The mud crab (Scylla paramamosain) is a marine decapod crustacean species which plays a significant commercial and ecological role in marine aquaculture. Scylla paramamosain is widely distributed and farmed in Indo-West-Pacific, including south and southeastern China. S. paramamosain has been the most productive marine crab species cultured in China since 2009, and its output reached 159, 433 t in 2020 (China FBOAMO, 2021). Nevertheless, due to the unreliability of the hatchery phase in aquaculture settings, the mud crab industry still highly relies on wild harvested mud crab juveniles, raising concerns over the sector’s sustainability. Unreliable hatchery supply is mainly attributed to the lack of optimized and standardized husbandry protocols during early development leading to high mortality, and variable growth performances (Chen et al., 2021a).

Light, as an important environmental cue that influence growth performance, behavior, and physiology of aquatic animals (Gao et al., 2016a; Takahashi et al., 2018; Nasr et al., 2019; Yang et al., 2020), has three core elements, i.e., intensity, spectrum, and photoperiod. The visible spectrum is composed of short-wavelength (violet, 380–440 nm and blue, 440–485 nm), middle wavelengths (green 500–565 nm and yellow, 565–590 nm), and long-wavelengths (orange, 590–625 nm and red, 625–740 nm) (Wu et al., 2021). Unlike terrestrial animals, photic conditions experienced by aquatic animals can be highly variable and depend on water properties including plankton, suspended particles, dissolved organic substances, and water depth acting as a chromatic filter (Mukai, 2011; Peng et al., 2019). In aquatic animals, understanding the impact of light and specifically spectrum on key physiological functions is complex and poorly characterized when compared to other abiotic factors. Studies performed in fish showed that shorter wavelengths (blue and green) can promote larvae growth performance in Atlantic cod (Gadus morhua), turbot (Scophthalmus maximus), and European sea bass (Dicentrarchus labrax) (Villamizar et al., 2009; Sierra-Flores et al., 2016). In crustaceans, enhanced growth was reported in the giant freshwater prawn (Macrobrachium rosenbergii) when exposed to green as opposed to red light (Wei et al., 2021). However, specific growth rate (SGR) was enhanced in the Chinese shrimp Fenneropenaeus chinensis reared under natural light while SGR was suppressed under blue light (Wang et al., 2003). In shellfish, previous studies on abalone Haliotis discus have shown improved hatching and larvae growth under blue and green light (Gao et al., 2015) contrasting with positive effect of orange and red light reported on growth performance of juvenile H. discus (Gao et al., 2016a). Such contrasting results reported in the literature clearly illustrate the species and stage of development specific effects of spectrum on aquatic animals and most studies so far have been performed in fish.

The effect of light on aquatic vertebrate animals is thought to be mainly mediated through melatonin (N-acetyl-5-methoxy-tryptamine), the light perception hormone which is remarkably conserved across vertebrate phyla (Falcón et al., 2010). In fish, studies have suggested that melatonin is involved in the circadian and seasonal entrainment of many essential physiological functions (Falcón et al., 1992; Oliveira et al., 2007). However, light regulation of pineal melatonin production diverged in teleost species as a result of evolution and striking differences in spectral sensitivities were reported between species (Migaud et al., 2007a; Vera et al., 2010). In crustaceans, light perception and transduction has not been studied much and while melatonin is mainly secreted by the eyestalks, it has also been detected in the hemolymph and nervous systems (Pape et al., 2008; Zhang et al., 2018). Melatonin was reported to impact on a range of biological processes in crustaceans including glucose metabolism, oxidative stress, limb regeneration, and molting (Sainath and Reddy, 2010a,b; Sainath et al., 2013; Girish et al., 2015). A recent study from our laboratory showed melatonin levels in the eyestalks of S. paramamosain were significantly affected by light intensity (Chen et al., 2021a), however, light spectral effects remain unknown in the species.

Many biological functions (e.g., reproduction, metabolism, growth, behavior) and life transition events (e.g., spawning, hatching, metamorphosis, smoltification) in fish are regulated by environmental signals including light with strong relevant applications to the sector. In crustaceans, molting is particularly interesting as it directly controls the growth of the animals (Kobayashi, 2012; Yang et al., 2018). Crustaceans molting is mainly controlled by ecdysteroids and molt-inhibiting hormone (MIH) secreted by the Y-organs and the complex X-organ–sinus gland (XO-SG) (Imayavaramban et al., 2007). The fundamental function of MIH is to suppress ecdysteroid biosynthesis to regulate molting (Qiao et al., 2018). Molting, and subsequently growth, in crab species are also regulated by many abiotic factors including temperature (Yuan et al., 2017), salinity (Gong et al., 2015), and light intensity (Li et al., 2011; Chen et al., 2021a). Previous studies indicated that light fluctuations from blue to green or yellow spectra could promote growth and molting frequency of L. vannamei (Guo et al., 2011). In contrast, the effect of light spectrum on mud crab molting remains little studied.

Oxidative stress is another critical function in animals which is impacted by external factors. The reactive oxygen species (ROS) such as superoxide anion radical (O^2–), hydroxyl radical (OH^–1), and hydrogen peroxide (H₂O₂) are produced as a result of metabolism and immune defense response in cells (Bogdan et al., 2000; Kohen and Nyska, 2002). Excessive accumulation of ROS may result in oxidative stress, cellular damage, and ultimately compromise cell functions (Guo H. et al., 2013; Guo Z.-X. et al., 2013; Cheng et al., 2020). In response to ROS, animals have evolved various antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and peroxiredoxins (Prx) to counteract oxidation (Chen et al., 2021b). Light spectrum have been suggested to influence the antioxidant capacity of aquatic organisms as reported in abalone (Haliotis discus hannai) (Gao et al., 2016b), whiteleg shrimp (Penaeus vannamei) (Fei et al., 2020a), and turbot (Scophthalmus maximus) (Wu et al., 2021). Furthermore, as an end-product of lipid peroxidation, malondialdehyde (MDA) is used as an indicator of oxidative damage in organisms (Liu et al., 2011). Recent studies have demonstrated that ROS and the resulting oxidative stress play a pivotal role in activating apoptosis (Kannan and Jain, 2000). Apoptosis plays an essential role in removing the excess, damaged, necrotic, and potentially dangerous cells (Wyllie et al., 1980). The expression levels of apoptosis-related genes such as bcl-2, p53, and caspase 3 can be used as biomarkers of tissue apoptosis. A recent study showed that exposure to dark condition suppressed apoptosis-related gene expression in Litopenaeus vannamei (Fei et al., 2020a). In addition, gene expression of bcl-2, p53, and cytochrome c in the hepatopancreas of L. vannamei was significantly reduced in shrimp reared under a full spectrum + UVA and full spectrum + UVB (Fei et al., 2020b).

In the present study, the Light Emitting Diodes (LED) were used to create seven different light spectra to investigate the effects of light spectrum on growth, molting, antioxidant capacity, and apoptosis-related gene expression in the mud crab S. paramamosain. The overall goal of this work is to identify optimal environmental conditions for S. paramamosain produced in land-based hatcheries and nurseries to boost juvenile outputs, performance and reduce the reliance of the sector on wild-harvested stocks.

In aquaculture, most research on the effects of light have been performed in fish with less studies done in invertebrates and especially crustaceans. Results from the current study showed that crabs reared under blue (460 nm) and cyan (510 nm) narrow bandwidth lights grew significantly better than for any other spectra tested, especially violet and yellow. Additionally, higher molting performance and lower oxidative stress and apoptosis were observed in the blue groups. The 4-SKM showed that the optimal light wavelength for maximum SGR in mud crab was 449.97 nm.

Published studies suggested that light spectrum can affect the feeding behavior of crabs impacting on feeding efficiency and growth (Cohen and Forward, 2002; Luchiari et al., 2009). However, contrasting results have been reported in the literature and the relationship between the animal spectral sensitivity and its behavior, depending on its stage of development, is not well understood. Spectral sensitivity of mud crab Scylla serrata was shown to peak at around 500 nm (Yang et al., 1985). A study in the swimming crab (Portunus trituberculatus) suggested that crabs are more sensitive to yellow and red light by looking at the microvillus of photoreceptors and number of organelles (Luo et al., 2006). However, a growth experiment showed feeding rate was suppressed in P. trituberculatus exposed to yellow light in contrast to blue which appeared to promote feeding but surprisingly growth did not correlate with feeding response (Wang et al., 2014). The visual sensitivity, background color preference, and growth and development are clearly not consistent, as observed in P. trituberculatus larvae (Shi et al., 2019) but also in fish species like barramundi (Lates calcarifer) (Ullmann et al., 2011). Light perception, sensitivity and biological efficiency change during the ontogenic development of any species and in crabs, it was suggested that juveniles may rely more on chemoreception to locate and identify their preys rather than light at the larval stage (Zimmer-Faust, 1989). This would explain mismatches reported between light sensitivity and physiological effects such as feeding and growth response. Growth effects identified under specific light conditions may be mediated through changes in feed conversion ratio as reported in fish species rather than feeding rate (Taylor et al., 2006; Karakatsouli et al., 2010). Characterizing the effect of light spectrum on crustacean physiology is therefore complex and will depend on species, stage of development but also light irradiance and photoperiod.

In the present study, the highest molting frequency was found in full-spectrum and cyan groups. The relative expression of mih gene was significantly upregulated in violet and yellow groups (3.1- and 2.4-fold, respectively, compared to full-spectrum group), and these two groups had the lowest molting frequency, suggesting spectrum may play an important role in growth by regulating molting in crabs. Similar results were also observed in Litopenaeus vannamei, with higher molting frequency in shrimp exposed to green rather than blue light (Guo et al., 2011). Molting in crustaceans can also be suppressed by stress factors (Mykles et al., 2010). The highest cortisol levels found in the present study were in crabs reared under violet and yellow light. As a stress indicator, cortisol is correlated to mortality and growth suppression (Tataranni et al., 1996). In crustaceans, the crustaceans hyperglycemic hormone (CHH) secreted by X-organ and sinus gland complex (XO-SG) has similar effects than cortisol and corticosterone, by regulating the conversion of glycogen into glucose (Elwood et al., 2009; Liu et al., 2019). Although CHH and MIH are different in structure, yet they appear to have similar functions in inhibiting ecdysteroid synthesis (Chung and Webster, 2003). Thus, higher cortisol levels could lead to delayed molting and subsequently reduced growth in crabs exposed to violet and yellow lights.

Heat shock proteins (HSPs) belong to a conserved class of molecular chaperones that play essential roles in growth, development, and stress response in all living organisms (Nie et al., 2017). HSPs are used as biomarkers of stress response, especially HSP70 and HSP90 which are involved in stress and immune responses (Fu et al., 2011). As a cochaperone for HSP70, HSP40 is in the regulation of ATP hydrolysis to maintain normal physiological functions and alleviate stress-related responses (Fan et al., 2003). In the present study, the relative expression of hsp40 in the violet group was significantly higher. Moreover, higher expression of hsp70 was detected in blue and yellow groups, and the relative expression of hsp90 was upregulated in all narrow bandwidth light treatments. These results suggest that monochromatic light may lead to stress in crabs.

Melatonin, the light perception hormone, is very conserved across animals and plays an important role in the entrainment of circadian rhythmicity although the underlying pathways remain unclear (Migaud et al., 2007b; McStay et al., 2014; Saha et al., 2019; Song Y. et al., 2020). In crustaceans, melatonin injection was shown to induce ecdysteroidogenesis and stimulate the synthesis of methyl farnesoate which is involved in the regulation of molting, leading to increased molting frequency in Scylla serrata (Sainath and Reddy, 2010b; Girish et al., 2015). Furthermore, melatonin was shown to suppress nitric oxide synthase activity and reduce nitric oxide (Kim et al., 2004; Lee et al., 2007), which are thought to inhibit ecdysteroidogenesis mediated by MIH (Nakatsuji et al., 2009). The present study showed that short-wavelength light (blue and cyan) suppressed melatonin contents of S. paramamosain eyestalks. Similar results were also observed in ornamental cleaner shrimp (Lysmata amboinensis) in which melatonin content in eyestalks of shrimp reared under blue light (455 nm) was significantly reduced compared to red (630 nm) and white (control) spectral treatments (Choi et al., 2019). However, a lower melatonin level was not shown in this study to impact on molting frequency.

Antioxidant capacity refers to compounds capable of protecting cells against oxidative stress involving ROS and RNS (reactive nitrogen species) that are by-products of the metabolism and immune system. Antioxidant capacity plays an essential role in maintaining homeostasis of oxidation/reduction by antioxidant enzymes such as SOD, CAT, and glutathione peroxidase (GPX) (Karadag et al., 2009; Wu et al., 2020). Excessive accumulation of ROS may cause oxidative damage, induce disease, and lead to death in animals. MDA, the end product of lipid peroxidation by ROS, is an essential indicator that reflects oxidative damage status (Chen et al., 2021b). While no significant differences in T-AOC, SOD, and CAT were detected between light treatments, H₂O₂ and MDA were significantly higher in hepatopancreas of crabs exposed to violet light, suggesting a higher oxidative stress under this spectrum. Correlation analysis showed that the content of H₂O₂ was negatively correlated with wavelength. These results contrast with previously published data obtained in goldfish (Carassius auratus) and bay scallop (Argopecten irradians) that showed a reduction in H₂O₂ content under short wavelength lights (Kim et al., 2014; Song J. A. et al., 2020). Contrasting results between studies may be due to differences between species and stage of development, light characteristics of the respective habitats and tissues analyzed. Thus, further studies are required to understand the mechanisms behind such distinctions.

In the present study, bcl-2 gene expression in cyan, and the cox IV and caspase 3 in green were upregulated. Bcl-2 is the founding member of the Bcl-2 family that can block apoptosis by preventing the release of cytochrome c into the cytoplasm to suppress the activation of the caspase cascade (Chen et al., 2019). The p53 is a crucial transcription factor for cell cycle arrest, cellular senescence, and apoptosis (Nuñez-Hernandez et al., 2018). Cytochrome c and cytochrome c oxidase (COX) could catalyze the terminal reaction of the mitochondrial electron transport chain, reducing oxygen to water by transfers electrons to molecular oxygen (Kadenbach et al., 2004; Hüttemann et al., 2012). The results from the present study suggest that cyan light could inhibit, while green light could promote apoptosis. On the contrary, previous studies in white leg shrimp and olive flounder (Paralichthys olivaceus) showed that green light could reduce oxidative stress and apoptosis via the mitochondria-mediated caspase-dependent pathway (Kim et al., 2016; Fei et al., 2020a). Unlike P. vannamei and P. olivaceus, S. paramamosain is an intertidal organism that migrates between that migrates between different tidal zones. Light requirements of mud crab may therefore be more complex. Thus, the results may suggest that cyan light (a mixture of blue and green light) and full-spectrum (a mixture of all spectrum) may have some advantage over other treatments in terms of stress and apoptosis for mud crab.

In summary, this study evaluated the effects of light spectrum on the growth performance, molting, antioxidant capacity, stress response, and apoptosis-related gene expression of S. paramamosain. The results suggested that crabs under full-spectrum, cyan and blue light performed better, while violet light appeared to reduce molting and growth of S. paramamosain via hormonal, oxidative stress, and apoptosis pathways. Notably, the current results suggested that the optimal light wavelength for SGR is 449.97 nm. Although more studies are required to describe and understand the underlying pathways behind light induced effects, these new results contribute to the optimization of rearing environment for mud crab farming while providing scientific hypotheses for further studies to characterize light perception and biological efficiency in crustaceans. Violet light should be avoided in the hatchery and nursery of S. paramamosain while blue, cyan, and full-spectrum light are recommended for juvenile on growing.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

SC: methodology, data curation, visualization, and writing—original draft. CS: conceptualization and review and editing. HM: writing—review and editing. CbS: resources and review and editing. CM: review and editing and supervision. YY: review and editing. CW: funding acquisition. ZR: resources. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.