Protein and amino acid profile of Ruditapes decussatus and Ruditapes philippinarum from Çanakkale Strait, Türkiye

Bivalves possess a nutritionally rich composition due to their high protein content, abundance of essential amino acids, and low lipid levels (Acarli et al., 2023; Prato et al., 2019; Vural, 2022; Vural & Acarli, 2021; Yildiz et al., 2021). These mollusks incorporate all amino acids essential for regulating lipid metabolism, supporting growth and repair processes, and preventing hepatic disorders and renal failure in the human body (Koral & Süleyman, 2017; Varlik et al., 2004). Among these, amino acids such as aspartic acid, glycine, and glutamic acid play a pivotal role in wound healing (Vural & Acarli, 2021; Zuraini et al., 2006).

Türkiye harbors two clam species with significant production potential among bivalve taxa. These are the native grooved carpet shell, Ruditapes decussatus, and the Manila clam, Ruditapes philippinarum. The origin of R. philippinarum is traced to the western Pacific coasts, with its natural distribution encompassing the Philippines, South and East China Seas, Yellow Sea, and Sea of Japan (Scarlato, 1981). Its introduction to the Adriatic and Aegean Seas was reported in 2004–2005 (Jensen et al., 2004, 2005). On the other hand, R. decussatus exhibits a distribution extending from Norway to Somalia, including the Iberian Peninsula and the Mediterranean Sea (Parache, 1982).

Various studies have been conducted in Turkish marine waters focusing on R. decussatus and R. philippinarum. These include investigations on the biochemical composition of both species (Çelik et al., 2014), heavy metal accumulation in R. decussatus (Bilgin & Uluturhan, 2015; Ulusoy, 2010), PAH contamination in R. philippinarum (Künili et al., 2021), spat collection in R. decussatus (Serdar et al., 2009), juvenile rearing in R. decussatus (Serdar et al., 2007), reproduction in R. decussatus (Serdar et al., 2010a), and morphological assessments of both species (Acarli et al., 2015; Doğan, 2005; Doğan et al., 2016). Additionally, research has addressed genetic traits and hybridization in R. decussatus and R. philippinarum (Acarli et al., 2015, 2018).

Monthly protein levels of R. decussatus in Türkiye have been analyzed in the Sufa Lagoon (Serdar & Lök, 2009; Serdar et al., 2009) and Çakalburnu Lagoon (Serdar et al., 2010a, 2010b) in the Aegean Sea. Additionally, Çelik et al. (2014) examined the seasonal protein levels of R. decussatus and R. philippinarum in the Marmara Sea (Bandirma). However, there is no research available on the extent of monthly variations in amino acid content for these two clam species in Türkiye. Determining such variations every month is particularly important for developing consumption strategies for these species.

This study aims to evaluate the monthly protein and amino acid contents of R. decussatus and R. philippinarum from the Çardak Lagoon, to identify their nutritional values, and to assess the extent to which these species meet human requirements for essential amino acids and protein. The findings will also help establish consumption recommendations and encourage the aquaculture of these species in the region based on the results obtained.

Proteins are the building blocks of life; they are essential for cell renewal, strengthening the immune system, protecting the body against external factors, and maintaining and strengthening muscle tissue. The nutritional value of certain bivalve species is superior to that of land animals such as beef, chicken, and pork in terms of protein content (Wright et al., 2018). The protein content in bivalves is affected by their reproductive period (Acarli et al., 2018; Vural & Acarli, 2021). During gonad maturation, an increase in protein content is observed in bivalves, continuing until spawning, after which it decreases (Berthelin et al., 2000; Gökoğlu, 2021). Additionally, protein can serve as an alternative energy source during gametogenesis in some bivalve species (Acarli et al., 2015; Beninger & Lucas, 1984; Galap et al., 1997). Vural and Acarli (2021) suggested that the decrease in protein levels in F. glaber from August to September could be related to the transition to the spawning and resting phases. Acarli et al. (2023) indicated that in Modiolus barbatus, protein could be used as an energy source during the onset and continuation of gametogenesis. Anibal et al. (2011) observed that in R. decussatus, gametogenesis began in January, ovulation continued from June to September, and the resting period lasted from October to December. Serdar and Lök (2009) reported that in R. decussatus from the Sufa Lagoon in Izmir, gametogenesis began in January, the peak reproductive period occurred between July and September, and gonad development ceased completely between October and December. Protein levels were highest in March, and decreased to their lowest in the month following gonad maturation. Lök et al. (2011) also reported that R. decussatus in both the Marmara and Aegean Seas had spawning in the summer months. Gözler and Tarkan (2000) observed that in R. decussatus from the Çardak Lagoon in the Çanakkale Strait, gonad development began in March and ended in October, with a resting phase between November and March. Mature individuals with ripe eggs were found in June. This study was conducted in the same region as the one by Gözler and Tarkan (2000). In this study, R. decussatus showed high protein levels in July and August, which decreased in September and continued to decline in October. There was a sharp increase in November, followed by a decrease in January, with a subsequent increase in February. Protein levels decreased in March and April, and after a sharp increase in May, another decline was observed in June (Fig. 2). When examining gonad maturation macroscopically, maturation in R. decussatus occurred in all months except March and May, with ripening in all months except autumn and June, and spawning in the autumn and winter months. For R. philippinarum, gonad maturation occurred in all months except spring, and ripening occurred in all months except September. Spawning took place in the autumn and winter months (Fig. 2). In this study, protein content in R. decussatus ranged from 63.44 ± 0.32% in April to 80.00 ± 0.00% in May, and in R. philippinarum, it ranged from 60.84 ± 0.16% in September to 72.91 ± 0.24% in December. The protein content in R. decussatus was reported to range from 39.82% to 64.24% in the Çakalburnu Lagoon (Aegean Sea) by Serdar et al. (2009), from 44.7% (December) to 50.8% (June) in Galicia (Spain) by Ojea et al. (2004), from 34.3% to 42.7% in Izmir (Mersin Bay) by Lök et al. (2011), and from 36.55% to 50.20% in Musakça (Balıkesir). The species, water temperature, available nutrients, and reproductive period can influence protein levels in bivalves (Acarli et al., 2018; Vural & Acarli, 2021). The higher protein levels observed in this study may be attributed to regional differences, reproductive phases, temperature, salinity, and nutritional factors.

Bivalves typically contain essential amino acids and bioactive peptides (Venugopal & Gopakumar, 2017). The levels and proportions of amino acids are crucial for the proper functioning of the human body. Changes in amino acid levels play a significant role in the formation of various diseases, making it important to address deficiencies in these amino acids for treatment. A balanced ratio of EAA to NEAA (>1.0), with a higher quantity of sulfur-containing amino acids, indicates that these species provide well-balanced protein deposition (Joy & Chakraborty, 2016). The EAA/NEAA ratio for Paphia malabarica and Villorita cyprinoides (Joy & Chakraborty, 2016), Senilia senilis (Inyang & Effong, 2017), and Crassostrea madrasensis (Chakraborty et al., 2016) is above the threshold of 1.0, demonstrating that these bivalve species offer high-quality, well-adjusted protein. The EAA/NEAA ratio for razor clams, Ensis siliqua, and F. glaber was found to be <1 (Baptista et al., 2014; Vural & Acarli, 2021). In this study, the EAA/NEAA ratio was found to be high in both R. decussatus and R. philippinarum in all months, indicating that they possess high protein quality.

Bivalves are known to be rich in essential amino acids (Abirami et al., 2015; Inyang & Effong, 2017; Vural & Acarli, 2021). Amino acids such as lysine, methionine, threonine, tryptophan, isoleucine, leucine, phenylalanine, and valine are essential for humans (Vural & Acarli, 2021). Lysine, which is absent in cereals but present in adequate amounts in bivalves, can complement the limiting amino acids in our daily diets (Chakraborty et al., 2015). Lysine is necessary for fat metabolism, B vitamins, calcium absorption, muscle protein formation, and the synthesis of hormones, antibodies, and enzymes (Greenstein & Winitz, 1961; Özden & Erkan, 2011). P. malabarica and V. cyprinoides were found to have high levels of lysine, an essential amino acid (Joy & Chakraborty, 2016). The daily required intake of lysine for humans is 1–1.5 g (Özden & Erkan, 2011). In this study, the average lysine content was 6.9 ± 0.0 mg · 100 g⁻¹ dw for R. decussatus and 4.8 ± 0.1 mg · 100 g⁻¹ dw for R. philippinarum. Based on these results, consuming 100 g · day⁻¹ of R. decussatus or 208 g · day⁻¹ of R. philippinarum would provide the daily lysine requirement for a human.

Histidine and arginine are semi-essential amino acids, meaning they are essential for children but not for adults (Baykara et al., 2019). Arginine plays a role in the synthesis of creatine, ornithine, putrescine, nitric oxide, urea, protamine, and histone compounds (Keleşoğlu, 2012). In this study, histidine levels were found to be 7.5 ± 0.00 mg · 100 g⁻¹ dw in R. decussatus and 5.8 ± 0.01 mg · 100 g⁻¹ dw in R. philippinarum. The recommended daily intake of histidine for pregnant women is 33 mg · kg⁻¹ (HMDB, 2019). Based on this study, 22 g · kg⁻¹ dw of R. decussatus and 33 g · kg⁻¹ dw of R. philippinarum would meet the recommended daily histidine requirement.

Threonine is another important amino acid, and its levels were found to be 16.0 ± 0.3 mg · 100 g⁻¹ dw in R. decussatus and 10.1 ± 0.2 mg · 100 g⁻¹ dw in R. philippinarum in this study. Threonine is recommended to be used at 1 g · day⁻¹ for the treatment of genetic spasticity disorders (HMDB, 2019). Therefore, 50 g · kg⁻¹ dw of R. decussatus and 100 g · kg⁻¹ dw of R. philippinarum could meet the daily threonine requirement. Specifically, for R. philippinarum, August and September, and for R. decussatus, August, are the most important months due to their peak values.

Valine, leucine, and isoleucine are used in the treatment of liver diseases and kidney failure. They are also recommended for stress and severe burns (Aydogan & Ekici, 2012; Cengiz, 2019). Leucine plays a critical role in protein synthesis and muscle repair. It can increase resistance to diseases, especially under stressful conditions such as reproduction and migration periods (Machado et al., 2020). In this study, leucine content was found to be 853.6 ± 2.8 mg · 100 g⁻¹ dw in R. decussatus and 676.8 ± 6.8 mg · 100 g⁻¹ dw in R. philippinarum. Baptista et al. (2014) suggested that the increased leucine levels in E. siliqua during spawning may be due to its involvement in the formation of gonads post-spawning. Leucine, threonine, and lysine amino acids showed a strong correlation, increasing during spawning and decreasing afterward (Wu et al., 2013). In this study, a positive correlation between leucine and lysine, histidine, isoleucine, methionine, phenylalanine, threonine, valine, alanine, glycine, serine, tyrosine, proline, and arginine was observed in R. decussatus (p < 0.05). In R. philippinarum, positive correlations were found between leucine and phenylalanine, alanine, and negative correlations with methionine, threonine, proline, and arginine (p < 0.05). When evaluating the results in conjunction with Wu et al. (2013), it was observed that the leucine levels did not show complete synchronization, suggesting that other factors also influence the use of leucine.

Glycine is a non-essential amino acid synthesized from serine, threonine, choline, and hydroxyproline (Wang et al., 2014). This amino acid plays a vital role in metabolic regulation, antioxidant reactions, and neurological functions. In animals, glycine serves as a critical component of extracellular structural proteins such as collagen and elastin, which are essential for maintaining tissue integrity and resilience (Wang et al., 2014; Wu, 2009; Wu et al., 2013). Additionally, glycine has been shown to enhance sleep quality, contributing to its importance in physiological regulation (Bannai & Kawai, 2012). Although glycine is categorized as a non-essential amino acid, its endogenous synthesis may be insufficient under certain physiological demands, making dietary supplementation necessary. As such, it is often regarded as a conditionally essential amino acid (Wang et al., 2014).

Bivalves, including species from the genera Ruditapes and Venerupis, demonstrate physiological adaptations to salinity fluctuations through changes in hemolymph osmolality, transmembrane ion transport, and the concentration of free amino acids (Lin et al., 2021; Mo et al., 2020). Free amino acids play a crucial role in cell volume regulation and osmoregulatory processes (Pourmozaffar et al., 2020). Specifically, glycine, taurine, alanine, proline, and glutamic acid are essential for bivalves to withstand salinity changes and other environmental stressors (Cao et al., 2022; Zhou et al., 2023). Prior studies, such as those by Chakraborty et al. (2016) on C. madrasensis and Vural and Acarli (2021) on F. glaber, have reported lower glycine levels during summer months, potentially linked to environmental factors.

In this study, glycine concentrations in R. philippinarum and R. decussatus were highest in autumn and lowest in spring. A significant positive correlation between glycine and alanine levels (p < 0.05) was observed, supporting the role of glycine in physiological adaptations to environmental conditions.

Arginine, another non-essential amino acid, is critically important for various physiological functions in aquatic organisms. It has been shown to enhance growth hormone synthesis (Paddon-Jones et al., 2004) and is the sole substrate for nitric oxide synthase, which catalyzes the production of nitric oxide—a key molecule in vasodilation and immune responses (Palmer et al., 1987). Additionally, arginine is required for the synthesis of polyamines, which regulate DNA replication and cell division. It also plays a role in the synthesis of micronutrients such as iron, folate, zinc, and magnesium, as well as nucleic acids (Calder, 2020).

Arginine, along with glutamine, threonine, and tryptophan, is essential for modulating immune responses in bivalves (Wu, 2009). Seasonal variations in arginine levels were evident in this study. In R. philippinarum, arginine concentrations peaked in spring and were lowest in summer. Conversely, in R. decussatus, arginine levels were highest in autumn and lowest in spring. These findings highlight the dynamic role of arginine in supporting the physiological and immunological needs of bivalves during different seasonal and environmental conditions.

R. decussatus and R. philippinarum, collected from the Çardak Lagoon, Çanakkale Strait, exhibit a protein and amino acid composition that suggests they are beneficial for consumption. However, bivalve aquaculture in Türkiye is currently limited to mussels. The findings of this study can pave the way for clam aquaculture in Türkiye, potentially generating new employment opportunities, reducing unemployment, and contributing to the national economy. Thus, studies like this are significant for better economic utilization, promotion of consumption, and encouragement of production for these clam species.

The data obtained in this study may also guide the identification of optimal periods for harvesting these species based on their amino acid and protein content. Among essential amino acids, leucine and isoleucine were the most abundant, while arginine, serine, and alanine dominated the non-essential amino acid profile. Leucine levels were consistently high throughout the year for both species, but the recommended optimal consumption periods are all months except September and April for R. philippinarum, and all months except spring for R. decussatus.

Furthermore, the EAA/NEAA ratio exceeding 1.0 underscores the high protein quality of these species distributed in the region. These findings support the economic and nutritional potential of these clams and highlight the importance of further research and development in their aquaculture and market utilization.