylková et al. 2013 (non-native Carassius in Europe); Halas et al. 2018 (diversity in North America); Khosravi et al. 2020 ( Iran).