Anterior aorta artery heart stema artery

posterior aorta posterior aorta ventral thoracic artery hepatic artery ventral abdominal artery

Fig. 3.4 The circulatory system of caridean prawns.

ventral thoracic artery hepatic artery ventral abdominal artery

Fig. 3.4 The circulatory system of caridean prawns.

Venous return occurs via distinct channels into the collecting sinuses that supply the gills and branchiostegal circulations, and then to the pericardial cavity and finally to the heart. As a result of this arrangement, the heart is directly supplied with oxygenated blood from the gills, which is pumped to the tissues.

McMahon & Burnett (1990) reviewed the physiology of several advanced crustacean open circulatory systems, suggesting that they are not poorly designed structures with sluggish performance, as is commonly assumed. These systems are, in fact, complex, very efficient and highly regulated.

3.2.2.2 Respiratory system

The gill structure of M. olfersii has been described in detail by Freire & McNamara (1995) and may be representative of other Macrobrachium spp. The gills are phyllobranchi-ate, i.e. there is an elongated axial structure with a series of lateral branches (respiratory lamellae). Each gill (Fig. 3.5) bears a very large number of lamellae that consist of flattened plates, in which the perfusing haemolymph flows from lateral efferent vessels to the outer marginal canals, across the haemilamella by means of gill capillaries to the inner marginal canals and back through the central afferent vessel.

The gills are enclosed in branchial chambers (Fig. 3.5), which are formed between the thoracic body wall and the inner surface of the carapace (branchiostegite). These chambers offer protection to the gills, as well as directing the water current over them. The beating of a leaf-like flap, gills gills

Fig. 3.5 General anatomy of the gills of M. olfersii (A and B are reproduced from McNamara & Lima 1997, with permission from the Marine Biological Laboratory, Woods Hole, MA.)

Key: A = general anatomy of the gill; B = general anatomy of a gill lamella; ev = lateral efferent vessels; av = central afferent vessel.

The decapod heart is a condensed muscular structure (ventricule), which has paired ostial valves (normally three) in its wall that communicate between the ventricular cavities and the surrounding areas, known as the pericardium (or pericardial sinus). The heart pumps haemolymph into a complex arterial system which perfuses the tissues. The arterial system comprises several distinct outflow systems:

• a single anterior aorta that provides blood to the eyes and supra-oesophageal ganglion;

• anterior lateral arteries that are the primary suppliers of blood to the cephalic appendages, foregut walls, musculature, antennal glands and carapace;

• hepatic arteries, leaving anterolaterally;

• a sternal artery that runs towards the ventral region and divides into anterior ventral thoracic arteries and posterior ventral abdominal arteries (Fig. 3.4).

Anterior Abdominal Arteries

the scaphognathite (the exopod of the maxillae), causes a current of water to enter into the branchial chamber. Water is drawn from openings between the bases of the pere-opods and the thoracic walls, flows through the gills within the branchial chambers and is expelled anteriorly. The gills are directly involved in the respiratory process, as well as in the osmo-ionic regulation and excretion. A connection between metabolic rate, as measured by resting oxygen consumption (ROC), was found in relation to social status in M. rosenbergii; effectively the outcome of aggressive interactions could be predicted on the basis of this (Brown et al. 2003a).

Oxygen uptake and its transport to tissues are facilitated by respiratory pigments, which in M. rosenbergii is haemocyanin, a copper-containing protein, as reported in several other decapods. According to Taylor & Funge-Smith (1994), the oxygen affinity ofhaemocyanin ofM. rosenbergii is sensitive to changes in temperature, and also in salinity to a lesser extent. These phenomena are of great importance, since this animal is exposed to large fluctuations in environmental parameters, especially as it migrates to estuaries for spawning. Until now, there is no information on whether adaptive changes ofthe properties ofhaemocyanin may occur during ontogenesis.

3.2.2.3 Digestive system

The alimentary canal of decapods is a tubular structure with an anteroventral mouth and runs dorsally along the body terminating in the anus, located in the base of the telson. The digestive system is divided into three distinct regions (foregut, midgut and hindgut) and includes the digestive gland (hepatopancreas), cecae and diverticulae. The foregut and hindgut are ectodermal in origin and hence lined with cuticle. The midgut derives from the endoderm so has no cuticular covering and its cells are directly in contact with the lumen (Icely &Nott 1992). The midgut starts where the digestive gland joins the main tract.

The foregut is a chambered structure situated in the cephalothorax and consists of the oesophagus and the proventriculus, which is formed of an anterior and posterior chamber. The two chambers ofthe proventriculus have historically been referred to as cardiac and pyloric stomachs but, since this terminology derives from vertebrate anatomy where the anterior most part of the stomach is closest to the heart, this makes no sense whatsoever in crustaceans where the heart is found at the most posterior end of the thorax. This has been fully expounded in McLaughlin (1983) and Dall etal. (1990). The process of food mixing with enzymes passed forward from the digestive gland is essentially quite complex, as described in Dall & Moriarty (1983); the basic principle is that food has to be broken down into particles small enough to pass through the filterpress that guards the entrance to the digestive gland. This mixing action is aided by the action of muscles moving the proventriculus walls; in higher decapods they also act on the gastric mill, particularly dentate structures found between the two chambers. However, the gastric mill is not found in some Caridea (Dall & Moriarty 1983) including M rosenbergii (Kihara 1997).

The midgut of decapods is the primary site of enzyme action and nutrient absorption (Dall & Moriarty 1983). It exhibits a well-developed glandular appendage, known as the digestive gland or hepatopancreas, forming a compact complex of ducts and blind-ending tubules, which occupy a large part of the cephalothorax. This glandular appendage is involved with the synthesis and secretion of digestive enzymes, the re-absorption ofnutrients, the storage ofenergy reserves, as well as ion transport for mono- and di-valent cations and anions (Dall & Moriarty 1983; Icely & Nott 1992). A sodium-dependent glucose transport mechanism in the midgut epithelium of M. rosenbergii was demonstrated by Ahearn & Maginniss (1977). The indigestible material is packaged into faecal pellets within the midgut and pushed by peristaltic contractions into the hindgut. This final region is relatively short and terminates in a muscle controlled anus.

3.2.2.4 Excretory system

The excretory system of decapods has been described by McLaughlin (1983), and Mantel & Farmer (1983), on whose work this section has been based.

Several tissues and organs participate in the excretion of metabolic wastes in crustaceans, but the primary organ of urine production in adult malacostracans is the antennal gland (or green gland), which is paired and located in the cephalothorax. The morphology ofthis gland follows abasic plan: each consists of an initial sack (coelomosac), a long excretory tubule (a labyrinth, and a nephridial canal) and a large bladder. The opening is an excretory pore near the base of the antennae (hence the name, antennal gland).

Urine is formed in the antennal gland by filtration and reabsorption, with tubular secretion added. The most studied antennal gland is that of crayfish (Mantel & Farmer 1983). The distal portion of its labyrinth contains cells with active endocytotic vesicles, which may absorb proteins. The bladder is the site of uptake and secretion of ions and water. Glucose re-absorption also occurs in the antennal gland, but the specific site of this process has not been identified. In freshwater hyper-regulators, the renal organ is effective in salt re-absorption, producing a diluted urine.

Sarver et al. (1994) hypothesised that only a portion of the crawfish antennal gland Na/K-ATPase is involved with renal salt re-absorption, with the consequential production of dilute urine. The other portion may power non-osmoregulatory transport functions, such as organic acid secretion into the urine, and sugar and amino acid reabsorption from it.

Although the antennal gland is denominated as the excretory organ, the site of nitrogenous waste excretion is actually the gills. In decapods, the end product of protein metabolism is mainly excreted as ammonia, with small amounts as amino acids, urea and uric acid (Regnault 1987). According to Chen & Kou (1996a), M. rosenbergii was found to excrete 70.2% of its total nitrogen at 32°C as ammonia-N, 25.6% as organic-N (mainly amino acids) and 4.2% as urea-N.

The effects of environmental (temperature, salinity and pH), and physiological (moult cycle) factors on nitrogen excretion have been described for M. rosenbergii (Armstrong et al. 1981; Stern & Cohen 1982; Stern et al. 1984; Chen & Kou 1996a,b; Schmitt & Uglow 1996), but are not discussed here.

3.2.2.5 Reproductive system

The internal reproductive structures of M. rosenbergii are located in the cephalothorax.

In females, the paired ovaries are located dorsally either side of the proventriculus and dorsal to the digestive gland. They give rise to a pair of oviducts which extend towards, and open into, the gonopores on the basal segment of the third pereopods (Fig. 3.6) (Sandifer & Smith 1985).

In males, they consist primarily of a pair of testes, which are fused and lie mid-dorsally in the cephalothorax, each giving rise to a vas deferens. The paired vasa deferentia are simple tubes that end in terminal ampullae, which contain the spermatophores and open at the gonopores on

Ovaries Heart

Ovaries Heart

(5th pereopods)

Fig. 3.6 The internal reproductive organs of M. rosenbergii. (Source: modified after Sandifer & Smith 1985.)

(5th pereopods)

Fig. 3.6 The internal reproductive organs of M. rosenbergii. (Source: modified after Sandifer & Smith 1985.)

the coxae of the fifth pereopods (Fig. 3.6). During mating, the ampullae extrude the spermatophores, containing the sperm mass (Sandifer & Smith 1985). On each of the vas deferens can be seen a distinct structure - the androgenic gland - and it is the presence of these two glands that determines the development of the male primary and secondary sexual characteristics. The development of the androgenic gland depends directly on male genotype (Charniaux-Cotton & Payen 1985). This important role was first suggested by Charniaux-Cotton (1954) and demonstrated in M. rosenbergii by implantation and ablation experiments by Nagamine et al. (1980a,b). It has been usedforsexreversalpurposes (Sagi etal. 1990; Malecha et al. 1992; Aflalo et al. 2006) and studied in relation to development of morphotype (Okumura & Hara 2004); this topic has been dealt with in more detail in Chapter 16.

3.2.2.6Nervous system

The crustacean nervous system consists of a large supra-oesophageal ganglionic mass (also referred to as the brain) and a ventral nerve cord with a pair of ganglia corresponding to each embryonic somite. The ganglia are joined longitudinally by connectives and transversely by commissures (Fig. 3.7a) (McLaughlin 1983).

(a) Supraesophageal ganglion (Brain) Ventral nerve

(a) Supraesophageal ganglion (Brain) Ventral nerve

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