Interactions Between Food, Feeding and Diets in Crustaceans: A Review

There are several literatures that cover different views of crustacean food, feeding and behavior aspects, but little was known on its interaction between them and it's also shown a different perspective. Thus, a better understanding of the interactions between food, feeding and diets in crustaceans is vital for developing better quality of seed or broodstock produced in hatchery and its adaptation to the aquaculture environment and system. The aim of the present review is to update the state of the art and to explicit the knowledge regarding food, feeding and diets in crustaceans and challenges and opportunities in the development of formulated diets.


Introduction
Interactions between food types, feeding behavior and formulated diets in the crustacean are important to improve the seed production and enhance fundamental knowledge of the cultured animals [1]. The development of pellet diets or aquafeed for aquaculture species has gained much interest nowadays as pellets offer many advantages compared to the natural feeds. In terms of nutrient content, artificial feed offers a nutritionally balanced diet with known nutrient content such as total lipid and protein that will promote growth and reproduction in the crustacean. By manipulating the level of protein and lipid important for growth and reproduction on crustacean's group, the formulated feed could provide sufficient nutrition to broodstock. Initially, no commercial broodstock diet was available, for some crustacean species. Most formulated feeds are focused on the larval stages. There are not much published studies focused on formulating a feeding diet, particularly to some crustacean broodstock. This is probably because of the unique habitat characteristics which respond differently to different environment conditions [2]. Several species are restricted to certain environments and aquaculture systems that give effect to the type of feeding selection [3][4][5][6].
The growing demand for animal protein from existing competition between human need and aquaculture feeding resulted in the decreasing of fish landings since fish are the sole provider of n-3 PUFA in the diet. This adds to the existing gap between the demand and the supply for fish and fish products. Regarding this issue, many studies have covered some adjustments to the feeding formulation by not depending too much from fish source as the source of protein and lipid. As a replacement to fish oil and fishmeal, the use of terrestrial animal proteins such as meat, poultry by-product, bone meal, and blood meal have confirmed their efficiencies in providing the animals with good protein level [7].
Besides that, the use of protein sources from plant-based materials are getting more attention nowadays. Plant protein sources such as camelina meal, canola meal, and soybean meal can be used as a substitute to fish meal without imposing negative effects to the growth and feed intake [8]. However, the major setbacks lie with the use of protein source from the terrestrial animals and plant origin include the lack attractants and palatability factors [9]. Compared to aquatic animals such as fishmeal, shrimp meal, and squid meal, the lacking of attractants component may result in poor ingestion of feeds thus reducing the rate of feed intake which consequently will retard growth in the animals [10].
In general, the physical form of the pellets depends on the species being cultured. High moisture contents in the pellets are often associated with nutrient leaching since it dissociates easily upon entering the water. Apparently, low pellet stability and durability resulting from high moisture content may not be suitable for the crustacean, which some species are aggressive in handling foods [11]. In addition to that, the proper storage and handlings of the final products are difficult to manage especially for the wet pellet. Since the wet pellets have high moisture content, having a prompt spoilage such as mold problems due to long storage period is unavoidable.
In this review, we attempt to identify the interaction between food types, feeding behavior and formulated diets in crustacean, by summarizing all the available information on the topic using the title of the review as keywords in the Web of Science Core Collection database. Table 1 summarized the crustacean's feed types with pellet and animal performances. Most studies on feeding of crustacean was done at the juvenile stage, especially for shrimp, crayfish, and crabs, meanwhile lobster and prawn (and some on crab), most studies focused on the adult or broodstock stages (Table 2). There are several types of feed for broodstock used in the hatchery for commercial purposes: the wet feed, dry feed, semimoist feed, and the moist feed. These feeds can be differentiated from each other in terms of moisture content where the moisture levels in each feed falls in the range of 45 -70%, 7 -13%, 25 -45% , 15 -25% for wet feeds, dry feeds, moist feeds, and semi-moist feeds respectively [12]. At the same time, water activity (aW) in the pellet defines better protection against bacterial growth where lower aW in the pellets are preferable. The aW differs from the moisture content where aW is defined as the ratio between the vapor pressure of the food in a completely undisturbed balance by the surrounding air media with the vapor pressure of distilled water under identical conditions. In most cases, pellets with aW of lower than 0.79 inhibits the growth of yeast whereas aW of lower than 0.65 successfully stops mold growth [13].Wet, moist and semi-moist diets are more effective in terms of promoting good growth and feed efficiency owing to their soft texture and palatability. In this review, only two basic types will be considered for intensive farming; the dry feeds and the moist feeds (semi-moist will be included as it falls under the same category with moist feeds).   [14] *N/A: Not available

Dry pellet
The use of dry pellets can be in variety form; the dry-sinking pellet, extruded sinking pellet, and extruded floating pellet. The suitable feed ingredients selection with proper manufacturing procedures such as the extrusion process ensure good water stability which is the main criteria in producing good feeds. The extrusion method is different from the steam pellet in a way that the extruder does not use any pellet binder to add adhesion to the particles. Extruded pellets are more brittle where they only expand through gelatinization of starch upon cook [51]. During the gelatinization process, the starch becomes activated and absorbs large volumes of water. Tuber starches such as potato and tapioca are popularly used as binding agents since they are high in amylase enzyme [52].
Overall, the production of dry-sinking pellets is more practical for bottom feeders such as crustaceans, particularly prawns, lobsters, and crabs. Among the necessary steps in the formulation of water-stable dry pellets include the use of good binding agents plus finely ground ingredients to ensure maximum adhesion of the binder molecules. Whereas, the application of extruded floating pellets is more suitable for fish species which predominantly feed in the water column such as tilapia, trout, grouper, sea bass, and carp. The use of floating pellets allows observation on feeding activity other than the fish well-being [53].

Moist pellet
Moist pellets or wet pellets consist of a combination of high moisture ingredients and dry pulverized ingredients. The use of moist feed is widely accepted among the aquaculture's practitioners for maturation of broodstocks [36]. Regardless of their acceptance to use in the hatcheries, no commercialization of moist feeds has been produced presently. Due to their high moisture content, the moist pellets have low water stability and are prone to mold problems. Meanwhile, the innovation of semi-moist pellets has been successfully developed at laboratory scale. Compared to moist pellets, the moisture content of semi-moist pellets is under the permissible level with the addition of chemical agents to avoid yeast and mold growth.

Palatability and attractability
Optimization of feed intake is determined by a good physical attribution of the pellet which includes the palatability and acceptability of the animals towards the feed, considering the species behavior and their physiological requirements as well [32]. Priority is given in ensuring that the nutrient is reached to the animal with minimum leaching. Absence of attractants and palatability features in the pellets resulted in the declining feed consumption hence resulted in the poor growth in the crabs. The palatability and the attractability of the feeds are thus necessary which will lead to good ingestion and utilization of the prepared nutrients. Palatability is defined as the acceptance of the animals towards the food, resulting in the increasing of the body weight whereas attractability involves the animal's orientation towards the presence of, one of the two feeds that have been offered [20].
Both palatability and attractability have become a primary factor in the development of cost-effective feed since animals have great sense of smell, taste and sight to search for food. Both physical features ensure higher feeding rates in the animals. Diets of low-palatability and attractability will result in crabs not being able to reach optimum nutritional requirements. Good palatability is determined through the feed intake [54] and low food conversion ratio (FCR) as indicator to the efficiency of the feed or feeding strategy [55]. A good attribution in the pellet such as strong smell and good binding factor will help the crabs find the pellet as well as reducing the risk of nutrient loss due to leaching problems. Insufficient levels of attractants factors can result in low feed intake which eventually resulted in poor growth of the organisms.

Type of binder
Aquatic feed formulation involves the use of good quality binding agents as the primary ingredient that help in stabilizing feed during exposure to water and at the same time enhance feed floatation time [56]. Vast binders have been used while formulating high durability pellets to increase the water stability and minimize nutrient leach by adding cohesion to the particles and reducing the void spaces. This includes agar, starch, gelatine, carrageenan, and carboxymethylcellulose (CMC). Good binder selection with correct inclusion level in the diet formulation will determine the overall pellet performance against nutrient leaching, water stability and turbidity of the ponds. Practically, binders that can be digested and assimilated are chosen. Polysaccharides such as starch plays an important role in the aquafeed development in providing the animals with necessary carbohydrates as well as a binder responsible in the adhesion of the feed components. Extruded pellets depend on the gelatinization in the starch since no binders are used in the formulation. Starch such as maize, millet, guinea corn, wheat, and cassava improve the pellet durability, contain high protein level and make a good binder in the extruded feed pellets [8]. These types of binder are capable of generating air traps in the formulated feeds thus improving the physical integrity of the feeds in the water.
On the other hand, the use of unbranched polysaccharide from the seaweeds such as the agar, sodium alginate, and carrageenan have been widely applied in the field of aquaculture nutrition, mainly as binder. Ruscoe et al. [32] compared the use of carrageenan, CMC, agar, and gelatin as the binders at different concentration in freshwater crayfish and concluded that carrageenan and CMC at 5% concentration were significantly better than both agar and gelatin. Meanwhile, research carried out by Paolucci et al. [57] regarded that agar performed better compared to both sodium alginate and carrageenan during feed manufacturing. Agar is usually activated when heated up to 80 -85°C and the binding of feed components generally begin once the solution cools down to gelling temperature of range 32 -43°C [57].

Water stability and durability
Compared to fish pellets, the pellet disintegration and nutrient leaching in crustacean pellets require more attention because of their nature as a benthic organism and a slow feeder [19]. Physical features such as the pellet stability and durability especially are more critical than other species where larger pellet sizes are used [32] with longer soaking hour and the least possible leaching of nutrients [58]. It is suggested that the crustacean pellets must maintain a minimum of 90% dry matter retention even after 1 hour exposure in water, thus the use of dry pellets is not suitable as it does not solve the nutrient leaching problems [59]. Crustaceans especially crabs and crayfish are very robust in terms of handling foods using their cheliped and the mouth appendages to grasp and break up the food to smaller bites prior to ingestion [32,34] in which sinking and water stable pellets are necessary.
The pellet water stability is defined as the ability of the pellet to retain its integrity and nutrients while in water until consumed by the animal [19]. Meanwhile, durability is defined as the ability of the pellet to maintain its shape while handling, transportation, and inflatable transmission, without breaking it to smaller particles [51]. In aquatic pellets, the stability of pellets while in water is determined by the type of binding agent that holds the pellet together. Good water stability in pellets defines its effectiveness in optimizing feed intake in the crabs from harsh handlings and vigorous mastication so the nutrients required for growth will be fulfilled. Internal factor such as slow feeding rate and external factors such as high water currents and strong aeration in the tank will accelerate pellet disintegration which can result in the nutrient leach [32,51] and consequently increase the water clarity and turbidity from the suspended materials. The use of binders helps in holding the feed components together, minimizing the void spaces, maintaining pellet integrity thus producing a more compact and durable pellet [32].

Buoyancy
For fish, floating feed is fundamental for optimum feed intake since fish are fast swimmers and naturally eat at the water column [60]. The use of different ingredients combinations particularly the binder agent exhibits greater pellet characteristics such as pellet buoyancy, good water stability, digestibility, minimum wastage of raw materials, as well as low water pollution [61]. Fish feeds specifically require good binding agents that will help in stabilizing the feed and prolong the feed floatation period when in water while maintaining its nutritional value. Sinking of the uneaten feeds to the bottoms of the pond as a result of short floatation time will eventually deteriorate the water quality and might end up as fertilizer which triggers the algal blooms from the high nutrient inputs [62]. Plus, additional cost may be incurred while maintaining good water quality due to low feed performance [63]. The good binding agents contribute to minimum wastage and provide the fish with optimum nutrient utilization [56]. The use of floating feeds are advantageous as they help the farmer to closely observe the feeding activity of the fish and the uneaten feed can be discarded immediately thus preventing the low water quality problems since they afloat at the water column [64].
Unlike fish, for a bottom feeder and slow eater particularly, the long-term sinking pellets are preferable, characterized by a less expanded structure and high densities. Compared to floatation fish feeds, the sinking pellets offer longer time taken to float to suit the slow, bottom feeder such as the crayfish [32], shrimp [65,66] and mud crabs [36,67]. For these reasons, the moist or dry sinking pellets are more appropriate since they are high in density compared to the floating pellets. Experiments on soft shell portunid crabs observed that crabs having a hard time grasping the floating feed with their claws signifying that the sinking pellet would be more appropriate [67]. Table 3 shows the macro and micronutrients at different crustacean life stages. The main group of nutrients in crustacean related diet studies are protein, carbohydrates and lipids considered as macronutrients, meanwhile vitamin, minerals and feed additives are the micronutrients group (Table 3). Nutrition plays an important role in the development of ovaries [68]. Although some crustacean species can survive a period of starvation due to insufficient food supply; either from the hatchery of wild, more lipid reserves are used to sustain energy metabolic functions thus retards growth and reproduction activities in the crustacean [69]. Selectivity of feed in the aquaculture will determine the time taken for the crustacean to reach sexual maturation. Lipid and protein are described to be the most important component of the nutrient classes that act as main source of nutrient for embryonic development [70].

Protein requirement for crustacean broodstock
Protein is one of macronutrients in crustacean feed ingredients that takes part in promoting growth, fattening, and reproduction of aquatic animals. Optimum protein levels are especially important in juvenile crabs since they actively grow through molting activities. Inadequate amount of protein supplies hinders growth [71], sometimes causes mortalities especially in the juvenile crabs from the prolonged intermolt period [41]. Yet, dietary protein surplus results in water deterioration from degradation of protein leftovers to form ammonia or urea [72]. Hence, information on the dietary protein requirement is of vital importance to ensure good growth and maturation. Many investigations have been carried out to determine the protein requirement in different crustaceans such as prawns, shrimps, swimming crabs, and mud crabs. The results showed that the protein requirements are species specific, ranging from 22% -60% [37,41,72] where the dietary protein intake in juvenile or early life stage of animals are usually higher compared to the matured animals for most crustacean species.

Lipid requirement for crustacean broodstock
Lipid encompasses various classes of organic molecules such as triacylglycerols, phospholipids, sterols, waxes, carotenoids, and fatty acids [10]. Lipids, along with proteins and carbohydrates share the same importance in terms of providing the body with energy. Lipid differs from protein and carbohydrates in a way that it provides energy twice than both proteins and carbohydrates, serve as the structural components of cell membranes, and as important signaling molecules [75]. Neutral lipids, particularly triacylglycerols or also known as triglycerides are the principal form of energy source found in the adult, egg, and larvae of most crustaceans [50]. The phospholipids primarily functions in the building of the cell membrane [76], whereas, the cholesterol is the best known sterols that serves as a precursor of physiological components including sex hormones, particularly ecdysone that regulates molting activities in the crustaceans [43,77]. Feedings containing dietary cholesterols are essential to ensure good growth and survival in the crabs. Fatty acids govern a wide range of physiological processes including the reproductive performance and egg quality in crustaceans [70].
Studies demonstrated that the lipid levels in most crustaceans increased with size where the adults have higher lipids than the juveniles. Some reserved lipids are catabolized as energy while others are stored in the gonad for structural purposes, such as maturation and eicosanoid synthesis [78]. The information on the lipid requirement is very important for the development of formulated feed to ensure the nutrients suffice for good growth and maturation [10]. Previous studies showed that the determination of lipid requirements are generally species specific and are basically different at different developmental stages. Nevertheless, collective studies on crustaceans concluded that optimum growth can be achieved with a total lipid level from 2 -10% [39] or from 2 -12% of diet dry weight [37].
The fatty acids can be further divided into several classes; the saturated fatty acids (SAFA), monounsaturated fatty acid (MUFA), and polyunsaturated fatty acids (PUFA). Fatty acids that have no double bond are grouped as SAFA, while MUFA are categorized as fatty acids that have a single double bond in their carbon chain. Unlike SAFA and MUFA, polyunsaturated fatty acids (PUFA) contain more than one double bond in its carbon backbone. They act as precursors for animal hormones and play an important role in regulating cell membrane. PUFA are commonly generated from the plant synthesis as the primary producers of carbon in marine ecosystems, synthesizing various important biological molecules such as carbohydrates, proteins and lipids [79]. Most animals do not have the ability to synthesize PUFA de novo except for their capacity to convert one form of PUFA to another form through elongation and desaturation. Such fatty acids are termed as the essential fatty acids (EFA) as they must be taken in through the diet. This includes linoleic and linolenic acid since not all animals have the ability to produce them [80].
Meanwhile, the highly unsaturated fatty acids (HUFA) are the subset of PUFA, having 20 or more carbon atoms with 3 or more double bonds. They are responsible for survival, maintaining high growth rates and reproductive rates as well as high food conversion in both marine and freshwater organisms [80]. Arachidonic acid (ARA, C20:4n6), eicosapentaenoic acid (EPA, C20:5n3), docosahexaenoic acid (DHA, C22:6n3) are among the derived omega-6 and omega-3 long chain of HUFA (n-3 and n-6 LC PUFA; C ≥ 20). The consumption of diet containing EPA and DHA help to optimize animal growth [80] while ARA functions as the precursor for eicosanoids that regulates reproductive success and sexual behaviour of females [81]. In general, EPA and DHA can be obtained from the consumption of plant materials or through series of elongation and desaturation of α-linolenic acid (ALA, C18:3n3). Whereas, elongation and desaturation of linoleic acid (LA, C18:2n6) will produce ARA. During the desaturation and elongation process, both n-3 and n-6 PUFA from ALA and LA will compete for the same desaturation enzyme to produce LC-PUFA [82,83].

Conclusions
The importance of pellet physical characteristics in aquaculture nutrition cannot be overemphasized. The advantages of good quality pellets not only depend on the binding agent alone, but the attractants that enhance palatability as well as the inclusion of the correct proportion of nutrients to boost animal performance. The updated knowledge on the possible interactions between food, feeding and diets in crustaceans needs to be improved to increase the quality of seed or broodstock produced in captivity, especially in commercial aquaculture. The studies performed so far have given the clear interaction between food, feeding and diets especially for the crustacean group, such being graphically explained in the graphical abstract.