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Freshwater Prawns Biology and Life History
Commercial production of
one of the species of freshwater shrimp or prawns (Macrobrachium rosenbergii)
has periodically been the subject of research and commercial enterprise in the
United States. Basic production techniques were developed in the late 1950's in
Malaysia, and in Hawaii and Israel during the last three decades. In 1984, the
Mississippi Agriculture and Forestry Experiment Station, Mississippi State
University, started an extensive research program to develop and evaluate
management practices that would ultimately establish commercial production
techniques for freshwater prawns.
During the past 6 years, new
management practices have dramatically increased the potential for economic
success of prawn culture. Research efforts have been complemented by
demonstration projects designed to evaluate methods under large-scale,
Freshwater prawns, like
all crustaceans, have a hard outer skeleton or shell that must be shed regularly
in order for growth to occur. The process of shedding the shell is called
"molting," and weight and size increases occur principally soon after each molt.
Because of these periodic molts, growth occurs in distinct increments, rather
Females generally become
reproductively mature within 6 months. Mating can occur only between
hard-shelled males and soft-shelled females, i.e., females who have just
completed a premating or prenuptial molt. The male deposits sperm into a
gelatinous mass that is held underneath the body of the female, between her
fourth pair of walking legs. Eggs are laid within a few hours after mating and
are fertilized by the sperm contained in the gelatinous mass attached to the
outside of the female's body. The female then transfers the fertilized eggs to
the underside of the abdominal (tail) region, in a "brood chamber," where they
are kept aerated and cleaned by movement of the abdominal swimming appendages.
Eggs remain attached to the abdomen until they hatch. The number of eggs
produced at each spawn is directly proportional to the size of the female. As
long as water temperature exceeds 70 °F, multiple spawns per female can occur
annually. Females carrying eggs are termed "berried females." The bright-orange
color of newly spawned eggs gradually changes to orange, then brown, and finally
gray about 2 to 3 days before hatching. At a temperature of 82 °F, the eggs
hatch approximately 20 to 21 after spawning. Newly hatched freshwater prawns
enter into a larval phase of growth and metamorphosis.
After hatching, larvae
are released and swim upside down and tail first. The larvae cannot survive in
freshwater beyond approximately 48 hours and migrate to brackish water with a
salinity of 10 to 14 parts per thousand (ppt). Larvae are aggressive sight
feeders and feed almost continuously, primarily on small zooplankton, worms, and
larval stages of other aquatic invertebrates. Larvae undergo 11 molts, each
representing a different stage of metamorphosis. Following the last molt, larvae
transform into postlarvae. Transformation from newly hatched larvae to
postlarvae requires 15 to 40 days, depending upon food quantity and quality,
temperature, and a variety of other water quality variables.
After metamorphosis to
postlarvae, the prawns resemble miniature adult prawns, about 7 to 10 mm
(0.3-0.4 inch) long and weighing 6 to 9 mg (50,000 to 76,000 per pound). The
prawns behaviorally change from living suspended in the water column to
principally bottom dwelling, crawling individuals. When they do swim, they move
like adults with the dorsal (back) side uppermost and in a head-forward
Postlarvae can tolerate a range of
salinities and migrate to freshwater upon transformation. In addition to the
food they ate as larvae, larger pieces of animal and plant materials will be
ingested. The diet includes larval and adult insects, algae, mollusks, worms,
fish, and feces of fish and other animals. At high densities, or under
conditions of food limitations, prawns become cannibalistic.
Postlarvae are translucent and may
have a light-orange-pink head. As they change to the juvenile stage, they take
on the bluish to brownish color of the adult stage. Juveniles are intermediate
in size between postlarvae and adults; however, no standard definition for the
juvenile stage exists.
Older juveniles and
adults usually have a distinctive blue-green color, although sometimes they may
take on a brownish hue. Color is usually the result of the quality and type of
diet. Adult males are larger than the females, and the sexes are easily
distinguishable. The second walking legs or claws (chela) and the head region of
males are larger than those of the females.
The base of the fifth or last pair
of walking legs (periopods) of males is expanded inward to form a flap or clear
"bubble" that covers the opening (gonopore) through which sperm are released.
The walking legs of males are set close together in nearly parallel lines, with
little open space between them, which helps distinguish immature males from
females. A wide gap exists between the last pair of walking legs in females, and
they have a genital opening on the base of the third pair of walking legs.
Three types of males have been
identified, based upon external characteristics. Blue-claw (BC) males are easily
distinguishable and are characterized by long, spiny blue claws. Two other
classes of non-blue-claw males exist, orange claw (OC) and strong orange claw
(SOC) males. The transformational sequence is from OC to SOC to BC males.
Smaller OC males (< 10g) grow slowly but are more reproductively mature than are
other OC males.
BC and some smaller OC males are
the most reproductively active and successful at mating. The BC maintains a
territory associated with a group of females that are ready for mating, and
protects them during the vulnerable period just before and after molting. Small
OC males eventually grow and transform to SOC (strong-orange claw) males before
becoming BC males. BC males undergo an extended period of nonmolting. As the BC
male ages, reproductive capacity is lost. Eventually, the BC male molts and
returns to a growth phase. Later, its reproductive capacity is regained.
There are three phases of
culture of the freshwater prawn -- hatchery, nursery, and pond grow out. If you
are contemplating starting a freshwater shrimp production enterprise, forego,
initially at least, the hatchery phase and possibly the nursery phase by
purchasing juveniles from a supplier. As production increases and you are
successful at pond grow out of the animals, you should begin plans to develop a
nursery, and possibly a hatchery. There are a limited number of juvenile prawn
suppliers, but increased demands will eventually lead to a need for more
enterprises that deal exclusively in the production and sale of seed stock.
For information on
request Extension Publication 2002,
and for information on pond production practices,
request Extension Publication 2003.
By Dr. Louis R.
D'Abramo, Professor, Dr. Martin W. Brunson,
Extension Leader/Fisheries Specialist, and Dr. William H. Daniels, former
Research Assistant, all in the Department of Wildlife and Fisheries.
Information Sheet 1525
Extension Service of Mississippi State University, cooperating with
U.S. Department of Agriculture.
Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A.
Copyright by Mississippi
State University. All rights reserved.
This document may be copied and distributed for nonprofit educational purposes
provided that credit is given to the Mississippi State University Extension