The market share of the crab, Portunus pelagicus, at the international level from year to year tends to increase, with the main markets in Singapore, Japan, the Netherlands, France, USA, Taiwan, Hong Kong, Korea, etc ^{1– 4} . The high market value of, and high demand for, crabs in the global market provide a strong stimulus for their intensive culture. Although the mass production of blue swimming crab seeds was first reported by the Research and Development Center for Oceanology (RDCO)-LIPI in Indonesia ⁵ , its culture industry has not developed as expected. A shortage of seeds, lack of suitable feed, lack of an appropriate culture system and poor management are recognized as the major bottlenecks in the development of the crab culture industry in this country. P. pelagicus seeds were previously caught in the wild, causing an insufficient seed supply for commercial firms and recreational fisheries. Currently, great progress in the artificial propagation of commercially important portunid crabs in the genus Portunus has been achieved and various methods have been adopted for mass-scale hatchery activities. However, farming this species has always been limited by the seasonal availability of spawning crabs from the wild and their inability to mature and spawn in culture ponds. The second major problem associated with the blue swimming crab culture is the high larval mortality during the zoeal and megalopa stages. Thus, it has become necessary to biologically test the supplementation of vitamin E for the formulated diets of broodstocks of females blue swimming crabs, Portunus pelagicus (Linnaeus, 1758).

Molting is the process of the replacement of old shell with new shell, and is a cycle that occurs in all types of arthropods, ranging from insects to crustaceans. It is important for growth, reproduction and metamorphosis ¹³ . Fujaya et al. ¹⁴ explained that the activities of seeding, growth and soft shell crab production become more efficient when the mechanisms of reproduction and growth of the animals in culture are understood. In this experiment, it is clear that an artificial diet at a dose of vitamin E 300 IU/kg (diet 2), 600 IU/kg (diet 3) and 900 IU/kg affects the physiological process of the test animal known as molting (ecdysis) ( Table 1). Fujaya et al. ¹⁴ and Kuballa et al. ¹³ mention that several factors stimulate the process of molting in crustaceans, namely, external information from the environment such as light, temperature and food availability. Studies using vitamin E as a nutritional source for crustaceans have been performed by several previous investigators, including Winestri et al. ¹⁵ on Scylla paramamosain and Nasution et al. ¹⁶ on Macrobrachium rosenbergii. These researchers mentioned that vitamin E-supplemented feed gives the best results for the growth of mangrove crabs and the fecundity of prawns.

The high absolute weight gained in the treatment with diet 2 compared to diet 1 and diet 4 is due to the improvement in nutritional quality by supplementation with tocopherols (such as vitamin E) being balanced with the composition of other nutrients in the composition of the parent artificial diet. Cahu et al. ¹⁷ found that vitamin E plays a role in the improvement of the reproductive performance of crustaceans. The addition of 300 mg/kg vitamin E in the diet of crustacean broodstocks is considered to increase the potential of reproduction that affects the growth of the absolute weight of aquatic animals ¹⁸ . Furthermore, Izqiuerdo et al. ¹⁹ explain that a lack of vitamin E results in the compromised development of reproductive organs toward mature gonads.

The difference in carapace length in those receiving diet 2 from those receiving diets 1, 3 and 4 is significant due to the higher percentage of females that undergo the molting process with diet 2 (80% at day 20 and 20% at day 30) compared to those with diet 1 (0% at day 40), diet 3 (60% at day 20, 20% at day 30), and diet 4 (40% at day 20 and 20% at day 30). According to Fujaya et al. ¹⁴ and Kuballa et al. ³ , the growth and reproduction of crabs and other crustaceans are closely related to the molting cycle phase and control stimulation (ecdysteroids) in its hemolymph. Furthermore, four phases are described in the molting cycle; intermolt, premolt (preparation for molting), ecdysis and postmolt (recovery from molting). During intermolt, exoskeletons formed perfectly, and the crustaceans accumulated stored calcium and energy. Premolt begins when the old exoskeleton begins to separate itself and a new epidermis begins to form. The newly formed exoskeleton is larger and is still pale and soft. Postmolt is the new exoskeleton’s hardening process. The low accretion of mean carapace length of diet 1 (1.00 mm) compared to diets 2, 3 and 4 is due to the crab not experiencing replacement of the skin (molting). The added carapace length is allegedly due to changes in the shape of the carapace. Sulaeman and Hanafi ²⁰ mention that carapace length changes in the female broodstocks that do not undergo molting are caused by changes in the curvature of the back shell margin, where the eggs mature making the rear carapace increasingly convex. According to the results reported for observations of the mud crab, S. serrata, accretion is obtained when the carapace length ranges between 0 and 3 mm during a maintenance period of 35 days.

The low-growth carapace width of the female parent in those receiving diet 1 compared with the other diets happened because the treatment did not induce molting in the maintenance period from 0 to 40 days. The increasing carapace width is supposed to have occurred because of the shape changes in the carapace, which can be seen in the change in curvature of the back of the carapace. The greater the weight of the parent crab, the more convex the carapace became. The absolute changes in carapace width are relatively large in the artificial diet treatment with a dose of 300 IU vitamin E (diet 1). The diet treatment causes the broodstock females to undergo molting. Maheswarudu et al. ²⁰ and Kuballa et al. ¹³ state that the differences in growth in the cultivation of crabs are caused by several factors, such as feeding, age, baseline weight, space and other factors. Furthermore, they explains that the more feed consumed, the larger the crab will become and the more frequent the replacement of the skin. Crab shell replacement last occurred between 17 and 26 days, and every crab molted, and increased by one-third of its original size. According to Fujaya et al. ¹⁴ and Kuballa et al. ¹³ , the molting process that occurs in crustaceans in principle is caused by two factors: internal and external (such as feed, light and other factors). Thus, both factors will affect the brain and stimulate the Y organs to produce hormone molting. Molting is controlled by a steroid circulating in the exoskeleton hemolymph that stimulates the synthesis of new and the regeneration of integument lost before molting ¹³ . Ecdysteroid is a crab molting hormone, which is secreted by the organs in the form of ecdysone-Y ²¹ . Inside the hemolymph, the hormone is converted into an active hormone, 20-OH-ecdysone hydroxylase, which is present in the epidermis organs and other body tissues ^{13, 21} .

Water quality is one of the limiting factors in the crab culture system. The crab is active entirely in water, where it carries out functions such as respiration, excretion of wastes, feeding habits, growth and reproduction. According to Habashy and Hassan ²² , the required water quality parameters for the maintenance of crustaceans are salinity, temperature and pH ²³ . Salinity affects the osmotic pressure of the water. The higher the salinity, the greater the osmotic pressure ²³ . For the cells of all animal organs to function properly, the cells must receive a liquid medium with the appropriate composition and concentration of ions. It appears that like other marine invertebrates, a marine crustacean living in water that is isotonic with the blood has a water medium and generally has a similar ionic composition, but this differs slightly; a large difference can even occur between the ionic composition of the fluid in the cell (haemolymph) with the sea water medium. Therefore, osmoregulation is required to ensure that the intracellular and extracellular composition and concentration of the ionic liquid remains normal or balanced ^{21, 24– 26} . Additionally, Stiarto et al. ²⁷ and Silvia et al. ²⁸ reported that hyperosmoregulation in crustaceans requires energy in the form of protein or lipids ^{29, 30} . Table 6 shows that the water salinity range of the maintenance media during the observation period was between 29.0 and 32.0 ppt. Salinity is still within the range that is highly favorable to the survival and reproductive activities of the crabs ^{17, 31, 32} . Environmental temperature also affects the growth and survival of aquatic organisms. Nearly all aquatic organisms are very sensitive to abrupt environmental temperature changes. At 5°C ambient temperature abrupt changes can cause stress or even death in some types of cultured organisms ¹² . In this study, the average water temperature during the observation ranged between 26.0 and 28.0°C. The temperature is in the support range for the activities of life, growth and reproduction of the crab ^{33, 34} . The pH of the water media during the study ranged from pH 7.26 to 8.00. This shows that the pH of the water was maintained within the range of neutral and slightly alkaline, and was thus suitable for living crabs. According to the Ministry of Environment in Anonymous ³⁵ , water with a pH range of 6.5–8.5 has a considerable potential for the development of aquaculture in terms of productivity. Taslihan et al. ³⁶ stated that alkaline waters would be more productive than acidic waters.

Dissolved oxygen plays an important role in the life of all living organisms. However, there is a difference between the oxygen required by living organisms in terrestrial and aquatic organisms. Terrestrial organisms consume oxygen contained in the air, while aquatic organisms take in dissolved or bound oxygen. The oxygen requirements for every type of aquatic biota differ depending on the species toleration of the rise and fall of oxygen. In general, all types of cultured organisms (fish, shrimp, crabs, clams, and sea cucumbers) are unable to tolerate fluctuations in oxygen that are too extreme ¹⁴ . Dissolved oxygen (O ₂) was relatively high in this experiment, ranging between 6,15 to 7,56 ppm. The amount of oxygen that must be maintained to ensure a good life for aquatic organisms is not less than 3 ppm. If the oxygen content drops to less than 2 ppm, some kinds of crustaceans will be under pressure and will even die ^{37, 38} . According to Millamena and Quinitio ⁹ . Crab cultivation requires dissolved oxygen at a concentration greater than 4 ppm. Although the role of carbon dioxide (CO ₂) is considerable for living aquatic organisms, very excessive levels would interfere, and are even toxic to cultivated biotas. The tolerance of each biota varies depending on its type and body size, and is generally no more than 15 ppm. A concentration of over 25 ppm of carbon dioxide is dangerous for cultured organisms ^{39, 40} because its presence in the blood can inhibit the binding of oxygen by hemoglobin. Furthermore, Dodd et al., ³³ Long et al., ⁴¹ and Landes and Zimmer ⁴² reported that acidification caused by CO ₂ has been found to strongly affect calcification rates, animal behavior, predator foraging behavior and the avoidance of predators by prey. Carbon dioxide measurements ( Table 6) in the study ranged between 3.78 and 6.10 ppm. Boyd and Tucker ⁴³ explain that free carbon dioxide is good for the crustacean no higher than 12 ppm and must not be less than 2 ppm.

The conclusions that can be drawn from the results of this experiment are as follows: (1) Diet 2, with supplementation of 300 IU/kg vitamin E formulated diet, provided for the highest absolute weight gain (45.38 g), absolute carapace length (6.23 mm), and absolute carapace width (13.06 mm); (2) supplementation of the formulated diet with 300 IU/kg vitamin E in the formulated diet also causes broodstock blue swimming crab molting, with a percentage of 40–80% on day 20 and 20% on day 30; and (3) a survival value of 100% was obtained for all treatments during the 40 days maintenance period.

Further studies are needed to observe and analyze the effects of formulated diet supplementation of vitamin E on the incubation period and reproductive performance of female broodstock blue swimming crabs, P. pelagicus (Linnaeus, 1758) in mass production.

The data referenced by this article are under copyright with the following copyright statement: Copyright: � 2019 Efrizal E et al.

Data associated with the article are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).

Dataset 1. Spreadsheet containing data associated with this study. Data include time of molting, carapace width and carapace length. DOI: https://doi.org/10.5256/f1000research.15885.d221868 ⁴⁴ .