PHENOTYPIC PLASTICITY AND PREDATOR EFFECTS ON MORPHOLOGY AND PHYSIOLOGY OF CRUCIAN CARP IN NATURE AND IN THE LABORATORY

Crucian carp Carassius carassius show great phenotypic plasticity in i ndividual morphology and physiology, and strong variation in populatio n density in different fish communities. Small fish with shallow bodie s and large heads are typical in overcrowded monospecific fish communi ties in small ponds, whereas deep-bodied, large fish are found in larg er, multispecies lakes. Crucian carp are especially vulnerable to pred ation by piscivorous fish and their greater relative body depth in mul tispecies fish communities has been proposed to be an induced defence against size-limited predation, and hence to be an adaptive feature. D ata are presented here on the two divergent body forms in field popula tions in eastern Finland, together with results of laboratory experime nts on predator effects on morphology and physiology (growth, respirat ion, heart rate). The deep body can be achieved in a few months by int roducing a low population density of shallow-bodied fish into a food-r ich environment with no piscivores. In the laboratory, both the presen ce of piscivores (chemical cues) and enhanced food availability increa sed the relative depth of crucian carp, but only to a modest extent wh en compared to natural variation. It is concluded that the deep-body f orm of crucian carp in the low density populations of multispecies fis h communities is the normal condition. Reproduction in monospecific po nds results in high intraspecific competition, low growth rate and a s tunted morphology. According to pilot tests, the mechanism behind the predator effect in the laboratory might be a behavioural reaction to c hemical cues (alarm substances/predator odour) causing changes in ener gy allocation: predator-exposed crucian carp adopt a 'hiding' mode wit h decreased activity (less swimming, lower respiration and heart rate) and with higher overall growth. Whether, and to what extent, this pre dator-induced mechanism works in nature is unclear. (C) 1997 The Fishe ries Society of the British Isles.