Control of hemolymph volume by adjustments in urinary and drinking rates in the crab, Cancer borealis.
T.M Greco, M.B. Alden and C.W. Holliday. Comp. Biochem. Physiol. 84A: 695-701, 1986.
(http://www.lafayette.edu/~hollidac/borealispaper.html)
Abstract
1. Cancer borealis rapidly and accurately regulates hemolymph volume by appropriate changes in urinary and drinking rates.
2. Hemolymph volume is probably sensed by receptors in the posterior cephalothorax, as section of the nerve cord caudal to the thoracic ganglion at the level of the fourth perieopod eliminates the crab's ability to control hemolymph volume.
3. It is proposed that stretch receptors in muscles of the posterior cephalothorax are the most likely sensors for hemolymph volume.
Introduction
Crustaceans are hard-bodied animals with open circulatory systems; such animals have limited ability to swell or shrink in response to changes in osmotic movements of water into or out of the hemolymph. Control of hemolymph volume in crustaceans by appropriate changes in urinary and drinking rates is well documented. Many workers have found that urinary rates in a variety of crustaceans are rapidly increased in dilute media (transiently in the case of osmoconformers) and are reduced in concentrated media (reviewed by Mantel and Farmer, 1983). In fact, urinary rate has been used in several studies as a first approximation of osmotic water entry, with the assumptions that urine is produced at a rate sufficient to keep hemolymph volume constant and that drinking is an insignificant component of water uptake from the medium. Conversely, drinking rates in several osmoregulating crustaceans have also been reported to decrease in dilute media and to increase in concentrated media, as would be expected if hemolymph volume were to be controlled (Lockwood, 1970; Hannan and Evans, 1973, Baldwin and Kirschner, 1976a,b). In addition, Mykles (1980) has shown that increased hemolymph volume during moulting in the lobster, Homarus americanus, is caused by drinking. Thus, accurate regulation of hemolymph volume by changes in drinking and urinary rates has long been assumed to occur. However, no published studies have actually documented accurate control of hemolymph volume by measuring changes in drinking rate in response to measured changes in hemolymph volume. Wolcott and Wolcott (1985) found that the ghost crab, Ocypode quadrata, accurately controls hemolymph volume by eliminating injected water or crab Ringer solution.
Little is known of the mechanism used by crustaceans to sense changes in hemolymph volume. Pilgrim (1974) has suggested that stretch receptors in muscles of the cephalothorax of hermit crabs might serve as sensors for changed hemolymph volume. This hypothesis is attractive because stretch receptors have been implicated in control of diuresis and, therefore, hemolymph volume in other arthropods (Gee, 1975; Stobbart, 1977; Kaufman et al.). Norfolk (1978) has suggested that urinary rate and hemolymph volume may be partially controlled in Carcinus maenas by monitoring the external salinity in a "feed-forward" fashion.
The present study was undertaken to determine how well hemolymph volume is regulated in Cancer borealis and to determine what role appropriate changes in urinary and drinking rates play in rectifying measured changes in hemolymph volume. In addition, we attempted to determine if stretch receptors are used to monitor hemolymph volume and to determine the level of entry of hemolymph volume sensory information into the crab's ventral nerve cord.
Discussion
Many workers have found that urinary rates of a variety of crustaceans are increased in dilute media (transiently in the case of osmoconformers) and are reduced in concentrated sea water (reviewed by Mantel and Farmer, 1983). Conversely, drinking rates in several crustaceans have also been reported to decrease in dilute media and to increase in hypersaline media, as would be expected if hemolymph volume were to be controlled (for example, Lockwood, 1970). Thus, accurate regulation of hemolymph volume in crustaceans by changes in drinking and urinary rates has long been assumed to occur. Wolcott and Wolcott (1985) have shown that injected water or crab Ringer solution is rapidly and nearly completely voided by Ocypode quadrata. Our results show that C. borealis also accurately regulates its hemolymph volume by appropriate changes in drinking and urinary rates.
Accurate regulation of hemolymph volume implies the existence of sensors which detect changes in this parameter. Pilgrim (1974) has suggested that stretch receptors in certain muscles in the cephalothorax of hermit crabs may serve as sensors for hemolymph volume. Pilgrim's hypothesis led us to look for evidence of such stretch receptors in C. borealis. Although our preliminary experiments did not demonstrate the existence of stretch receptors which serve to monitor hemolymph volume, we have demonstrated that sensory information about hemolymph volume enters the crab's ventral nerve cord between the anterior abdomen and the level of the fourth perieopod. Because anuric crabs well most noticeably at the junction between the carapace and abdomen, we believe that stretch receptors which monitor hemolymph volume, 
if they exist at all, are most likely to be found in muscles which originate on the endophragmic skeleton and insert on the posterior carapace. This notion is reinforced by our finding that sensory information about hemolymph volume enters the nerve cord in the posterior cephalothorax. Another possibility for muscle stretch receptors which might serve as hemolymph volume sensors are the many muscles which originate on the carapace and insert on the dorsal wall of the gill chamber.
Crustaceans are known to drink in a variety of sea water media (Lockwood, 1970; Hannan and Evans, 1973; Baldwin and Kirschner, 1976a,b) and, therefore, ingested water must be the source of some of the urine produced in all media. These authors have found that drinking is suppressed in dilute media and increased in concentrated media, as would be expected if hemolymph volume were to be controlled in osmoregulating crustaceans which experience osmotic gain or loss of water. The suppression of drinking during high rates of volume loading may account for the reduced difference we found between urinary rate and infusion rate at high rates of infusion in C. borealis (Fig. 2). This hypothesis is further reinforced by Holliday's finding (1978) that the crab, Cancer magister, has a lower urinary rate (as measured by the blocked nephropore method) when both ends of the gut are blocked to prevent drinking.
Norfolk (1978) has proposed that urine production in Carcinus maenas is controlled not by sensing changes in hemocoelic pressure (and, hence, hemolymph volume) but by monitoring the external salinity. This mechanism would have the added advantage of increasing urinary rate more quickly in dilute media and preventing excessive osmotic swelling. Mantel and Farmer (1983) suggest that in C. maenas a second "feed forward" mechanism of control of urinary rate and hemplymph volume may have been added to the basic mechanism postulated by Holliday (1978) in C. magister (and, as we have demonstrated, in C. borealis). This hypothesis is attractive because C. maenas is an osmoconformer; the latter two crabs are subtidal in distribution and probably experience smaller changes in salinity in nature.
With regard to the mechanism by which urinary rate is increased in dilute media, the elegant experiments of Norfolk and Craik (1980) provide good evidence that urinary rate is increased by increased pore size at the filtration site in the antennal gland. Spaargaren (1973) has proposed that increased heart rate and, presumably, arterial hemolymph pressure, could increase urinary rate in dilute media. However, hemplymph pressure in the heart of C. maenas (Norfolk, 1976) and the antennal artery of C. magister (Holliday, 1978) are unchanged in dilute media, indicating that urinary rate is not controlled by changing arterial hemolymph pressure in these crabs.
To the best of our knowledge, ours is the first study of the mechanism of control of drinking in crustaceans. Matthews (cited in Lockwood, 1967, p. 40) and Holliday (1978) have reported that lost hemolymph is rapidly replaced by drinking in C. maenas and C. magister, respectively. Lost hemolymph is also rapidly and nearly completely replaced by drinking in C. boralis (Fig. 4). The "trigger" for drinking appears to be a reduction of hemolymph volume by approximately 2.5-3.0% body weight (Table 1). Drinking rate is apparently also controlled by hemolymph volume sensory information which enters the nerve cord between the anterior abdomen and the level of the fourth perieopod, as section of the cord at the latter level abolishes drinking in response to reduced hemolymph volume.
Our current working hypothesis for the control of hemolymph volume by reciprocal control of drinking and urinary rates in crabs is summarized in Fig. 6. Further work must be done to localize the receptors for hemolymph volume and demonstrate their function.
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