INTRODUCTION
Within populations, individual animals often exhibit behavioural
differences that are consistent across time and context . Although this
phenomenon, widely referred to as animal personality , has been
demonstrated across very diverse animal taxa , the empirical literature
is dominated by vertebrate studies . In comparison, personality
variation in invertebrates generally, and aquatic invertebrates in
particular, has been less well studied . This omission matters because
understanding personality variation is not only central to fundamental
research on animal behaviour, but also increasingly relevant across more
applied fields such as welfare , ecotoxicology and invasion biology .
Here we investigate the presence and structure of among-individual
variation in the red cherry shrimp (Neocaridina heteropoda ). This
is a popular ornamental species that, being easy to maintain and breed
under laboratory conditions, is an emerging model for decapod research
with relevance to ecosystem stability, ecotoxicology, and aquaculture .
From a fundamental perspective, among-individual variation in behaviour
is widely assumed to have functional importance, impacting fitness via
effects on survival and/or reproduction . While quantitative genetic
studies have also shown that among-individual differences are heritable
, understanding the evolutionary causes and consequences of this
variation remains a challenge. Why does variation persist? Are
among-individual differences adaptive, or do they reflect evolutionary
constraints (e.g. trade-offs)? Why can’t all individuals adjust
behaviour plastically to be optimal for the conditions they find
themselves in? Hypothesised answers to these questions often invoke
state-dependence, predicting that personality will covary with other
traits (e.g. metabolic rate, body size) or intrinsic variables (e.g.
age, sex) as a consequence of feedback between behaviour and state . For
example, if small individuals face greater starvation risk, they may
need to be ‘bolder’ (less risk averse) and more explorative to find
resources faster, even if this behaviour increases predation risk .
State-dependence means behaviour is likely to be integrated within
broader life history strategies , and differ systematically between
sexes . For example, in some systems males, on average, need to adopt
riskier behaviour than females in order to obtain mating opportunities .
Conversely, female behaviour may be selected for to reduce costs from
male harassment .
Although evolutionarily motivated studies dominate the literature on
animal personality , this phenomenon is increasingly recognised as
having wider implications and applications. For example, Prentice et al.
(2022) argues how the integration of personality traits with stress
physiology means artificial selection on behavioural biomarkers could be
used to improve welfare in fish aquaculture (see also e.g. .
Ecotoxicology is another applied field in which the potential importance
of among-individual differences in behaviour has been recently
highlighted . For example, great tits (Parus major ) with high
levels of lead in their blood and high levels of multiple metals in
their feathers, showed lower explorative behaviours on average , while
insecticide exposure lowered behavioural repeatabilities in spiders ,
reducing the relative importance of among-individual differences.
Furthermore, found long-term fluoxetine exposure in guppies
(Poecilia reticulata ) erodes variation in activity levels between
individuals. Ubiquitous contamination may therefore impair behaviour and
future adaptive potential of phenotypic variation to
anthropogenic-induced alterations within both terrestrial and aquatic
landscapes.
In decapod crustaceans, such as the species we investigate here, several
applications of personality variation have been suggested. First, just
as in fishes, personality traits may be relevant to welfare outcomes in
captivity, which are under increasing scrutiny following recognition of
sentience . Second, decapod behavioural change following sub-lethal
exposure to environmental pollutants could contribute to bioassays
relevant for monitoring ecosystem health and susceptibility of benthic
and/or sediment dwelling invertebrates to pollutants . Third, since
decapods demonstrate trait-biased dispersal, with bolder individuals
outcompeting conspecifics, variation in boldness and activity may link
to invasive success . Finally, many billions of decapod crustaceans are
harvested from wild fisheries and raised in aquaculture systems for
human consumption annually . Personality differences have been shown to
predict trappability in decapods just as in vertebrates , and are also
expected to be integrated with life-history traits important for
production in aquaculture .
Here we characterise personality variation in the red cherry shrimpNeocaridina heteropoda (syn. N. davidi ), a small
(<30mm) caridean species that is emerging as a model for
pharmaceutical and ecotoxicological research . A short generation time
and fast development also make it amenable to genetic studies and
potentially a convenient model system for decapod aquaculture . Cherry
shrimp are of commercial aquaculture importance themselves as an
ornamental species . Unfortunately, release by aquarists combined with a
wide tolerance of water and temperature parameters means they have
become invasive outside their native range . Despite this, basic
biological information on this species is scarce (but see e.g. . In
particular there is a lack of baseline behavioural data that may, for
example, impede use of ‘behavioural endpoints’ in ecotoxicology . Very
little is currently known about the amount or structure of behavioural
variation among-individuals. Nor is it known whether state-dependent
behavioural variation, if present, is linked to intrinsic variables such
as size or sex.
We focus specifically on ‘shy-bold’ variation , an aspect of personality
that describes differences in behavioural response to (perceived) risk .
We use two simple testing paradigms, Open Field Trials (OFT) and Food
and Shelter trials (FST), coupled with multivariate behavioural
phenotyping and a repeated measures design. Our specific aims are to (i)
test for repeatable among-individual differences of the behavioural
measures of boldness across the OFT and FST, (ii) determine whether the
structure of multivariate behavioural variation observed is consistent
with expectations given an underlying shy-bold among-individual axis,
and (iii) determine whether size and/sex explains behavioural variation
among-individuals.