Osmoionic regulation

M. rosenbergii inhabits both freshwater and brackishwater environments during its life cycle. Therefore, each larval developmental stage, in addition to juveniles and adults, must be able to face the problems related to a hypo-osmotic medium, i.e. control of the volume of the haemolymph, prevention of ion loss and compensatory uptake of ions from the medium.

Although knowledge about larval osmoregulatory capability of this species is of great importance, there is a general lack of information on this topic. In a recent review, Charmantier (1998) summarised different types of the ontogeny of osmoregulation in crustaceans and noted that in decapod larvae, only a few species exhibited the same adult ability to osmoregulate, immediately after hatching. Like M. rosenbergii, M. petersi also migrates to estuaries to spawn and exhibits this pattern, since first-stage larvae are able to hyper/hypo-osmoregulate (Read 1984). According to this author, there is a relationship between the osmoregulatory capability of the developmental stages and their field distribution. The first zoeal stage of M. petersi is found both in freshwater and brackishwater, and has a strong ability to os-moregulate. From the second stage onwards, the larvae lose the ability to osmoregulate in freshwater and thus are confined to estuaries. The ability to osmoregulate is recovered when larvae reach metamorphosis.

The first zoeal stage of M. rosenbergii is able to survive for a few days in freshwater, suggesting that these larvae also display a strong ability to osmoregulate. Evidence for a progressive adaptation to freshwater during the ontogeny of M. rosenbergii is indicated by a decrease in the slope of the osmoregulation curves and a decrease in the isos-motic value (Harrison etal. 1981). Further evidence of such adaptation was demonstrated by Agard (1999), who found that the larvae of M. rosenbergii show a tendency to increase the rate of ammonia excretion as salinity decreases, under experimental conditions. Sandifer et al. (1975) showed that early PL are able to hypo/hyper-regulate the osmotic pressure of their haemolymph, in a wide range of salinity. However, the hyporegulatory ability is lost during the development to adulthood (Castille & Lawrence 1981; Stern et al. 1987; Funge-Smith et al. 1995). In fact, such loss can be an adaptive advantage, since the juveniles migrate to the adult habitat, i.e. to a freshwater environment. The difference in the patterns mentioned above suggests changes in the ontogeny of osmoregulatory capacity in M. rosenbergii, as reported by Read (1984) for M. petersi.

M. rosenbergii is also an effective regulator of the major ions, when exposed to a wide salinity range (Stern et al. 1987; Funge-Smith et al. 1995; Wilder et al. 1998). Haemolymph sodium and chloride concentrations are hyper-regulated in salinities below the iso-ionic point (0-15p.p.t.), and in salinities above the iso-ionic point the concentration of both ions follows that of the medium. Calcium andpotassium ions are hyper-regulated at all salinities, whereas magnesium ions are always maintained at a lower concentration than that of the medium. This species can also regulate oligo-elements, such as bromide and strontium (Funge-Smith etal. 1995). Thehaemolymphbromide concentration was hyper-regulated from freshwater up to the iso-ionic point, whereas strontium was hyper-regulated up to 18 p.p.t. In higher salinities, bromide levels followed those of the medium, whereas strontium was maintained hypo-ionic. These data suggest that both ions may have a physiological role in this species.

Examining the crystalline structure and ionic composition of the cuticle, Wilder et al. 1998) found that it was comprised principally of an a-chitin-like material, and cal-cite (calcium carbonate). Calcite accounted for 25% of total bulk weight in freshwater, while total sodium, potassium and magnesium compounds comprised less than 2.5% of this total. Although contents of these three ions increased nearly two-fold in response to changing salinity, calcium levels remained relatively constant. Wilder etal. (1998) considered that this may signify a possible role in preventing excessive titres from arising in the haemolymph under initial exposure to high salinity. However, these authors postulated that the cuticle might also serve as a reserve for maintaining calcium levels in the haemolymph.

The production of hypotonic urine, with respect to blood, is one of the important mechanisms that allow the achievement of high blood ion concentration in dilute media. According to Stern et al. (1987), M. rosenbergii may produce hypo-osmotic urine when exposed to freshwater, whereas in brackishwater they may produce iso-osmotic urine.

The isosmotic point of several Macrobrachium species is also directly related to species biotopes. Thus, species found in freshwater and brackishwater environments show higher values of their isosmotic points when compared to species found exclusively in freshwater (Moreira et al. 1983). The isosmotic point of M. rosenbergii adults is about 440 mOsm (Singh 1980); this low value, correlated with its pattern of osmoregulation, follows a trait shown by truly freshwater crustaceans (Mantel & Farmer 1983).

The osmotic and ionic concentration maintenance processes in freshwater crustacean haemolymph, through mechanisms of ion uptake and re-absorption, are regulated by neuroendocrine factors. In hyper-regulating crabs, a water-soluble factor found in the thoracic ganglion decreases water influx, while an acetone-soluble factor increases water influx. Dopamine, found in the pericardial organ, and cyclic AMP (adenosine 3',5'-cyclic monophosphate) increase sodium uptake in the gills (Kamemoto 1991; Pequeux 1995). A series of studies carried out on the prawn M. olfersii corroborate the evidence for hormonal control of salt transfer and water permeability.

McNamara et al. (1990) demonstrated that eyestalk ablation affects haemolymph Na concentrations during acute salinity exposure and suggested that the influx of sodium into the haemolymph, through the Na/K-ATPases located in the gill of M. olfersii, is at least partially modulated by a factor located in the eyestalks (or released from them). McNamara et al. (1991) suggested that neurofactors located within the thoracic ganglion may alter the apparent ionic permeabilities of this freshwater prawn.

Santos & McNamara (1996) proposed a model of os-moregulation for M. olfersii facing a freshwater environment. The animal hyper-regulates the osmolality and ionic concentrations of its haemolymph primarily through active salt uptake by means of Na/K-ATPases located in the gills and salt re-absorption from the urine by the antennal glands. Passive water entry is restricted by low osmotic permeability and water gained osmotically is excreted by the antennal glands by means of dilute urine.

How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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