**1. Introduction**

Bivalve aquaculture is common in coastal areas worldwide and is highly important for food production and ecosystem services [1]. In particular, oysters are a major bivalve aquaculture species; they comprised ~30% of global marine mollusc aquaculture in 2016 [2]. In general, shellfish aquaculture does not require artificial food supplements for the cultured organisms and is considered more environmentally friendly and sustainable than other feeding aquaculture species, such as finfish [3].

Bivalves are a rich source of highly unsaturated fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) [4]. Consuming an adequate amount of these omega-3 fatty acids is important for human health because they have important roles in regulating biological functions [5,6]. These fatty acids are mainly synthesised by aquatic algae and are transferred to humans via the food chain [7,8]. However, due to the increasing demand for these essential fatty acids due to population growth, an estimation of the global supply of EPA and DHA for humans has indicated that adequate amounts of these fatty acids cannot be provided sustainably [9]. Therefore, the provision of EPA and DHA by aquaculture will become more important [4].

Biofouling is one of the most critical issues in suspended bivalve farming and substantially raises the costs of maintaining culture equipment and farming operations [10,11]. The

**Citation:** Fujibayashi, M.; Nishimura, O.; Sakamaki, T. The Negative Relationship between Fouling Organisms and the Content of Eicosapentaenoic Acid and Docosahexaenoic Acid in Cultivated Pacific Oysters, *Crassostrea gigas*. *Mar. Drugs* **2021**, *19*, 369. https://doi.org/ 10.3390/md19070369

Academic Editors: Philippe Soudant and Maria do Rosário Domingues

Received: 19 May 2021 Accepted: 22 June 2021 Published: 25 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

biomass of fouling organisms devalues the products and increases the weight of culture equipment, creating difficulties in maintenance and farming operations [12]. Fouling organisms growing on bivalve shells are generally considered to negatively affect bivalve growth and survival [13,14] by weakening the water movement around the cultured bivalves and reducing the advective influx of food resources to them [12,15,16]. Moreover, numerical modelling studies on carrying capacity in bivalve aquaculture have demonstrated that food limitations owing to the overharvesting of the cultivated species can suppress growth of the cultivated species [17,18]. This implies that dietary competition with fouling suspension-feeding organisms may decrease the growth rate of these organisms. In fact, dietary competition has been highlighted as one of the main mechanisms of negative effects on the growth of cultivated species in laboratory experiments [19]. Since EPA and DHA are obtained from dietary sources, dietary competition may lead to a reduction of EPA and DHA in cultivated species. However, to our knowledge, no study has yet evaluated the effect of fouling organisms on the content of EPA and DHA in cultivated species.

*Crassostrea gigas* is one of the most economically important cultured oyster species in Northeast Asia [20]. Fouling marine organisms on oyster shells comprise various taxonomic groups, such as molluscs, bryozoans, barnacles, sponges, algae, ascidians, hydrozoans, and polychaetes [21–23]. Although the positive effects of fouling organisms on the growth of *C. gigas* have been reported in one case, fouling organisms can negatively affect the growth of *C. gigas* by food and space competition and require additional cost for maintenance to operators [23]. The objective of this study was to examine how fouling organisms affect the content of EPA and DHA in *C. gigas*. Furthermore, body condition, which is expressed as the relative weight of whole soft tissue to shell volume, is considered an important index of the product value of the cultivated oyster [24]. We collected oysters and fouling organisms from oyster farming sites in a temperate bay, located in Northeast Japan, and analysed the fatty acid composition and body condition of the *C. gigas* oysters.

#### **2. Results**

The wet weight of fouling organisms ranged from 567 to 4177 g cluster−<sup>1</sup> (Figure 1), and the mean value was 1795 g cluster−1. The relative weight of fouling organisms by taxonomic group differed between the 15 sampled clusters (Figure 2). The major components were generally sponges and macroalgae, which contributed 3–53% of the total wet weight of the clusters, and the mean value was 29.7%. *M. galloprovincialis* was the second most dominant fouling organism, ranging from 2 to 32% of the total wet weight of the clusters, and the mean value was 14.7%. Other organisms, consisting of mainly polychaetes, cirripedians, and decapod crustaceans, made a minor contribution to the clusters in our study sites (below 0.5%).

**Figure 1.** Total wet weight of fouling organisms of each sample. The values given in parentheses represent collecting water depth.

**Figure 2.** The contribution of each component within the sampled clusters from each sampling site. The "Other" component comprised mainly polychaetes, cirripedians, and decapod crustaceans. The mean values of *Crassostrea gigas*, *Mytilus galloprovincialis*, sponge and algae, and others are 55.5, 14.7, 29.7, and 0.1%, respectively. The values given in parentheses represent collecting water depth.

The condition index and content of EPA in *C. gigas* individuals were negatively correlated with the wet weight of *M. galloprovincialis* (Table 1). Negative correlations were also detected between the DHA content in *C. gigas* individuals and the wet weight of sponges and macroalgae (Table 1). For the relative weight of fouling organisms, *M. galloprovincialis* correlated negatively with EPA content in *C. gigas* individuals and CI (Table 1). Sponges and algae correlated negatively with the total wet weight, EPA with clusters, DHA with individuals, and DHA with clusters of *C. gigas*. The CI of *C. gigas* and *M. galloprovincialis* had a significant positive relationship with the EPA content (Figure 3). In addition, a significant positive relationship between CI and DHA was detected for *C. gigas* (Figure 3). The EPA content in *C. gigas* showed a significant positive relationship with the ratios of palmitoleic acid (16:1ω7) to palmitic acid (16:0) in *C. gigas* (Figure 4).


**Table 1.** The r values of the correlation analysis between cultivated oysters and the main fouling organisms. Spearman rank correlation analysis was applied for *Mytilus galloprovincialis* and Pearson's correlation analysis was applied for sponges and algae.

Bold represents a significant relationship, \*: *p* < 0.05, \*\*: *p* < 0.01.

**Figure 3.** The relationship between the condition index and EPA or DHA content in *Crassostrea gigas* and *Mytilus galloprovincialis*. Each plot is the average of five individuals from each cluster.

**Figure 4.** The correlation of the EPA content and the ratios of palmitoleic acid (16:1ω7) to palmitic acid (16:0) in *Crassostrea gigas*. Each plot is the average of five individuals from each cluster.
