Evolution of spiders from nocturnal to diurnal gave spider silks mechanical resistance against uv irradiation

ABSTRACT During their evolutionary history, some species of spiders have changed from a nocturnal to a diurnal lifestyle, and the important change in their environment was irradiation by

sunlight. Orb webs of diurnal spiders may be markedly affected by exposure to ultraviolet (UV) irradiation, whereas those of nocturnal spiders may be unaffected. It is of great interest to

consider the effects of UV rays on the mechanical properties of spider silks from the viewpoint of evolution. The webs of orb-weaving spiders must trap insect prey, which closely relates to

the breaking energy of the spider silk constituting the orb webs. The effects of UV rays on the mechanical properties, particularly the mechanical breaking energy, of the silks of diurnal

and nocturnal spiders were investigated. UV rays mechanically strengthened the draglines of diurnal _Nephila clavata_ and two other kinds of diurnal spiders (_Leucauge blanda_ and _Argiope

bruennichii_), suggesting that the ability of the orb web to capture insects requires less maintenance. However, UV rays mechanically weakened the draglines of nocturnal _Yaginumia sia_ and

one other kind of nocturnal spider (_Neosona nautica_), suggesting a decrease in the ability of the orb web to capture insects. The results provide strong support that diurnal spiders are in

a more evolved stage than nocturnal spiders, so they secrete silks with an evolved mechanical resistance against UV irradiation. This study suggests a means to search for biological

materials with resistance to UV rays. SIMILAR CONTENT BEING VIEWED BY OTHERS FREE-STANDING SPIDER SILK WEBS OF THE THOMISID _SACCODOMUS FORMIVORUS_ ARE MADE OF COMPOSITES COMPRISING MICRO-

AND SUBMICRON FIBERS Article Open access 19 October 2020 SPIDROIN PROFILING OF CRIBELLATE SPIDERS PROVIDES INSIGHT INTO THE EVOLUTION OF SPIDER PREY CAPTURE STRATEGIES Article Open access 24

September 2020 SUBSTRATE THERMAL PROPERTIES INFLUENCE VENTRAL BRIGHTNESS EVOLUTION IN ECTOTHERMS Article Open access 04 January 2021 INTRODUCTION Ultraviolet (UV) rays are generally harmful

to living organisms, including animals and humans,1 as well as to specific biological materials, such as silkworm silk.2, 3, 4 As a result, the mechanical functions of silks that constitute

the orb webs of diurnal spiders for capturing insect prey outdoors could be expected to be markedly affected by exposure to UV irradiation. Because UV irradiation rapidly decomposes

silkworm silk, it is very important to search for biological materials with resistance to UV rays, as the quantity of these rays has increased recently owing to ozone layer depletion.5

However, there are no appropriate methods for searching for them. In their evolutionary history, some species of spider have changed from nocturnal to diurnal creatures.6, 7 An important

environmental feature of the diurnal lifestyle is irradiation by sunlight. The visible and UV reflectance of spider silks has been previously reported.8, 9, 10 It is of great interest to

study how spiders changed from nocturnal to diurnal habits in the evolutionary process from the viewpoint of the effects of UV rays on spider silks. Despite the potential importance of

studying the effects of UV rays on spider silks, the implications concerning the evolution of a diurnal lifestyle have not yet been explored. In this study, the difference between nocturnal

and diurnal habits of spiders is investigated with respect to the effects of UV rays on spider silks. Naturally occurring UV rays from sunlight irradiate living creatures that are active not

at night but during the daytime. The daylight performances of orb webs of diurnal spiders thus are markedly affected by UV irradiation. However, the orb webs of nocturnal spiders are not

affected because no UV rays reach the earth at night. The webs of orb-weaving spiders must be able to trap insect prey. This ability is closely related to the breaking energy of the orb

webs, which is dependent upon the mechanical breaking stress (BS) and mechanical breaking strain (BN) of the spider silk. If UV rays were to disrupt the mechanical function of orb webs, they

can fail to effectively trap insects. Therefore, it would have been necessary for spiders to build orb webs that were resistant to UV rays to allow them to become active during the daytime.

However, it is unknown whether spider silks have properties that protect them against UV rays. Previously, the effects of UV rays, including UV-A (320–400 nm), UV-B (290–320 nm) and UV-C

(200–290 nm) rays, on spider draglines were studied using electron spin resonance,11, 12 and it was shown that UV-A rays mechanically strengthened the draglines of diurnal _Nephila clavata_

(L. Koch) spiders (Japanese golden web spiders) (Figure 1a),13 although the impact on the mechanical breaking energy was not investigated. Determining whether the silk of nocturnal spiders

is comparatively weaker or stronger in mechanical resistance to UV irradiation, with respect to the mechanical breaking energy, could help to clarify this stage in the evolutionary history

of spiders. To explore this issue, UV irradiation experiments were performed using UV rays with an intensity distribution similar to sunlight. A difference in the resistance to UV rays

between the silks of diurnal and nocturnal spiders would elucidate the evolutionary history of spiders. It is important to know whether UV rays maintain, weaken or strengthen the mechanical

function of silks secreted from diurnal and nocturnal spiders. Such knowledge will provide an important way to search for biological materials with resistance to UV rays. This study

describes findings regarding the mechanical resistance to UV rays of the silks of nocturnal and diurnal spiders, provides evidence to support the evolution of spiders and also provides an

important clue for obtaining spider silks with mechanical resistance to UV rays from an evolutionary standpoint. MATERIALS AND METHODS SAMPLE PREPARATION The spiders used here were mainly

diurnal _N. clavata_ and nocturnal _Yaginumia sia_ (STRAND) spiders. Diurnal _Leucauge blanda_ (L. KOCH) and _Argiope bruennichii_ (SCOPOLI), and nocturnal _Neosona nautica_ (L. KOCH)

spiders were also used for comparison. Female diurnal _N. clavata_ spiders, which are active in the daytime, rebuilt approximately half of their orb webs every night. In other words, both

halves of the orb webs were rebuilt every 2 days. _L. blanda_ and _A. bruennichii_ spiders were also active on the orb web in the daytime outdoors. In contrast, nocturnal _Y. sia_ spiders

that rebuild their orb webs every evening were active on the webs only at night and retreated to a sheltered position every morning. Broken webs where there were no _Y. sia_ spiders were

usually observed in the daytime. _N. nautica_ spiders were active at night and often stayed on their orb webs even in the daytime. Orb webs are mechanically supported by a main frame of

radial threads, which are similar to the draglines that act as ‘mechanical lifelines’ for falling spiders by supporting their body weight.14, 15 Although the radial threads should be used to

study the effects of UV rays on the orb webs, it is not appropriate to use the radial threads cut directly from the built orb web as samples because of the mechanical hysteresis associated

with cutting and building. Thus, it is appropriate to estimate the mechanical strength of the orb web using the draglines instead of the radial threads, because both types of threads are

secreted from the same ampullate gland.6 In this study, samples of the draglines produced by diurnal _N. clavata_, _L. blanda_ and _A. bruennichii,_ and nocturnal _Y. sia_ and _N. nautica_

spiders were investigated. Live spiders just after being captured from the orb webs were used in the experiments. A spider was made to fall from a wooden bar under laboratory conditions to

avoid the mechanical hysteresis of the draglines ascribed to the extra stress. Long draglines were obtained from the spider by sticking the draglines to both sides of a plastic frame that

was ∼50 cm long and covered with an adhesive, and then the draglines were cut into eight specimens stuck to both sides of a paper frame covered with an adhesive for mechanical measurements.

One specimen was designated for scanning electron microscope observation and the other specimens were used for UV irradiation for fixed periods. This sampling procedure was repeated more

than five times for each live spider. Scanning electron microscope (JEOL, Tokyo, Japan) images were used to ascertain whether the samples were true draglines consisting of double filaments

and to determine the cross-section area of each dragline. MECHANICAL MEASUREMENTS The stress–strain curves of the silk draglines were carefully measured using a Tensilon UTM-IIIL instrument

(Instron Japan, Tokyo, Japan) at a stretching velocity of 3.3 × 10−4 m s−1.13, 15 The 4-cm long dragline samples for UV irradiation were prepared by cutting the long dragline secreted from

each spider. Mechanical measurements were repeated for five dragline samples irradiated by UV rays for the same amount of time, and an average value and a standard deviation were calculated.

The BS and BN, defined, respectively, as the stress and strain at the breaking point, were determined from the stress–strain curves of the draglines. The mechanical breaking energy, which

is defined as the work necessary to break the spider silk, of the orb webs depends on the BS and BN of the silk. The mechanical breaking energy can be roughly estimated as the BS multiplied

by the BN, although it can be exactly determined from the integral area of the stress–strain curves of the spider silks. UV RAYS UV rays reaching the ground from the sun are mainly UV-A rays

(320–400 nm). The intensity of the UV-B rays (290–320 nm) reaching the ground from the sun is very weak, and no UV-C rays (270–290 nm) reach the earth's surface. Thus, UV-A rays were

the most appropriate radiation for these experiments. UV-A rays (hereafter referred to as UV-A* rays) were prepared by excluding the wavelengths shorter than 320 nm from a 660-W xenon-arc

lamp (Suntester XF-180, Shimadzu, Japan) with a special filter, thereby providing an intensity distribution similar to sunlight. The intensity of the UV-A* rays was similar to that of

natural UV-A rays. In the UV irradiation instrument, samples were set 25 cm away from the xenon-arc lamp and an air fan was installed to avoid overheating. ELECTRIFICATION The

electrification of the draglines was measured after UV irradiation using a Faraday cage (KQ-1400; Kasuga Denki Inc., Tokyo, Japan). RESULTS DIURNAL SPIDERS UV-A* irradiation changed the

stress–strain behavior of draglines secreted by _N. clavata_, a diurnal spider, leading to a marked increase in the BS.11 The time-dependent changes in the BS and BN values of the draglines

of a diurnal _N. clavata_ spider weighing 385 mg (Figure 1a) after UV-A* (>320 nm) irradiation are shown in Figures 2 and 3, respectively. Here, the normalized BS and BN used in Figures 2

and 3 were obtained by dividing the observed values of BS and BN by the initial values before UV irradiation. The BS increased rapidly shortly after UV-A* irradiation commenced, peaked

relatively quickly and then decreased gradually as it approached an asymptotic value (see Figure 2). The UV-A* rays were found to mechanically strengthen the draglines. The UV-A* rays caused

the BS to remain elevated compared with the baseline value (that is, the start of the irradiation period) for ∼28 h, and the peak level was ∼35% higher than the baseline. However, UV-A*

irradiation for >28 h, which corresponds to a sunlight-irradiation period of about 2 days, decreased the BS below the initial value, suggesting the destruction of the mechanical functions

of the orb webs. The asymptotic value was lower than that at the baseline. In contrast, UV-A* irradiation had no effect on the BN of draglines secreted by a diurnal _N. clavata_ spider,

which remained relatively constant during the fixed period (Figure 3). The observed increase in the BS and the maintenance of the BN under UV-A* irradiation may be related to the mechanical

strengthening of the orb webs in terms of the breaking energy. UV-A* irradiation rapidly increased the BS values of the draglines of a diurnal _L. blanda_ spider weighing 71 mg and a diurnal

_A. bruennichii_ spider weighing 601 mg. Small peaks in the time-dependent changes in the BS values appeared, and then the peaks gradually decreased. The peaks were relatively small

compared with those for the _N. clavata_ spider. Thus, UV-A* irradiation also mechanically strengthened the draglines of diurnal _L. blanda_ and _A. bruennichii_ spiders (Figures 2 and 3).

This result suggests that the silks of diurnal spiders are strong or at least not weakened by UV-A rays, although all diurnal spiders were not examined. UV rays, including UV-A and UV-B rays

with wavelengths >290 nm (referred to as UV-B*), reportedly cause a dramatic decrease in BS13 compared with that observed for UV-A*. However, relatively little natural UV-B in sunlight

reaches the Earth's surface. Thus, UV-A appears to be more important for mechanically strengthening spider draglines, and for maintaining the mechanical functions necessary for

capturing insects in orb webs during a fixed period of exposure to sunlight. NOCTURAL SPIDERS In contrast to the diurnal spiders, UV-A* irradiation affected the stress–strain behavior of

draglines secreted by a nocturnal _Y. sia_ spider weighing 153 mg (Figure 1b) by markedly decreasing both the BS and the BN values. Figures 2 and 3 show the time-dependent BS and BN changes

of the _Y. sia_ draglines after UV-A* irradiation, respectively. The BS and BN both decreased exponentially as the period of irradiation increased, suggesting that the orb webs of _Y. sia_

had relatively weak mechanical resistance to UV-A* rays in terms of their mechanical breaking energy. This result suggests that UV-A* rays decrease the BS and BN of the orb webs of _Y. si_a

spiders during the daytime, thereby weakening their mechanical function and making them unsuitable for capturing insects. Indeed, every evening these spiders emerge from their sheltered

resting places, collect their previous orb webs and build new ones. The BS and BN of the orb webs of _Y. sia_ spiders remain constant during the night because they are not exposed to UV

irradiation. Thus, their mechanical properties do not decline overnight unless the orb webs are damaged by invaders. UV-A* irradiation decreased the BS and BN values of the draglines of a

nocturnal _N. nautica_ spider weighing 203 mg, supporting the hypothesis that the draglines of nocturnal spiders are relatively weak against UV-A rays. However, the degree of decrease for

the _N. nautica_ spider was small compared with that of the _Y. sia_ spider. Dragline samples from both diurnal and nocturnal spiders, which were used in this study and kept under laboratory

conditions without UV irradiation showed no changes in BS or BN over a period of 3 months. This constancy demonstrates that the absence of UV irradiation does not affect the BS and BN of

the draglines. LIFESTYLE CHANGE Our findings imply that it is unnecessary for nocturnal spiders to secrete silks with strong resistance to UV rays. However, if such a species changes its

lifestyle and adopts diurnal habits, they need to produce silk with sufficient UV resistance to preserve the mechanical function of the orb webs and allow them to trap insects. Though

nocturnal _Y. sia_ and _N. nautica_ spiders are active at night, _Y. sia_ spiders leave their orb webs and retreat to a sheltered position during the daytime, but _N. nautica_ spiders are on

their orb webs even in the daytime. This lifestyle difference may reflect the difference in the time dependence of the BS and BN after UV-A* irradiation. The silk of diurnal spiders could

thus have developed their mechanical functional resistance to UV rays in association with a lifestyle change. DISCUSSION The experimental results indicate that diurnal _N. clavata, L.

blanda_ and _A. bruennichii_ spiders use UV-A rays to mechanically strengthen the silks of their orb webs or to lengthen the period of time spent repairing and rebuilding their webs, whereas

nocturnal _Y. sia_ and _N. nautica_ spiders do not employ these mechanisms. The particular relationship between the change in the BS of silks of _N. clavata_ spiders and the period of

rebuilding their orb webs is discussed below. The time to reach a peak in the BS from the starting point showed a close correlation with the span, which is defined as the time during which

the BS is higher than that at the starting point of UV irradiation. The span may give an indication as to the time period during which the orb webs should be rebuilt. Even though UV rays

were continuously irradiating the spider's draglines in the experiments presented here, UV rays actually irradiate the orb webs discontinuously due to the cycle from day to night and

due to clear and cloudy weather. If sunlight in Japan is, on average, incident on the orb web for about 14 h in summer and about 11 h in autumn per day, the spans of 28 h for UV-A* rays may

be estimated to be about 2 days. The estimated span corresponds roughly to the actually observed period of 2 days needed to rebuild the orb web by _N. clavata_ spiders outdoors.13 UV-A rays

contribute to producing crosslinks between protein molecules in spider silks for _N. clavata_ spiders, and diurnal spiders effectively utilize UV-A rays to either mechanically strengthen the

orb webs or lengthen the period between rebuilding to capture insects. This period is be affected by UV-A irradiation. However, it is unnecessary for the silks of nocturnal _Y. sia_ spiders

to be strengthened by UV rays during the night. Indeed, the BS and BN of the silks of nocturnal _Y. sia_ did not change under the absence of UV rays corresponding to the conditions at

night, although the BS and BN decreased markedly with UV rays corresponding to the conditions in the daytime. This change suggests that nocturnal spiders never utilize UV rays for

strengthening orb webs and also never use previous orb webs. Thus, nocturnal _Y. sia_ spiders have to rebuild their orb webs every evening because UV rays destroy the orb webs in the

daytime. In this study, three kinds of diurnal spiders were studied. However, nocturnal _N. nautical_ spiders were neglected, except for _Y. sia_, because it was very difficult to accurately

determine which spiders are nocturnal using only eyesight at night. UV-A* irradiation induced the electrification of draglines even though no cracks in the surface were detected using

scanning electron microscope photos. Actually, UV rays produce radicals11, 12, 16 ascribed to the chemical cleavage of proteins. If UV-A* rays easily induce many cracks in the surface of

draglines, it will not be easy for diurnal spiders to live safely during sunlit hours. The results suggest the chemical composition12 that does not strongly alter the surface structure of

draglines. Another theory should be considered. The stronger BS of the silk of diurnal _N. clavata_ spiders could be attributed to crosslinks between protein molecules.10 This theory could

account for the observed peaks of the BS as a consequence of the superposition of two factors: a decrease in the molecular weight caused by UV-induced chemical composition17, 18 and an

increase in the molecular weight caused by UV-induced crosslinks.19, 20 Infrared and Raman spectra were obtained to ascertain the existence of crosslinks in the draglines after UV

irradiation. However, no signals ascribed to crosslinks in the spectra were detected using these methods, and it is very difficult to detect crosslinks even with infrared and Raman spectra.

Thus, it will be difficult to deny the theory that the difference in the mechanical strength between two kinds of spider silks may be ascribed to crosslinks. The difference in the mechanical

behavior after UV irradiation may also be ascribed to the difference in the amino-acid composition or fine structure; or this difference may be ascribed to the hardening due to evaporation

of water contained in spider draglines.21 In a previous paper,12 electron resonance measurements revealed that photo-irradiation produced Cα-centered radicals of the protein molecules

constituting the draglines, which was attributed to the breaking of chemical bonds. Greater numbers of radicals were induced by UV irradiation in mature spiders than in juveniles. The low

levels of absorbance of UV radiation by the draglines of juveniles of _N. clavata_ might be responsible for their stronger resistance to UV irradiation. The yellowing of draglines ascribed

to the absorption at ∼275 nm may induce the production of radicals. In this study, however, the color of the draglines from nocturnal _Y. sia_ and _N. nautical_ spiders was white, and no

absorption at wavelengths between 320 and 400 nm corresponding to UV-A* rays was observed. Thus, the peptide bonds ascribed to the amino acids constituting draglines secreted from nocturnal

and diurnal spiders should be considered. It is possible that UV-A* rays markedly destroy the specific peptide bonds constituting draglines, although the peptide bonds were not examined

here. The difference in the mechanical resistance to UV rays between diurnal and nocturnal spiders may be ascribed to the difference in the sequence of amino acids. The results provide

evidence for the hypothesis that diurnal spiders are at a more evolved stage than nocturnal spiders, because they produce silks that are mechanically strong against UV rays, even though the

difference in the UV resistance is not yet clear in terms of the chemistry. CONCLUSION The findings imply that diurnal spiders cope with sunlight irradiation by effectively utilizing UV-A

rays to mechanically strengthen their orb webs or by lengthening the period of time spent repairing and rebuilding their webs, whereas nocturnal spiders do not employ these mechanisms. The

period of UV-A irradiation is important for these processes. In addition, if the quantity of UV-B rays reaching the ground is increasing due to the breakdown of the ozone layer and could

have a significant effect on diurnal spider species in the future. To conclude, this study provides strong evidence to support the hypothesis that _N. clavata_ spiders have adapted to a

diurnal lifestyle through mechanisms that are absent in nocturnal _Y. sia_ spiders, such as the evolution of the strong mechanical functional resistance of their silk against UV rays, and

further provides an important clue for obtaining silks with such mechanical functions from the evolutionary standpoint. Furthermore, knowledge of the evolution of mechanical functions

provides an important way to search for biological materials with resistance to UV rays. It is possible that UV-A* rays markedly destroy the specific peptide bonds constituting draglines,

but the peptide bonds were not examined. The difference in the mechanical resistance to UV rays between diurnal and nocturnal spiders may be ascribed to the difference in the design for the

sequence of amino acids. The process of chemical decomposition ascribed to UV-A* irradiation is now being studied in terms of the molecular weight.22 In the near future, the hypothesis will

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ACKNOWLEDGEMENTS We are grateful to Ms Riye Ishikawa and Yumi Ida for helping in mechanical measurements. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * Department of Chemistry, Nara Medical

University, Kashihara, Nara, Japan Shigeyoshi Osaki * Department of Neurology, Tokyo Kyosai Hospital, Meguro, Tokyo, Japan Masao Osaki Authors * Shigeyoshi Osaki View author publications You

can also search for this author inPubMed Google Scholar * Masao Osaki View author publications You can also search for this author inPubMed Google Scholar CORRESPONDING AUTHOR

Correspondence to Shigeyoshi Osaki. RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Osaki, S., Osaki, M. Evolution of spiders from nocturnal to diurnal

gave spider silks mechanical resistance against UV irradiation. _Polym J_ 43, 200–204 (2011). https://doi.org/10.1038/pj.2010.119 Download citation * Received: 20 July 2010 * Revised: 14

October 2010 * Accepted: 15 October 2010 * Published: 08 December 2010 * Issue Date: February 2011 * DOI: https://doi.org/10.1038/pj.2010.119 SHARE THIS ARTICLE Anyone you share the

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Nature SharedIt content-sharing initiative KEYWORDS * diurnal spider * evolution * mechanical resistance * nocturnal spider * spider silk * UV rays