Captivating color: evidence for optimal stimulus design in a polymorphic prey lure

Many species – humans included – employ color as an instrument of deception. One intriguing example of this resides in the conspicuous abstract color patterns displayed on the bodies of female orb weaving spiders. These displays increase prey interception rates and thereby function at least as visual lures. Their chromatic properties however vary extensively, both across and within species, with discrete forms often co-existing in the manner of a stable polymorphism. Variation is principally expressed in terms of signal hue (color per se), but it is unclear how attractiveness scales with this property and if extant morphs are maximally attractive relative to a graded range of potential alternatives. We examined these questions by assessing catch rates among color-manipulated females of the dimorphic jeweled spider Gasteracantha fornicata in their natural webs. The manipulation altered dorsal appearance in a manner akin to adding six new variants of their existing white/yellow phenotypes. This magnified the natural variation in stimulus hue independently of chroma (saturation) across a range spanning most of the color spectrum. Catch rate varied across treatments in simple accordance with how greatly stimulus hue deviated from either of the two extant spider phenotypes. Predictions based on fly-perceived chromatic and achromatic background contrast were clearly unsupported despite dipterans constituting ~60 % of identifiable prey. This study supports the importance of signal coloration per se in G. fornicata and suggests that extant lure phenotypes reside in a broadly optimal spectral range for stimulating their aggregate prey community.

Predators as Agents of Selection and Diversification

Predation is ubiquitous in nature and can be an important component of both ecological and evolutionary interactions. One of the most striking features of predators is how often they cause evolutionary diversification in natural systems. Here, we review several ways that this can occur, exploring empirical evidence and suggesting promising areas for future work. We also introduce several papers recently accepted in Diversity that demonstrate just how important and varied predation can be as an agent of natural selection. We conclude that there is still much to be done in this field, especially in areas where multiple predator species prey upon common prey, in certain taxonomic groups where we still know very little, and in an overall effort to actually quantify mortality rates and the strength of natural selection in the wild.

Phylogeography of the widespread Caribbean spiny orb weaver Gasteracantha cancriformis

BACKGROUND

Modern molecular analyses are often inconsistent with pre-cladistic taxonomic hypotheses, frequently indicating higher richness than morphological taxonomy estimates. Among Caribbean spiders, widespread species are relatively few compared to the prevalence of single island endemics. The taxonomic hypothesis Gasteracantha cancriformis circumscribes a species with profuse variation in size, color and body form. Distributed throughout the Neotropics, G. cancriformis is the only morphological species of Gasteracantha in the New World in this globally distributed genus.

METHODS

We inferred phylogenetic relationships across Neotropical populations of Gasteracantha using three target genes. Within the Caribbean, we estimated genetic diversity, population structure, and gene flow among island populations.

RESULTS

Our findings revealed a single widespread species of Gasteracantha throughout the Caribbean, G. cancriformis, while suggesting two recently divergent mainland populations that may represent separate species, diverging linages, or geographically isolated demes. The concatenated and COI (Cytochrome c oxidase subunit 1) phylogeny supported a Caribbean clade nested within the New World. Genetic variability was high between island populations for our COI dataset; however, gene flow was also high, especially between large, adjacent islands. We found structured genetic and morphological variation within G. cancriformis island populations; however, this variation does not reflect genealogical relationships. Rather, isolation by distance and local morphological adaptation may explain the observed variation.

Color lures in orb-weaving spiders: a meta-analysis

Lures are deceptive strategies that exploit sensory biases in prey, usually mimicking a prey’s mate or food item. Several predators exploit plant–pollinator systems, where visual signals are an essential part of interspecific interactions. Many diurnal, and even nocturnal, orb-web spiders present conspicuous body coloration or bright color patches. These bright colors are regarded as color-based lures that exploit biases present in insect visual systems, possibly mimicking flower colors. The prey attraction hypothesis was proposed more than 20 years ago to explain orb-web spider coloration. Although most data gathered so far has corroborated the predictions of the prey attraction hypothesis, there are several studies that refute these predictions. We conducted a multilevel phylogenetic meta-analysis to assess the magnitude of the effect of conspicuous orb-web spider body coloration on prey attraction. We found a positive effect in favor of the prey attraction hypothesis; however, there was substantial heterogeneity between studies. Experimental designs comparing conspicuous spiders to painted spiders or empty webs did not explain between-studies heterogeneity. The lack of theoretical explanation behind the prey attraction hypothesis makes it challenging to address which components influence prey attraction. Future studies could evaluate whether color is part of a multicomponent signal and test alternative hypotheses for the evolution of spider colors, such as predator avoidance and thermoregulation.

Prey and predators perceive orb-web spider conspicuousness differently: evaluating alternative hypotheses for color polymorphism evolution

Color polymorphisms have been traditionally attributed to apostatic selection. The perception of color depends on the visual system of the observer. Theoretical models predict that differently perceived degrees of conspicuousness by two predator and prey species may cause the evolution of polymorphisms in the presence of anti-apostatic and apostatic selection. The spider Gasteracantha cancriformis (Araneidae) possesses several conspicuous color morphs. In orb-web spiders, the prey attraction hypothesis states that conspicuous colors are prey lures that increase spider foraging success via flower mimicry. Therefore, polymorphism could be maintained if each morph attracted a different prey species (multiple prey hypothesis) and each spider mimicked a different flower color (flower mimicry hypothesis). Conspicuous colors could be a warning signal to predators because of the spider’s hard abdomen and spines. Multiple predators could perceive morphs differently and exert different degrees of selective pressures (multiple predator hypothesis). We explored these 3 hypotheses using reflectance data and color vision modeling to estimate the chromatic and achromatic contrast of G. cancriformis morphs as perceived by several potential prey and predator taxa. Our results revealed that individual taxa perceive the conspicuousness of morphs differently. Therefore, the multiple prey hypothesis and, in part, the multiple predator hypothesis may explain the evolution of color polymorphism in G. cancriformis, even in the presence of anti-apostatic selection. The flower mimicry hypothesis received support by color metrics, but not by color vision models. Other parameters not evaluated by color vision models could also affect the perception of morphs and influence morph survival and polymorphism stability.

Age variation in the body coloration of the orb-weaver spider Alpaida tuonabo and its implications on foraging

Spiders show a repertoire of strategies to increase their foraging success. In particular, some orb-weaver spiders use attractive body colorations to lure prey. Interestingly, coloration varies with age in many species, which may result in ontogenetic variation of foraging success. By using field observations, laboratory experiments and spectrophotometric analysis, we investigated whether pale juveniles and bright adults of the orb-weaver Alpaida tuonabo use different foraging strategies due to ontogenetic variation in coloration. Field observations revealed that foraging success of juveniles and adults was influenced by web properties. However, foraging success increased with body size only in adults, supporting the idea that larger individuals produce a stronger visual signal for prey. The attractiveness of the adult coloration for prey was confirmed in the laboratory with frame-web-choice experiments, in which webs bearing a spider intercepted more bees than empty webs. Our spectrophotometric analysis suggests that the yellow coloration may produce the deceiving signal for prey. Moreover, we identified potential alternative foraging strategies: cryptic juveniles at higher heights and 'attractive' adults at lower heights. This study reveals how ontogenetic colour variation may favour the use of alternative foraging strategies in orb-weaver spiders and reduces intraspecific competition.

Artificial night light alters nocturnal prey interception outcomes for morphologically variable spiders

Artificial night light has the potential to significantly alter visually-dependent species interactions. However, examples of disruptions of species interactions through changes in light remain rare and how artificial night light may alter predator–prey relationships are particularly understudied. In this study, we examined whether artificial night light could impact prey attraction and interception in Nephila pilipes orb weaver spiders, conspicuous predators who make use of yellow color patterns to mimic floral resources and attract prey to their webs. We measured moth prey attraction and interception responses to treatments where we experimentally manipulated the color/contrast of spider individuals in the field (removed yellow markings) and also set up light manipulations. We found that lit webs had lower rates of moth interception than unlit webs. Spider color, however, had no clear impact on moth interception or attraction rates in lit nor unlit webs. The results show that night light can reduce prey interception for spiders. Additionally, this study highlights how environmental and morphological variation can complicate simple predictions of ecological light pollution’s disruption of species interactions.

Evidence of bird dropping masquerading by a spider to avoid predators

Masquerading comes at various costs and benefits. The principal benefit being the avoidance of predators. The orb-web spider Cyclosa ginnaga has a silver body and adds a white discoid-shaped silk decoration to its web. The size, shape and colour of C. ginnaga's body resemble, when viewed by the human eye against its decoration, a bird dropping. We therefore hypothesized that their body colouration might combine with its web decoration to form a bird dropping masquerade to protect it from predators. We measured the spectral reflectance of: (i) the spider's body, (ii) the web decoration, and (iii) bird droppings, in the field against a natural background and found that the colour of the spider bodies and decorations were indistinguishable from each other and from bird droppings when viewed by hymentopteran predators. We monitored the predatory attacks on C. ginnaga when the spider's body and/or its decorations were blackened and found that predator attack probabilities were greater when only the decorations were blackened. Accordingly, we concluded that C. ginnaga's decoration and body colouration forms a bird dropping masquerade, which reduces its probability of predation.