Regulation of hemolymph volume by uptake of sand capillary water in desiccated fiddler crabs, Uca pugilator and Uca pugnax.
W.E. Thompson, P.J. Molinaro, T.M. Greco, J.B. Tedeschi and C.W. Holliday (1989). Comp. Biochem. Physiol. 94A: 531-538.
(http://www.lafayette.edu/~hollidac/ucapaper.html)
Abstract
1. Desiccated Uca pugilator and Uca pugnax can rehydrate on damp sand.
2. The crabs use setal tufts between the second and third pairs of walking legs and branchial chamber suction of up to -43 mmHg to extract sand capilary water, which is absorbed by the gills and/or drunk.
3. Desiccated crabs of both species have a limited ability to generate "free water" from sand capillary water which they obtain from the substratum by suction; thus, although they can regulate hemolymph volume, they cannot regulate hemolymph osmotic pressure by using sand capillary water.
4. The most likely use of this mechanism by the crabs in nature is for obtaining water from the substratum for flotation deposit-feeding.
Introduction
Several species of flotation deposit-feeding intertidal crabs have been shown to use interstitial water from the substratum to concentrate and consume organic material freshly deposited on the sand or mud by the receeding tide. These crabs circulate water through the buccal area and use it to separate organic detritus from the heavier sand or mud particles by flotation (Miller, 1961). Several authors have postulated that hydrophilic setae in various locations on the crab's ventral surfaces take water from the substratum by capillary action and lead the water into the gill chamber, 
where it is pumped out on to the buccal area by the scaphognathites (Bliss, 1968; Hartnoll, 1973; Powers, 1975; Quinn, 1980). In contrast, Wolcott (1976, 1984) has shown that the ghost crab, Ocypode quadrata, uses special setal tufts and branchial chamber suction of up to -50 mmHg to obtain sand interstitial water when it is desiccated. Furthermore, Wolcott and Wolcott (1985) have, in an elegant study, shown that Ocypode uses its gills to reabsorb salts from isosmotic urine which it voids on to its gills when it is given volume loads of distilled water, but not when it is volume-loaded with physiological saline. This process is functionally equivalent to making a dilute urine and it is thought to enable the crab to overwinter in burrows for up to 6 months with access to only very dilute sand capillary water.
Many members of the family Ocypodidae have setal tufts between the second and third pairs of walking legs, in particular the fiddler crabs. The present study was undertaken to determine if two species of fiddler crabs which possess these setal tufts, Uca pugilator and Uca pugnax, also have a suction mechanism similar to that of Ocypode and to determine if they can also use sand capillary water to rehydrate when they are desiccated. Preliminary data on a third species, Uca minax, are also presented.
Discussion
The present study clearly demonstrates that desiccated fiddler crabs, Uca pugilator and Uca pugnax, can obtain water from damp sand by a mechanism identical to that reported for the ghost crab, Ocypode quadrata (Wolcott, 1976, 1984). All three species of crab use pairs of hydrophilic setal tufts between the second and third pairs of walking legs to make contact with sand capillary water. Branchial chamber suction of up to -40 to -50 mmHg is then used to suck water from the sand into the branchial chamber, where it appears to be absorbed into the hemolymph at the gills. In addition, when sufficient water has accumulated in the branchial chamber, water which is pumped out of the exhalant openings can be passed to the mouth and/or anus and swallowed. Interestingly, Wolcott (1976) found that Ocypode uses only one branchial chamber at a time to obtain sand capillary water. Although no fiddler crabs were successfully cannulated in both branchial chambers in the present study, the fact that dye in sand capillary water sometimes appeared in only one branchial chamber of desiccated crabs makes it likely that fiddler crabs also use suction in only one branchial chamber at a time. As Wolcott (1984) notes, Ocypode alternates suction between branchial chambers and there may be good hemodynamic reasons for this behavior.
Wolcott and Wolcott (1985) discussed the adaptive value of Ocypode's ability to obtain sand capillary water within the context of water balance of crabs overwintering for up to 6 months in their burrows in North Carolina sand dunes, well removed from the shore. These animals are exposed to osmotic flooding by very dilute interstitial water and must make and void urine to keep from swelling. When voided, the urine is isosmotic with the hemolymph and it flows onto the gills. Ocypode has evolved a unique mechanism by which it uses its gills to reabsorb salts from the urine; the crab discards from the branchial chamber a final excretory product ("P", Wolcott and Wolcott, 1985) which is quite dilute. Although it is possible that Uca can also absorb salts from urine voided on to its gills, there does not seem to be any ecologically significant reason for it to do so. We have observed that the U. pugnax population examined in the present study overwinters in coastal New Jersey in intertidal burrows below the frost line. These burrows are regularly flushed by the tide and it seems unlikely that the fiddler crabs would suffer either osmotic flooding or salt loss under these conditions. Measurements of burrow water salinity in this population of U. pugnax from September 1987 to April 1988, yielded values ranging from 500-1000mOsm (M.A. Connaughton, unpublished data).
The most likely adaptive value of the water extraction mechanism in Uca is that it gives the crabs the ability to get water for flotation deposit-feeding without the necessity of returning to the burrow or to the shoreline. In this respect, it is of interest that Ocypode has been observed using flotation deposit-feeding in Georgia on sandy beaches, where it is nearly as efficient as U. pugilator in removing algae from the sand (Robertson and Pfeiffer, 1982). It is also possible that water obtained from the substratum by suction serves to keep the fiddler crabs cool while they are feeding (Edney, 1960; Smith and Miller, 1973). This may be very important for U. pugnax, which is quite dark and which would tend to heat up quickly when feeding in the daytime.
At high sand water contents, Uca can rehydrate on damp sand without the ability to produce branchial chamber suction and without the setal tufts (Figs 3c and 4c). Thus, fiddler crabs appear to have a second mechanism for extracting sand capillary water. This mechanism may act by the capillary of the setae which project ventrally all around the ventral and posterior margins of the branchial chamber and which act to seal its junction with the body. Other setae, such as those at the lateral margins of the abdomen and those surrounding the coxae of the walking legs, may also be involved in this mechanism, which, if it exists, is similar to that proposed by Quinn (1980) for the soldier crab, Mictyris.
Although fiddler crabs can use their suction mechanism to replace lost hemolymph volume when desiccated, they have a limited ability to use this water to lower the increased hemolymph osmotic pressure which also results from desiccation (Tables 2 and 3). Both species were able to keep their hemolymph osmotic pressure well below that of any fluid in the branchial chamber, but both had hemolymph osmotic pressures significantly higher than those of crabs in a "natural" environment. Fiddler crabs apparently need access to large volumes of standing 100% sea water in order to be able to hyporegulate hemolymph osmotic pressure below that of the medium (D'Orazio and Holliday, 1985; Holliday, 1985). Thus, small volumes of sand capillary sea water are not sufficient for normal osmoregulatory performance in these two species.
It might be expected that desiccated fiddler crabs would choose to rehydrate on sand dampened with dilute sea water and this appeared to be the case (Table 4), although the apparent preference for dilute media was seen over a broad range of salinities (fresh water to 80% sea water). Thus, the crabs may be able to sense the osmotic pressure of sand capillary water and to choose water of the appropriate salinity to ameliorate the high hemolymph osmotic pressure as well as the lowered hemolymph volume which result from desiccation. Obviously, further study is necessary to document and characterize this ability, if it exists.
In closing, it is interesting to note that a third species of fiddler crab, Uca minax, from the eastern shore of Maryland, was not able to rehydrate when desiccated and placed on damp sand in the present study. U. minax is always found on muddy substrata in the upper ends of estuaries (Williams, 1984) and, although it has well-developed setal tufts between the second and third walking legs, it does not use them to rehydrate when on damp sand. Preliminary measurements of branchial chamber pressure in desiccated crabs on damp sand and on mud did not reveal any attempts by this species to use the suction mechanism seen in the other two Uca species. Desiccated U. minax were, however, able to rehydrate when placed on mud and they did this whether or not the setal tufts were removed or the branchial chamber was fenestrated. This may indicate that they depend upon the capillary attraction of setae on the ventral surface of the cephalothorax and/or the abdomen (i.e., the second mechanism discussed above) for obtaining water from the mud substratum, which contains much more water (30-40% by weight in the present study) than does damp sand.
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