DIGESTIVE PHYSIOLOGY OF SHRIMPS

 

Numerous anatomical descriptions of the digestive apparatus of crustaceans have been published over the past centuries. They poorly represent the complexity of specialized organs involved in digestion of species in this group. In addition, they do not take into account-except in a very partial manner-the immensity and diversity of the subbranch of the crustacean. The first studies of digestive physiology are rather ancient but from the basis of our current understanding. However, they did not become universally applicable until more recent times. While studies of nutritional requirements of crustaceans, using spesies of commercial interest and especially penaeid shrimps, have multiplied during recent decades, the same effort has not been deployed on fundamental research; comparative physiology remained the main line of interest for biologists and zoologists, and in the phyla of interest to aquaculture to which we are limited, only a few studies have been devoted to enzymology and digestive physiology. In this chapter, the term ‘shrimps’ is used to cover all decapod crustaceans commonly referred to as ‘shrimp’ or ‘prawn’.

 

This chapter is devoted to the decapods and concentrates on the penaeid family. The so-called inferior crustaceans, including the Branchiopods, are not considered, despite the considerable importance of artemia salina in aquaculture (chapter 13).

 

In many studies, aspects of anatomy and physiology are ignored. The unique characteristics of the crustaceans in contrast to higher animals must be emphasized in order to allow a better comprehension of the principal mechanisms that form the basis of their nutrition; technicians of shrimp, giant freshwater shrimp or crayfish have to take them into account. These characteristics, of which the last five will be specially mentioned, are:

  • Existence of moults which obligates cycles of laying down of reserves; reutilization of nutrients;
  • Existence of periods of physiological fasting during these moults;
  • Renewal of internal walls of part of the digestive tract;
  • Unique anatomy of mastication and absorption organs;
  • Specific mechanisms for digestion and absorption;
  • Existence of certain unique enzymes, alongside enzymes that are very similar to those found in vertebrates;
  • Involvement of the digestive gland in various functions and in particular in nutrient storage.

ANATOMY AND HISTOLOGY OF THE DIGESTIVE TRACT

 

This tract (figure 15.1) is classically divided into five parts: mouth, oesophagus, stomach, hepatopancreas and midgut and hindgut. While this subdivision is entirely justified, the naming of the different parts by reference to the higher vertebrates is much less so and sometimes leads to a certain amount of confusion. Thus, the stomach in crustaceans only remotely resembles that of vertebrates and the digestive gland or midgut gland or hepatopancreas, which is neither a liver nor a pancreas, is not even the equivalent of that found in certain fish in which the liver is penetrated by pancreatic acini.

 

Mouth

As in other arthropods, the mouth is surrounded by several pairs of appendages which are specialized for chemoreception and prey capture and manipulation: maxillae, maxillulae, mandibles and maxillipeds. These appendages allow the animal to bring food close to mouth, to commence breaking up the food, especially in the decapods, using the maxillipeds to sort particular sizes of food which are the right size for the buccal cavity, and finally to swallow them. The mouth it self has a relatively hard labrum which mainly plays the role of a value. Note that in very young larvae, the antennae, antennules and mandibles are used primarily for swimming.

 

Oesophagus

As in other animals, the lumen of the digestive tract is part of the external environment and its walls constitute a continuation of the epidermis. In crustaceans, this situation is particularly obvious since the walls of the anterior as well as the posterior parts of the digestive tract are covered by a thin layer of chitin, a major constituent of the exoskeleton. This cuticle is renewed during each moult. The oesophagus thus appears to be a conduit with walls made up of supple chitin-protein complexes. It is relatively short and straight in species important to aquaculture; its cross section is generally X-shaped. Waves of contraction run through it that are quite similar to, though simpler than, those found in the vertebrate oesophagus.

 

Stomach

In crustaceans the stomach, like the oesophagus, forms part of the anterior gut in broad terms (figure 15.2). Its exterior structure is relatively simple and relatively rigid. During larval development, the presence of hard structures (bristle, spines and brushes) on the internal walls of the stomach have been described, which masticate the food in the same way as teeth do in vertebrate mouths. The development of these structures results in the ‘gastric mill’. In the decapods, the stomach is made up of two parts separated by a marked constriction which has a sort of valve; its complexity varies between groups. Extending the usage of the terminology employed for the higher vertebrates, these two parts have been called the cardiac chamber and the pyloric chamber. Like the oesophagus, the cross-section of the cardinal chamber is X-shaped as a result of the presence of folds in the form of grooves and thickenings of the walls. The antero-ventral part of this chamber contains a ridge with a row of hard, pointed projections called ossicules, or teeth where they are large (figure 15.2). These calcified and articulated structures are moved by the action of specific muscles situated on the external walls of the stomach bag. Their number and complexity are characteristic of the species – there are 14 in the penaeids.

 

Neurophysiological studies have demonstrated a complex innervations (six groups of different nerves) allowing the movement of the different parts of the gastric mill, contractions of the cardio-pyloric valve as well as peristalsis of the main part of the pyloric stomach. Chemoreceptors and mechanoreceptor organs also play their part in these processes. All these parts, along with the specialized muscles, constitute the gastric mill, a crushing organ which also has a powerful unpaired dorsal tooth. The teeth and ossicules are shed at each moult, at the same time as the exoskeleton, the walls of the oesophagus and the posterior intestine. The complexity of the gastric mill varies between families and appears to be inversely linked to that of the mandibular organs: the more effective they are at breaking down the food, in addition to simple prey capture and manipulation, the simpler the gastric mill, and vice versa. It is well developed in the brachyurids (crabs), moderately developed in the penaeids and very poorly developed in the carids, which include the common prawn and the giant freshwater prawn (macrobranchium).

 

Calcified structures extend into the pyloric sac that forms the start of the midgut or mescuteron. Hard structures also exist here in the form of comb teeth as well as bristles which are sometimes very complex, and tubercules. The sac it self also contains tight folds. Folds, spines and bristles only allow the finest particles of the food bolus to pass; in other words, there is crushing by the teeth, compression by the stomach movements and filtration through the combs. The pyloric stomach compresses, sorts and filters. The finest particles thus cross a ventral, posterior filter sometimes called the filter-press from whence they are directed towards the hepatopancreas an absorptive organ. Because of this, some zoologists consider crustaceans to be filter-feeding organisms, under the same heading as other marine invertebrates such as bivalve mollusks. If mastication and filtration were carried out by the mandibles, a large part of the food would be lost by dissolving into the surrounding environment.

 

The path of food particles to the interior of the stomach is complex and cannot be reduced to simple sequences: crushing, compression and filtration. Despite the difficulties involved in observation, detailed studies have been able to describe part of the particles path. The most likely paths are shown in figure 15.2. Food coming from the oesophagus moves firstly from front to back in the upper groove until it reaches the gastric mill. After grinding and a first mixing with digestive enzymes, the chime returns to the forward part, but during this movement the fine particles separate off towards the rear in the ventral gutter until they reach the filter-press. The finest particles, only a few micrometres in size, can pass this and reach the opening of the digestive gland or hepatopancreas in the pyloric chamber, where

they are digested and then absorbed through the internal lumen of the tubules. Instead of falling into the ventral channel, particles which have not been sufficiently crushed are subjected to another cycle: they reach the anterior part of the cardiac chamber, then pass again towards the rear in order to go through the gastric mill again. This reprocessing of food can continue until it is sufficiently crushed. Particles that are too hard or indigestible, however, finally enter the pyloric chamber, but they cannot pass across the filters and pass into the midgut; they constitute the faeces.

 

The other unique features of the stomach are an external musculature, a quasiautonomic innervations and a particular kind of coating. In most species, the pH of the stomach contents is neutral or slightly alkaline, which is not surprising when the pH of seawater is about 8.3. The organ does not possess any glands or cells that secrete acids or enzymes. Slightly acidic pHs have, however, been indicated in the lobster. Despite this, crustacean stomachs do not correspond to the usual definitions for vertebrates to which physiologists are accustomed.

 

Digestive gland

The hepatopancreas, is a massive organ made up of two symmetrical lobes. It is situated in the dorsal part of the body, immediately below the heart. It represents 2-6% of the body mass and in the adult is formed of hundreds of blind-ending tubules opening into two symmetrical chambers which open into the pyloric chamber oof the stomach. Each tubule has a double network of longitudinal and circular muscles that allow peristaltic and contraction movements, ensuring the movement of the liquid phases within the tubules.

 

In the larva, the hepatopancreas becomes differentiated in the form of diverticuli that appear from each side of the primitive stomach, which is then a simple bag. During larval development, these tubes elongate and multiply greatly. Connective tissue surrounds all these structures and a network of muscle fibres forms to shape the definitive gland. The fundamental structure of the tubules never differs very much from the primitive organization and the gland remains as a collection of blind-ending tubules that converge towards the mesenteron, connected by a collecting bulb (figure 15.3).

 

Within the tubules, as in the acini of secretory glands or in the intestinal villi of vertebrates, a distal zone can be distinguished in which there is cellular differentiation from embryonic cells, then one or more functional zones, secretory and Absorbent and finally, at the proximal part of the tubules, a zone in which the cells degenerate and are rejected into the lumen of the digestive tract. Nomenclature, classification and function of the cells making up the tubules have yet to be agreed unanimously. However, a certain consensus has been reached around the interpretations described below.

 

E cells (embryonic) in the apical proliferation zone give rise to three main cell types: F cells (fibrillar), B cells which are secretory (from the German Blasenzellen, cells in bulbs) and R cells (from the German Resizellen, reserve cells). Other cell types have been described in the tubules, in particular M cell (midget cells). According to the most recently accepted theory, there might be only two types of cell family produced from the proliferation zone: F and B cells on the one hand, and M and R cells on the other. F cells might be intermediate between E and B cells and M cells intermediate between E and R cells. F cells appear to secrete enzymes. At a subsequent stage, that of young B cells, they are the site of endocytosis. When they are mature, these same cells are characterized by a very large central, ovoid vacuole which occupies almost the whole cellular space; there after they break down and their contents are released into the tubular lumen. B cells are thus secretory. R cells contain significant amounts of lipids, as well as glycogen, calcium, zinc and copper and thus certainly play a role in storage, for nutrients either originating from food or from the destruction of the animal’s own during the biological cycle prior to moulting. These cells, like their precursor M cells, also appear to be capable of significant absorption. The energy metabolism of crustaceans relies much more on lipid utilization than on a glycogenic cycle.

 

Movements of particles within the interior of each tubule are ensured by the successive contraction and relaxation of the organ and of each tubule. The mechanisms involved are still not fully understood, since the fluids contained in the tubules are partly discharged into the stomach to allow the start of enzymatic digestion while, apparently almost simultaneously, the chyme being prepared for digestion is aspirated into the tubules for enzymatic digestion and can then straight away be followed by the absorption of the products of biochemical digestion. Recent observations on larvae show rapid passage of food particles in the digestive process from some tubules that contract strongly towards tubules on the opposite side, due to the action of the valves situated beside the filter-press. It is not known if similar mechanisms exist in the juveniles.

 

Midgut

This rectinlinear part of the digestive tract extends from the pylorus to the rectum. Although its surface area is increased by the presence of quite large caeca or diverticuli and the apex is covered in well-developed microvilli, it is not comparable to the vertebrate intestine with its convolutions and villosities. Nerve cells, haemocytes and probable endocrine cells can be distinguished in the epithelium. The great uniqueness of the epithelium in this area is its ability firstly to secrete mucus which coats the solid waste produced by stomach and then, by means of an annular structure, a pellicule of chitin forms a cylinder which envelopes the faecal mass; this is the peritrophic membrane of the excreta. This permeable membrane does not impede the absorption of residual nutrients from the faeces. Its role has not yet been elucidated, but may be analogous to that of the glycocalyx in vertebrates.

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