A PLURALISTIC ANALYSIS OF CHARACTER RELEASE IN PUMPKINSEED SUNFISH (LEPOMIS GIBBOSUS)

Abstract

While the importance of competition in adaptive divergence has recently been demonstrated in a number of systems, its interaction with other ecological factors has been neglected. We attempt a more pluralistic approach in this comparative study of divergence in pumpkinseed sunfish (Lepomis gibbosus) from the Adirondack region of New York. In some lakes, pumpkinseed forms that use shallow‐ and open‐water environments coexist. We tested four predictions relating to an hypothesis of character release: (1) Pumpkinseed divergence is favored in the absence of the closely related bluegill sunfish (L. macrochirus), a known competitor elsewhere, (2) divergence is favored when the combined effects of other planktivorous fish taxa that may impose diffuse competition is reduced, (3) divergence is positively related to lake size because larger lakes provide more distinct and diverse environments, and (4) divergence is favored in lakes dominated by open‐water habitat. In two lakes with introduced bluegill (sympatry), bluegill appear to limit pumpkinseed divergence, as they do elsewhere. Pumpkinseeds were neither found in the open‐water habitat, nor did they eat significant amounts of open‐water prey. In nine allopatric lakes selected for their similarity to the sympatric lakes, pumpkinseeds were ecologically and morphologically divergent, consistent with a character release hypothesis. Morphological divergence followed functional expectations. Open‐water pumpkinseeds generally had longer caudal peduncles, shorter heads, thinner bodies, and smaller paired fins compared to shallow‐water pumpkinseeds (heritable variation in body shape has been demonstrated elsewhere). Among 26 allopatric lakes, other planktivores had a negative effect on intrapopulation divergence as if they collectively imposed diffuse competition. Surprisingly, lake size was unrelated to divergence, and the amount of open‐water habitat had an unexpected negative effect. Mean body size was also negatively related to divergence. Our findings illustrate that a variety of ecological factors, in addition to competition, may influence the divergence of pumpkinseeds, and possibly other fishes in postglacial north temperate lakes.

Body size evolution under character release in the ground beetleCarabus japonicus

“…Schluter & McPhail, 1992;Radtkey et al, 1997;Pfennig & Murphy, 2000;Taylor & McPhail, 2000;Grant & Grant, 2006;Rice & Pfennig, 2008), but there are few convincing cases of morphological character shifts under character release (Grant, 1972;Simberloff et al, 2000;Meiri et al, 2007Meiri et al, , 2011. This may be because a character state, in the absence of interacting species, can vary among populations owing to environmental differences (Robinson et al, 2000), and because the same character state can be equally adaptive in both sympatric and allopatric habitats. Therefore, to understand character evolution under character release, it is important to examine the effects of environmental factors other than the presence/absence of interacting species on the character state in question (Meiri et al, 2011).…”

Feeding ecology of non-native centrarchids (Actinopterygii: Perciformes: Centrarchidae) in two Iberian reservoirs with contrasting food resources

“…Although experimental studies suggest that pumpkinseed above 80 mm use Daphnia with less profitability than other prey, such as gastropods (Mittelbach 1984), some investigations conducted in North America have showed that adult pumpkinseeds can eat large quantities of zooplankon in lakes without the bluegill or other zooplanktivorous species, sometimes developing a pelagic morphotype (Robinson et al 1993(Robinson et al , 2000. The adult bluegill is morphologically better adapted than the pumpkinseed to eat zooplankton and when the two species are sympatric the former usually precludes the utilisation of zooplankton by the pumpkinseed (Osenberg et al 1988.…”

Contemporary and historical evolutionary processes interact to shape patterns of within‐lake phenotypic divergences in polyphenic pumpkinseed sunfish,Lepomis gibbosus

“…Its functional morphology includes specialized traits for feeding on large armored invertebrates such as snails (Lauder 1983;Wainwright et al 1991), which likely provides a competitive refuge from other sunfish (Mittelbach 1984). In the absence of bluegill sunfish (L. macrochirrus), a zooplankton generalist, some pumpkinseed sunfish are found in open water (pelagic) habitats of postglacial lakes, where they feed on zooplankton and are phenotypically divergent from local pumpkinseed sunfish that inhabit the shallow inshore littoral habitat (Robinson et al 1993;Robinson et al 2000;Gillespie and Fox 2003;Jastrebski and Robinson 2004). Phenotypic diversity within numerous lakes in New York State (USA) and the province of Ontario (Canada) is thus represented by a trophically related polyphenism bounded by sympatric littoral and pelagic ecomorphs (Robinson et al 1993;Robinson et al 2000;Gillespie and Fox 2003;Jastrebski and Robinson 2004).…”

“…We created a balanced sampling design by choosing three lakes within each of four contemporary drainage systems, two drainages in each of two regions (Ontario and New York, approximately midway between glacial refugia in the Mississippi drainage, and along the Atlantic seaboard). Approximately 50 individuals from each littoral and pelagic habitat in every lake were collected using sampling techniques consistent with previous studies (Robinson et al 2000;Gillespie and Fox 2003;Jastrebski and Robinson 2004). Fish were collected by angling, or captured in fish traps (35-cm diameter × 90-cm long; 8-cm diameter aperture).…”

“…Previous studies documenting parallel aspects of phenotypic divergence among lakes containing polyphenic populations of pumpkinseed (Robinson et al 2000;Jastrebski and Robinson 2004;Gillespie and Fox 2003;Riopel et al 2008) have assumed that lake populations are evolutionarily independent and that historical effects have little affect on phenotypic diversity. Our analyses above show that neither of these assumptions is strictly justified because all populations are related, although in a nested hierarchy that affords less evolutionary independence between lakes within regional genetic clusters and greater independence among lakes from different genetic clusters.…”