The Crucian carp is known for its remarkable hardiness (Muus and Dahlstrom 1978). Historical accounts report the species can live for hours out of the water (Seeley, 1886). The Crucian Carp is tolerant of low-oxygen conditions, high turbidity, low ph (down to 4), and a broad temperature range. It can tolerate temperatures from 0 to 38 C (optimum at 27 C) (Holopainen & Oikari 1992 and Lenkiewicz 1964 and Horoszeqich 1973 in Holopainen et al. 1997). Survival has been documented at water temperatures below 0°C, and individuals may even survive for a few days with a frozen integument (Szczerbowski and Szczerbowski 2001). In Finland, the shift in energy allocation from growth and reproduction to the accumulation of glycogen occurs in July. These carbohydrate reserves, along with a shift to anaerobic metabolism that decreases metabolic rate to 20-30% of the aerobic rate, help C. carassius survive prolonged periods of anoxia and low temperatures (Holopainen et al. 1997; Piironen and Holopainen 1986; Holopainen and Hyvärinen 1984). In response to hypoxia, this species’ gill will display a protruding lamellae that increases the respiratory surface area allowing the Crucian Carp a greater capacity for oxygen uptake. The efficiency of Crucian Carp haemoglobin in extracting oxygen from the water allows this species to reduce the surface area of its gill filaments during normoxic conditions,which reduces osmoregulatory costs and improves its tolerance to toxic substances (Poleo et al. 2017; Sollid et al. 2003). In their native range, feeding may stop for several months as the fish rest in a state of "suspended animation" during winter months when ponds become anoxic and covered with ice (Zhadin and Gerd 1963, Penttinen and Holopainen 1992). Carassius carassius is a freshwater species, but lab experiments have noted that this species can survive for at least 6 hours in hypersaline water (16 ppm). Carassius carassius are relatively inactive fish that inhabit littoral zones and tend to locate themselves within habitat structure, especially in the presence of predators (Greenberg et al. 1995 in Holopainen et al. 1997). Crucian Carp thrive in shallow ponds with dense, emergent vegetation such as Typha spp. and Phragmites spp. (Wheeler 2000).
Crucian Carp actively feed during the summer months (Penttinen and Holopainen 1992). While planktonic and benthic invertebrates typically form the dominant part of the diet, Carassius carassius will also feed on phytoplankton plant material, and detritus (Penttinen and Holopainen 1992, Paszkowski et al. 1996 in Holopainen et al. 1997).
Crucian Carp typically have a lifespan of 10 years with males reaching sexual maturity at 3 years and females at 4 in Central and Eastern Europe (Freyhof and Kottelat 2008).
Crucian Carp spawn intermittently from late spring through mid-summer, after water temperatures have reached the required threshold of 17-20 C (Seymour 1981 in Holopainen et al. 1997). This species is a phytophilous, open substrate spawner with eggs that adhere to submerged structures and macrophytes (Holopainen et al. 1997). Relative and absolute fecundity varies among populations and environments and increases with size. Average relative fecundity of Crucian Carp in Lake Varaslampi was 129.2 eggs/g of fish, which would amount to an absolute fecundity of 121,822.7 for an average sized female in that lake (942.9 g wet mass) (Moisander 1989 in Holopainen et al. 1997). Fertilized eggs were found to develop normally in laboratory conditions at 15-28 C, but died at 5, 10, and 30 C. Temperatures of 24-28 C resulted in the quickest (>3 days) incubation from fertilization to the free embryo stage. In Scandinavian ponds, hatching took six days at 18-19 C (Laurila et al. 1987 in Holopainen et al 1997).
Growth rates can be highly variable and are believed to be strongly influenced by population densities and food availability. In their first year of life, Carassius carassius have been observed to range in size from 2.2 to 10.9 cm depending on the population density, environment, and when an individual was born during the spawning season (Skora 1982, Piironen and Holopainen 1988 in Holopainen et al. 1997).
This species occurs regularly in small ponds (often as a monoculture assemblage) as well as in larger lakes with high species richness. Crucian Carp exhibit two distinct morphologies which are reported to be driven predominantly by the fish community assemblage. Particularly, the existence of “pond” and “lake” forms of Crucian Carp are believed to be strongly related to the presence or absence of predators more than environmental or physical characteristics of the water body. In Swedish lakes, Crucian Carp population density and size were significantly smaller (mean size 9.2 cm, 20 g) in ponds and lakes without piscivores than in water bodies with piscivores (mean size 31.3 cm, 735 g) (Bronmark et al. 1995). However, growth-stunted, low-bodied (‘pond’ form) Crucian Carp were observed to shift their deep-bodied form (‘lake’ form) when introduced into a fishless, productive pond. This suggests that two different conditions can induce Crucian Carp to grow into the deep-bodied lake forms: 1) the presence of piscivorous fish and 2) increased food availability in the absence of piscivorous fish (Holopainen et al. 1997).