Density-dependent costs of an inducible morphological defense in crucian carp
- Division aquatic ecology
be permanent. Thus, in the absence of predators and with limited resources, defended
individuals are expected to be outcompeted by individuals lacking the costly defense.
Crucian carp, Carassius carassius, increase in body depth in response to chemical stimuli
from piscivorous fish, and the deeper body constitutes a morphological defense against
gape-limited piscivores. Here, we have performed a field experiment investigating effects
of population density on the relative competitive performance of shallow-bodied and deepbodied
crucian carp. We assigned groups of six individually marked fish (the ‘‘focal’’
individuals) of either morph into enclosures with either low or high crucian carp density,
and monitored changes in body mass, length, and body depth of these fish for 4 mo. In
addition, we determined sex, and final liver and gonadal mass, of all focal fish.
At high population density, there was a reduction in food resources (zooplankters),
which resulted in reduced gonad mass, reduced gonadosomatic indices, and reduced growth
in fish of both morphs. Unexpectedly, the relative allocation to livers was higher at high
density. There was no difference in body mass gain between morphs at the low density,
whereas at the high density shallow-bodied fish gained twice as much body mass as deepbodied
fish. Hence, when predators were absent and food resources were limited, deepbodied
individuals suffered a substantial cost when competing with shallow-bodied conspecifics.
This study verifies the theoretical prediction that an inducible defense results in fitness
costs in the absence of predators. Further, as we found no measurable disadvantage of the
deep-bodied morphology at high food availability but a strong disadvantage under intense
competition for food, our results also demonstrate the importance of relating defense costs
to levels of intraspecific competition.
- body size
- Carassius carassius
- density dependence
- field-enclosure experiment
- inducible defense
- phenotypic plasticity
- resource allocation
- ISSN: 0012-9658