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 orangepink 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 pincerended 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.

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

SOURCE: METHIL NARAYANAN KUTTY

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.
Postlarval prawns are usually a greenishbrownish 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.

TABLE1
Body segments (somites) in Macrobrachium rosenbergii and appendage function

SOURCE: DERIVED FROM PINHEIRO AND HEBLING (1998)

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

FIGURE3
How to sex juvenile Macrobrachium rosenbergii

SOURCE: EMANUELA D’ANTONI, AFTER MARIO PEDINI

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).
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,

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

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

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).