3.3.1 Sexual dimorphism

According to Nagamine & Knight (1980), M. rosenbergii can be sexually distinguished with the first appearance of gonopores in juveniles, at 5.9 mm (carapace length) for males and 7.6 mm for females. Male gonopores are situated at the base ofthe coxae ofthe fifth pereopods andare covered by flaps, while female gonopores appear as oval apertures on the coxae of the third pereopods and are covered with a membrane. New & Singholka (1985) illustrated the fact that the ventral side of the first abdominal somite in male M. rosenbergii has a central lump or point, which can be felt with the finger. This feature is absent in females. The presence of the appendix masculina can also be a useful diagnostic feature, as described in section

Mature females (Plate 2, facing p. 254) have proportionally smaller heads and claws than males (Sandifer & Smith 1985). They exhibit a typical brood chamber, formed by the first, second and third abdominal pleurae; this characteristic does not exist in penaeid shrimps, as they never carry their eggs, but spawn them free in the water. Macrobrachium also exhibit reproductive setae on the pleopods and thorax, which are functionally distinct: the ovipositing setae, which are mostly permanent, on the coxae of the last three pairs of pereopods and pleopods guide the eggs during spawning, and the ovigerous setae, which only occur following a pre-spawning moult are used to secure the eggs to the pleopods for brooding (Nagamine & Knight 1980). According to New & Singholka (1985), female M. rosenbergii can lay between 80 000 and 100 000 eggs in each spawning when fully mature. However, their first broods are often not more than 5000-20 000. Ovaries frequently ripen again while eggs are being carried in the brood chamber. On the basis of a study of 80 berried females Sureshkumar & Kurup (1998) found that fecundity was strongly correlated with total weight, total length, and carapace length and were able to predict fecundity of870 per gram oftotal body weight ofthe mother with similar predictions possible in relation to total length and carapace length (452 eggs/mm and 1625 eggs/mm, respectively). Male prawns, except small males (SM), are easily recognised by longer and stronger chelipeds with larger spines than the females. Apart from these characteristics, and the presence of gonopores, males may also be recog-nisedby the appendixmasculina, a spinous process adjacent to the appendix interna on the endopod of the second pleo-pod (Sandifer & Smith 1985), as illustrated in Figure 3.2.

Sexually mature populations of M. rosenbergii are composed of three distinct male morphotypes: blue claw (BC) males are relatively large and possess long blue claws, covered with spines, with high clawto body length ratio. Orange claw (OC) males possess medium-sized, spineless and often orange-coloured claws, with lower claw to body length ratio. SM possess short delicate claws, often with little pigment and translucent, with the lowest claw to body length ratio (Kuris et al. 1987). The characteristics ofthe various male morphotypes, and their impact on the management of grow-out systems for M. rosenbergii, are fully discussed and illustrated in Chapter 16.

3.3.2 Mating and spawning

Freshwater prawns can reproduce either continuously or periodically, depending on their geographical distribution. In some regions (e.g. Malaysia) reproduction is possible throughout the year (Ling 1969) but in others, particularly monsoon regions, it is seasonal (George 1969).

Mating behaviour has been discussed by several authors, including Ling (1969), George (1969), Sandifer & Smith (1985) and Valenti (1985, 1987). After ovarian maturation, M. rosenbergii females experience a moult, known as a pre-spawning orpre-mating moult, which usually occurs at night. After this moulting process occurs, courting and mating commence. Courting behaviour involves stroking, movements of the antennae and male chelipeds touching the female, and raising the body, which lasts a few minutes until the female accepts the male. Mating is preceded by a cleaning act, in which the male holds the female and carefully cleans her ventral thoracic region. Following this behaviour, copulation begins; the male depositing a sperm mass into the pre-cleaned area of the female. The reproductive behaviour of freshwater prawns is also discussed in Chapter 16.

Thomas (1998) identified six distinct stages in mating behaviour but more recent work using video recording has refined this to five (Al-Mohsen 2008). Previously, very little work had been conducted on female mating behaviour but these studies outline the characteristic mating response of a pre-copulatory Drach stage A (see p. 28) moulted female.

• The female approaches the most dominant male BC while at the same time the BC male will only approach the newly moulted female. However, in communal tanks, the pairing would usually occur prior to the moult with the female seeking out the male.

• Antennal contact is maintained at all times by sweeping movements between male and female at this stage.

• The female either orientates herself to the front of the male and moves backwards until she is between his chelae or the male moves towards her from the rear to position her telson under his cephalothorax region.

• The male mounts the female, and begins a rubbing action with his pleopods on the ventral lower cephalothorax and on the dorsal and sides of the abdominal segments of the female. The female appears to become torpid.

• Using the chelate pereopods the male begins to turn the female body over. Once she has been turned on to her dorsal side her chelae are stretched out in front and she remains torpid. In this position the reproductive organs are aligned.

Once mating has occurred, the male moves off the female, who recovers and reverts to normal behaviour. When observed and recorded under laboratory conditions, the entire process of copulation lasts 2 to 3 minutes without disturbance. The fact that the female is rotated in this manner may be the reason why the female chooses the largest BC male. In this position she is vulnerable, andthe BC male will defend her aggressively from other males. The rubbing and cleaning behaviour may play a role in creating the trance like torpor observed in the female. According to R. Smullen (pers. comm. 1999), it has been demonstrated in Panulirus cygnus in Western Australia that scratching the region between the cephalothorax and the abdomen results in the crayfish becoming torpid. In this state, it can be positioned ventral side up forbetween 2 and 3 minutes. However, when placed in its normal position, the crayfish reverts to normal behaviour. It is possible that the similar action by the male Macrobrachium, described above, induces torpor in the female. However, this needs further investigation, as it is quite possible that this behaviour may be an attempt by the male to remove the spermatophore of a previous partner.

It had been previously believed that male M. rosenbergii are attracted to females when their ovaries are in a late maturing stage. Thomas (1998) demonstrated that this is not the case. Newly moulted stage IV females (Drach moult stage A) are very strongly attracted to BC males. Under experimentation, when BC males are enclosed in a perforated box, the attraction time of a female to a BC male is totally dependent on ovarian stage, with the stage described above demonstrating the most rapid pairing. Attraction time decreases in descending order from premoult ovarian stages IV to I, the latter being attracted least to the BC males. Recent work has shown that females will show attraction to BC males most strongly but will also respond to SM males but not to OC males (Al-Mohsen 2008) When the attraction experiments were reversed, i.e. males were free to move and could approach enclosed females, only the SM were attracted to the females, again with attraction times corresponding to the ovarian stages, i.e. soft postmoult females were the most strongly attractive to the SM. This would cor-respondto the normal observedbehaviour ofthe SM, which attempts sneak copulation with females that are guarded as part of a BC harem. This study also demonstrated fighting behaviour between BC males in the presence of newly moulted females. If females of differing ovarian stages were placed in an opaque perforated box, the BC males would fight only in the presence of a late stage IV female or a newly moulted mature female. This topic needs further investigation because the BC males could be separated into two categories: some fight all the time, while others fight only in the presence of females. These studies indicated that pheromones play a major role in attraction, both between the sexes and the different morphotypes. Further work on this subject is clearly necessary, since the identification of male and female pheromones could play a major role in improving the aquaculture and husbandry of this species.

M. rosenbergii, like other decapods, has aflagellate and non-motile spermatozoa (Felgenhauer & Abele 1991), which are enclosed in packets called spermatophores formed by a secretion of the male accessory sex glands. These are rich in glycoprotein and acid mucopolysaccha-rides (Dougherty et al. 1986), which may keep the delicate

Medulla externa Medulla interna

Medulla terminalis

Optic nerve

Medulla externa Medulla interna

Medulla terminalis

Optic nerve

Sinus gland

Neurosecretory cells of the ganglionic X-organ

Fig. 3.8 Internal eyestalk structures in M. acanthurus. (Source: adapted from Correa et al. 1996b, with permission of the Brazilian Journal of Biology.)

spermatozoa viable until fertilisation (Subramoniam 1991). Within a few hours after copulation, spawning is initiated through the gonopores into the brood chamber, guided by the ovipositing setae; hence fertilisation occurs externally. The eggs remain adhered to the female ovigerous setae during the whole embryonic development, which lasts around 3 weeks (depending on temperature). Finally, hatching occurs and the embryos hatch into free-swimming zoeae. In contrast, penaeid shrimps, once spawned and fertilised, release their eggs into the environment where they hatch. The embryonic development of penaeids is shorter than carideans, and the eggs hatch into nauplii.

At spawning, the eggs of M. rosenbergii have low water (55%) and ash (0.9% DW [dry weight]) contents, and the remainder is a protein-rich yolk (31.1 |ig protein and 14.4 |g lipid); together, the protein and lipid contents occupy virtually all of the organic content of the egg (Clarke et al. 1990). According to these authors, both lipids and proteins are utilised in the developmental processes, but the former provide the greater proportion of energy (0.21 J from lipids as compared to 0.12 J from protein, between the fifth day of development and the last day before hatching). During this period the developing embryo utilises 4.8 |g protein and 5.3 |g lipid. After hatching, the first zoeal stage larvae have enough energy to go through their first moult without feeding.

3.3.3 Physiology

Endocrine control of reproduction in crustaceans is a very complex process, which has been extensively reviewed by Adiyodi & Adiyodi (1974), Adiyodi (1985), Fingerman (1987), Huberman (2000) and Chang et al. (2001). It remains an active area of research that cannot be fully covered in this chapter but is essentially summarised as follows.

The X organ-sinus gland (XOSG) complex, located in the eyestalks, contains two distinct hormones that inhibit moulting (MIH) and gonadal development (GIH). Another neurohormone, found in the brain and thoracic ganglia, is the gonad-stimulating hormone (GSH). When MIH and GSH levels in the haemolymph are low, and the levels of GIH and the moult hormone (MH) secreted by the Y organs are high, moulting is induced. Low titres of GIH start the vitellogenic and spermatogenic processes.

The ganglionic XOSG complex is the major site of neuroendocrine control in crustaceans, and is involved with several physiological processes, such as moulting, growth, sexual maturation and the regulation of metabolism (Adiyodi 1985). The nerve fibres from the ganglionic X organ run to the sinus gland, a neurohaemal organ that stores and releases the neurohormones. The ganglionic X organ morphology and the neurosecretory system of freshwater prawns were investigated by Correa et al. (1996b). According to these authors, the eyestalk of M. acanthurus follows the typical standard found in the majority of the decapods, i.e. the nervous tissue is concentrated into three lobes: the most proximal is the medulla terminalis (a brain centre) that is connected with the protocerebrum by the optic lobe peduncle (Fig. 3.8). The other two lobes, the medulla interna and externa, are optic centres. The medulla externa is connected with the lamina ganglionaris, the neuropil lying immediately behind the retina. Eight cellular groups were identified around the neuropil ofthe medulla externa, interna and terminalis. Correa et al. (1996a) identified five different types of neurosecretory terminal axons in the neu-rohaemal organ, the sinus gland, of M. acanthurus. They suggested that the neurosecretory granules might be released mainly by exocytosis.

The ovarian development of M. rosenbergii can be classified into five stages by external observation through the carapace, based on their size and colour (Chang & Shih 1995). Such stages reflect vitellogenesis, i.e. egg yolk production and accumulation. In stage I, the ovaries are white, corresponding to pre-vitellogenesis; in stage II a small yellow mass of ovarian tissue can be observed dorsally in the carapace, near the epigastric tooth, consisting of pre- and vitellogenic oocytes; in stages III—IV, the ovaries are orange, corresponding to vitellogenesis; and stage V is characterised by reddish ovaries, extending from behindthe eyes up to the first abdominal segment. At this time, the ovaries are composed ofyolk globules fully occupyingthe oocytes. After the ovaries reach stage V, the females are ready to moult and spawn. This classification has been validated by histological studies through the ovarian cycle by Martins et al. (2007), allowing confidence in this non-invasive technique for classification. Likewise, Meeratana & Sobhan (2007) classified the stages of differentiation of oocytes histologically and described five stages of ovarian development that fitted with the externally visible signs. The description of the ovarian stages of M. rosenbergii is similar to that described for M. acanthurusby Carvalho &Pereira (1981).

Sagi et al. (1995) suggested that an extra-ovarian source of vitellogenin existed in M. rosenbergii. Oogenesis occurs continuously, the ovary redeveloping even when the females are 'berried'. Induction of spawning, in order to achieve the synchrony which would be beneficial in hatcheries, has been achieved by Zhao et al. (1995). These workers dropped 40 |il of 1% juvenile hormone analogue-ZR515 on each prawn body surface. The hormone permeated through the integument to stimulate maturation and spawning. After 16 to 17 days, the spawning rate reached 41.8%, which was over 50% higher than routine. Large amounts of high polarity immunoreactive products (HPP) accumulate in the ovaries and eggs of M. rosenbergii during vitellogenesis and embryogenesis (Wilder & Aida 1995). These workers also found that methyl farnesoate (MF) was present but, although its level fluctuated during the moult cycle (Wilder et al. 1995), were uncertain whether it is involved in reproduction. This topic was reviewed by Wilder (1998). Evidence does now point to MF having a role not only in crustacean reproduction (Laufer et al. (1993) but also with ecdysteroids in morphogenesis (Laufer & Biggers 2001).

Okumura & Aida (2001) showed during bilateral eyestalk ablation in M. rosenbergii that in the absence of hormones produced by the eyestalk the moult cycle continued and the link between vitellogenesis and the moult cycle was maintained. Martins et al. (2007) looked for vertebratelike steroids in M. rosenbergii but could only point to their having a possible role in endocrine regulation ofthe ovarian cycle.

Like moulting, the reproductive processes require significant energy mobilisation. Sensory neurons send information to the endocrine systems in the form ofneurosecretory hormones. The XOSG complex plays a central role in the regulation of metabolism. The hyperglycaemic hormone (HGH), more recently referred to as the crustacean hyper-glycaemic hormone (CHH) (Chang 1992), is synthesised and released from the XOSG and is primarily concerned with carbohydrate metabolism. Lin et al. (1998) showed that an injection of XOSG extract evoked a hyperglycaemic response which peaked in 1 hour. Hyperglycaemia is a characteristic response to cold shock in M. rosenbergii, but CHH may not be the only factor (Kuo & Yang 1999); norepinephrine and/or octapamine may also be involved.

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