Omega-3 polyunsaturated fatty acids (PUFA) including α-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) play important roles in our body as components of phospholipid (PL), which forms cell membranes, and eicosanoids. Eicosanoids are locally acting signaling molecules that control inflammation, vasoconstriction, and platelet aggregation. Eicosanoids synthesized from omega-6 PUFA, such as arachidonic acid (AA), are generally more potent mediators than those synthesized from omega-3 PUFA, although there are a few exceptions [
1]. Omega-3 and omega-6 fatty acids generally compete for the same enzymes; especially, EPA and DHA compete with AA for the synthesis of eicosanoids [
2]. Intake of omega-3 PUFA improves the balance of eicosanoids and consequently helps reduce inflammation [
3]. Moreover, omega-3 PUFA has been reported to reduce the risk of cardiac death in several epidemiological and clinical trials, partially due to its anti-inflammatory effects [
4,
5,
6]. Therefore, various recommendations for the daily intake of omega-3 PUFA have been proposed.
Among vegetable oils, flaxseed oil is rich in ALA (49–60%) and contains a modest level of linoleic acid (LA, 12–17%) [
7]. Flaxseed oil is thus one of the richest omega-3 PUFA sources among edible fats and oils. The bioconversion rate of ALA to EPA or DHA in humans is generally poor [
8], reaching only 8% and <0.1%, respectively [
9]. Several factors are thought to influence the bioconversion to EPA and DHA, especially the competition of LA with ALA for Δ6-desaturase, which is a rate-limiting enzyme in the bioconversion of LA to AA, and ALA to EPA and DHA [
10]. Therefore, to reduce the risk of coronary heart disease, intake of EPA and DHA instead of ALA is recommended by the Food and Drug Administration (FDA) and the American Heart Association (AHA) [
11,
12].
EPA and DHA are present in several dietary supplement formulations, including fish oil, krill oil, and algal oil. A typical fish oil supplement consists of about 1000 mg/person/day fish oil, inclusive of 180 mg EPA and 120 mg DHA [
13]. Reports on health-promoting functions of EPA and DHA have led to an increase in the demand for oils containing EPA and DHA. As fish is a restricted resource, there is growing interest in exploring alternative sources of EPA and DHA. From the perspective of fish resource protection, krill oil and algal oil are attractive alternatives. The biomass obtained from krill (
Euphausia superba) is generally between 125 and 750 million metric tons [
14], thus being attractive for commercial harvest. Although krill oil contains both EPA and DHA, their concentration is less than that of anchovy fish oil [
15]. Algal oil, on the other hand, has advantages, such as acceptability to vegetarians and improved palatability and smell, but is relatively expensive [
16]. Previous studies have demonstrated that the hepatopancreas of Japanese giant scallop (
Patinopecten yessoensis) has extremely high omega-3 PUFA content, especially EPA, as compared to typical fish oils [
17,
18]. In Japan, quantities of around 500,000 tons of Japanese giant scallops are landed annually, but there is only about 15% (
w/w) of the adductor muscle, which is the edible part. The internal organs of scallop, such as hepatopancreas, gonads, mantles, and gills, are generally discarded during processing and account for 32,000 and 6000 tons/years in Hokkaido and Aomori, respectively. The global production of
P. yessoensis was 2200 thousand tons in 2016, and their hepatopancreas were discarded [
19]. Thus, the abundance of scallop internal organs makes them attractive alternative sources of omega-3 PUFA. However, in particular, the scallop hepatopancreas contains large amounts of toxic components, such as arsenic (As) compounds, cadmium (Cd), and sometimes diarrhetic shellfish toxin (DST) [
20,
21]. For this reason, the scallop internal organs have not been utilized as sources of EPA and DHA. Recently, we successfully prepared high-quality scallop oil (SCO) that satisfies the specifications for use as food from the scallop internal organs, by removing toxic components, including As compounds, Cd, and DST [
22]. Omega-3 PUFA accounts for 40% of the fatty acid composition of SCO, which is much higher than that of fish oil and krill oil. In this study, we prepared SCO using the previously described method [
22] from the scallop internal organs obtained from two different processing areas and referred to them as SCO-M (SCO from Mutsu bay, Aomori, Japan) and SCO-U (SCO from Uchiura bay, Hokkaido, Japan). Although SCO-M and SCO-U are novel alternative sources of EPA and DHA, the safeties of SCO-M and SCO-U are uncertain due to the presence of toxic substances in hepatopancreas. Our previous study demonstrated that SCO-M and SCO-U did not exhibit genotoxicity in in vitro (Ames test) and in vivo (micronucleus test) studies [
22]. However, no acute and sub-acute toxicity studies have been carried out in animal models. Therefore, further safety assessments of SCO-M and SCO-U are required for their use as food ingredients or supplements. In this study, the safeties of SCO-M and SCO-U were assessed in ICR mice by single oral dose toxicity test and repeated oral dose toxicity studies for 28 days and 13 weeks. In the repeated oral dose toxicity studies, the effects of tuna oil, which is already available in the market, were also investigated.