Since crustaceans have an external exoskeleton they can only increase in size by shedding this and growing a new exoskeleton which they can expand to a larger size. The actual shedding of the old exoskeleton or exuviae is known as ecdysis (moulting) but this is only the visible sign of what is a more or less continuous process in the life of crustaceans -the moult cycle.
The decapod moult cycle can be divided into stages and substages originally classified by Drach (1939). Drach used 4 stages, beginning at A (immediately after ecdysis) and continuing to D (immediately preceding the moult), based on the hardness of the exoskeleton in specific regions of the crustaceans he studied. In order to understand the changes fully it is necessary to understand the basics of the structure of the exoskeleton or cuticle. The description given here draws substantially from Stevenson (1985) and Skinner (1985). The changes through the moult cycle can be seen in changes in the cuticle itself. The cuticle has an outer layer, the epicuticle, composed of protein, lipids and calcium salts. This layer contains no chitin but the main layer below this, the procuticle, is largely made up of chitin. Chitin is a polymer of N-acetyl-glucosamine and glucosamine. It is a highly resistant polymer but can be converted to chitosan if treated with strong alkali at a high temperature (Stevenson 1985). The procuticle can be divided into two layers, one being formed in preparation for the next moult, the pre-ecdysial procuticle, and the other layer underneath this, the postecdysial procuticle which is formed after the moult. The postecdysial layer consists of a principal layer which is essentially the same as the pre-ecdysial procuticle and a further underlying layer, the membraneous layer which is always free of calcium salts. The procuticle also contains protein and calcium salts, the proportion of which will relate to the degree of hardness of the exoskeleton at that point (e.g. for the chelipeds there will be very high levels of calcium salts amongst the chitin chains, while in the underside of the abdomen these spaces will be filled with protein to maintain maximum flexibility and little or no calcium for hardness) (Fig. 3.9).
While Drach (1939) took the stages from immediately after the moult to the moment prior to the next moult, it is easier to understand the process if the starting point is taken as the intermoult (Drach stage C4). As the animal prepares to moult, the first visible sign of this is that the epidermis,
which forms the new cuticle, splits from the cuticle that is to be shed in a process known as apolysis. The membraneous layer of the cuticle is digested as the epidermis produces a moulting fluid containing enzymes or proenzymes. This is the start of premoult (Proecdysis; Drach stage D). The epidermal cells enlarge and start to produce the new epicu-ticle and pre-ecdysial procuticle. Material is resorbed from the old exoskeleton and because pigments (astaxanthin) are resorbed at the same time the colour of the blood reflects this change, becoming markedly pink. Calcium is resorbed and stored in the digestive gland and gastroliths on the proventriculus walls. Immediately prior to ecdysis the animal does not eat. At ecdysis the exoskeleton splits on the dorsal surface and the animal struggles out, dorsal surface first.
Immediately after the moult, the animal is effectively unable to move. In stage A Peebles (1977) describes how the chelae droop if the animal is held out of water. However, rapid uptake of water provides hydrostatic pressure that gives some rigidity to the limbs, so movement is possible even while the exoskeleton remains soft and malleable (Taylor &Kier 2003).
In this stage of post moult (Metecdysis; Drach A-C3) the uptake of water allows the animal to increase in size and the epidermis produces the postecdysial procuticle; both layers of the procuticle are calcified at this stage. Further economy of material is shown by the animal normally eating its exuviae (the cast off exoskeleton). The membraneous layer of the postecdysial procuticle is formed and this marks the start ofthe intermoult period (Drach C4).
A technique for identifying each stage and sub-stage of the cycle in live M. rosenbergii adults was devised by Peebles (1977) based on the hardness of the exoskeleton for the fundamental distinction between postmoult, intermoult and premoult and for distinguishing the intermediate stages by examination of the setal development within the antennal scale or, for some specific stages, of the pleopod tip.
It is important to consider the moult cycle as a continuous process. In some reptantian Crustacea the intermoult period can last for a considerable period of time, constituting a very significant part of the moult cycle; however, in mature M. rosenbergii the intermoult period is relatively short (29-79 days) (Peebles 1977)) and this is the stage where the animal must build reserves of glycogen and lipid. Moulting is of considerable metabolic cost to the individual crustacean. Amongst the less obvious costs is the necessity for the muscles of the chelipeds to atrophy in order for the legs to be extricated from the old exoskeleton. Thus, while knowing the stages of the moult cycle of individuals under farming conditions is not feasible, it can be useful for breeding programmes to know when a female will be ready for mating. It is also essential during any experimental treatments to ensure that results obtained are not being affected by the stage of the moult cycle the experimental animals are in. In experiments with PL or juveniles it is possible to quickly record what stage each animal is in, even if only on the basis of pre- or post-moult. For example, in the trials of tagging PL, Brown et al. (2003b) recorded moult stages to check if this affected mortality or the outcome of the tagging itself, while Taylor et al. (2002) showed that the metabolic rate as measured by the ROC was highest in postlarval prawns during the premoult, as had already been shown in other crustaceans.
More specific changes in the biochemistry and physiology of various tissues occur during the crustacean moult cycle, and have been reviewed by Chang (1995). Some specific aspects for M. rosenbergii follow. In the premoult stage, ionic changes occur in different body compartments. Calcium, which is transferred from the old exoskeleton to the extracellular fluid, is excreted or carried to the digestive gland for storage. As a consequence, an increase in the concentration of calcium in the digestive gland and haemolymph is observed, rather than a decline in the ion concentration ofthe whole body (Fieber & Lutz 1982). In the early postmoult stage, exoskeleton calcium increases while calcium levels in the extracellular fluid and digestive gland are depleted. Such changes during the moult cycle reflect the importance of calcium in the exoskeleton (Fieber & Lutz 1984).
Changes in netwatertransport in M. rosenbergii perfused midgut during the moulting cycle were demonstrated by Mykles & Ahearn (1978). The water flux rates were elevated in animals approaching ecdysis and much reduced in the postmoult stage. According to these authors, the midgut may be the major site of water absorption at ecdysis.
Ecdysis and growth in crustaceans are affected by several extrinsic factors, such as temperature, photoperiod, water alkalinity, food availability and others (for a detailed review, see Chang 1995; Hartnoll 2001). Reproductive moulting in M. rosenbergii is enhanced in 12L:12D, 32°C conditions (Justo et al. 1991). Prawns held in high alkalinity water moult more frequently, but no enhancement ofgrowth rate is observed (Latif 1992 apud Latif etal. 1994). PLandjuve-niles show optimum growth in the temperature range 29 to 31°C (Díaz-Herrera & Buckle-Ramirez 1993; Díaz-Herrera et al. 1993a) and growth rate is reduced with increasing salinity (from 0 to 20p.p.t.) (Díaz-Herrera etal. 1993b).
The moult cycle is under hormonal control (as reviewed by Skinner 1985; Fingerman 1987; Hopkins 1992; Lachaise et al. 1993; Chang 1995; Hartnoll 2001). In brief, a moult-inhibiting hormone (MIH) is secreted in the ganglionic X organ and transported to the sinus gland for storage and release. Moult-inhibiting hormone acts upon an epithelial endocrine gland localised in the maxillary segment, which is called the Y organ; this secretes ecdysteroids syn-thesised from dietary cholesterol. Evidence supports the hypothesis that 20-hydroxyecdysone is the moulting hormone (Huberman 2000). Wilder & Aida (1995) reported that an ecdysteroid 'surge' occurs in the late premoult stage; the predominant ecdysteroid was observed to be 20-hydroxyecdysone, followed by high polarity immunoreac-tive products (HPP). Spaziani etal. (1997) found that MIH released from the eyestalk occurs by means of an ecdysteroid feedback mechanism. The Y organ is negatively regulated, i.e. when the output of MIH by the XOSG complex is reduced, then the moulting hormone is secreted in greater abundance, and moulting events start.
In addition to the inhibitory regulation of the crustacean Y organ, there is evidence for the presence of a stimulatory factor. This factor, methyl farnesoate (MF), which is the equivalent of the insect juvenile hormone, is synthesised and secreted in the mandibular organ and has already been identified in M. rosenbergii (Sagi et al. 1991). However, its involvement in the moulting of this species remains unclear (Wilder & Aida 1995; Wilder et al. 1995). There are difficulties in administering MF in experimental work, since it is hydrophobic. These have been overcome by Abdu et al. (1998a), who showed that this hormone could be introduced into M. rosenbergii larvae through a vector, Artemia, which had been cultured in a lipid medium enriched with MF. These authors found that the administration of MF retarded larval growth (carapace length) and the stage-specific morphological features between larval stages V and IX. Furthermore, Abdu at al. (1998b) found that MF significantly affected the patterns of metamorphosis and the appearance of intermediate individuals that exhibit both larval and postlarval morphology and behaviour. The relative abundance ofthese intermediate forms increased from 2% in the control to 32% when the MF concentration was high, suggesting that this hormone has a juvenoid effect.
The complexity of the crustacean endocrine system is such that it cannot be said to be fully elucidated yet. The changes in the physiology just occasioned by the process of moulting require control of a large number of factors; knowledge on these control systems is slowly growing. A further hormone, crustacean hyperglycaemic hormone described in the sinus gland (Huberman 2000) has also been shown to have a role in the water uptake necessary to increase the animal's size at ecdysis when produced from endocrine cells of the foregut and hindgut in Carcinus maenas (Chung etal. 1999).
Generally, Macrobrachium males grow larger than females (Holthuis 1980), except for M. ohione (Truesdale & Mermilliod 1979). Bertalanffy mathematical models have been used to describe the growth rate of several species (Guest 1979; Bond & Buckup 1983; Valenti et al. 1987, 1994; Kurup et al. 1992). These equations suggest that the life span of small species (e.g. M. borelli and M. acanthurus) is about 2 years. M. rosenbergii can reach at least 3 years, while in M. carcinus the life span varies from 6 to 8 years.
Kurup et al. (1992) observed that M. rosenbergii males and females can reach 330 mm and 290 mm, respectively, somewhat larger than observed by Holthuis (see section 126.96.36.199). Generally, female growth in decapod crustaceans is retarded after they reach maturity, due to the fact that part of their energy is diverted to egg production and incubation (Hartnoll 1982). According to Ra'anan et al. (1991), the growth rate of immature females is relatively high, initially exponential, but after sexual maturation this rate reduces considerably. Thus, size variation decreases with time, an initial highly variable female size distribution being replaced by a relatively normal distribution, with a mean value above the size threshold (~18-26 g) for maturation. Male growth exhibits a very complex model which is fully discussed in Chapter 16. Growth rate and its consequences for M. rosenbergii production in ponds are discussed in Chapter 9.
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