AQUIDEAS

     World crustacean production in 2016 might well represent a small proportion of total fisheries and inland aquatic farming catches (around 1%), but its commercial value was far from being negligible ...totalizing some 15% of them. Among these crustaceans, shrimps are an important output.

     All shrimps belong to the order of Decapoda (literally "ten-footed") and are all Natantia¹ or swimming organisms. Marine shrimps are classified as Penaeidea and freshwater prawns² as Caridea. Interests have been directed towards these tropical invertebrates for their high value and their fast growth rate as compared to temperate prawns. They generally take only four to six months to reach market sizes.

     Mating occur off-shore for marine species, but up-stream in the rivers for freshwater ones. Penaeid females only becomes mature during their second year. Ovary maturation is particularly visible by light contrast during the night: with the development of the gonads, there is a change in the color of the ovary, from transparent to green, then to greyish green and becoming dark green when fully mature. A mature female has its ovary occupying nearly the whole of the dorsal side of the body.

     In controlled environment, females are eye-ablated to accelerate the process: the removal of one eye eliminates the supply of inhibitory hormones produced and stored in the Sinus gland and X-organ situated in the eye stalk. However this practice shorten the life span of these breeders and limits the re-maturation of their ovaries. The number of eggs which are released varies from species to species, but are generally estimated between 250,000 and 1,600,000... ! Hatching occurs 24 hours later.

     Copulation of adults of Macrobrachium species, the major freshwater prawns, results in the deposition of semen in a gelatinous mass on the underside of the thoracic region of the female's body - between the walking legs. Successful molting can only take place between hard-shelled males and ripe females which have just completed their pre-mating molt and are soft-shelled. Egg-laying occurs within a few hours of copulation, the eggs being fertilized on extrusion by the attached semen. The eggs are transferred to a brood chamber of the underside of the abdominal region of the female, held in place by a thin membrane and kept aerated by vigorous movements of pleopods or abdominal appendages.

     Egg incubation changes are better observed in Carideans as they are yellowish orange at the beginning and grey at the end. The length of time that eggs are carried by the female in this way varies but is normally no longer than 3 weeks (generally 15 days). Freshwater prawns lay between 50,000 and 100,000 eggs per spawning when fully matured.

     Both larval cycles spend their development in the estuaries. Brackish water is required for freshwater prawns and these conditions are recreated in laboratories. Penaeid shrimps are passing by several larval stages which can be classified into four main shapes: nauplius, (proto)zoea, mysis and postlarva (PLs) having the adult profile. Its larval cycle takes between 11 and 18 days. Caridean prawns pass by 12 steps characterized by the appearance of a morphological structure. Its larval cycle generally takes 3 to 4 weeks. From egg to post-larvae, survival is low (5%) for marine shrimps compared to 45% for freshwater prawns, the latter compensating the lower amount of eggs released at spawning.

     Hatcheries require clean water for reproduction to avoid diseases. Therefore filtration and water treatment are important. Feed also accounts for a great deal in the operating cost of the activity. Main ingredients are chicken yolk, baby milk, micro-algae, Artemia species (a micro-crustacean), and some artificial compounds which are generally used as complements. Breeding infrastructures include simple plastic tanks up to sophisticated incubators in cement and polyester.

     Grow-out is achieved in earthen ponds along the coast for marine shrimps and inland for freshwater prawns. Some farms may cover several thousand hectares and are mostly found in Asia and Latin America where water temperature fluctuates between 28^o and 32^oC. Feed comprises pellets (or concentrated), specifically designed for these aquatic organisms.

     However due to its quick and huge return, research in this luxurious field is also found at the highest level of education in temperate countries. Facilities do not need to be very expensive: a simple greenhouse with some raceways of plastic sheet on a wooden frame can do the trick. A building can also be chosen, using a closed system which recirculates water between several floors: on the top reproduction can take place, lower grow-out can be achieved, and on the basement the filtration process and pumps can be set. Water can be heated artificially by the sun, electricity, geothermal source or hot waste water coming from the industry.

     Research should be more emphasized to larval stages where mortality strikes heavily for a number of reasons. One fundamental aspect which is frequently neglected by many researchers (for the difficulty of larval rearing) is the right diet to use at each stage. Some considerations should include more specifically physical presentation and energetic budget.

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Annex 1 : "Prawn" versus "Shrimp"

• Prawns are larger in size and have larger legs with claws on three pairs. They have branching gills.
• Shrimps are smaller, have shorter legs and have claws only on two pairs. Their gills are lamellar, i.e. plate-like.
• Prawn and shrimp are both decapod crustaceans i.e. that they have exoskeletons and 10 legs.
• They can be found in salt water and fresh water all over the world, typically swimming in search of food.
• Both shrimp and prawns tend to stay near the ocean floor.
• They also have similar flavors, and come in a wide range of sizes from minuscule to quite large.
• In commercial farming and fisheries, the terms shrimp and prawn are often used interchangeably. But of late, the term "prawn" only signifies freshwater forms of palaemonids and "shrimp" for the marine penaeids.
• In the United Kingdom, the word “prawn” is more common on menus than “shrimp”, while it’s the opposite in the United State of America.
• The term “prawn” is also loosely used to describe any large shrimp, especially those that come 15 (or fewer) to the pound³ (such as “king prawns”, yet sometimes known as “jumbo shrimp”).
• Australia and some other Commonwealth nations follow this British usage to an even greater extent, using the word “prawn” almost exclusively.

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     3  1 lbs (Pound) = 453,5924 g or 0,4535924 Kg

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Annex 2 : Références

     - Courses/lectures, Dept.of Fisheries and Allied Aquaculture, Auburn University, U.S.A.

     - Internal documents, Marine Sciences Laboratories, University of Wales, U.K.

     - fao.org

     - merriam-webster.com/dictionary

     - en.wikipedia.org

     - Shrimp & prawn farming in the Western hemisphère, J.A.Hanson & H.L.Goodwin, Dowden,

       Hutchinson & Ross, Inc., 1977

     - Frehwater prawn farming, FAO fisheries technical paper No.225, 1985

     - Personal notes