The rayed trough-shell Mactra stultorum (= M. corallina) (Linnaeus 1758) is a commercially important benthic clam that inhabits sandy bottoms at depths between 5 and 30 m. It is a medium-sized marine bivalve with a very thin and delicate shell with concentric growth lines (Guarniero et al., 2010). M. stultorum is distributed along the coastlins of the Black Sea, Mediterranean Sea and the Eastern Atlantic Ocean from Norway to Senegal (FAO, 1987). This species occupies an important trophic level in the food chain, and is a source of food for crustaceans, gastropods and macrophage fish (Mnari, 2000). M. coralline was reported in Northern (Gulf of Tunisia) and Southern (gulf of Gabes) Tunisian Coasts by Enzenros (2001) and Seurat (1934). This clam is commercially exploited and consumed in the Atlantic Sea (e.g Manche coasts) (Direction Départementale des affaires maritimes de la Manche, 2013), while it is virtually unexploited and not yet commercialized in the Mediterranean Sea.

Sea organisms, in particular, bivalves are considered a good source of omega 3 fatty acids such as eicosapentanoic (EPA, 20:5n-3) and docosahexanoic (DHA, 22:6n-3) acids that are crucial for human nutrition and health (Shanmugam et al., 2007). It has been well demonstrated that these fatty acids are beneficial for human health and for the prevention of many diseases (coronary artery disease, inflammatory and autoimmune diseases etc.) (Harris and Schacky, 2004; Calder and Grimble, 2002; Gil, 2002). Shellfish are also rich in protein, amino acids, vitamins (e.g. B12), essential elements (Calcium, Magnesium, Zinc…) and are frequently considered a healthy food for human consumption (Ghribi et al., 2018; Dong, 2001).

Lipids are considered essential compounds for the formation of cells, tissue membrane and egg production and are also used in chronic stress (Napolitano et al., 1992; Nevejan et al., 2003). Lipids are a major source of metabolic energy and support animal growth when the food supply from the environnement is limited (Holland, 1978). According to Teshima et al., (1988), the distribution of lipids in different organs and tissues depends on their metabolism (digestion, absorption, modification, transport and storage). Lipids and fatty acids in the soft tissue of marine bivalves undergo seasonal fluctuations, which are controlled by exogenous factors (food availability, temperature, salinity etc.) and by endogenous factors (physiological status) (Prato et al., 2010).

Previous studies on M. stultorum investigated the Mediterranean Sea (1) and populations (2). They included research on genetics and ecology (Chetoui et al, 2012; et al., 2016) and biology (Chetoui et al., unpublished data).

The lipid content of this species was described by Hanus et al., (2009) in the plasmogen fraction of Mactra corallina but no data are available on its fatty acid composition and nutritional value during an annual cycle. To the best of our knowledge, the fatty acid composition and nutritional quality indices in different edible organs (mantle, foot and adductor muscle) of Mactra stultorum are examined for the first time.

The aim of this work was to study the seasonal variations in the fatty acid profiles of the M. stultorum population from the Tunisian Coasts and to examine the potential influence of environmental parameters and reproductive cycle on its nutritional quality. The results provided by the current study will be useful for indicating periods of the year that are more suitable for the consumption and marketing of M. coralline as a new marine resource in Tunisia.

The present study showed that the total lipid content observed in M. stultorum was similar to those found in M. verneriformis (Qiaozhen and Qi, 2013) f and in M. chinensis (Li et al., 2011) from the Eastern Coast of China.

For aquatic organisms such as bivalves, lipids are considered an important dietary constituent. In stressful environmental conditions and after the depletion of glycogen, the total lipid contet is used as an energy source during the reproduction process (Couturier and Newkirk, 1991). In the current work, a significant decrease in TL, especially in the adductor muscles of M. stultorum was observed. We suppose that this reduction could be related to the reproductive cycle of this species.

It is likely that this animal mobilizes the lipids from the muscle to be stored in the gonads for use during the maturation stage. This was confirmed by the reduction in the TL content in M. stultortum during the spawning phase that occured from summer to autumn (Chetoui, 2016). In contrast, no significant differences were observed in the TL content of the M. stultorum foot and mantle. In general, the digestive gland, gonads and adductor muscles of bivalve species were considered as organs of energy storage during the reproductive cycle (Darriba et al., 2005)

Fatty acids are vital components needed by the body for different metabolic and structural functioning. In our study, the fatty acid compositions in M. stultorum are in agreement with several investigations carried out on M. chinensis (Teshima et al., 1988) and Ruditapes decussates (Pazos et al., 2003). In addition, our results on the dominance of SFA and PUFA in M. stultorum were in agreement with the findings of Teshima et al. (1988) in M. chinensis and Besnard (1988) in P. maximus. Other investigations have demonstrated that n-3 PUFA proportion was accumulated at high levels in the muscle of Megangulus zyonoensis and Megangulus zyonoensis (Kawashima and Ohnishi, 2003). However, SFA was shown to be the more dominant in Glycymeris glycymeris tissues than PUFA and MUFA (Galap et al., 1999).

The seasonal variation in SFA (16:0 and 18:0) in M. stultorum was marked by a significant reduction during spring. This decrease was followed by a slight increase in MUFA content in all the studied organs. It could be due to the action of delta-9 desaturase that probably transformed SFA to MUFA (e.g. palmitoleic acid (16:1n-7) and Oleic acid (18:1n-9)) (Sprecher, 2000).

Indeed, many studies have showen that the PUFA level in bivalves reflect their good nutritional quality (Fernández et al., 1996).

Other investigtions have reported that this important proportion of PUFA probably resulted from their transfer from other reserve tissues such as the gonads and digestive glands to the mantel (Le Pennec et al., 2001; Darriba et al., 2005).

The seasonal variation in ARA for M. stultorum did not present any differences in all seasons among the organs except in winter for the Mantle. This exception was characterized with an increase in the ARA level associated with a decrease in LA level. This was probably due to the stress effects on the animals following a drop in water temperature during this period (10.33 ± 3 °C) (Pernet et al., 2007). In fact, LA is considered the main precursor of ARA, which can confirm their elevation.

n-3 PUFA, including EPA (C20:5n-3) and DHA (C22:6n-3), are considered essential fatty acids and dietary fats which are beneficial to human health (Dong 2001). During the spring, the foot of M. stultorum appeared as the richest organ in EPA and DHA, coinciding with a máximum level of chlorophyll a (4.28mg/l). AM and MT did not show a significant variation during the studied period. The results of essentials fatty acids in our study from the Mediterranean Coast reminded us of those found in M. chinensis from the Pacific Coast (Teshima et al., 1988). Likewise, other authors have shown that an increase in n-3 PUFA; especially EPA and DHA may reduce cardiovascular risk in consumers of marine products by up to 50% (Angerer and Sclaky, 2000). In addition, these fatty acids were of great interest for cardiovascular and some cancer treatments (Fenton et al., 2000).

The edible portion of M. stultorum was found to be rich in these essentials fatty acids (EPA and DHA). Their important nutritional quality was in accordance with other bivalves such as M. chinensis, M. veneriformis and M. murchisoni which are considered important commercial seafood (Ryou, 1997; Wu et al., 2002; Nottinghram and White, 2015).

For the Non-methylene-interrupted dienoic (NMID; C22:2i+C22:2j), the adductor muscle and mantle showed important percentages during all periods (> 2%). Fatty acid Non-methylene-interrupted dienoic (NMID) are polyunsaturated fatty acids, of 22 carbon atoms (C22:2i and C22: 2j), synthesized by shellfish, in particular bivalves (Zhukova, 1991). Their structures were established as C20:2∆5-11, C20:2∆5-13, C22:2∆7-13 and C22:2∆7-15. It was suggested that these fatty acids in animals are derived almost exclusively from food sources and are biochemically inert (Paradis and Ackman, 1975). For M. corallina, levels of C22:2i were lower than those of C22:2j and their sums varied between 1% and 3.74%. These levels were reversed in M. chinensis, which showed higher values which can reach 5% (Hiroaki, 2007). Our results were comparable to those reported in oysters Crassostrea gigas (Dridi et al., 2007); in mussels Mytilus galloprovincialis (Garrido and Medina, 2002) and in pectenidae (Telahigue et al., 2010).

Concerning the lipid quality index, Kraffe et al., (2008) showed that EPA+DHA in Manila clams were closer to our results (14.9%). However, EPA+DHA reflected more functional and structural modifications of mitochondrial membranes of this bivalve than others such as scallops, oysters, and mussels (Kraffe et al., 2008). So, our values for the PUFA/SFA ratio were higher than those suggested by the Department of Health and Social Security (1994); which were estimated at 0.45. In addition, the (n-3) / (n-6) ratio calculated in our study was close to the one indicated for M. chinensis (Teshima et al., 1988). Compared to other bivalve from the Mediterranean Sea, the n-3/n-6 ratio was higher than those reported for oyster Crassostrea gigas (2.8) and lower than those recorded for the Pectinidae family (Dridi et al., 2007; Telahigue et al., 2010). According to the UK Department of Health, an ideal proportion of n–6/n–3 is recommended at 4.0 (HMSO, 2001). So, a ratio greater than 4.0 was considered harmful to human wellbeing (Andrade et al., 1995). In the present study, n–3/n–6 ratio is greater than 2 and the n–6/n–3 ratio is lower than 1. Therefore, M. stultorum can be considered as part of a healthy diet.

In the light of these results, Mactra stultorum was shown to be a good source for some important fatty acids such as high levels of n-3 PUFA, EPA and DHA. The seasonal variation in fatty acid composition showed a difference among the three studied organs of Mactra stultorum (foot, mantle and adductor muscle). Moreover, spring was considered a remarkable season which was characterized by higher fatty acid levels due to the food availability in the natural environment. According to our results Mactra stultorum may be considered a good resource for manufacturing animal foodstuff and breeding.