Fish-induced variation in abdominal spine length of Leucorrhinia dubia (Odonata) larvae?

On dangerous ground: the evolution of body armour in cordyline lizards

Animal body armour is often considered an adaptation that protects prey against predatory attacks, yet comparative studies that link the diversification of these allegedly protective coverings to differential predation risk or pressure are scarce. Here, we examine the evolution of body armour, including spines and osteoderms, in Cordylinae, a radiation of southern African lizards. Using phylogenetic comparative methods, we attempt to identify the ecological and environmental correlates of body armour that may hint at the selective pressures responsible for defensive trait diversification. Our results show that species inhabiting arid environments are more likely to possess elaborated body armour, specifically osteoderms. We did not find any effect of estimated predation pressure or risk on the degree of body armour. These findings suggest that body armour might not necessarily evolve in response to direct interactions with predators, but rather as a result of increased habitat-mediated predation risk. Furthermore, we discuss the possibility that osteoderms might have been shaped by factors unrelated to predation.

Phenotypic plasticity in specialists: How long-spined larval Sympetrum depressiusculum (Odonata: Libellulidae) responds to combined predator cues

Phenotypic plasticity is a common defensive strategy in species experiencing variable predation risk, such as habitat generalists. Larvae of generalist dragonflies can elongate their abdominal spines in environments with fish, but long spines render larvae susceptible to invertebrate predators. Long-spined specialists adapted to fish-heavy habitats are not expected to have phenotypic plasticity in this defence trait, but no empirical studies have been undertaken. Moreover, in comparison to prey responding to multiple predators that induce similar phenotypes, relatively little is known regarding how species react to combinations of predators that favour opposing traits. We examined plasticity of larval dragonfly Sympetrum depressiusculum, a long-spined habitat specialist. In a rearing experiment, larvae were exposed to four environments: (i) no predator control, (ii) fish cues (Carassius auratus), (iii) invertebrate cues (Anax imperator), as well as (iv) a combination of (ii) and (iii). Compared with the control, fish but not invertebrate cues resulted in longer spines for two (one lateral, one dorsal) of the six spines measured. Interestingly, the combined-cue treatment led to the elongation of all four dorsal spines compared with the fish treatment alone, whereas lateral spines showed no response. Our experiment provided evidence of morphological plasticity in a long-spined specialist dragonfly. We showed that nearly all spines can elongate, but also react differently under specific predator settings. Therefore, while spine plasticity evolved in direct response to a single predator type (fish), plasticity was maintained against invertebrate predators as long as fish were also present. Selective spine induction under the combined condition suggests that S. depressiusculum can successfully survive in environments with both predators. Therefore, phenotypic plasticity may be an effective strategy for habitat generalists and specialists. Although more studies are necessary to fully understand how selection shapes the evolution of phenotypic plasticity, we demonstrated that in dragonflies, presence or absence of a specific predator is not the only factor that determines plastic defence responses.

Phylogeography and larval spine length of the dragonfly Leucorhinia dubia in Europe

Presence or absence of predators selects for different kind of morphologies. Hence, we expect variation in traits that protect against predators to vary over geographical areas where predators vary in past and present abundance. Abdominal larval spines in dragonfly larvae provide protection against fish predators. We studied geographical variation in larval spine length of the dragonfly Leucorrhinia dubia across Western Europe using a phylogenetic approach. Larvae were raised in a common garden laboratory experiment in the absence of fish predators. Results show that larvae from northern Europe (Sweden and Finland) had significantly longer larval spines compared to larvae from western and central Europe. A phylogeny based on SNP data suggests that short larval spines is the ancestral stage in the localities sampled in this study, and that long spines have evolved in the Fenno-Scandian clade. The role of predators in shaping the morphological differences among the sampled localities is discussed.

Predation risk increases immune response in a larval dragonfly (Leucorrhinia intacta)

Predators often negatively affect prey performance through indirect, non-consumptive effects. We investigated the potential relationship between predator-induced stress and prey immune response. To test this, we administered a synthetic immune challenge into dragonfly larvae (Leucorrhinia intacta) and assessed a key immune response (level of encapsulation) in the presence and absence of a caged predator (Anax junius) at two temperatures (22 degrees C and 26 degrees C). We hypothesized that immune response would be lowered when predators were present due to lowered allocation of resources to immune function and leading to reduced encapsulation of the synthetic immune challenge. Contrary to our expectations, larvae exposed to caged predators had encapsulated monofilaments significantly more than larvae not exposed to caged predators. Levels of encapsulation did not differ across temperatures, nor interact with predator exposure. Our results suggest that the previously observed increase in mortality of L. intacta exposed to caged predators is not driven by immune suppression. In situations of increased predation risk, the exposure to predator cues may induce higher levels of melanin production, which could lead to physiological damage and high energetic costs. However, the costs and risks of increased allocations to immune responses and interactions with predation stress remain unknown.

Relaxed predation results in reduced phenotypic integration in a suite of dragonflies

Although changes in magnitude of single traits responding to selective agents have been studied intensively, little is known about selection shaping networks of traits and their patterns of covariation. However, this is central for our understanding of phenotypic evolution as traits are embedded in a multivariate environment with selection affecting a multitude of traits simultaneously rather than individually. Here, we investigate inter- and intraspecific patterns of trait integration (trait correlations) in the larval abdomen of dragonflies as a response to a change in predator selection. Species of the dragonfly genus Leucorrhinia underwent a larval habitat shift from predatory fish to predatory dragonfly-dominated lakes with an associated relaxation in selection pressure from fish predation. Our results indicate that the habitat-shift-induced relaxed selection pressure caused phenotypic integration of abdominal traits to be reduced. Intraspecific findings matched patterns comparing species from both habitats with higher abdominal integration in response to predatory fish. This higher integration is probably a result of faster burst swimming speed. The abdomen holds the necessary morphological machinery to successfully evade predatory fish via burst swimming. Hence, abdominal traits have to function in a tight coordinated manner, as maladaptive variation and consequently nonoptimal burst swimming would cause increased mortality. In predatory dragonfly-dominated lakes, no such strong link between burst swimming and mortality is present. Our findings highlight the importance of studying multivariate trait relationships as a response to selection for understanding patterns of phenotypic diversification.

Habitat-specific morphological variation among threespine sticklebacks (Gasterosteus aculeatus) within a drainage basin

Habitat-specific morphological variation, often corresponding to resource specialization, is well documented in freshwater fishes. In this study we used landmark based morphometric analyses to investigate morphological variation among threespine sticklebacks (Gasterosteus aculeatus L.) from four interconnected habitat types within a single lowland drainage basin in eastern England. These included the upper and lower reaches of the river, the estuary, a connected ditch network and a coastal salt marsh. We found significant habitat-specific differences in morphology, with three axes of variation describing differences in orbit diameter, body depth, caudal peduncle shape and pectoral fin positioning as well as variation in relative dorsal and pelvic spine size. Interestingly, the ditch system, an artificial and heavily managed habitat, is populated by sticklebacks with a characteristic morphology, suggesting that human management of habitats can in some circumstances lead to morphological variation among the animals that inhabit them. We discuss the mechanisms that conceivably underlie the observed morphological variation and the further work necessary to identify them. Finally, we consider the implications of habitat-specific body shape variation for the behavioural ecology of this ecologically generalist species.

A review of chemosensation and related behavior in aquatic insects

Insects that are secondarily adapted to aquatic environments are able to sense odors from a diverse array of sources. The antenna of these insects, as in all insects, is the main chemosensory structure and its input to the brain allows for integration of sensory information that ultimately ends in behavioral responses. Only a fraction of the aquatic insect orders have been studied with respect to their sensory biology and most of the work has centered either on the description of the different types of sensilla, or on the behavior of the insect as a whole. In this paper, the literature is exhaustively reviewed and ways in which antennal morphology, brain structure, and associated behavior can advance better understanding of the neurobiology involved in processing of chemosensory information are discussed. Moreover, the importance of studying such group of insects is stated, and at the same time it is shown that many interesting questions regarding olfactory processing can be addressed by looking into the changes that aquatic insects undergo when leaving their aquatic environment.

Chemical ecology of predator–prey interactions in aquatic ecosystems: a review and prospectusThe present review is one in the special series of reviews on animal–plant interactions

The interaction between predator and prey is an evolutionary arms race, for which early detection by either party is often the key to success. In aquatic ecosystems, olfaction is an essential source of information for many prey and predators and a number of cues have been shown to play a key role in trait-mediated indirect interactions in aquatic communities. Here, we review the nature and role of predator kairomones, chemical alarm cues, disturbance cues, and diet cues on the behaviour, morphology, life history, and survival of aquatic prey, focusing primarily on the discoveries from the last decade. Many advances in the field have been accomplished: testing the survival value of those chemicals, providing field validation of laboratory results, understanding the extent to which chemically mediated learning may benefit the prey, understanding the role of these chemicals in mediating morphological and life-history adaptations, and most importantly, the selection pressures leading to the evolution of chemical alarm cues. Although considerable advances have been made, several key questions remain, the most urgent of which is to understand the chemistry behind these interactions.

A phylogenetic perspective on larval spine morphology in Leucorrhinia (Odonata: Libellulidae) based on ITS1, 5.8S, and ITS2 rDNA sequences

Leucorrhinia (Odonata, Anisoptera, Libellulidae) consists of 14-15 species with a holarctic distribution. We have combined the morphological characters of a previous study with sequence data from the ITS1, 5.8S rDNA, and ITS2 regions of the nuclear ribosomal repeat. Cloning was used to investigate the intra-individual variation and such variation was found in all investigated species. Parsimony jackknifing was used to identify supported groups. The effect of sequence alignment and gap coding was explored by a modified sensitivity analysis. Loss of spines in Leucorrhinia larvae has occurred twice: once in Europe and once in North America. The role of spines as a defence against predation is discussed in a phylogenetic context.

Reaction norms and production costs of predator-induced morphological defences in a larval dragonfly (Leucorrhinia dubia: Odonata)

To understand the evolution and ecology of inducible defence we need to understand the genetics and costs underlying this phenomenon. It has been suggested that the abdominal spines of odonate larvae work as a defensive trait, and that the presence of fish predators induces the production of longer abdominal spines. This study was designed to answer the following questions: (i) What is the shape of the reaction norms of spine length in the larvae of the dragonfly Leucorrhinia dubia reared in the presence and absence of fish? (ii) Does the production of longer spines imply that production costs are incurred in terms of development time or size? I performed a laboratory experiment in which I raised 30 families of L. dubia larvae in the presence and absence of fish. In general, the presence of fish induced the production of longer abdominal spines in the larvae, and there was a genotype × environment interaction, suggesting the potential for evolution of plasticity of the traits. No production costs could be found with respect to development time and size at final instar.

Induced morphological defence: evidence from whole-lake manipulation experiments

Predator-induced defences are activated by cues associated with predators and confer some degree of resistance to subsequent attacks. Laboratory studies of many taxa have revealed such induced defences, and these data often conform to large-scale surveys of defence levels in habitats with and without predators. However, there have been no studies that make the direct connection between these laboratory studies and field surveys. We conducted a large-scale field manipulation of predators to provide this connection. Previous laboratory experiments on dragonfly (Leucorrhinia dubia) larvae have demonstrated that the presence of fish predators induces the development of elongated abdominal spines that serve to reduce mortality risk. In this study we determine the effect of whole-lake predator manipulation on this induced morphological defence of L. dubia. We monitored the spine lengths of final-instar larvae in two experimental lakes for 7 consecutive years. Fish were present during the first 2 years and then removed for the remaining 5 years. Results demonstrate that the spine lengths of L. dubia larvae decreased significantly in both lakes after the removal of fish. In contrast, there was no corresponding change in the spine lengths of larvae in reference lakes, and we found little change in food supply for larvae. Our results suggest that the plastic response in spine length is strong and attributable to the presence of predators.

Fish predation, cannibalism, and larval development in the dragonfly Epitheca cynosura

We manipulated the risk of fish predation and cannibalism in semi-field and laboratory experiments with larvae of the dragonfly Epitheca cynosura. Results of a 22-week semi-field experiment showed that larvae from different densities were indistinguishable in mean size by the end of the experiment, but that individuals from low densities were larger until week 12. Larvae from low densities varied less in size than did those from high densities. Larvae showed rapid convergence of densities in the absence of fish. Fish predation also generated rapid convergence of densities. The presence of fish initially reduced larval size variation. There was a significant trend for the presence of fish scent to increase larval survival at low larval densities over survival in the absence of fish. Behavioral observations in the laboratory supported the results of the semi-field study in that (i) dragonfly larvae reduced their activity under daytime conditions, (ii) the presence of large larvae caused small larvae to increase their hiding inside a refuge and inhibited their crawling while exposed, (iii) small larvae ate fewer prey in the presence of fish. The results of our study suggest that the interaction between intra- and inter-specific predation risk may prove to be important for survival and development of animals in natural systems.