Stano Pekár

Evolution of Stenophagy in Spiders (Araneae): Evidence Based on the Comparative Analysis of Spider Diets

Stenophagy (narrow diet breadth) represents an extreme of trophic specialization in carnivores, but little is known about the forces driving its evolution. We used spiders, the most diversified group of terrestrial predators, to investigate whether stenophagy (1) promoted diversification; (2) was phylogenetically conserved and evolutionarily derived state; and (3) was determined either by geographical distribution and foraging guild. We used published data on the prey of almost 600 species. Six categories of stenophagy were found: myrmecophagy, araneophagy, lepidopterophagy, termitophagy, dipterophagy, and crustaceophagy. We found that the species diversity of euryphagous genera and families was similar to stenophagous genera and families. At the family level, stenophagy evolved repeatedly and independently. Within families, the basal condition was oligophagy or euryphagy. Most types of stenophagy were clearly derived: myrmecophagy in Zodariidae; lepidopterophagy in Araneidae; dipterophagy in Theridiidae. In contrast, araneophagy was confined to basal and intermediate lineages, suggesting its ancestral condition. The diet breadth of species from the tropics and subtropics was less diverse than species from the temperate zone. Diet breadth was lower in cursorial spiders compared to web‐building species. Thus, the evolution of stenophagy in spiders appears to be complex and governed by phylogeny as well as by ecological determinants.

Mate quality, not aggressive spillover, explains sexual cannibalism in a size dimorphic spider

Sexual cannibalism particularly before mating is costly for the male victim but also for the female aggressor if she risks remaining unmated. The aggressive spillover hypothesis explains the persistence of this behavior as a maladaptive side effect of positive selection on aggressiveness in a foraging context. The hypothesis predicts that the occurrence of sexual cannibalism is explained by female aggressiveness but is not related to male phenotype or behavioral type. An alternative hypothesis invokes sexual selection and makes the opposite prediction namely that sexual cannibalism is an expression of female choice and should hence mainly target males of low quality. We tested the above hypotheses on a sexually dimorphic nephilid spider Nephilengys livida, known for male monopolization of females via genital damage, female genital plugging, and mate guarding, by staging mating trials during which we recorded mating behaviors and occurrences of pre- and postcopulatory cannibalism. We did not restrict assessment of aggressiveness to the mating and foraging context but also included aggression against same sex conspecifics. To assess female personalities, i.e., consistent individual differences in behavior including aggressiveness, we repeatedly tested them for intra-sex aggression, voracity towards prey, locomotory activity, and boldness. Females exhibited consistent differences in intra-sex aggressiveness, latency to attack prey, and boldness. Aggressive females had shorter latencies to attack prey and were more active than non-aggressive ones. In contrast to the predictions of the aggressive spillover hypothesis, females that were aggressive towards prey and towards other females were not more likely to attack a male than non-aggressive females. In support of the mate choice hypothesis, less aggressive males were more likely attacked and cannibalized than more aggressive ones. This hints at sexual selection for aggressiveness in males and raises the question of mechanisms that maintain variation in male aggressiveness.

Sperm storage and copulation duration in a sexually cannibalistic spider

Female St Andrew’s Cross spiders control copulation duration by timing sexual cannibalism and may thereby control paternity if cannibalism affects sperm transfer. We have investigated the effect of copulation duration on sperm transfer and documented sperm storage patterns when we experimentally reduced the ability of females to attack and cannibalise the male. Virgin males and females were paired and randomly allocated either to a control treatment, where females were allowed to attack and cannibalise the male during copulation, or to an experimental treatment, where females were unable to cannibalise the male. The latter was achieved by placing a paintbrush against her chelicerae during copulation. Our experimental manipulation did not affect copulation duration or sperm storage. However, the number of sperm stored by the female increased with copulation duration only if the male was cannibalised, suggesting that cannibalism increases relative paternity not only through prolonged copulation duration following a fair raffle model but also through the cannibalism act itself. Future studies should explore whether cannibalised males ejaculate more sperm or whether females selectively store the sperm of cannibalised males.

Selective olfactory attention of a specialised predator to intraspecific chemical signals of its prey.

Prey-specialised predators have evolved specific cognitive adaptations that increase their prey searching efficiency. In particular, when the prey is social, selection probably favours the use of prey intraspecific chemical signals by predatory arthropods. Using a specialised ant-eating zodariid spider, Zodarion rubidum, which is known to prey on several ant species and possesses capture and venom adaptations more effective on Formicinae ants, we tested its ability to recognise chemical cues produced by several ant species. Using an olfactometer, we tested the response of Z. rubidum towards air with chemical cues from six different ant species: Camponotus ligniperda, Lasius platythorax and Formica rufibarbis (all Formicinae); and Messor structor, Myrmica scabrinodis and Tetramorium caespitum (all Myrmicinae). Z. rubidum was attracted to air carrying chemical cues only from F. rufibarbis and L. platythorax. Then, we identified that the spiders were attracted to airborne cues coming from the F. rufibarbis gaster and Dufours gland, in particular. Finally, we found that among several synthetic blends, the decyl acetate and undecane mixture produced significant attraction of spiders. These chemicals are produced only by three Formicine genera. Furthermore, we investigated the role of these chemical cues in the communication of F. rufibarbis and found that this blend reduces their movement. This study demonstrates the chemical cognitive capacity of Z. rubidum to locate its ant prey using chemical signals produced by the ants. The innate capacity of Z. rubidum to olfactory detect different ant species is narrow, as it includes only two ant genera, confirming trophic specialisation at lower than subfamily level. The olfactory cue detected by Zodarion spiders is probably a component of the recruitment or trail pheromone.

Discovery of a monophagous true predator, a specialist termite-eating spider (Araneae: Ammoxenidae)

True predators are characterised by capturing a number of prey items during their lifetime and by being generalists. Some true predators are facultative specialists, but very few species are stenophagous specialists that catch only a few closely related prey types. A monophagous true predator that would exploit a single prey species has not been discovered yet. Representatives of the spider family Ammoxenidae have been reported to have evolved to only catch termites. Here we tested the hypothesis that Ammoxenus amphalodes is a monophagous termite-eater capturing only Hodotermes mossambicus. We studied the trophic niche of A. amphalodes by means of molecular analysis of the gut contents using Next Generation Sequencing. We investigated their willingness to accept alternative prey and observed their specific predatory behaviour and prey capture efficiency. We found all of the 1.4 million sequences were H. mossambicus. In the laboratory A. amphalodes did not accept any other prey, including other termite species. The spiders attacked the lateral side of the thorax of termites and immobilised them within 1 min. The paralysis efficiency was independent of predator:prey size ratio. The results strongly indicate that A. amphalodes is a monophagous prey specialist, specifically adapted to feed on H. mossambicus.

Intersexual Trophic Niche Partitioning in an Ant-Eating Spider (Araneae: Zodariidae)

Background Divergence in trophic niche between the sexes may function to reduce competition between the sexes (“intersexual niche partitioning hypothesis”), or may be result from differential selection among the sexes on maximizing reproductive output (“sexual selection hypothesis”). The latter may lead to higher energy demands in females driven by fecundity selection, while males invest in mate searching. We tested predictions of the two hypotheses underlying intersexual trophic niche partitioning in a natural population of spiders. Zodarion jozefienae spiders specialize on Messor barbarus ants that are polymorphic in body size and hence comprise potential trophic niches for the spider, making this system well-suited to study intersexual trophic niche partitioning. Methodology/Principal Findings Comparative analysis of trophic morphology (the chelicerae) and body size of males, females and juveniles demonstrated highly female biased SSD (Sexual Size Dimorphism) in body size, body weight, and in the size of chelicerae, the latter arising from sex-specific growth patterns in trophic morphology. In the field, female spiders actively selected ant sub-castes that were larger than the average prey size, and larger than ants captured by juveniles and males. Female fecundity was highly positively correlated with female body mass, which reflects foraging success during the adult stage. Females in laboratory experiments preferred the large ant sub-castes and displayed higher capture efficiency. In contrast, males occupied a different trophic niche and showed reduced foraging effort and reduced prey capture and feeding efficiency compared with females and juveniles. Conclusions/Significance Our data indicate that female-biased dimorphism in trophic morphology and body size correlate with sex-specific reproductive strategies. We propose that intersexual trophic niche partitioning is shaped primarily by fecundity selection in females, and results from sex-differences in the route to successful reproduction where females are selected to maximize energy intake and fecundity, while males switch from foraging to invest in mating effort.

Long-term changes in communities of native coccinellids: population fluctuations and the effect of competition from an invasive non-native species

We assessed the changes in abundance and community composition of native species of coccinellids (Coleoptera: Coccinellidae) on deciduous trees that occurred between 1970s and 2010s, in the Czech Republic. As the composition of adult communities varies with host plant and season, coccinellids were sampled in May–June from Acer, Betula and Tilia trees using a standardised sweeping method. This was done before (1976–1986) and after (2011–2014) the arrival of Harmonia axyridis in 2006, with interim samples from a period immediately before it arrived in the Czech Republic (2002–2006). Twenty‐one native species were identified in the total sample of 2674 adults. The abundance of Adalia bipunctata, Coccinella quinquepunctata and Propylea quatuordecimpunctata decreased over the whole period sampled. Declines in abundance of these species were already evident prior to the arrival of H. axyridis. Recent declines in Adalia decempunctata and Calvia quatuordecimguttata followed the arrival H. axyridis. Their abundance was increasing prior to the arrival of H. axyridis, but decreased following its invasion and the latter species might have affected their decline. The abundance of only one species, Calvia decemguttata, increased. Although the abundance of many species decreased and the frequency of some species varied, the diversity of native coccinellid populations (Shannon index) was similar over the 40 years of this study. The changes in species composition can in part be attributed to H. axyridis, the role of other factors (e.g. climate change, habitat degradation) in the long‐term fluctuations in abundance of coccinellids should be considered in future assessments.

Comparative analysis of passive defences in spiders (Araneae).

Being frequent prey of many predators, including especially wasps and birds, spiders have evolved a variety of defence mechanisms. Here I studied patterns of passive defences, namely anachoresis, crypsis, masquerade, aposematism and Batesian mimicry, in spiders. Using published information pertaining more than 1000 spider species, the phylogenetic pattern of different passive defences (i.e. defences that decrease the risk of an encounter with the predator) was investigated. Furthermore, I studied the effect of foraging guild, geographical distribution and diel activity on the frequency of defences as these determine the predators diversity, presence and perception. I found that crypsis (background matching) combined with anachoresis (hiding) was the most frequent defence confined mainly to families/genera at the base of the tree. Aposematism (warning coloration) and Batesian mimicry (imitation of noxious/dangerous model) were found in taxa that branched later in the tree, and masquerade (imitation of inedible objects) was confined to families at intermediate positions of the tree. Aposematism and Batesian mimicry were restricted to a few lineages. Masquerade was used particularly by web-building species with nocturnal activity. Aposematism was rare but mainly used by web-building diurnal species. Batesian mimicry was frequently observed in cursorial species with diurnal activity. Cryptic species were more common in temperate zones, whereas aposematic and mimetic species were more common in the tropics. Here I show that the evolution of passive defences in spiders was influenced by the ecology of species. Then, I discuss the evolutionary significance of the particularly defences.

Local trophic specialisation in a cosmopolitan spider (Araneae).

Trophic specialisation can be observed in species with long-term constant exploitation of a certain prey in all populations or in a population of a species with short-term exploitation of a certain prey. While in the former case the species would evolve stereotyped or specialised trophic adaptations, the trophic traits of the latter should be versatile or generalised. Here, we studied the predatory behavioural adaptations of a presumed myrmecophagous spider, Oecobius navus. We chose two distinct populations, one in Portugal and the other in Uruguay. We analysed the actual prey of both populations and found that the Portuguese population feeds mainly on dipterans, while the Uruguayan population feeds mainly on ants. Indeed, dipterans and springtails in Portugal, and ants in Uruguay were the most abundant potential prey. In laboratory trials O. navus spiders recognised and captured a wide variety of prey. The capture efficiency of the Portuguese population measured as components of the handling time was higher for flies than for ants, while that of the Uruguayan population was higher for ants. We found phenotypic plasticity in behavioural traits that lead to increased capture efficiency with respect to the locally abundant prey, but it remains to be determined whether the traits of the two populations are genetically fixed. We conclude that O. navus is a euryphagous generalist predator which shows local specialisation on the locally abundant prey.

The Spider Assemblage of Olive Groves Under Three Management Systems

ABSTRACT Olives, Olea europaea L., are one of the most important crops in Spain. They are currently produced under three management systems that involve different aspects of soil and pest management, productivity, and crop economy: organic, (integrated pest management—IPM), and conventional. Here, we studied how these systems affect the spiders, the natural enemies of olive grove pests, and performed a detailed analysis of their assemblage. The study was performed during one season in 18 olive groves in Andalusia, Spain, and included both ground-dwelling and canopy species.We found that the organic system supported a significantly higher level of abundance and diversity of canopy spiders than the IPM and conventional systems. Plowing had a negative effect on spider abundance and diversity. However, the presence of hedge vegetation had a positive effect on the spiders. The practices affected the guild structure differently, with some guilds supported by organic and others by IPM. It is suggested that sustainability (in terms of pest control) in olive grove agroecosystems may be obtained by maintaining hedge vegetation regardless of the management system.