Spiders have melanin after all

Abstract

Melanin pigments are broadly distributed in nature -from bacteria to fungi to plants and animals. However, many previous attempts to identify melanins in spiders were unsuccessful, suggesting that these otherwise ubiquitous pigments were lost during spider evolution. Yet, spiders exhibit many dark colours similar to those produced by melanins in other organisms, and the low solubility of melanins makes isolation and characterization difficult. Therefore, whether melanins are truly absent or have simply not yet been detected is an open question. Raman spectroscopy provides a reliable way to detect melanins in situ, without the need for isolation. In this study, we document the presence of eumelanin in diverse species of spiders using confocal Raman microspectroscopy. Comparisons of spectra with theoretically calculated data falsify the previous hypothesis that dark colours are produced solely by ommochromes in spiders. Our data indicate that melanins are present in spiders and further supporting that they are present in most living organisms.

Spiders have rich pigmentary and structural colour palettes

“…4D). We previously showed that this black colour is produced by eumelanin using Raman spectroscopy (Hsiung et al, 2015a). The Raman signals that we detected must be from eumelanin, not false-positives due to carbonization of burnt specimens, because we only obtained positive Raman signals from black spider body patches, not from nearby regions that were not black (Hsiung et al, 2015a).…”

“…The nanomorphology of structural colours has been investigated recently for some spiders (Foelix et al, 2013;Hsiung et al, 2015b;Ingram et al, 2011;Land et al, 2007;Simonis et al, 2013;Stavenga et al, 2016). However, our knowledge of spider pigment biochemistry has remained stagnant for almost 30 years (Holl, 1987, but see Hsiung et al, 2015a). Thus, we still know very little about pigments and the colour production mechanisms of spiders in general.…”

“…Thus, we still know very little about pigments and the colour production mechanisms of spiders in general. The cumulative body of literature suggests that spiders lack some of the most common pigments that are widespread in other organisms, such as melanins and carotenoids, and only three pigments have been identified in spiders: ommochromes, bilins and eumelanin (Hsiung et al, 2015a;Oxford and Gillespie, 1998). However, given the recent discovery of eumelanin in spiders (see below), it is time to reassess pigmentary colours in spiders.…”

“…Initial development of the workflow led to our discovery of eumelanin in spiders using Raman spectroscopy (Hsiung et al, 2015a), a pigment that is nearly ubiquitous in animals but was previously assumed to be absent in spiders. This unexpected finding motivated us here to look for another common pigment, carotenoids, in spiders using Raman spectroscopy.…”

The ocelli of Archaeognatha (Hexapoda): Functional morphology, pigment migration and chemical nature of the reflective tapetum

“…They, in turn, resemble the Raman spectra of xanthopterin and isoxanthopterin (Saenko et al, 2013;Feng et al, 2001). There is little correspondence with the Raman spectra of melanin, carotene and the simulated spectrum of the ommochrome ommin (Hsiung et al, 2015).…”