1. Introduction
The nutritional contribution of crustaceans to their larvae can significantly influence the profile of nutritional parameters, such as lipids, at hatching and during the first days of life [
1]. Lipids are transferred from the ovary to the eggs; therefore, lipid content in eggs can be used to infer the physiological status of breeding females [
2,
3]. Lipids represent one of the most common energy sources in animal cells [
1], which are stored in the form of fatty acids (triacylglycerol). Another important source of energy is glucose, stored as glycogen. Free carbohydrates, in the form of glucose, play a key role in energy metabolism [
4]. Glucose is highly mobilized and metabolized because, among other things, a large amount of it is needed after molting to produce the exoskeleton [
5]. Proteins, on the other hand, are essential metabolites for the production of hormones and tissues and provide structure and support to the cells [
6].
A diet rich in lipids and protein is essential to stimulate sexual maturation and copulation activity in crustaceans [
7]. The ideal protein level varies in ranges between 23 and 60% depending on factors such as age, feeding strategies, quality, protein source and energy levels [
8]. On the other hand, biochemical changes that occur in larvae during feeding and starvation are indicators of their nutritional requirements and are an important basis for determining appropriate diets during larval culture [
9].
Prawns, also called river shrimp or freshwater shrimp, are freshwater crustaceans valued for their nutritional properties and good taste. Worldwide, research regarding nutritional requirements and reproduction in prawns focuses on
M. rosenbergii, especially females, because it is the most cultivated species globally and economically important. In this species, it is reported that diets with protein levels of 42% and lipids of 9% promote reproductive performance, egg composition, hatching efficiency and larval survival compared to a commercial diet [
10,
11]. Regarding Latin American species of prawns, in a study with
M. carcinus broodstock where different experimental formulations with variation in proteins and lipids were evaluated, greater growth was reported for a diet with 13% lipids and a greater gain in muscle protein for diets with 40 to 45% protein [
12]. In
M. acanthurus, a lipid inclusion of between 15 and 17.5% in the diet of females could be optimal for egg maturation and production and increase their protein and lipid content [
13]. In
M. amazonicum, it is reported that a diet containing 35% protein optimizes the use of energy channeled to growth and minimizes its loss through excretion, which is why it is considered a promising protein level for a diet in this species [
14]. Meanwhile, for
M. tenellum, a protein requirement below 35% is suggested for maturation, gonadal development and spawning [
15].
One of the most important shrimp species in the Mexican Pacific is
Macrobrachium tenellum, due to its importance as a food resource, its local commercial value and its cultural importance [
16,
17]. At present, adequate protocols and technologies have not been established to develop aquaculture models and commercial-scale production of
M. tenellum [
18]. The lack of postlarvae production of this and other shrimp species is often compensated for by obtaining seed from the wild, but this practice, together with overfishing, could have a devastating impact on the natural populations of this shrimp [
19]. To address this problem, several researchers have focused on studying the nutritional requirements in relation to reproduction in freshwater prawns [
10,
11,
15], as well as biochemical profiles of their eggs and tissues [
20]. Despite this, information is still limited and scarce, especially in the specific case of
M. tenellum.
Considering that M. tenellum is a native species and that it has attractive characteristics for potential cultivation, it is extremely important to identify its nutritional requirements and to know its biochemical composition. The present study evaluated the effect of feeding different levels of proteins and lipids in M. tenellum females in relation to the biochemical composition of their eggs, larvae, gonad and hepatopancreas. The results obtained provide valuable information to increase knowledge of the elaboration of diets that cover the energetic demands to improve the reproductive quality and production of this species. The data on biochemical components also provide an insight into the metabolic processes and nutritional needs during the early stages of development.
4. Discussion
The range of the overall biochemical composition, taking into account the results of the estimations of all diets, were, in eggs, TP 11 to 18 µg/egg, TC 1.5 to 3 µg/egg and TL 3.5 to 7 µg/egg; in larvae, TP 515 to 545 µg/mg, TC 32 to 65 µg/mg and TL 55 to 88 µg/mg; in the gonad, TP 413 to 570 µg/mg, TC 7.5 to 31 µg/mg and TL 198 to 245 µg/mg; and in the hepatopancreas, TP 258 to 619 µg/mg, TC 16 to 84 µg/mg and TL 142 to 507 µg/mg. A variation in TP composition was observed in eggs, larvae and tissues; this has already been documented in muscle and body composition (whole organism) of juvenile and adult marine crabs [
30,
31,
32] and the prawns
Macrobrachium acanthurus and
M. nipponense [
13,
33]. The same situation occurred with TC, and in this sense, intraspecific variations of this macronutrient are reported in the muscle of other decapods, such as
Charybdis smithii [
34],
Scylla serrata [
30] and
Scylla tranquebarica [
32]. TL was not the exception, it were found to be present in a different range of proportions, as reported in the muscle of the crabs
Portunus trituberculatus [
35] and
S. tranquebarica [
32] and the whole body of the prawns
Macrobrachium rosenbergii [
10,
36],
M. acanthurus [
13] and
M. nipponense [
33]. The above demonstrates that decapods can manifest different proportions of proteins, lipids and carbohydrates in their tissues, structures and organs, even if they are of the same species. It is known that the biochemical composition of aquatic animals may vary according to season, animal size, maturity stage, temperature, genetic factors, level of domestication and availability and quality of food [
32,
37].
This study showed that lipid and protein levels in diets fed to
M. tenellum broodstock affect the biochemical composition of their eggs, larvae, gonad and hepatopancreas. It has been shown that in some decapods the type of food and protein levels in the diets influence the body protein content [
32], as is the case of cultured penaeid shrimp, since they are organisms that have the quality of using protein as a source of energy [
38]. In
Macrobrachium americanum, diets with different lipid and protein composition influence TP, TL and TC content [
39] and lipid content increases when the lipid level in the diet increases from 6 to 14%. On the other hand, in
M. amazonicum, the level of protein in body composition is reduced when dietary protein increases from 20 to 35% [
40].
In the eggs of
M. tenellum at the intermediate stage of development, the major organic component is protein, followed by lipids and, to a lesser extent, carbohydrates. Similar proportions have been reported for the eggs of other crustaceans [
41,
42,
43,
44,
45] and congeneric species [
20,
46]. Proteins are usually the major component of crustacean eggs, as they are essential for tissue synthesis and serve as a source of energy [
47,
48]; it is known that the continuous synthesis and degradation of proteins for morphogenesis processes constantly produce variations in their biochemical composition [
49,
50]. It is typically reported that carbohydrates are the minority component of decapod eggs [
51,
52]; despite this, they play a key role in metabolic processes and are the main source of energy in the early stage of embryonic development [
4,
46,
49]. In turn, lipids serve as a source of metabolic energy for early tissue formation [
13,
46,
49]. Considering the above, a higher content of proteins, lipids and carbohydrates in eggs could translate into higher egg quality, which would be advantageous for their development and survival under variable environmental conditions.
In the present study, it was demonstrated that lipid and protein levels influence the TP and TL of eggs; within the same lipid levels, TP composition increases when protein in the diets is higher than 35%. For
M. acanthurus, it is reported that the type of protein and lipids in the diets affect the proximal composition of the egg [
53]; this composition is similar to that found in the present study in terms of TP and TL. In contrast, it was also found that for
M. acanthurus, the egg protein content is not affected by lipid levels [
13], which differs with
M. tenellum. These differences may be due to the specific environmental and feeding conditions for each study, as these variables may affect lipid and protein content to a lesser or greater extent [
54,
55].
Diets with different proportions of lipids and proteins do not affect the TP composition of newly hatched larvae of
M. tenellum, but they do affect the composition of TC and TL. It was also shown that the level of protein affects TL, but no relationship was found in the lipid–protein combination. As in eggs, protein is the macronutrient with the highest fraction in larval composition, followed by lipids and finally, carbohydrates. The reason that proteins are noticeably higher than lipids may be because protein catabolism may be more important in the early stages of larval development, while lipid catabolism is a more important energy source for the later stages [
56]. The protein composition of larvae in this study is notably higher than reported for spiny lobster (
Jasus edwardsii), which is 31%, but similar in TC [
1]. The lipid proportions found in spiny lobster larvae range from 7 to 12% [
1,
57,
58] and 12% in prawns
(M. amazonicum) fed L8P43 diets [
59]; these percentages are higher than the TL ranges estimated in the present study. In contrast, the percentage of TP found in
M. amazonicum [
59] is notably lower than that reported in
M tenellum. This study demonstrates that the contribution of the broodstock to the newly hatched larvae through the egg can significantly influence their nutritional parameters, which coincides with what has been reported in other investigations [
1,
10]. Thus, the quality and quantity of nutrients remaining after embryonic development will have a great impact on larval development and survival, as newly hatched larvae may rely on yolk energy reserves in case food is not immediately available [
4,
60,
61].
The biochemical composition of the gonad followed the same proportion of eggs and larvae, although it is noteworthy that a notoriously higher percentage of TL was found. Similar fractions in TP, TC and TL have been reported in other decapod crustaceans [
62,
63,
64]. The reason that proteins are the major component in this tissue may be due to the fact that intense protein synthesis occurs during vitellogenesis and oocyte development [
65,
66], leading to an increase in protein content [
22,
67]. The reason that TL is higher than in eggs and larvae may be because generating the energy reserves necessary for embryonic development requires high energy expenditure [
68]; lipids and, to a lesser extent, carbohydrates will be responsible for covering the energy requirements during ovarian maturation [
66,
67].
The effect of dietary lipids on the biochemical composition of the ovary has been documented in some decapods. In females of
Cherax quadricarinatus fed L6P38 diets, values of 47% TL and 33% TP were found [
69], while in
Eriocheir sinensis fed L8P38 diets, TL fractions of 26% and TP of 58.5% were found [
64]; in both species, TL was higher than in
M. tenellum. Studies with the shrimp
M. rosenbergii report variations in TP as a function of L8 to L10 levels in the diets [
10,
36], which differs from the present study. In
M. acanthurus, there is a lower concentration of TP and a higher concentration of TL when dietary lipids are higher, and values of 14 to 15% in TL and 53 to 65% in TP were found in diets with L10 to L12 [
13], which indicates that in this species, lipid levels have a clear effect on the biochemical composition of the gonad, contrary to what was found in this study.
The hepatopancreas was the organ that presented the most marked differences in the effect of lipid levels on the composition of TP and TL. Higher TP values were found in L4, while the highest TL means belonged to the diets with L12. This coincides with what was reported for
M. acanthurus where it was shown that there is a tendency for proteins to decrease in the hepatopancreas when lipid levels increase in the diets [
13]. Generally, the proportions of TP and TL in decapods range from 15 to 43% and 45 to 67%, respectively [
35,
36,
55,
64], similar to that reported in the present study. The cause of an increase in the proportion of lipid composition compared with other tissues may be due to the fact that this is the organ where, among other functions, lipid absorption, secretion, and metabolism are performed.