Introduction

The ability to regulate rhythmic processes through zeitgebers or synchronizers is preserved throughout the evolution of organisms (Beck, 1980), with photoperiod and thermoperiod as the most important zeitgebers, influencing many functions in an organism (Beck, 1980; Mishra, Nayak, & Samal, 2019). For example, in arthropods, photoperiod is involved in daily circadian mechanisms that regulate physiological functions and ecological adaptations (Beck, 1980; Goettel & Philogène, 1978; Kannan, Reveendran, Hari Dass, Manjunatha, & Sharma, 2012), wherein alterations to these synchronizers can cause a slower growth rate in some insects (Cayrol, 1975; Danilevskii, Johnston, & Waloff, 1961; Geyspitz, 1953). In a species of scorpion Gigantometrus swammerdami (Simon, 1872) these synchronizers can influence groups of neurosecretory cells that are involved in ecdysis and water regulation (Rao & Habibula, 1973). Additionally, the relationship between photoperiod and thermoperiod is a factor of stress and has implicated an oviposition behavior that decreases with constant light and temperature regimes in collembolan Orchesella cincta (Linnaeus, 1758) (Liefting, Cosijn & Ellers, 2017). Goettel and Philogène (1978) suggested that the dark period is more important than the light period, and reported that it influences the development of moth Pyrrharctia isabella (J. E. Smith, 1797), a nocturnal feeder.

Despite the importance of synchronizers in many arthropod groups, such as arachnids, the biological rhythms have not received much attention, with studies being concentrated on locomotor activity and physiological rhythms (Cloudsley-Thompson, 1978). Studies, mainly locomotory, on the Order Scorpiones, were limited because about 67% of them were conducted on a single species, the Chersonesometrus fulvipes (C. L. Koch, 1837) (Warburg, 2013). Scorpions’ biological rhythm ensures that they stay in total darkness during the day, allowing them to be sheltered (Cloudsley-Thompson, 1978). In these animals, an increase in natural illumination by the moon can cause a decrease in foraging behavior, possibly because the illuminated night in the field can make the species more vulnerable to predation (Skutelsky, 1996).

Therefore, in this study, we speculated that juvenile scorpions of Jaguajir rochae (Borelli, 1910), which are large in size (50–100 mm), abundant, and widely distributed in the Caatinga vegetation in the Brazilian northeast region (Esposito, Yamaguti, Souza, Pinto-da-Rocha, & Prendini, 2017; Lira, De Souza, & Albuquerque, 2018), may be more active during all or most of the time under a full dark regime than those under a normal light-dark cycle. To the best of our knowledge, no study has investigated the effects of photoperiod on body mass and mortality rate in scorpions. This study aimed to verify the effect of photoperiod on the biomass and survivorship using J. rochae as a study system.

Material and methods

Effects of photoperiod on biomass and mortality

The experiment was performed with 71 juvenile scorpions in the 2^nd instar from three females collected in a Caatinga vegetation in the village of Ameixas (8°06′11″ S, 35°46′30″ W), Municipality of Cumaru, Pernambuco State, Brazil. All groups were maintained in a laboratory at the Universidade Federal de Pernambuco. The juvenile specimens were individualized into plastic terrariums (14 × 10 × 8 cm) with pieces of cardboard for shelter and cotton wool soaked in water at 24 ± 2°C, 70 ± 5% relative humidity. All scorpions were fed at night (6:00pm-7:00pm) with cockroaches Nauphoeta cinerea (Oliver, 1789), approximating 0.2 ± 0.1 g twice per week. The juveniles were randomly divided into two different photoperiods, wherein group A (n = 36) was subjected to a light and dark (LD) cycle 12L:12D, and group B (n = 35) was subjected to a total dark cycle 0L:24D. The animals were weighed at night every 15 days until the 120^th day of observation, under red light using an Analytical Balance model AUW220D, Shimadzu, Barueri, São Paulo State, Brazil. At end of experiment juveniles reached the 3^rd – 4^th instar and none sexual dimorphism in species was noted (see Esposito et al., 2017). To analyze the influence of photoperiod on mortality, all occurrences of death were recorded. All voucher specimens were deposited in the Arachnological Collection of Universidade Federal de Pernambuco.

Data analyses

Correlation analyses were used to verify the relationship between body mass and time (every 15 d) in both groups (groups A and B). In this analysis, body mass was expressed by subtracting the body mass value recorded in a week from that of the previous week. Comparisons between the body mass of the groups (15, 30, 45, 60, 75, 90, 105, and 120^th day) were performed using the Kruskal–Wallis test using PAST 3.14 software for windows (Hammer, Harper, & Ryan, 2001). For mortality analysis, we used the log-rank (Mantel-Cox) test to assess survival between the groups, which was performed using GraphPad Prism version 8.0.0 for Windows, GraphPad Software, San Diego, California, USA.

Results

The observations were performed until 120^th day because of the high mortality rate of juveniles. The juveniles of group A exhibited a gain in body mass until the first 60th day of observations, while animals from group B exhibited a gain in body mass until the 45^th day (Figure 1). However, group A animals showed a significantly higher (H = 518.2, p < 0.0001) gain in body mass compared to that of group B throughout the experiment (group A = 0.0028 ± 0.0051 g; group B = 0.0021 ± 0.0028 g). Scorpions from group A exhibited one peak of mortality at 75–90^th day that represented 27,78% (n = 10) individuals of this group, while group B exhibited three peaks at 1^st –15^th (n = 9), 75–90th (n = 7), and 105–120th day (n = 3). Despite these, no differences were found through the log-rank (Mantel-Cox) test (p = 0.1957) for the mortality rate between the groups (Figure 2).

Discussion

Our results showed a considerable relationship between weight gain and photoperiod in scorpion development. Group B, which was reared in darkness, had a slower weight gain and showed a peak fall in weight gain starting from the 45^th day, while group A, which was reared in 12L:12D, showed a peak fall only on the 60^th day. Previous studies on a caterpillar, Pterourus (Papilio) glaucus (Linnaeus, 1758), also showed a slower growth under total darkness than those that were placed under a different photoperiod (12L:12D and 16L:8D) (Muehlhaus & Scriber, 2006). Similarly, in this study, J. rochae reared in darkness had a slow mass gain, and their survival showed a non-significant difference compared to that of the individuals kept under natural conditions. According to Muehlhaus and Scriber (2006), the larval stage in P. (P.) glaucus exhibits a compensatory response and that this behavior and the physiological adjustments for rapid growth in the feeding of neonates is a response to longer periods of experimental stress such as low temperatures, poor host plant conditions, and long periods under total darkness. In our findings, after the 60^th day, group B had compensatory gain weight, reaching the biomass of group A, suggesting an adaptation. Therefore, this possibly occurred as a result of an energy-saving mechanism, which was a consequence of the decreased activity. Furthermore, this may indicate the anticipation of the scorpions for these environmental conditions, as a result of the regulation of the circadian rhythms and this system’s fundamental role in the initiation and interruption of the activities of organisms (Cruz, 2007).

Scorpions, in general, have decreased activity with conditions of increased brightness (Polis, 1979; 1990; Brown & O’Connell, 2000); thus, we hypothesized that the absence of light as well as any natural alteration could directly affect the development of scorpions. In our experiments, the change in the photoperiod affected group B more, and they had a lower overall biomass gain due to the absence of exposure to light. We suggest that the increased stress conditions in group B was an important factor in this physiological change, given that scorpions can increase their activity time, culminating in greater energy expenditure in these laboratory conditions.

Additionally, in our experiments, we found no differences in the mortality between groups A and B, with both groups showing a high juvenile scorpion mortality. At the end of the experiment were registered a total of 21 (58.33%) deaths in group A and 24 (68.57%) in group B. Indeed, the initial instars present a high mortality rate (Sarmento, Souza, Meiado, & Albuquerque, 2008). This is particularly true in species of the genus Jaguajir (Esposito et al., 2017), which produces a large number of small young individuals (Sarmento et al., 2008). This indicates that scorpions of this genus invest in quantity instead of quality, classifying them as R-strategists. R-strategist species can explore temporary resources, exhibiting high ecological plasticity (Southwood, 1962; Begon, Townsend, & Harper, 2006). Our assumptions may be supported by the strong seasonality of the habitat of J. rochae (Caatinga vegetation) and by their wide range of colonized microhabitats (Esposito et al., 2017; Lira et al., 2018). Therefore, the high mortality found in our study could not be attributed to the photoperiod. Similar to that described for tick, Amblyomma maculatum Koch, 1844 wherein different photoperiods indicated that survival could not be attributed to the effects of light and dark periods (Lohmeyer, Pound, & George, 2009).

Even with the evidence that the circadian rhythm in organisms is composed of several circadian oscillators, the structure and interaction of this system with the environment are poorly understood, especially for invertebrate organisms because studies are concentrated on vertebrates, specifically, on mammals (Cruz, 2007).

Conclusion

With this study, it was possible to better understand how the alteration in photoperiod is connected to weight gain and mortality in scorpions. The results obtained from this study could be used for future research that seeks to understand the expression and functioning of circadian regulation mechanisms and how luminosity interferes in different organisms.

Acknowledgements

We thank to the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for granting a scholarship to J.R. Lima. We also thank the Fundação de Amparo a Ciências e Tecnologia de Pernambuco (FACEPE) for granting a master scholarship to M.A. Silva and a post-doc scholarship (BFP-0010-2.05/19) to A.F.A. Lira. We also thank to Michiel Cozijn and an anonymous referee for their valuable comments in early version of this manuscript.

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