Mosquitofish were raised in environments with and without dragonfly nymphs to observe phenotypic plasticity. Statistical analysis found mosquitofish developed a longer, thinner posterior region and more lateral eye placement in the presence of dragonfly nymphs. These traits likely aid in detecting and evading attacks. The study demonstrates non-contact chemical cues can induce vertebrate developmental shifts, representing a novel example of phenotypic plasticity in response to invertebrate predators.

1. Mosquitofish, Gambusia affinis, is a small live-bearing
fish that lives in shallow, stagnant or slow-moving water,
with abundant vegetation mostly absent of predatory fish.
Their mouths are up-turned to feed from surface waters
and they primarily eat invertebrates and zooplankton. Two
main predators of mosquitofish are bluegill sunfish and
dragonfly nymphs. Bluegill chase down their prey
whereas nymphs are lie-in-wait predators.
The purpose of this study was to observe if chemical cues
from dragonfly larvae induce morphological change in
mosquitofish. This is one component of a larger study
looking also at the affect of bluegill on mosquitofish
development. Because bluegill and dragonfly nymphs
have different methods of predation, it was expected
mosquitofish would develop a body shape in the presence
of dragonflies distinct from the body shape in the presence
of bluegill. Such environmentally-contingent development
is called phenotypic plasticity, or simply “plasticity”.1
Plasticity can be an adaptive strategy in variable
environments whereas fixed development of adapted
phenotypes is expected in invariant environments.1
Dragonfly nymphs are variable in presence and
abundance, both seasonally and among years, in
freshwater environments. As a lie-in-wait predator, it is
unknown whether the fast-swimming morphology in
mosquitofish might be adaptive. Such morphology,
involving a longer caudal area (thrust generating region)
of the body, is known for mosquitofish from habitats with
bluegill.2
Thus we conducted the following experiment to
test whether mosquitofish could be induced fast-
swimming or any other unique forms of morphology in the
presence of dragonfly nymphs.
Methods Conclusions
Insect pheromone alters development of mosquitofish
Kaylee Pickwell, Briana Lindsley, Chris Roberts, Katie Bridge, Thomas J. DeWitt
BESC Undergraduate Research Scholars program, Department of Plant Pathology and Microbiology, Texas A&M University
Pregnant mosquito fish were collected from two populations in Brazos
Valley, TX: Hensel Park (30 36.6’N, 96 17.6’W), a drainage canal⁰ ⁰
with no piscivorous fish, and White Creek (30°36'N, 96°21'W)
containing predatory dragonflies. Collected organisms were placed in
brood chambers to allow them to deliver offspring. The offspring were
then randomly assigned to rearing tanks. Rearing tanks were 1 of 4
conditions (Fig. 1).
×3
Fig.1. Experimental design. Four rearing environments (A, no predators; B,
dragonfly larvae; C, bluegill; D, both predator species) were replicated three times
each. Twelve mosquitofish were reared in each of the 12 tanks, although only 90
have so far been measured.
The rearing experiment was conducted in a 4x3 arrangement with 12
mosquitofish per tank. Fish were reared for 63 days and then pictures
were taken of the lateral side of each individual. Geometric
morphometric analysis was conducted by digitizing the 10 landmarks
below using the software tpsUtil (Fig. 2).
Introduction
Results
Fig. 2. Lateral view of a mosquitofish with landmarks used for analysis.
Shaded region of interest is the caudal (thrust-generating) region.2
BA C D
Prey: Western mosquitofish, Gambusia affinis
Predators:
Bluegill sunfish, Dragonfly larvae
Lepomis macrochirus Anax sp.
1.Phenotypic Variation from Single Genotypes.
(2004). In T. DeWitt & S. Scheiner
(Eds.), Phenotypic Plasticity: Functional and
Conceptual Approaches (p. 2). New York, New
York: Oxford University Press.
2.Langerhans, B., & DeWitt, T. (2004). Predator-
driven phenotypic diversification in Gambusia
affinis. Evolution, 58(10), 2305–2318-2305–2318.
3.Template used from
http://osp.ua.edu/URCAnewFAQ.html
ReferencesAcknowledgements
The DeWitt and Winemiller lab were essential in
the understanding and execution of the work in
this study. Sponsors for high impact
experiences for BESC and the BESC poster
symposium include the Department of Plant
Pathology and Microbiology, the College of
Agriculture and Life Sciences, the Office of the
Provost and the Executive Vice President for
Academic Affairs.
Statistical analysis and thin-plate spline visualization
indicated that mosquitofish developed a longer, thinner
thrust-generating posterior region the presence of
dragonfly nymphs. It seems likely that this morphology
would allow greater thrust to either avoid attacks that
could be detected in progress, or to rip free from the
insect’s grasp if attacks could not be evaded in progress.
Surprisingly, placement of the eye, which is generally
considered a deeply canalized trait in vertebrate
development, was distinctly more lateral when
mosquitofish were reared with nymphs. Because
dragonflies typically attack the side of mosquitofish, it is
likely the lateral eye placement allows the fish to better
detect nymphs lying in wait or to better see oncoming
attacks. In the absence of nymphs, more anterior eye
placement would likely increase foraging efficiency
(spotting prey and targeting suction streams). So on logical
grounds, it appears likely the phenotypic plasticity
observed in response to nymphs represents an evolutionary
adaptation.
Our data demonstrate that cues from an invertebrate,
presumably chemical cues, induce developmental shifts in
a vertebrate animal. Although invertebrate prey are known
to induce trophic (jaw structure and musculature)
differentiation in vertebrates (e.g. several species of fish),
through use and disuse of the jaws, the present result is
unique in that no physical contact occurred between
species. Mosquitofish must have a mechanism that can
accept, process, and respond to non-mechanical stimuli
during development. Thus we believe we found a novel
occurrence that will add to the growing breadth of case
studies on phenotypic plasticity.
Landmark data of 90 mosquitofish from the rearing experiment were
superimposed (translated, scaled, and rotated) to yield separate size
and shape variables using tpsRelw software. Shape variables were
subjected to principal components analysis for dimension reduction.
Sixteen principal components of shape variation were used as
dependent variables in shape analysis.
To test for morphological divergence in body shape between predatory
environments, we performed a multivariate analysis of covariance
(MANCOVA). Morphological data (16 principal components) were
tested for effects attributable to dragonfly presence and fish presence
while statistically controlling for the affect of size on shape (i.e.
allometry). Statistical analysis was conducted using JMP Pro 12.
Variation in landmark positions due to the dragonfly effect were
visualized with tpsRegr software. This program maps deformations
among the shapes from one treatment to another, in this case from no
predator to the presence of dragonfly nymphs.
Phenotypic Plasticity in Mosquito Fish
Figure 3. Visualization of morphological divergence between predator regimes for
mosquitofish. Thin-plate spline transformations depict morphological differences
in no predator and dragonfly regimes as described by canonical axes
derived from MANCOVA.
Table 1. MANCOVA results for the effects of size and dragonflies on body shape in mosquito fish.
Source F dfnum dfdenom P
Size 5.20 16 63 <.0001
Dragonfly 2.16 16 63 0.016
No Predator Dragonfly Present
Dragonfly nymphs were found to induce body shape
differentiation in mosquitofish (Table 1). In the presence of
dragonfly nymphs, mosquitofish developed a longer caudal
(thrust-generating) area .and more lateral position of the eye.
Allometry, as expected, was present and accounted for the
standard vertebrate scaling of body proportions, wherein
juveniles have relatively larger heads and trunks, with more
axial features developing as development toward fully adult
morphology progresses.