Temperature can directly affect the respiration and energy metabolism of crustaceans. Under low temperature stress, on the one hand, energy consumption increases. On the other hand, neurohormone secretion and digestive enzyme activity decrease, and energy metabolism-related enzyme activity and metabolic modes are altered, resulting in crustacean metabolic disorder (Anestis et al., 2008). Low temperatures are believed to have widespread effects on marine organisms’ behavioral and physical traits, including their metabolism. Generally, crustaceans lack efficient regulators, making them sensitive to reduced temperatures.

In shrimp and crab, proteins are the primary energy source (Cuzon et al., 2010). In cold-adapted L. vannamei, fat absorption and digestion, and the protein pathways were enhanced significantly (He et al., 2018). Similarly, under cold stress (23°C), plasma lipids (especially total cholesterol and triglycerides) and total proteins increased significantly; there were no significant changes in glucose levels (Wu et al., 2020). Therefore, it was speculated that in crustaceans under acute cold-stress, lipids and proteins are the main energy sources (Wang et al., 2019). A metabolic study of the black tiger shrimp (Penaeus monodon) cultured under low temperature revealed that its amino acid and trehalose contents increased significantly (Jiang et al., 2019). Under low temperature stress, in addition to the fatty acid composition of tissues and cells, the content of free amino acids (FAA) in tissues also changed. The content of total FAA increased in spring and autumn, but decreased rapidly in winter. As an important nutrient in the body, protein may be automatically decomposed into amino acids under low temperature stimulation. On the one hand, amino acids are used for protein synthesis and turnover, and on the other hand, they might have anti-stress functions. To improve the metabolic rate and oxygen carrying capacity of the body, or to meet the needs of protein synthesis, the structure and synthesis rate of hemocyanin in shrimp and crab will change significantly in response to stress.

The fatty acid metabolism of crustaceans is sensitive to temperature. Cold temperature mainly affects membrane fluidity by affecting the saturation of fatty acids in the cell membrane. Membrane fatty acid desaturation is considered an important mechanism by which crustaceans adapt to low temperature, and is crucial to maintain membrane fluidity, enzyme activity, and normal cell function (Pruitt, 1990; Suprayudi et al., 2004). Cold stress leads to a change in the fatty acid (FA) composition in crustacean cells, which usually leads to the decrease in the saturated fatty acid (SFA) ratio and a rapid increase in the unsaturated fatty acid (UFA) ratio, which is conducive to the maintenance of cell membrane fluidity (Azra et al., 2020a, b). In Scylla serrata, Cancer pagurus, and Carcinus maenas, the SFA content decreased significantly at low temperature (Cuculescu et al., 1995; Wang et al., 2007). In the crayfish cultured at low temperatures, the haemolymph cholesterol and triglyceride contents were reduced significantly, suggesting that under cold stress, these two substances are consumed to release energy (Wu et al., 2020). UFAs are important components of cellular membranes and participate in energy metabolism (Nemeth et al., 2014). Under cold stress, UFA levels increased in L. vannamei (Fan et al., 2019), the Chinese fleshy shrimp (Fenneropenaeus chinensis) (Meng et al., 2019), and the kuruma shrimp (Marsupenaeus japonicas) (Ren et al., 2020). Desaturase enzymes play an important role in the synthesis of unsaturated fatty acids. In C. quadricarinatus low temperature treatment increased Δ6 desaturase mRNA expression and enzyme activity with decreasing water temperature (Wu et al., 2018). However, the mechanisms for the induction of Δ6 desaturases at low temperature remain unclear.

As an important energy source, sugar plays a vital role in the low temperature stress of shrimp and crab. A decrease in temperature led to an increased blood glucose content and a decreased glycogen content in M. rosenbergii, S. serrata, Pachygrapus crassipesran Dall, Paranephrops planfrons, and L. vannamei (Hsieh et al., 2006; Kong et al., 2008; Valle et al., 2009; Zhou et al., 2011). This change in sugar levels in shrimp and crab is an adaptation to low temperature. During cold stress, glucose is consumed as a fast energy source, and the hepatopancreas continuously decomposes glycogen to meet the needs of maintaining the metabolic energy supply. When the temperature rises, or the crustacean adapts to low temperature, the haemolymph glucose level will gradually recover.

The crustacean immune system mainly functions via innate immune mechanisms comprising humoral and cellular responses. Cellular innate immunity comprises all hemocyte-mediated reactions (e.g., phagocytosis, nodule formation, and encapsulation). Humoral innate immunity comprises mainly lysozyme, phosphatases, antimicrobial peptides (AMPs), protease inhibitors, agglutinins, and the prophenoloxidase-activating system (Kenneth and Lage, 1992; Kulkarni et al., 2020). In the humoral response, AMPs, lysozyme, or phenoloxidase (PO) concentrations increase markedly under stress conditions, e.g., invasive pathogens, disease outbreaks, and environmental hazards. Hemocytes comprise the major component of the crustacean cellular immune system, and their levels will change according to the condition of the organism and the environment (Wang and Chen, 2006). Thus, stress-induced immune system activity is conveniently assessed using the total haemocyte count (THC) (Xu et al., 2019). Fan et al. (2013) found that the THC in L. vannamei was reduced when the temperature decreased from 28 to 13°C. These results indicated that the THC of crustaceans is closely related to temperature. The lower the temperature, the lower the enzyme activity and the lower the THC. In lobsters, the hemocyte phagocytic activity was affected negatively by low temperature (Steenbergen et al., 1978). The evolutionarily conserved cellular process of autophagy involves maintaining homeostasis by recycling damaged or excess cellular components (e.g., misfolded proteins, intracellular pathogens, damaged organelles, and damaged DNA) (Bolliet et al., 2017). In L. vannamei, autophagy is associated with low temperatures (Liang et al., 2020).

In invertebrates, the important innate immune response mechanism, melanization, functions via the prophenoloxidase (proPO)-activating system and is catalyzed by PO (Amparyup et al., 2013). In shrimp, melanization has been suggested to be an antiviral response (Zhao et al., 2020). Meanwhile, PO functions in cellular defense in association with phagocytosis-enhancing factors; therefore, PO is used frequently to assess the effect of environmental stress on the invertebrate immune system (Ellis et al., 2011). In brown shrimp (Penaeus californiensis) exposed to increasing temperature (18−32°C), the hemocyte proPO system activity decreased at 32°C (Vargas-Albores et al., 2008). In the crab (Carcinus aestuarii), when incubated at 4°C, the PO activity in cell-free haemolymph was significantly higher than that in the control crabs incubated at 17°C (p < 0.05) (Matozzo et al., 2011).

In addition, immune parameters, such as antibacterial activity, are suppressed by low temperature. Taken together, these previous studies show that low temperature has important effects on shrimp disease tolerance and survival. However, to date, there have been few studies investigating the immune regulatory mechanisms in shrimp exposed to low temperature. Lysozyme (LSZ), as a kind of hydrolase, is the basis of phagocyte sterilization, existing widely in different tissues, body fluids, and secretions of various organisms, and can be used to measure the non-specific immune capacity of organisms (Mock and Peters, 1990). Low temperature can affect the activity of LSZ. Ding et al. (2010) reported that temperature change could inhibit the LSZ activity of S. serrata. In the red claw crayfish, LSZ was inhibited significantly following low temperature exposure (Wu et al., 2019). Hemocyanins are extracellular negatively charged proteins that are involved in numerous physiological functions, such as protein storage, osmoregulation, oxygen transport, and enzyme activities (Ishwarya et al., 2018; Coates and Costa-Paiva, 2020). In the crayfish P. clarkii and P. zonangulus, the acclimation temperature directly affected the hemocyanin binding affinity (Powell and Watts, 2006). Thus, it is believed that shrimp are more susceptible to pathogens under low temperature conditions.

In healthy organisms, the production and elimination of free radicals are in a dynamic balance; however, in adversity, stress will induce a reaction from the enzyme systems and non-enzyme systems of mitochondria, microsomes, and the cytoplasm, resulting in the production of excess reactive oxygen species (ROS) and oxygen free radicals, thus breaking the balance of reactive oxygen metabolism (Wade et al., 2017). In cells and tissues, oxidative stress’s effects on cellular damage can be indicated by the level of lipid peroxidation (Mensah et al., 2012). To reduce oxidative stress and repair damaged cells, the primary defense response comprises the production of enzymatic and non-enzymatic antioxidants to scavenge ROS and free radicals (El-Gendy et al., 2010). In all organisms, the main antioxidative enzymes that detoxify ROS are glutathione S-transferase (GST), glutathione reductase (GR), catalase (CAT), glutathione peroxidase (GPx), and superoxide dismutase (SOD), in addition to the non-enzymatic antioxidant molecule, reduced glutathione (GSH) (Lesser, 2006; Zheng et al., 2019).

In mud crabs subjected to cold stress, the CAT, SOD, and GPX activities increased over 2 h, and then decreased gradually; the content of malondialdehyde (MDA) also increased gradually under cold stress (Kong et al., 2007). In S. paramamosain acclimated at 5, 10, 15, and 27°C (control group), the SOD, CAT, and GPx activities, and the MDA content decreased gradually with lowering temperatures and were significantly reduced at 5 and 10°C compared with those in crabs incubated at 27°C (Kong et al., 2012). Qiu et al. (2011) evaluated the physiological effects of continuous temperature decrease on L. vannamei. The MDA level increased when water temperature decreased from 23 to 12°C.

It has become clear that organisms share a common adaptation mechanism, termed the heat shock response (HSR), to cope with temperature-induced stress, which results in a dramatic change in gene expression patterns and leads to the elevated synthesis of a range of molecular chaperones and the induction of other cell-protective pathways (Richter et al., 2010). The heat shock protein (HSP) and heat shock factor (HSF)- mediated regulation pathways play crucial roles in the HSR, and have been studied intensively in terms of HSR mechanisms and the cold-tolerance of organisms (Gbotsyo et al., 2020).

HSPs are regulated by heat shock elements (HSEs), HSFs, and other factors to control their cellular levels (Morimoto and Santoro, 1998). HSF1 is an important transcription factor that regulates the heat shock response, and is expressed widely in eukaryotes, playing an important role in maintaining intracellular homeostasis during heat stress (Anckar and Sistonen, 2011). When the body is subjected to cold stress, it combines with HSE. In addition, HSPs are conserved at the evolutionary level. In a study of high temperature stress of Penaeus monodon, PmHSF1 expression was elevated. The expression levels of HSPs and other heat tolerance related genes in P. monodon changed significantly after the PmHSF1 gene was knocked down (Sornchuer et al., 2018). In M. japonicas, MjHSF1 transcription was upregulated under heat stress (Zheng et al., 2020). To date, most of the studies on the related functions of HSF1 have focused on the interaction between HSF1 and HSPs, and there are few studies on the expression of immune related factors associated with HSF1. Several HSP genes are downstream targets of HSF1, which are involved in crustacean resistance to adverse environments.

HSPs comprise molecular chaperones that are produced during the exposure to, and recovery from environmental or physiological stress, including cold stress (Johnston et al., 2018). HSPs, also referred to as molecular chaperones or stress proteins, comprise a group of highly conserved proteins that are present ubiquitously in both prokaryotic and eukaryotic organisms (Roberts et al., 2010). HSPs protect cellular functions and structures and from the effects of stress and have important functions in the maintenance of cellular homeostasis (Morimoto and Santoro, 1998). Based on their molecular weight, HSPs are generally classified into five families, HSP100, HSP90, HSP70, HSP60, and small HSPs (Ahn and Im, 2020). In F. chinensis, the levels of FcHSP90 mRNA were induced sensitively in response to heat shock (from 25 to 35°C), reaching a maximum level after 6 h of heat shock (Li et al., 2009). In other crustaceans (S. serrata and L. vannamei) mRNA levels of HSP40, HSP70, or HSP90 were increased in response to cold or heat shock (Fu et al., 2013; Chen et al., 2018; Sung et al., 2018; Fan et al., 2019).

Apoptosis, a cell death process, has a crucial function in maintaining tissue hemostasis and disease protection. As a component of inflammatory reactions, the physiological function of apoptosis helps to remove damaged or harmful cells from immune tissues (Johnstone et al., 2002). Li et al. (2014) evaluated the effect of continuous temperature decrease on hemocyte apoptosis of L. vannamei, which showed an increase in the apoptotic cell ratio and a decrease in caspase-3 activity when the water temperature was reduced from 27 to 17°C. Cold temperature led to increase caspase-3 expression in the swimming crab (Portunus trituberculatus) (Meng et al., 2014). A previous study from our group demonstrated that in M. japonicus, the expression of p53 increased significantly under cold stress, which suggested that cold-induced apoptosis might involve p53 (Ren et al., 2020). Significant changes in p53 signaling pathways under cold stress were also observed in the hepatopancreas of the red claw crayfish under cold stress (Wu et al., 2019).

During cold acclimation (or low temperature adaptation), shrimps and crabs change the composition and concentration of intracellular ions by regulating the number and distribution of various ion channels on the cell membrane and changing the composition and concentration of intracellular ions to maintain normal physiological activities (Masroor et al., 2018). On the gill cell membrane of S. serrata, four kinds of adenosine triphosphatases (Ca²⁺/Mg²⁺-ATPase, Ca²⁺-ATPase, Mg²⁺-ATPase, and Na⁺/K⁺-ATPase), which are involved in ion uptake and osmotic pressure regulation, were upregulated during the process of adaptation to a lower temperature (Kong et al., 2012). In the hepatopancreas of M. nipponense, the Na⁺-K⁺ ATPase activity in the temperature range 16−22°C was enhanced by 1.38-fold compared with that in the temperature range 25−32°C (Wang et al., 2006). In Procambarus clarkia, exposure from room temperature (23°C) to 4°C for 28 days resulted in a significant increase in Ca²⁺-ATPase activity (Gao et al., 2009). Thus, in a cold environment, shrimps and crabs can reduce heat loss by adjusting the ionic concentration and osmotic pressure of their body fluid to reduce the difference between their body temperature and that of the outside water.

The embryonic development of crustaceans is a dynamic physiological process. In addition to the influence of the parents, the external environmental conditions also have an important impact on embryonic development. In particular, the temperature not only affects the time of embryonic development, but also affects the quality and speed of embryo development (Yamamoto et al., 2017). Studies have shown that only when the temperature of organisms is above zero can they begin to develop and grow (Hartnoll and Abele, 1982). The biological zero of embryonic development of Exopalaemon carinicauda, S. serrata, and P. clarkii are 12.18°C, 11.70°C, and 5.60°C, respectively (Wu, 1991; Lv et al., 2004; Liang et al., 2013).

In the suitable temperature range, the higher the temperature, the faster the embryo develops. Wang et al. (1998) found that the embryonic development time of Thenus orientalis was shortened from 43 to 21 days with an increase in temperature from 22 to 31°C. Liang et al. (2013) found that in the ridgetail white prawn E. carinicauda, the incubation time of embryos shortened with the increase in temperature when the temperature was between 18 and 28°C. Cooler water retards growth and delays maturity, causing crabs to begin maturation when they are at larger sizes (Azra et al., 2020a).

During evolution, crustaceans have formed a relatively perfect reproductive regulation system, involving neuropeptides, hormones, neurotransmitters, and other hormones (Nguyen et al., 2016). The levels of these hormone are adjusted with the changes in temperature, salinity, and other environmental factors, such that crustaceans can reproduce under the best environmental conditions. Among them, temperature is involved in gonadal maturation by regulating hormone synthesis and secretion (Qian et al., 2015).

Xu et al. (2008) studied P. clarkii and found that an increase in water temperature from 22 to 28°C could promote gonadal maturation. Carmona-Osalde et al. (2004) found that in the range of 16–25°C, the ovary development of P. llamasir could be promoted by increasing the temperature. When the water temperature was between 15 and 25°C, the egg holding rate of S. serrata increased as the temperature increased (Yao et al., 2005). These studies confirmed that the water temperature is a major factor that influences crustacean gonadal development. In a certain temperature range, the higher the water temperature, the better the quality of gonadal development of crustaceans.

Behavioral modifications comprise changes in microhabitat use, abundance and distribution, feeding, predation, migration and spawning behaviors. In crustaceans grown under low temperatures, decreased activity and a decrease or cessation of feeding are the most frequently observed in behavior (Matheson and Gagnon, 2012). Fighting behavior increases the heart rate and metabolic rate of animals, and has a certain impact on their ability to withstand high temperature (Wang et al., 2020). Crustaceans are intolerant to low temperature and lack the ability to regulate their body temperature. In a low temperature environment of 9°C, the body of L. vannamei lost its balance and was slow to respond to external stimuli. Temperature has more complicated effects on locomotor activities (e.g., swimming or walking), which form part of the normal behavior of an animal, and are thus controlled by the central nervous system (Lagerspetz and Vainio, 2006).

Temperature is a growth limiting factor for all living things, but especially for aquatic organisms (Lushchak, 2011). All shrimps and crabs have a temperature tolerance range (see Table 2). When the water temperature exceeds the regulatory capacity of shrimp and crab, low temperatures will slow down their growth rate and even cause death. Temperature optima can be defined as the temperature at which shrimp grow fastest and most efficiently (González et al., 2010). The tolerance of different crustaceans to temperature is shown in Table 1. At low temperatures, shrimp and crabs need more energy to cope with stress, resulting in a significant reduction of reserves used for the growth process. Studies have shown that temperature is closely related to the growth of L. vannamei (Wyban et al., 1995) P. monodon (Deering et al., 1995), and Macrobrachium nipponense (Wang et al., 2006).

Temperature can affect the growth of crustaceans by altering two factors, the molt increment (the increase in duration between successive molts) and the intermolt period (the time interval between successive molts). Increasing temperature usually decreases the intermolt period; however, its effect on the molt increment is unknown. In early juvenile mud crabs, S. paramamosain, temperature-induced autotomy influenced the molting of early juvenile mud crabs, and changes in the levels of mRNA encoding the ecdysone receptor (EcR) seemed to play an important regulatory role in the molting process (Gong et al., 2015). Juvenile dungeness crabs (Metacarcinus magister) at different stages of molting (12, 19, or 26 days post-molting) were moved from ambient temperature (15°C) to temperatures of 5°C and 20°C for 14 days. From 5 to 20°C, survival ranged from 97 to 100% Molt stage progression increased from 5 to 15°C, but not at 20°C (Wittmann et al., 2018). L. vannamei incubated at 13°C showed significant reductions in swimming and feeding behaviors, and more deaths were observed at this temperature (Huang et al., 2017).