Tales of two fish: the dichotomous biology of crucian carp (Carassius carassius (L.)) in northern Europe

Abstract

Crucian carp, a common Eurasian cyprinid fish, shows striking dichotomies in several aspects of its physiology and ecology, at both the individual and population levels. These dichotomies consistently reflect the communities and ecosystems in which they occur, contrasting crucian carp that occur in (single-species fish assem blages) in ponds with those occurring in multi-species assemblages, primarily in lakes. Dichotomies also occur in the physiological state of individuals between summer and winter. All these dichotomies, involving differences in morphology and population structure, population dynamics and life history, reflect in an integrative way the adap tive strengths and limitations of a unique species. In numerous northern ponds, dense monocultures of crucian carp are sealed in by thick ice and snow to live more than six months in dark, anoxic waters at near freezing temperatures. In summer, these populations experience temperatures of up to 30°C during a relatively brief period of vigorous growth and repeated bouts of reproduction in almost continuous daylight. Crucians in lakes experience a more benign abiotic environment and, with densities that can be orders of magnitude lower than those in ponds, are likely unaffected by intraspecific interactions. However, co-occurring species present a challenging biotic environment. Crucian carp is exceptionally vulnerable to predation and populations persist via three kinds of refugia. Aided by extreme physiological adaptations, crucian carp can be the sole piscine inhab itant of seasonally harsh but productive small ponds, the refugium habitat, where they form dense monocultures of stunted individuals. The structural complexity offered by dense beds of macrophytes in productive larger lakes, the other common habitat (habi tat refugium) of crucian carp, ensures survival of a few offspring, even in the presence of piscivores. The risk of predation still remains high for crucians until a certain length (and/or body depth), the size refugium, is attained. Crucian carp's unique anoxia toler ance, accompanied by many exceptional structural and functional features, and the species' suitability for laboratory studies, has recently raised crucian carp to a status of a physiological model species like its relative, the goldfish. Moreover, the dichotomy found in the structure of natural populations has made crucian carp an attractive model for ecological studies in competititon and predation. Crucian carp's resistance to ad verse environmental conditions further suggests promising economical use in aquaculture to produce hybrids endowed with higher survival capacity than common carp.