Biological Journal of the Linnean Society, 2007, 90, 517–523. With 2 figures © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 517–523 517 Blackwell Publishing LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066© 2007 The Linnean Society of London? 2007 903 517523 Original Articles GUT LENGTH PLASTICITY IN PERCH J. OLSSON ET AL . *Corresponding author. E-mail: jens.olsson@ebc.uu.se †Current address: Ecology Unit, Department of Biology of Organisms and Systems, Oviedo University, 33071 Oviedo, Spain. ‡Current address: Department of Zoology, University of British Columbia, 6270 University Boulevard, Vancouver BC, V6T 1Z4, Canada. Gut length plasticity in perch: into the bowels of resource polymorphisms JENS OLSSON 1 *, MARIO QUEVEDO†, CELINE COLSON and RICHARD SVANBÄCK‡ Limnology/Department of Ecology and Evolution, Evolutionary Biology Centre, Uppsala University, Norbyv. 20, SE-752 36 Uppsala, Sweden Received 21 November 2005; accepted for publication 20 June 2006 Resource polymorphisms, intraspecific variation in morphology due to differential resource use, are common across a wide range of animal taxa. The focus in studies of such polymorphisms has been on external morphology, but the differential use of food resources could also influence other phenotypic traits such as the digestive performance. In the present study, we experimentally demonstrate that Eurasian perch (Perca fluviatilis L.) display adaptive plas- ticity in gut length when exposed to different food types. Perch fed a less digestible food type developed relatively longer guts compared to fish fed a more easily digested food type. This divergence in gut length was also apparent under natural conditions because perch inhabiting the littoral and pelagic habitats of a lake differed in resource use and relative gut length. Despite that the digestive system in perch is plastic, we found that individuals switching to a novel food type might experience an initial fitness cost of the diet switch in the form of a temporary reduction in body condition. These results show the importance of gut length plasticity for an ontogenetic omnivore but also a cost that might prevent diet switching in polymorphic populations. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 517–523. ADDITIONAL KEYWORDS: adaptive individual variation – body condition – diet switch – fitness cost – food digestibility – food type – gastrointestinal tract length – phenotypic plasticity – stable isotope analysis. INTRODUCTION It is widespread among several vertebrate taxa that species utilizing food resources that differ in quality have digestive systems adapted to the digestive demands of their food (Piersma and Lindström, 1997; Starck, 1999). For example, species feeding on food of low quality usually have a larger digestive tract com- pared to species feeding on more easily digested food resources (Sibly, 1981). An increase in size of digestive organs has been thought to facilitate a more efficient utilization of the food because larger digestive systems allow longer transit times and a larger diameter per unit body for food processing (Sibly, 1981; Magnan and Stevens, 1993). The proposed cost of having a large digestive tract is increased requirements of nutrients and energy for maintenance of the tract (Sibly, 1981; Moss, 1983). Animals are thus expected to trade-off the size of their digestive tracts between the digestive requirements of their food and the costs involved in maintaining a large organ system. It has been hypothesized that plastic responses are ubiquitous in unstable environments (Stearns, 1989; Pfennig, 1992; Scheiner, 1993), whereas phenotypic variation in more stable environments is under strong genetic control (Hori, 1993; Smith, 1993). Because optimal digestion theory suggests a tight link between the morphology of the digestive system and food qual- ity (Sibly, 1981), plasticity in the outline of the diges- tive organs is expected among animals experiencing temporal or spatial differences in food quality (Ham- mond, 1993; Magnan and Stevens, 1993; Peirsma and Lindström, 1997). Several studies have also demon- strated an individual variation in the morphology of