TALES OF 2 FISH - THE DICHOTOMOUS BIOLOGY OF CRUCIAN CARP (CARASSIUS-CARASSIUS (L)) IN NORTHERN EUROPE

Crucian carp, a common Eurasian cyprinid fish, shows striking dichotom ies in several aspects of its physiology and ecology, at both the indi vidual and population levels. These dichotomies consistently reflect t he communities and ecosystems in which they occur, contrasting crucian carp that occur in ''monocultures'' (single-species fish assemblages) in ponds with those occurring in multi-species assemblages, primarily in lakes. Dichotomies also occur in the physiological state of indivi duals between summer and winter. All these dichotomies, involving diff erences in morphology and population structure, population dynamics an d life history, reflect in an integrative way the adaptive strengths a nd limitations of a unique species. In numerous northern ponds, dense monocultures of crucian carp an sealed in by thick ice and snow to liv e more than six months in dark, anoxic waters at near freezing tempera tures. In summer, these populations experience temperatures of up to 3 0 degrees C during a relatively brief period of vigorous growth and re peated bouts of reproduction in almost continuous daylight. Crucians i n lakes experience a more benign abiotic environment and, with densiti es that can be orders of magnitude lower than those in ponds, are like ly unaffected by intraspecific interactions. However, co-occurring spe cies present a challenging biotic environment. Crucian carp is excepti onally vulnerable to predation and populations persist via three kinds of refugia. Aided by extreme physiological adaptations, crucian carp can be the sole piscine inhabitant of seasonally harsh but productive small ponds, the refugium habitat, where they form dense monocultures of stunted individuals. The structural complexity offered by dense bed s of macrophytes in productive larger lakes, the other common habitat (habitat refugium) of crucian carp, ensures survival of a few offsprin g, even in the presence of piscivores. The risk of predation still rem ains high for crucians until a certain length (and/or body depth), the size refugium, is attained. Crucian carp's unique anoxia tolerance, a ccompanied 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 rel ative, the goldfish. Moreover, the dichotomy found in the structure of natural populations has made crucian carp an attractive model for eco logical studies in competition and predation. Crucian carp's resistanc e to adverse environmental conditions further suggests promising econo mical use in aquaculture to produce hybrids endowed with higher surviv al capacity than common carp.