The foraging ecology of fish is often considered to be the primary determinant of body shape due to tight links between morphology, swimming performance, and foraging efficiency. Fish foraging on littoral benthic macroinvertebrates typically have a deeper body shape than those foraging on pelagic zooplankton in the water column. However, morphological traits often have multiple ecological functions, which could result in performance trade-offs between functions. Here, we provide the first examination of body shape and diet in a species with alternative reproductive tactics, in this case, bluegill sunfish (Lepomis macrochirus Rafinesque, 1819). Bluegill males mature into either “parental” or “cuckolder” reproductive tactics. Parentals build nests and provide sole parental care and defense of the young. Cuckolders instead act as “sneakers” darting into the nests of parental males while mating is occurring and then later in life become “satellites,” mimicking female appearance and behavior. Using stable carbon and nitrogen isotopic analysis of diet, we found that parentals and females consumed primarily pelagic zooplankton yet were the deepest in body shape. Sneakers consumed more littoral resources but were the most streamlined. Satellite males also consumed predominately littoral resources but had a deeper body form that was more similar to females than to size-matched juveniles. Our results differ from past studies of foraging ecomorphology and suggest that other selection pressures, such as sexual selection in species with alternative reproductive tactics, may also be an important factor influencing shape.
We collected a total of 142 bluegill from Lake Opinicon (Ontario, Canada, 44°34′N, 76°19′W). During the period of 8–26 June 2009, we used daily snorkel surveys of the littoral habitat to collect 102 bluegill using dip nets; sampling began on the first day parental males began to form colonies and continued until the first day after spawning had occurred. These fish sampled in 2009 were previously used in an examination of bluegill morphology and swim performance (Colborne et al. 2011). An additional 40 fish (parental males and females only) were collected from multiple locations in Lake Opinicon by angling with a small piece (2–3cm) of earthworm suspended from the side of our research boat during the period of 24 May–30 June 2010 and were added to the original data set. Classification of fish into juveniles, females, parental males, satellites, or sneakers was initially based on observations of behavior in the field (i.e., immediately prior to collection; 2009 only) and the results of subsequent dissections. Juveniles have a gonadosomatic index (GSI; the ratio of gonad mass to body mass) of less than 1%, sneakers have a total body length of less than 100mm and GSI of about 4%, and satellites have a total length of 100mm or greater and a 3% GSI (Gross 1982; Colborne et al. 2011).
All collected fish were taken to the Queen’s University Biological Station and held in flow-through aquariums with water drawn directly from Lake Opinicon for no longer than 6h before being euthanized with clove oil. We first assessed morphological variation in body shape among the 5 bluegill groups (n = 25 juveniles, 43 females, 36 parentals, 18 satellites, and 20 sneakers) by taking photographs of the left side of each fish with either a Canon PowerShot A95 (5.0 megapixels) or Olympus Stylus Tough-6000 (10 megapixels) digital camera. Using tpsDig software (Rohlf 2008), we placed 20 homologous landmarks on the image of each fish (see Colborne et al. 2011 for landmark locations). These homologous landmarks were then used to compare body shape, independent of size, across the 5 bluegill groups using partial warp analysis (Zelditch et al. 2004; Colborne et al. 2011).
Stable isotope analysis
Following photographing, we collected tissue samples for stable isotope analysis of diet. A sample of white muscle was removed from the right side of each fish immediately underneath the posterior portion of the dorsal fin. The liver was then removed and both tissue samples were stored at −20 °C. White muscle and liver tissues were selected because they are commonly used in stable isotope studies of fish diet (reviewed in Boecklen et al. 2011) and provide the opportunity to infer diet over different time frames because of tissue-specific differences in metabolic activity; white muscle reflects diet over a period of months, compared with several weeks for liver tissue (Hesslein et al. 1993, Perga and Gerdeaux 2005, Guelinckx et al. 2007).