Colour polymorphism in spiders

Spiders come in a wide variety of colours and patterns and the purpose and function of these different colourations includes crypsis, mimicry, aposematism, and even thermoregulatory considerations. The maintenance of cryptic colouration is of significant survival value to spider populations that are subject to intense selection via predators with a high visual acuity and colour vision, such as that by insectivorous birds and spider-hunting wasps. This has led to the convergent evolution of similar colourations in unrelated species – indeed, one can easily recognise the specific adaptations (be it colour, morphology or behaviour) required for crypsis on flowers, leaves, grass, twigs, bark, the ground and other strata. Spiders not only employ cryptic colouration to hide from their predators but also to hide from their potential prey, as is the case with a number of spiders that exhibit a sit-and-wait or ambush strategy. They may also avoid detection by mimicking parts of their environment, or even other organisms – a great number of spiders mimic ants as ants are considered unpalatable by many predators.

However, some web-building spiders are often brightly coloured and very conspicuous. Work on both diurnal (see Bush et al. 2008) and nocturnal (see Chuang et al. 2007 and Blamires et al. 2011) spiders with bright colouration suggests they are attractive to insect prey. If this results in greater prey intake it may offset the benefit of being cryptic, especially if their colouration disrupts or confuses predators. Bright and iridescent colours have also been shown to be important in signalling and sexual identification, especially in salticids, where males are often vibrantly coloured (Taylor & McGraw 2007). Aposematic colouration in spiders has not been studied in detail, but some spiders exhibit warning colouration such as the striking red and black markings on spiders in the genus Latrodectus (Theridiidae). As far as thermoregulation goes, many spiders, especially those that inhabit a web during the day, possess silver or reflective colours that are known to assist in preventing overheating. With such a wide range of functions and pressures on spider colouration alone, it is no wonder that multiple colour morphs often exist for a single species.

These colour polymorphisms are widespread in the animal kingdom (Gray & McKinnon 2006), and their proximate and ultimate causes are as interesting as they are varied. In regards to spiders, Oxford & Gillespie (1998) published a wonderful review on the subject, and even though their work is more than a decade old, it is still highly pertinent and provides a great summary on the purposes of spider colouration and its underlying physiological and evolutionary processes.

The chromatic pigments that give rise to the different colours in spiders are due to the presence of a number of organic compounds (many of which have not been studied or determined) that include ommochromes (yellows, reds and browns), bilins (blues and greens) and guanine (whites and silver). The presence, absence and interaction between these and other compounds produces the startling array of colours we see in the thousands of different spider species alive today.

While the colour of many spiders is genetically fixed, reversible colour changes also occur and are mediated by environmental cues. Colour change can be passive (food induced) or active (physiological, morphological or behavioural) in nature. Colour change in spiders has mostly been studied in relation to the ability of a spider to match its background. This phenomenon is best known in the flower-dwelling crab spiders (Thomisidae), who may alter their colouration to match the colours of the flowers on which they ambush their prey (see Théry & Casas 2009 for a review on the subject). Additionally, these spiders may alter their UV reflectance to match that of the flowers they inhabit, thus making themselves less perceptible to their prey (e.g. bees).

For the majority of spiders that cannot alter their colouration, their polymorphisms are genetically determined. The maintenance of distinct, discontinuous colour genotypes within a population are generated by the segregation of two or more alleles at (usually) one major locus. Morphs for the locus in question can be represented equally between sexes (autosomal), have some morphs limited to one sex (sex limited), or differ between the sexes (sex linked). One of the most famous examples of genetic colour polymorphism in spiders is that of the Hawaiian happy-face spider, Theridion grallator (Theridiidae), which has a morph with abdominal patterns that resemble a smiling face, amid a plethora of other morphs. The mechanisms underlying the maintenance of such variation are discussed in Franks & Oxford (2009) but see also Gray & McKinnon (2006) for a more general review. It is often difficult to untangle the interactions between genetic drift and selection within a population and that of gene flow between populations.

The photos above are of two different individuals of the nocturnal orb weaving spider, Araneus eburneiventris (or a closely related species). Although these individuals are of the same species they differ markedly in their abdominal colours and patterns. Another two colour morphs of this species are also commonly observed; one has an abdomen that is mostly orange and yellow, while another is predominantly green with a yellow stripe running longitudinally down the centre. These colour morphs, by all accounts, seem to be discontinuous in nature suggesting that a number of distinct genotypes exist, all of which are maintained by processes similar to those discussed in the above papers.

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