CHAPTER 1
BIOLOGY

THE FOLLOWING NOTES contain background information on the genus Macrobrachium and some basic details about the biology of M. rosenbergii. This section of the manual has mainly been derived from Holthuis (2000), the work of Ling (1969), and a review by Ismael and New (2000), and is intended to provide basic background information for extension workers and students.

1.1 Names, natural range, and characteristics of freshwater prawns

NAMING FRESHWATER PRAWNS (NOMENCLATURE)

All the freshwater prawns that have been cultured so far belong to the genus Macrobrachium, Bate 1868, the largest genus of the family Palaemonidae. About 200 species have been described, almost all of which live in freshwater at least for part of their life.

The giant river prawn, Macrobrachium rosenbergii, was one of the first species to become scientifically known, the first recognisable illustration appearing in 1705. The nomenclature of freshwater prawns, both on a generic and a species level has had quite a muddled history. In the past, generic names have included Cancer (Astacus) and Palaemon. Previous names of M. rosenbergii have included Palaemon carcinus, P. dacqueti, and P. rosenbergii and it was not until 1959 that its present scientific name, Macrobrachium rosenbergii (De Man 1879) became universally accepted.

Some taxonomists recognize a western sub-species (found in the waters of the east coast of India, Bay of Bengal, Gulf of Thailand, Malaysia, and the Indonesian regions of Sumatra, Java and Kalimantan) and an eastern sub-species (inhabiting the Philippines, the Indonesian regions of Sulawesi and Irian Jaya, Papua New Guinea and northern Australia). These are referred to as Macrobrachium rosenbergii dacqueti (Sunier 1925) for the western form and Macrobrachium rosenbergii rosenbergii (De Man 1879) for the eastern form. However, from the perspective of freshwater prawn farmers, exact nomenclature has little relevance, especially because the species M. rosenbergii has been transferred within its natural geographical range and been introduced into many other zones where it may become established.

THE NATURAL HOME OF FRESHWATER PRAWNS (DISTRIBUTION)

Species of the freshwater prawn genus Macrobrachium are distributed throughout the tropical and subtropical zones of the world. Holthuis (1980) provides useful information on the distribution, local names, habitats and maximum sizes of commercial (fished and farmed) species of Macrobrachium.

They are found in most inland freshwater areas including lakes, rivers, swamps, irrigation ditches, canals and ponds, as well as in estuarine areas. Most species require brackishwater in the initial stages of their life cycle (and therefore they are found in water that is directly or indirectly connected with the sea) although some complete their cycle in inland saline and freshwater lakes. Some species prefer rivers containing clear water, while others are found in extremely turbid conditions. M. rosenbergii is an example of the latter.

There is a wide interspecific variation in maximum size and growth rate, M. rosenbergii, M. americanum, M. carcinus, M. malcolmsonii, M. choprai, M. vollenhovenii and M. lar being the largest species known. M. americanum (Cauque river prawn) is found naturally in western watersheds of the Americas while M. carcinus (painted river prawn) is found in those connected with the Atlantic. M. choprai (Ganges river prawn) is found in the Ganges and Brahmaputra river systems. M. lar (Monkey river prawn) is native from East Africa to the Marquesas Islands of the Pacific (and was introduced into Hawaii). M. malcolmsonii (monsoon river prawn) is found in the waters of Bangladesh, India and Pakistan. M. rosenbergii (giant river prawn) is indigenous in the whole of the South and Southeast Asian area as well as in northern Oceania and in the western Pacific islands. M. vollenhovenii (African river prawn) is naturally distributed in West Africa, from Senegal to Angola.

Many Macrobrachium species have been transferred from their natural location to other parts of the world, initially for research purposes. M. rosenbergii remains the species most used for commercial farming and consequently is the one which has been introduced to more countries. Following its import into Hawaii from Malaysia in 1965, where the pioneer work of Ling (1969) was translated into a method for the mass production of postlarvae (PL) by Fujimura and Okamoto (1972), it has been introduced into almost every continent for farming purposes. M. rosenbergii is now farmed in many countries; the major producers (>200 mt) are Bangladesh, Brazil, China, Ecuador, India, Malaysia, Taiwan Province of China, and Thailand (FAO 2002). More than thirty other countries reported production of this species in the year 2000. Viet Nam is also a major producer, according to New (2000b). In addition, there are also valuable capture fisheries for M. rosenbergii, for example in Bangladesh, India, and several countries in Southeast Asia.

IDENTIFYING MACROBRACHIUM ROSENBERGII FROM OTHER FRESHWATER PRAWN SPECIES

Macrobrachium rosenbergii (Figure 1) can be distinguished from other species in the genus by the following characteristics (the morphological terms used below are explained in the glossary – Annex 11):

• it has a very long rostrum, with 11-14 dorsal teeth and 8-10 ventral teeth (the ventral characteristics are especially important);
• the tip of its telson reaches distinctly beyond the posterior spines of the telson;
• the adult male has very long second chelipeds in which all segments are elongate and have blunt spines;
• the movable finger of the second chelipeds of the adult male is covered by a dense velvet-like fur (except the extreme tip) but this fur is absent from the fixed finger and the rest of the cheliped; and
• it is the largest known of all Macrobrachium species, adult males having been reported with a total body length of up to 33 cm, and adult females of up to 29 cm.

1.2 The shape (external morphology) and other characteristics of freshwater prawns

The following information deals with the general external anatomy of the freshwater prawn M. rosenbergii, and provides some notes on the function of various major parts of the body. Internal morphology (circulatory, respiratory, digestive, excretory, reproductive and nervous systems) is not covered in this manual, which concentrates on farming, but further information is available in the references cited in the introduction to this section.

Freshwater prawn eggs of this species are slightly elliptical, with a long axis of 0.6-0.7 mm, and are bright orange in colour until 2-3 days before hatching when they become grey-black. This colour change occurs as the embryos utilize their food reserves.

Most scientists accept that the larvae go through 11 distinct stages (Uno and Kwon 1969) before metamorphosis, each with several distinguishing features which are described and illustrated in Annex 1. However, from stage VI onwards their size is variable, which has led to some workers, notably Ling (1969) to describe only eight stages. Stage I larvae (zoeae) are just under 2 mm long (from the tip of the rostrum to the tip of the telson). Larvae swim upside down by using their thoracic appendages and are positively attracted to light. By stage XI they are about 7.7 mm long. Newly metamorphosed postlarvae (PL) are also about 7.7 mm long and are characterized by the fact that they move and swim in the same way as adult prawns. They are generally translucent and have a light orange-pink head area.

The body of postlarval and adult prawns consists of the cephalothorax (‘head’) and the abdomen (‘tail’). The bodies of freshwater prawns are divided into twenty segments (known as somites). There are 14 segments in the head, which are fused together and invisible under a large dorsal and lateral shield, known as the carapace. The carapace is hard and smooth, except for two spines on either side; one (the antennal spine) is just below the orbit and the other (the hepatic spine) is lower down and behind the antennal spine. The carapace ends at the front in a long beak or rostrum, which is slender and curved upwards. The rostrum extends further forward than the antennal scale and has 11-14 teeth on the top and 8-10 underneath (Figure 1). The first two of the dorsal (top side) teeth appear behind the eye socket (orbit).

The front portion of the cephalothorax, known as the cephalon, has six segments and includes the eyes and five pairs of appendages. The final three of these six segments can be seen if the animal is turned upside down and the appendages of the thorax (see below) are moved aside. The cephalon segments therefore support, from the front of the animal:

• the stalked eyes;
• the first antennae, which each have three-segment peduncles (stalks) from which three tactile flagella emerge;
• the second antennae, which each have five-segment peduncles and a single, long flagellum;
• the mandibles, which are short and hard and are used to grind food;
• the first maxillae, which are plate-like (lamelliform), hidden below the second maxillae, and used to transfer food into the mouth; and
• the second maxillae, which are similar to the first maxillae but have an additional function. Part of these appendages are constantly beating, thus producing a current of water through the gill chamber to promote the respiratory function of the latter.

The two pairs of antennae are the most important sites of sensory perception; the peduncles of the first antennae contain a statocyst, which is a gravity receptor. The mandibles and first and second maxillae form part of the six sets of mouthparts (see below).

The rear portion of the cephalothorax, known as the thorax, consists of 8 fused segments which have easily visible pairs of appendages. These appendages consist of 3 sets of maxillipeds and 5 pairs of pereiopods, as follows:

• the first and second maxillipeds are similar to the first and second maxillae and function as mouthparts (see above);
• the third maxillipeds, which are also mouthparts but look rather like legs;
• the first and second legs (pereiopods), which have pincers (chelae). These pincer-ended legs are also called chelipeds. The first legs are slender but the second pair bear numerous small spines and are much stronger than any other leg. The second chelipeds are used for capturing food, as well as in mating and agonistic (fighting) behaviour; and
• the third, fourth and fifth legs (pereiopods), which are much shorter than the second cheliped, have simple claws (not pincers), and are sometimes called walking legs. Eggs are extruded from oval gonopores in the base of the third pereiopods of females, which are covered with a membrane. In males, sperm is extruded from gonopores which are covered by flaps, situated in the base of the fifth pereiopods.

The pereiopods include chemoreceptor cells, which are sensitive to aqueous extracts of food and to salts (and may therefore be involved in migratory and reproductive processes). The left and right second legs (chelipeds) of M. rosenbergii are equal in size, unlike some other Macrobrachium spp. In adult males they become extremely long and reach well beyond the tip of the rostrum. Some extreme examples are shown in Figure 2.

The tail (abdomen) is very clearly divided into 6 segments, each bearing a pair of appendages known as pleopods or swimmerets (as this name implies, they are used for swimming, in contrast to the walking legs). The first five pairs of swimmerets are soft. In females they have attachment sites for holding clusters of eggs within the brood chamber (see below). In males, the second pair of swimmerets is modified for use in copulation. This spinous projection is known as the appendix masculina. The sixth pair of swimmerets, known as uropods, are stiff and hard. The telson is a central appendage on the last segment and has a broad point with two small spines which project further behind the point. The telson and the uropods form the tail fan, which can be used to move the prawn suddenly backwards.

A summary of the segments and the functions of each appendage is provided in Table 1.

FIGURE 2
These very large Macrobrachium rosenbergii males were obtained
from a fisheries enhancement
programme (India)

SOURCE: METHIL NARAYANAN KUTTY

Postlarval prawns are usually a greenish-brownish grey and sometimes blue. Normally there are irregular brown, grey and whitish longitudinal streaks on the body. Orange spots may be visible where the tail segments bend. The lateral ridge of the rostrum may be red. The antennae are often blue. The chelipeds are generally blue but the second chelipeds may also be orange (see below). The colour of the bodies of prawns tends to be brighter in younger animals and generally darker and blue or brownish in older prawns (they become red when cooked).

Mature male prawns are considerably larger than the females and the second chelipeds are much larger and thicker. The head of the male is also proportionately larger, and the abdomen is narrower. As noted above, the genital pores of the male are between at the base of the fifth walking legs. The head of the mature female and its second walking legs are much smaller than the adult male. The female genital pores are at the base of the third walking legs. An alternative technique for sexing juvenile prawns is shown in Figure 3. The pleura (overhanging sides of the abdomenal segments) are longer in females than in males, and the abdomen itself is broader. These pleura of the first, second and third tail segments of females form a brood chamber in which the eggs are carried between laying and hatching. A ripe or ‘ovigerous’ female can easily be detected because the ovaries can be seen as large orange-coloured masses occupying a large portion of the dorsal and lateral parts of the cephalothorax.

Female prawns are sometimes referred to as virgin females (V or VF), berried (egg carrying) females (BE or BF) and open brood chamber (spent) females (OP). Egg-carrying females are shown in Figure 4. There are three major types of freshwater prawn males and a number of intermediate forms, which were not fully described in the original FAO manual. All three major types of males are illustrated in Figure 5. The ability to distinguish between these forms is important in understanding the need for size management during the grow-out phase of culture (Annex 8). The first type consists of blue claw males (BC), which have extremely long claws. The second type of males, sometimes known as runts, have small claws and are now called small males (SM). Although this type is similar in size to younger juveniles, the prawns are much older. The third type of males are known as orange claw males (OC). OC males have golden coloured claws, which are 30 to 70% shorter than the claws of BC males. The three major types of males can generally be distinguished by sight. However, more reliable ways of determining which type males are can be found in Karplus, Malecha and Sagi (2000). As mentioned, a number of intermediate male forms have also been recognized, including weak orange claw (WOC), strong orange claw (SOC) and transforming orange claw (TOC) males. The relationship and transformation of these various male types, and their importance in size management is covered later in this manual (Annex 8).

TABLE 1

Body segments (somites) in Macrobrachium rosenbergii and appendage function

SOURCE: DERIVED FROM PINHEIRO AND HEBLING (1998)

Many people find it hard to distinguish between Macrobrachium and penaeid (marine) shrimp, once they have been harvested and the heads have been removed. If the ‘tail’ still retains its shell there are, in fact, two easy ways of distinguishing them (Fincham and Wickins, 1976). Firstly, Macrobrachium spp., have a smooth rounded dorsal surface to the abdomen while penaeids have a simple or complex ridge at the dorsal apex of the abdomen (Figure 6). Secondly, the second pleuron of the abdomen (or tail) of Macrobrachium (in common with all caridean prawns, including some marine shrimp such as Crangon spp., Pandalus spp., and Palaemon spp.) overlaps both the first and the third pleuron. In penaeids the second pleuron overlaps the third pleuron only and is itself overlapped by the first (Figure 7).

FIGURE 4
Notice that the abdominal pleura of the two females with
this BC male Macrobrachium rosenbergii are enlarged to accommodate eggs (Brazil)

SOURCE: EUDES CORREIA

1.3 Life history

All freshwater prawns (like other crustaceans) have to regularly cast their ‘exoskeleton’ or shell in order to grow. This process is referred to as moulting and is accompanied by a sudden increase in size and weight. There are four distinct phases in the life cycle of the freshwater prawn, namely eggs, larvae, postlarvae (PL) and adults. The time spent by each species of Macrobrachium in the different phases of its life cycle (and its growth rate and maximum size) varies, not only specifically but according to environmental conditions, mainly temperature.

The life cycle of M. rosenbergii can be summarized as follows. The mating (copulation) of adults results in the deposition of a gelatinous mass of semen (referred to as a spermatophore) on the underside of the thoracic region of the female’s body (between the walking legs). Successful mating can only take place between ripe females, which have just completed their pre-mating moult (usually at night) and are therefore soft-shelled, and hard-shelled males. All of the various types of males are capable of fertilising females but their behaviour is different (see Annex 8). Detailed descriptions of the mating process are given in Ismael and New (2000) and Karplus, Malecha and Sagi (2000). Under natural conditions, mating occurs throughout the year, although there are sometimes peaks of activity related to environmental conditions. In tropical areas these coincide with the onset of the rainy season, whereas in temperate areas they occur in the summer.

FIGURE 5
The major male morphotypes of Macrobrachium rosenbergii are called blue claw (BC), orange claw (OC), and small male (SM) (Israel)

SOURCE: ASSAF BARKI, REPRODUCED FROM NEW AND VALENTI (2000) WITH PERMISSION FROM BLACKWELL SCIENCE

Within a few hours of copulation, eggs are extruded through the gonopores and guided by the ovipositing setae (stiff hairs), which are at the base of the walking legs, into the brood chamber. During this process the eggs are fertilized by the semen attached to the exterior of the female’s body. The eggs are held in the brood chamber (stuck to the ovigerous setae) and kept aerated by vigorous movements of the swimmerets. This is in contrast to penaeid shrimp, whose fertilized eggs are released into the sea, where they hatch. The length of time that the eggs are carried by female freshwater prawns varies but is not normally longer than three weeks. The number of eggs which are laid depends also on the size of the female. Female prawns of M. rosenbergii are reported to lay from 80 000 to 100 000 eggs during one spawning when fully mature. However, their first broods, (i.e. those which are produced within their first year of life), are often not more than 5 000 to 20 000. Females normally become mature when they reach 15-20 g but berried females have been observed as small as 6.5 g (Daniels, Cavalli and Smullen 2000). Under laboratory conditions, where a breeding stock of both males and females was kept, it has been noted that egg incubation time averaged 20 days at 28°C (range 18-23 days). Ovaries frequently ripened again while females were carrying eggs. Pre-mate intermoults were separated by as little as 23 days (i.e. females on some occasions hatched two batches of eggs within a one-month period). It is unlikely that this would happen under natural conditions but it does show the potential fecundity of the animal.

As the eggs hatch, a process which is normally completed for the whole brood within one or two nights, the larvae (free-swimming zoeae) are dispersed by rapid movements of the abdominal appendages of the parent. Freshwater prawn larvae are planktonic and swim actively tail first, ventral side uppermost (i.e. upside down). M. rosenbergii larvae require brackishwater for survival. Those which hatch in freshwater will die unless they reach brackishwater within a few days. There are a number of microscopically distinct stages during the larval life of freshwater prawns, which lasts several weeks (Annex 1). Individual larvae of M. rosenbergii have been observed, in hatchery conditions, to complete their larval life in as little as 16 days but reaching this stage may take much longer, depending on water temperature and other factors. The importance of this observation is fully discussed later in the manual. Larvae eat continuously and, in nature, their diet is principally zooplankton (mainly minute crustaceans), very small worms, and the larval stages of other aquatic invertebrates.

On completion of their larval life, freshwater prawns metamorphose into postlarvae (PL). From this point onwards they resemble miniature adult prawns and become mainly crawling rather than free-swimming animals. When they do swim it is usually in a normal (dorsal side uppermost) way and in a forward direction. Rapid evasive movement is also achieved by contracting the abdominal muscles and rapid movement of the tail fan. Postlarvae exhibit good tolerance to a wide range of salinities, which is a characteristic of freshwater prawns.

Postlarvae begin to migrate upstream into freshwater conditions within one or two weeks after metamorphosis and are soon able to swim against rapidly flowing currents and to crawl over the stones at the shallow edges of rivers and in rapids. They can climb vertical surfaces and cross land, provided there is abundant moisture available. In addition to using the foods available to them as larvae, they now utilize larger pieces of organic material, both of animal and vegetable origin. Postlarval freshwater prawns are omnivorous and, as they grow, their natural diet eventually includes aquatic insects and their larvae, algae, nuts, grain, seeds, fruits, small molluscs and crustaceans, fish flesh and the offal of fish and other animals. They can also be cannibalistic. Further reading on this topic may be found in Ling (1969).

1.4 Sources of further biological information

The polymorphism of male prawns, which is particularly relevant to the management of prawn farming, is covered in Annex 8 of this manual. However, the internal morphology, reproductive physiology, and osmo-ionic regulation of freshwater prawns and the nature of ecdysis (moulting), autotomy (shedding of parts of the body) and the regeneration of appendages, are topics that are beyond the scope of this manual. New and Valenti (2000) have provided a review of these subjects.