INTRODUCTION
A major challenge facing all organisms is to adapt to environments that
vary within their lifespan. A route to responding to and surviving such
variation is phenotypic plasticity, the ability of individual genotypes
to change phenotype when exposed to different environments during their
life-cycle . One pervasive, natural source of environmental variation is
the risk of mortality from predation, known as predation risk. Predation
risk induces a suite of changes in the behaviour, life history and
morphology of many plants and animals . Of particular interest are
morphological responses to predation risk which range from the
production of spines to changes in the shape of a portion of the body or
the entire body plan of an organism.
Many studies of predator-induced shape change have focussed on linear
assessments of shape, which measure the distance between two points. Key
examples include changes in defensive dorsal spine length , body depth
that affects vulnerability to gape-limited predators and morphological
features associated with behavioural swimming escape responses .
However, this type of analysis only captures a subset of the overall
shape variation.
Measurement of overall shape is a multivariate analysis (i.e. it
involves multiple different variables). A well-established method for
assessing multivariate plasticity in shape is geometric morphometrics ,
which uses anatomical coordinates as shape variables to measure relative
differences in shape. This approach has been used extensively to measure
predator-induced changes in shape for a wide range of organisms, such as
fish , amphibians and snails .
Geometric morphometrics not only allows the measurement of shape
overall, but also the extent of modularity and integration between
different aspects of shape (Klingenberg, 2014). Modularity refers to the
level of covariation between different traits within morphological
structures, or modules, relative to the level of covariation between
these structures. Integration refers to the level of co-variation
between different traits throughout a morphological structure or even a
whole organism (Klingenberg, 2014). Therefore, modularity exists if the
level of integration within modules is strong compared to the
integration between modules (Klingenberg, 2009).
In addition to considering multivariate shape, there has been a shift
from the standard ”two-environment” approach for assessing phenotypic
plasticity and estimating reaction norms (Roff 1999) to analysing
changes along a gradient. More recently, it has become standard to
evaluate plasticity in multiple traits and along environmental gradients
of more than two environments .
Despite such work, shape has rarely been assessed as a plastic trait in
water fleas (Daphnia species), an iconic organism for the study
of size-selective, predator-induced phenotypic change. Instead, research
has largely focused on assessing the production of inducible
morphological defences, such as the head spikes of Daphnia pulex ,
called ‘neckteeth’, which develop in response to predator cues
(kairomones) released from their midge larvae predators . Although there
has been some research into the dorsal pattern of induced morphology inD. pulex , the question of how overall shape changes in response
to predation risk remains unanswered.
Other examples of predator-induced changes in Daphnia suggest
that overall shape may change in response to predation risk.
Considerable variation exists in many features of the Daphniaspp. body plan including body width , alignment , shoulder shape and
carapace strength . Furthermore, changes in body size are
well-documented in the context of size selective predation theory and
empirical assessment of life history responses to predation risk .
Together with associated research on fish, this suggests that overall
shape might increase survival and therefore provide important fitness
benefits to plasticity.
In this study, we evaluate shape plasticity along a gradient of
increasing predation risk in three genotypes of D. pulex which
differ in their sensitivity to predator cues. We apply morphometric
landmark-based analysis to photographs of Daphnia taken by , in
which D. pulex were exposed to six levels of predation risk from
their midge larvae predator, Chaoborus flavicans . We combine
geometric morphometrics with phenotypic trajectory analysis to formally
evaluate the multivariate change in shape and estimate measures of both
modularity and integration to evaluate if there are coherent units of
the body plan that respond to predation risk, and whether these units
change independently or together .
In advance of the analysis, we predict narrower bodies and bigger heads
will form part of the predator-induced response in D. pulex .
Narrower bodies enhance the predator escape response in fish and
amphibians and we expect that bigger heads are more likely to interfere
with predation linked to the neckteeth defence. Given these two
predictions, we might also predict modularity and / or integration of
these responses. From a modularity perspective, we predict that the head
region and lower body are separated in their response to predation due
to the nature of the induced morphological change in the head region.
This means that the neckteeth defence and associated shape changes of
the head would be localised to that part of the animal and relatively
independent of changes to the body. Furthermore, under the modularity
hypothesis, we might also predict that changes in the dorsal region,
where the neckteeth form, respond independently to the ventral portion
of the daphnid. As for the integration hypothesis, we predict some level
of integration between the head and lower body regions driven by a
negative developmental correlation, where the head gets larger and the
body gets narrower and longer.