According to our results, the average RR (at 80–100% saturation O2) at 18°С was 67.81 ± 11.13 µgО2·h− 1·gww− 1. Previously published data, the average daily RR level for S. inaequivalvis at 20°С was 75 ± 25 µgО2·h− 1·gww− 1 and the maximal value of the respiratory dynamics was observed at daytime (Van den Thillart et al., 1992; ). The oxygen consumption of the blood clam was substantially lower compared to other benthic bivalves (De Zwaan et al., 1991; Stolbov & Vialova, 2001; Shin et al., 2006). Scapharca broughtonii (whole weight 5.8 ± 0.4 g) consumed 40 µgО2·h− 1·gww− 1 at the same conditions, which was 5 times lower compared to the Mediterranean mussels, Mytilus galloprovincialis (220 µgО2.h− 1gww− 1) (Stolbov & Vialova, 2001). Kang et al. (2007) reported that maximal RR level for S. broughtonii (189 ± 0.003 µgО2·h− 1·gww− 1) was observed for clams with shell length 11.8 ± 2.1 mm at 20°C. The larger individuals (> 30 mm) showed two times lower RR (90 ± 0.012 µgО2·h− 1·gww− 1). Similarly, Buhadi et al. (2013) reported that RR of Anadara spp. decreased as the weight of the mollusk increased.
Blood clams are typical inhabitants of bottom biocenoses, where oxygen deficiency often occurs because of insufficiently intensive mixing of water and the accumulation of organic compounds in sediments and the bottom layer (Qian et al., 2002). The different resistance of A. kagoshimensis to hypoxia and other adverse factors is primarily associated with the physiological and ecological peculiarities of these animals. Thus, the results of studying the intensity of oxygen consumption showed that the studied mollusks with the same mass of soft tissues under normoxia and at 17–20°C differed significantly: the blood clams had 7 times lower oxygen consumption than mussels (Stolbov & Vialova, 2001; Santini et al., 2002). Thus, even under the conditions of external normoxia, anaerobic processes actively take place in the blood clam tissues and oxygen consumption is low. It was found that mussels exposed to oxygen concentrations decreasing from 9 to 2 mgO2·l− 1 resulted in a slow reduction in the respiration rate, while the filtration rate remained high and constant (Tang & Riisgård, 2018). Thus, a decrease of oxygen concentration by 78% only resulted in a 25% decrease in respiration rate of M.edulis. The lowest RR of A. kagoshimensis (5.0-7.7 µgО2·h− 1·gww− 1) obtained in our experiments at 76% oxygen saturation was unexpected. The drop of respiration rates was more than 94%. Such low values are usually observed in bivalves undergoing prolonged hypoxia and anoxia (De Zwaan et al, 1991; Miyamoto & Iwanaga, 2012) and when they tightly close shells, that prevented water flow through the gills (Riisgård & Larsen, 2015). The closed valves of S. inaequivalvis can to depress its energy metabolism to less than 30–40% of the standard metabolic rate measured when the valves were open (Van den Thillart et al., 1992). Our results suggest the features of the organization of tissue metabolism of blood clams and their ability to rapidly transition from aerobic to anaerobic metabolism under conditions that are not critical for them.
In this case low rate of aerobic metabolism in А. kagoshimensis is functionally determinant and possesses an adaptive role for survival of this benthic species. The blood clam lives at depths of up to 30 m, mainly on mud, mud-shell or mud-sand bottom; at certain periods of during the life cycle it may burrow periodically into substrata (Crocetta, 2012; Strafella et al., 2017).
Starvation is considered an extremely nutritive stressor, and bivalves typically decrease metabolic rate to a minimum value (required to vital functions) to cope with it. Thus, the strategy allows the animal to reduce energy consumption and preserve energy under the starvation period. For example, food limitation of > 15 days decreased respiration of the marine clams Mya arenaria by 80% (Haidera et al., 2020). However, we did not find this in present experiments with blood clams A. kagoshimensis. According to our results, the effect of starvation was expressed in an increase in oxidative processes, which required additional oxygen. The early work found that during initial stages of starvation (6 day) A. inaequivalvis used a resource of tissue lactate in a direction of oxidative decarboxylation reactions (Andreenko et al., 2009). Significant increase in activity of lactate dehydrogenase on a background of lactate content reduction and rise in pyruvate level in tissues was determined. The process of A. inaequivalvis adaptation to starvation involves using the amino acids reserves during tissue biosynthesis. It has been shown, that various stress-factors induce synthesis of low molecular weight proteins, the “acute phase proteins” (Andreenko, 2012; Gostyukhina & Andreenko, 2019, 2020). Protein synthesis is also confirmed by the increase of RNA/DNA index observed in the hepatopancreas of A. kagoshimensis during the 15-day starvation period (Shcherban, 2012). Hepatopancreas of the blood clams is a specific organ, which uses the resource of amino acids for biosynthesis and energy supply of tissues at the initial state of other negative factors, as an anoxia and hypoxia (Andreenko et al., 2009; Andreenko, 2012). In unfavorable conditions clams activate antioxidant enzyme cascade and demonstrate a complex of tissue responses, which regulates the intensity of free radical oxidation (Golovina et al., 2016). Some authors note an increase in the number of erythrocytes and hemoglobin content in the initial period of starvation of hydrobionts (Oubella et al., 1993; Borah & Yadav, 1996). During starvation, the bivalve haemocytes maintained a homeostasis in phagocytic efficacy and nitric oxide generation ability with respect to the control. The mollusk maintained a significantly high protein content in its haemolymph and tissues under the nutritional stress with respect to the control. The dietary stress had no significant impact on the activity of digestive tissue till sixteenth day (Mahapatra et al., 2017). A similar increase in oxygen consumption and amino acid oxidation for energy production was observed in Scapharca subcrenata under the negative influence of heat stress (Jiang et al, 2020). It postpones the use of carbohydrate reserves of the body to the later stages of starvation.
The prolonged starvation did not cause significant inhibition of aerobic metabolism in some species (Choromytilus meridionalis, Modiolus barbatus) (Griffiths, 1980; Glavic et al., 2018). During fifteen days of starvation, oxygen consumption and clearance rate of mussel M. barbatus and ark Arca noae also showed an increase (Glavić et al., 2018).
At the beginning of the experimental period the average weight of A. kagoshimensis was 5.19 ± 0.63 g for the control group and 4.86 ± 0.67 g for the experimental group. The total weight of individuals in both groups did not significantly change during the experiment (P > 0.05). At the end of the experiment the decrease in wet weight of soft tissues of starved clams was less than 4%, but water content of tissues increased from 81–85%. Starvation tolerance in the mussel Limnoperna fortune has been observed and the specimens that were not fed showed no weight loss (Cordeiro et al., 2016; Mahapatra et al., 2017). During starvation, the bivalve haemocytes maintained a homeostasis in phagocytic efficacy and nitric oxide generation ability with respect to the control. The mollusc maintained a significantly high protein content in its haemolymph and tissues under the nutritional stress with respect to the control. The dietary stress had no significant impact on the activity of digestive tissue (Mahapatra et al., 2017). This phenomena has also been observed in other invasive bivalves, such as the freshwater zebra mussel Dreissena polymorpha and the marine mussel Mytilus edulis (Chase & McMahon 1995; Harbach & Palm, 2018). The total shell linear parameters of clams A. kagoshimensis did not change during the experiment.
Benthic organisms are subject to prolonged seasonal food limitation in the temperate shallow coastal waters that can cause energetic stress and affect their performance. Sediment-dwelling marine bivalves cope with prolonged food limitation by adjusting different physiological processes that might cause trade-offs between maintenance and other fitness-related functions. We investigated the effects of 14-days food deprivation on energetic metabolism rate of alien species A. kagoshimensis.
Survival at unfavorable trophic conditions depends on the physiological state of the organism, i.e. complex interaction between the level of internal energy supply, stable metabolic rate and the process of gametogenesis. The energy reserves are primarily used to support the metabolism, growth and maturation of gonads (Bayne et al., 1978; Pipe, 1985; Robinson, 1992; Cannuel & Beninger, 2005; Liu et al., 2010). During the starvation of mollusks in the winter, when the process of gametogenesis is enhanced, energy needs are provided by protein catabolism, while specialized cells of the somatic tissue continue to accumulate glycogen. Starvation during summer months enhances catabolism of carbohydrates in bivalves. The responses of organisms to a deficiency or lack of available food can be largely determined by the size of the animal. For example, according to Rodhouse & Gaffney (1984) small oysters, C. gigas, loose much more soft tissue weight (estimated by a percentage) in comparison to larger individuals. The use of energy depends on the opportunistic or conservative strategy. The result of the study shows that the conservative strategy has been applied. It can be attributed to the fact that the blood clam does not lose weight for a long time (Acarli et al., 2015).