Energetic consequences of an inducible morphological defence in crucian carp

Crucian carp (Carassius carassius) increases in body depth in response to c hemical cues from piscivores and the deeper body constitutes a morphologica l defence against gape-limited piscivores. In the field, deep-bodied indivi duals suffer a density-dependent cost when competing with shallow-bodied co nspecifics. Here, we use hydrodynamic theory and swimming respirometry to i nvestigate the proposed mechanism underlying this effect, high drag caused by the deep-bodied morphology. Our study confirms that drag is higher for d eep-bodied crucian carp, both in terms of estimated theoretical drag and po wer curve steepness. However, deep-bodied fish swimming at the velocity ass ociated with minimum cost of transport, U-mc, did not experience higher cos ts of transport than shallow-bodied fish. Deep-bodied crucian carp had sign ificantly lower standard metabolic rates, i.e. metabolic rates at rest, and also lower U-mc, and the resulting costs of transport were similar for the two morphs. Nevertheless, when deep-bodied individuals deviate from U-mc, e.g. when increasing foraging effort under competition, their steeper power curves will cause substantial energy costs relative to shallow-bodied cons pecifics. Furthermore, there is evidence that reductions in standard metabo lic rate incur costs in terms of lower stress tolerance, reduced growth rat e, and life history changes. Thus, this work provides links between hydrody namics, a cost-reducing mechanism, and a density-dependent fitness cost ass ociated with an inducible defence.