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Fig. 3.1 Life cycle of M. rosenbergii.

fused to form the cephalothorax under a dorsal and lateral covering, the carapace, but the somites can be seen from the ventral surface. The head or cephalon is formed of 5 somites while the thorax has 8 and the abdomen 6.

The carapace ends anterodorsally in the rostrum. The rostrum of this species is long, reaching beyond the anten-nal scale, except in very large specimens, where it may be proportionately shorter. The rostrum is slender and slightly sigmoid, with the distal part curved upward. Dorsally the rostrum bears 11 to 14 teeth (the first 2 of which are placed behind the orbit, while the others extend over the full length of the rostrum) and 8 to 10 below; this large number of ventral teeth distinguishes M. rosenbergii from practically all other species of the genus; also its length is quite characteristic. The rest of the carapace is smooth except for the presence of two spines on either side. The strong antennal spine is placed on the anterior margin of the carapace just below the lower limit of the orbit. The second spine, the hepatic spine, lies below the antennal and far behind the anterior carapace margin; a closed groove, the branchioste-gal line, connects the hepatic spine with this anterior margin. The somites of the cephalon and thorax are completely fused and no segmentation is visible.

The cephalon has the following five appendages:

1) the antennules (or first antennae);

2) the antennae (or second antennae),

3) the mandibles;

4) the first maxillae or maxillules;

5) the second maxillae or 'maxillae'

The eyes are found at the very front of the head on a vestigial segment known as the acron, but which is not considered a true somite (McLaughlin 1980; Stachowitsch 1992). They are stalked and consist of a short sturdy peduncle which carries the rounded and darkly coloured cornea, in which are placed the ophthalmic elements that form the composite eye. The antennules are formed of a three-segmented peduncle, the top of which carries three flagella and are used as tactile organs. The antennae have a five-segmented peduncle that carries a long and strong single flagellum; on the outer margin of the third peduncular segment a large scale, the antennal scale or scaphocerite, is implanted. The mandibles and the first and second maxillae are placed near the mouth opening and are termed mouthparts; they play a role in the ingestion of food. The mandibles are short, heavily calcified molar-like jaws and serve for grinding up the hard parts in the food; the first and second maxillae are thin and lamelliform. These mouthparts are small and are entirely covered by the appendages posterior to them.

Of the eight pairs of appendages of the thorax the first three, named maxillipeds I, II and III, can also be termed mouthparts; maxillipeds I and II are lamelliform like the maxillae, while maxilliped III is pediform, and sometimes may be mistaken for a pereopod. The remaining five legs are the pereopods or true legs as typical of the Decapoda, consisting of seven segments: coxa, basis, ischium, merus, carpus, propodus (propus) and dactylus. The first two of these carry chelae or pincers and are termed chelipeds. The last three pairs end in a simple claw. The first cheliped is narrow and slender, it is smooth and of about the same width throughout; it is distinctly shorter and thinner than the second cheliped which is the largest of all the pereopods. The second cheliped is strong, bears numerous spinules and is far more robust than any other leg and both are similar; there is no differentiation in M. rosenbergii as can sometimes be seen in some other species of Macrobrachium (e.g. M. olfersii) and large reptantian chelipeds, such as Homarus spp. In the dominant adult males this leg becomes excessively long and may reach with a large part of the merus (plus carpus and chela) beyond the antennal scale. In females and younger males it surpasses the scale with only part ofthe carpus. In both sexes the segments ofthis leg are slender. The carpus is always longer than the merus. Numerous spinules are present on all segments; in the large males the carpus and merus have especially large and strong spinules. The fingers of the chelae are slightly shorter than the palm, and each ends in a sharp inward tip. A characteristic of the species is the fact that the mobile finger is covered with a dense, though rather short pubescence, which leaves only the very distal part of the finger naked; no such pubescence is found on the fixed finger or any other part of the che-liped. This gives an appearance to the naked eye of a velvet covering to the tip of the second cheliped but is made up of tufts of setae in close groups. The following three pairs of legs are rather similar; they have no chelae, are slender and in either sex are much shorter than the second cheliped.

In contrast to the cephalothorax, the segmentation ofthe abdomen is very distinct viewed from all aspects. This part of the body consists of six somites, each of which bears a pair ofventral appendages, called pleopods or swimmerets, which consist of a peduncle (protopod), with two supple lamelliform blades. As the name indicates, these swim-merets are used for swimming, while the pereopods are used mainly for movement on the ground. The swimmerets of the sixth abdominal somite are stiff and hard and, with the telson, serve as a tail fan. The telson is a median appendage on the posterior margin of the sixth abdominal somite. It is elongate, triangular and has two pairs of dorsal spinules; its posterior margin is produced in the middle into a strong broad point, which reaches distinctly beyond the two pairs of spinules that are implanted on the posterior margin; in most other species of Macrobrachium this point is overreached by the spinules. The tail fan is used for making a rapid forward motion to propel the prawn backwards in an escape response. The female, when 'berried' carries the small and very numerous eggs under the abdomen, where they are attached to the swimmerets.

3.2.1.2 Function of external structures

While external morphology has been fully described in section 3.2.1.1, some brief comments on the function of the various structures, based on the work of McLaughlin (1980, 1982); Cavalcanti et al. (1986) and Kaestner (1993), are appropriate here.

In spite of several chemo- and mechano-receptors identified on the chelae, ambulatory legs and mandibles of a number ofCrustacea, the antennules and antennae are considered the most important sites of sensory reception. The statocyst (gravity receptor) is localised in the basal segment of the antennular peduncle. The chemoreceptor cells in the pereopods of M. rosenbergii have, on average, broader response spectra than the chemoreceptor cells in the anten-nules and pereopods of lobsters (Derby & Harpaz 1988). They are sensitive not only to aqueous extracts of food but also to salts. Such sensitivity might be involved with reproductive behaviour, since gravid females move into estuaries, where their eggs hatch, and in their return to freshwater after spawning. The detection of salts might be also important for the migration of juveniles into adult habitats (Derby & Harpaz 1988).

Three sets of cephalic appendages are concerned with feeding: the mandibles, maxillules and maxillae. The mandibles, being well developed with incisor and molar processes and a three-segmented palp, are specialised for cutting and crunching. Each of the first maxillae consists of an endopod, which is bilobed at the top, and two endites. The exopod of each second maxillae is developed as a scaphognathite whose constant beating produces a water current through the gill chamber, promoting respiratory gas exchange. The second maxillae also include an endo-pod and two endites; as in the case of the first maxillae these assist in the handling of the food towards the mandibles. The first three thoracic appendages (maxillipeds), which are located between the mouth appendages and the locomotor appendages, are also utilised for feeding or food handling. The remaining five pairs of pereopods are what give the name to the order Decapoda but atypically of this order, the first pair in the Caridea is much smaller than the second pereopods; they are chelate and are utilised in capturing food. The second pair is chelate and is involved with several functions in addition to food capture, including agonistic and mating behaviour. The final three thoracic appendages (pereopods three to five) are typical walking legs.

The first five of the six abdominal appendages consist of pairs of well-developed supple natatory appendages (pleopods). These five pleopods have biramous structures, adapted for swimming. In females, these are developed with attachment sites for egg clusters. In males, the second pair of pleopods is modified for copulation (Fig. 3.2) by possession of an additional outgrowth from the endopod, the appendix masculina. This is a very useful means for sexing young prawns (Tombes & Foster 1979), being visible as a side bud from animals approximately 30 mm in length; by the time the animals are 70 mm in total length

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