Environmental salinity and dietary lipid nutrition strategy: Effects on flesh quality of the marine euryhaline crab Scylla paramamosain

Highlights

• Low salinity is new used in mud crab culture to expand the inland saline aquaculture.

• Dietary lipid source, but not salinity, influenced nutritional values of flesh of mud crab.

• Low salinity changed the myofiber structure and provided new texture for consumers.

• Low salinity and dietary soybean oil weakened the umami taste and aroma of crab flesh.

Abstract

The quality of crustaceans’ flesh has direct impact on consumers’ purchase choices, with water environment and dietary nutrition being effective ways to regulate flesh quality. The aim of present study was to investigate the impacts of water salinity (low, 4 and medium, 23) and dietary lipid source (fish oil and soybean oil) on nutritional values, texture, taste and odor of flesh of mud crab. While water salinity had no significant influence on nutritional values of crab flesh, crabs fed soybean oil displayed significantly lower contents of amino acids and n-3 PUFAs in muscle. However, crabs reared at low salinity showed reduced flesh hardness, chewiness and gumminess likely related to altered myofiber structure, that impacted muscle texture. Furthermore, low salinity and dietary soybean oil weakened umami taste and aroma characteristics of crab flesh associated with decreased contents of free amino acids, flavor nucleotides, inorganic ions and odor active compounds in flesh.

Introduction

Flesh quality, a complex trait crucial for aquatic products, is determined not only by species and genetic attributes, but is also greatly affected by environmental factors and feed in aquaculture (Cheng et al., 2020). As one of the most important environmental factors, salinity of seawater profoundly influences the physical and chemical properties of crustaceans, impacting flesh quality (Chen et al., 2014, Li et al., 2017). Due to the increasing frequency of typhoons, heavy rains or other types of extreme weather events related to global climate change, both short- and long-term alterations in seawater salinity are unavoidable, particularly in estuarine and coastal areas (IPCC, 2013). In addition, aquaculture production of euryhaline crustaceans has expanded recently from the coastal environment to inland regions that have a supply of underground low saline water or saline-alkali land (water salinity around 1–6) in order to take the advantage of the cleaner environment and increasing economic development in these regions (Li et al., 2017). However, the change of ambient salinity affects the physiological responses of euryhaline crustaceans and, thus, low salinity has become a problem that intensive farming of coastal crustaceans must address (Li et al., 2017). Previous studies have largely focused on the effects of salinity on growth, development, reproduction, osmoregulation and energy metabolism in order to improve production performance of commercially important crustaceans (Thabet, Ayadi, Koken, & Leignel, 2017). In contrast, studies on the influence of salinity on flesh quality of farmed crustaceans are scarce and mostly restricted to assessing impacts on chemical composition (Chen et al., 2014, Wu et al., 2019, Zhang et al., 2021).

Fish oil, with a high content of n-3 long-chain polyunsaturated fatty acids (LC-PUFA, ≥ C₂₀), especially eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), has traditionally been the most important lipid source used in commercial feeds for most aquatic species owing to its effective supply of energy and essential fatty acids (EFA) (Tocher, Betancor, Sprague, Olsen, & Napier, 2019). However, the global supply of fish oil is finite and, therefore, the rapid expansion of aquaculture in the last decades and competition with other industries (e.g. nutraceuticals) has prompted increasing interest in exploring the use of alternative lipid sources in aquafeed formulations (Shepherd, Monroig, &Tocher, 2017). Soybean oil, with high and stable production, moderate price and generally low content of harmful substances, can bring economic benefits and has been widely used as a lipid source in aquaculture feeds (Turchini, Ng, & Tocher, 2011). While soybean oil can supply energy effectively, it is rich in linoleic acid (18:2n-6), but has low n-3 PUFA and is devoid of EPA and DHA resulting in a low and imbalanced n-3/n-6 PUFA ratio that can result in several nutritional issues (Turchini, Ng, & Tocher, 2011). To address this problem, many studies have investigated the effects of different dietary lipid sources on growth, physiology, metabolism, intestinal health and overall health status of aquatic animals (NRC, 2011, Turchini et al., 2011). In addition, dietary lipid influences the chemical composition of edible portions like muscle of aquatic animals including crustaceans such as Chinese mitten crab (Eriocheir sinensis) and swimming crab (Portunus trituberculatus) (Tu et al., 2020, Wu et al., 2019, Yuan et al., 2020).

The quality of aquatic products directly influences consumer tendencies and consumption, which then determine the final commercial value of farmed aquatic animals, with the overall quality defined by a combination of both nutritional value and sensory qualities (Grigorakis, 2007). Nutritional value is determined by the content and composition of biochemical components include protein, lipid, amino acids, fatty acids, vitamins and minerals (Yuan et al., 2020), while sensory qualities are usually evaluated by parameters including color, texture, taste and smell/odor (Grigorakis, 2007, Song et al., 2020, Tang et al., 2020, Yuan et al., 2020). Taste is the result of the complex interaction of non-volatile substances including free amino acids, flavor nucleotides, inorganic ions, betaine, free sugars and organic acids, and taste receptors during the tasting process (Tu et al., 2020). The odor components of flesh products include mainly volatile substances such as aldehydes, ketones, esters and alkanes among others (Wu, Fu, et al., 2019). However, few studies have investigated the combined effects of environmental factors in combination with diet on flesh quality of farmed aquatic animals. In particular, detailed information on the impact of interaction between salinity and dietary lipid on flesh quality of crustaceans is scarce.

In consideration of the practical needs of aquaculture production, we herein investigated the impacts that ambient salinity and dietary lipid source have on the nutritional quality of a major crustacean species in China, the mud crab Scylla paramamosain. Due to its delicious flavor and high nutritional value, mud crab has become one of the most important crustacean species farmed in Asia, and is currently the marine crab species with greatest commercial production in China (FAO, 2011, Li et al., 2018). Important physiological features that have contributed to expansion of mud crab farming include its tolerance to a wide range of salinities and its capacity to utilize a variety of dietary lipid sources (Li et al., 2018). Therefore, the overarching aim of the present study was to characterize flesh quality in mud crab subjected to different culture regimes combining different ambient salinities and dietary lipid sources. The results of the present study will provide vital insight to the feasibility of low salinity culture and dietary fish oil replacement in mud crab from the perspective of flesh quality.