Hyperosmotic Regulation Research Articles

A note on the hyperosmotic regulation in the brackish-water clamRangia cuneata

The brackish-water clamRangia cuneata maintains the body fluids hyperosmotic to diluted sea-water of osmolalities less than 200–300 mOsm (Fig. 1). Analyses of pericardial fluid pH, protein concentration and chloride concentration (Table 2, Figs. 3, 2) negate the participation of a Gibbs-Donnan effect in the establishment of the hyperosmolality. The protein concentration of the body fluids decreases with decreasing water osmolality from a mean of 1.3 mg/ml at 416 mOsm to a mean 0.3 mg/ml at 27 mOsm.

Osmoregulation of the grass shrimp Palaemonetes pugio exposed to polychlorinated biphenyls (PCBs). I. Effecton chloride and osmotic concentrations and chloride-and water-exchange kinetics

Grass shrimp, Palaemonetes pugio, were capable of hypo- and hyper-osmotic regulation of body fluids. Hemolymph chloride and osmotic concentrations were maintained at relatively stable levels over a wide salinity range. Following an abrupt transfer from intermediate (14 and 17‰) to high (31 and 35‰) or low (1 and 2‰) salinities, hemolymph chloride levels exhibited initial overshoot and undershoot, respectively, of new steady-state levels. Osmotic concentrations exhibited an initial undershoot at low, but not overshoot at high salinity. Chloride space in salinity-acclimated shrimp was relatively stable at salinities from 1 to 35‰. Changes in chloride space following salinity transfer paralleled those of hemolymph chloride levels, and are discussed in the light of alterations in intracellular sodium concentrations reported earlier. Rate constants for chloride turnover indicated independent exchanges of sodium and chloride ions. Water-turnover measurements showed that permeability of P. pugio was greatest at the isosmotic salinity (17‰) and reduced at salinities which were associated with active osmoregulation. Exposure to sublethal and 96-h LC50 levels of Aroclor® 1254 did not seriously alter hemolymph chloride and osmotic concentrations, chloride space or chloride-exchange kinetics in adult shrimp. Disruption of hemolymph chloride regulation in juvenile shrimp was associated with large mortalities not observed in adults. Shrimp exposed to Aroclor 1254 at 17‰ S exhibited reduced water permeability similar to levels previously observed in controls at high and low salinities in response to osmotic or ionic gradients. Exposure to PCBs did not result in further reduction in permeability at the latter salinities.

Resistance to freezing by antarctic fauna: Supercooling and osmoregulation

Abstract o 1. Body fluids of six antarctic invertebrates are hyperosmotic to their normal environmental sea water during the late austral summer and winter. Pelagic and benthic species respond as osmoconformers to changes in salinity but regulate slightly hyperosmotically when returned to normal sea water. Adjustment of the amphipod Orchomene plebs Hurley to a concentration gradient requires a small but significant increase in energy output. 2. Tolerance to supercooling and the extent of hyperosmotic regulation is greater in pelagic than in benthic species. 3. Body fluids of antarctic invertebrates show equilibrium freezing points similar to ideal solutions while blood serum of the fish Trematomus bernacchi shows marked thermal hysteresis. The role of these resistance adaptations of antarctic marine fauna is discussed.

Holeuryhalinity and its mechanisms in a cirriped crustacean, Balanus improvisus

Abstract 1. The barnacle Balanus improvisus succeeds in fresh water as well as in sea water. The euryhalinity depends partly on hyperosmotic regulation of the haemolymph and partly on cell volume regulation. 2. The animals osmoconform in waters above 500 mOsm and osmoregulate in more dilute sea waters, showing strict homoiosmoticity below 100 mOsm. Haemolymph and maxillary gland fluid are isosmotic and have equal chloride concentrations. 3. Seventeen free amino acids are found in thorax muscle tissue in amounts varying with haemolymph osmolality. Proline reaches unique values of 0.7 M at hypersaline sea water. 4. The relative water content of muscle tissue varies little with changes in haemolymph osmolality pointing to a regulation of cell volume. The adjustments of the intracellular amino acids, especially proline, assist in this regulation. 5. The intermoult cycle does not significantly influence the measured parameters.

SOLUTE ADJUSTMENTS IN THE COELOMIC FLUID AND MUSCLE FIBERS OF A EURYHALINE POLYCHAETE,NEANTHES SUCCINEA, ADAPTED TO VARIOUS SALINITIES

1. The solute composition of the coelomic fluid and ventral musculature of a euryhaline polychaete, Neanthes succinea, acclimated to various salinities was examined.2. The coelomic fluid of these worms conforms osmotically and ionically (with the exception of potassium) with salinities as low as 35% SW. Below this point hyperosmotic regulation of the coelomic fluid occurs as a result of the retention of NaCl. Calcium and magnesium are also slightly concentrated in the regulating range but they do not contribute significantly to the hyperosmotic plateau. The means of solute retention in the coelomic fluid are considered.3. The extracellular space of the muscle tissue remains unchanged over the salinity range, although the tissue does become more hydrated. Thus, the water gain upon adaptation to dilute conditions must be proportionally distributed between the intra- and extracellular spaces.4. The sum of the apparent concentrations of the intracellular solutes measured was not a constant proportion of the coelomic fluid osmotic activity, possibly indicating that either the solvent volume or the extent of solute binding varies with the external osmotic activity.5. Intracellular solute concentrations fall with decreasing salinity. The reduction in concentration of K, PO4 fractions, Ca, and Mg may be explained merely on the basis of increases in cell hydration, while the lower concentrations of Na, Cl, and Nin + N are not wholly accounted for by the influx of water. The removal of a large fraction of the free amino acid pool is the major osmotic adjustment observed in the muscle fibers of this worm.

Osmoregulation in Pisidia longicornis (L.) and Porcellana platycheles (pennant) (decapoda, anomura) subjected to reduced salinities

1. 1. Pisidia longicornis and Porcellana platycheles exhibit hyperosmotic regulation to a slight extent in reduced salinities. 2. 2. P. platycheles has a greater tolerance of reduced salinities than P. longicornis. 3. 3. This tolerance, rather than regulation, accounts for their capacity to penetrate waters of low salinity.

Experimental studies on physiological and behavioural response mechanisms of Nitocra spinipes (Crustacea: Harpacticoidea) from brackish-water rockpools

Volume regulation and salinity-preference tests have been made on Nitocra spinipes Boeck (Crustacea, Harpacticoidea). This species is often dominant in Baltic brackish-water rockpools. The investigation attempts to evaluate the relative importance of some of the different response mechanisms of this species to salinity changes in relation to the unstable environmental conditions of the rockpools. Volume-regulation experiments have shown that N. spinipes is capable of hypoosmotic and probably hyperosmotic regulation in the tested salinity range of 1 to 20‰ S. In laboratory cultures, reproduction, hatching and moulting oecurred in salinities ranging from 0.5 to 30‰ S. Preference experiments showed that N. spinipes has a very weak behavioural response even to very large variations in salinity concentrations between the alternative; a significant choice could only be found under conditions which never occur in the natural biotope. It is therefore concluded that, in a biotope such as the rockpool, where salinity changes will affect the whole biotope rather than produce microclimatic variation, regulation and adaptation must have a higher ecological importance than escape responses.

Osmoregulatory Ability of Blue Crabs in Different Temperature-Salinity Combinations

Adult male and immature, mature, and ovigerous female blue crabs,Callinectes sapidus, prefer different portions of estuaries. Total osmotic concentrations of blood samples from crabs held in 5, 50, or 100% sea water (salinity 34 o/oo) at 10, 20, or 30 C indicated that some differences in osmoregulatory capabilities were related to differences in distribution. Ovigerous females did not regulate as well as mature females or adult males at 5 or 50% sea water, at all temperatures. In 50 or 100% sea water, osmotic concentrations of immature females generally were less than those of mature females. At almost all temperature-salinity combinations, however, differences in the osmoregulatory ability of adult males and mature females were not significant. The blue crab showed good hyperosmotic regulation in 5 and 50% sea water but regulated its blood hyposmotically in 100% sea water.

Salinity—stress and Desiccation in Intertidal Worms

SYNOPSIS. Intertidal worms (oligochaetes, polychaetes, sipunculids) inhabiting beaches and tidal fiats both on the open sea coast and in estuaries may be exposed to significant tidal as well as seasonal variations in salinity. However, there are very few measurements of the actual variations in salinity encountered by worms in nature. Behavior (irrigation of burrow, vertical movements in burrows, migration in gradients of salinity)may be important in determining to which of the available salinities in a tidal cycle the worms may be exposed. In response to rapid lowering of salinity, worms gain water and lose salts, these processes combining in diluting the internal fluids. Internal ilution occurs more slowly in euryhaline species than in stenohaline species. Worms fully adapted to salinities lower than 30 % sea water may be hyperosmotic (demonstrated for six species of Nereidae ). Mechanisms involved in hyperosmotic regulation include active transport of salts(demonstrated in Nereis diversicolor), reduction ofthe permeability of the body surface to salts and perhaps to water, and perhaps production of hypo—osmotic urine. Sipunculids can tolerate considerable loss of water rom dehydration and concomitant increases in osmotic concentration of the body fluids. It is suggested that worms exposed to significant tidal variations in salinity may seldom be in osmotic equilibrium with their external medium.

The body fluids of some centropagid copepods: Total concentration and amounts of sodium and magnesium

1. 1. Calamoecia salina, which inhabits athalassic saline waters and has a salinity range of 14–130%, is essentially poikilosmotic over the lower half of its range, but there is evidence of feeble hyperosmotic regulation towards the extreme low end. 2. 2. The marine brackish water species Centropages hamatus is completely poikilosmotic over the range 60–110% sea water, but strongly regulates magnesium (about 40% ambient concentration), and to a lesser extent sodium (slightly above ambient concentration). 3. 3. The body fluid of the fresh and brackish water species Limnocalanus macrurus has a freezing point depression of 0·36–0·38°C when in fresh water ( Δ < 0·01°C). In the body fluid of this species, sodium is the dominant cation and magnesium is present only in very low concentrations.

Aspects of Osmotic and Ionic Regulation inCorophium Volutator(Pallas)

Experiments have been made to elucidate the problem of hyperosmotic regulation in the mud-dwelling euryhaline amphipod,Corophium volutator.The animal produced urine hypo-osmotic to the blood when acclimatized to low salinities, and isosmotic urine at salinities above 20%. The restricted permeable areas of the cuticle have been localized by silver staining. Over a range of salinities from 1 to 35 %C. volutatorwas found to maintain Na+, K-−, Ca2+, Cl-−, more concentrated than the medium, and Mg2~less concentrated. The importance of these mechanisms in assisting hyperosmotic regulation is discussed.

RESPONSES OF AN ESTUARINE POPULATION OF THE POLYCHAETE NEREIS LIMNICOLA TO OSMOTIC STRESS

1. The ecology of an estuarine population of the euryhaline nereid polychaete Nereis limnicola is briefly described. Seasonally this population is exposed to salinities varying from nearly fresh water to at least 85% sea water. Tidal variation is minimal.2. The pattern of chloride regulation in the body fluids is similar to that described for four other nereid species: osmotic and ionic conformity at salinities above 30% sea water; and hyperosmotic regulation in lower salinities, down to a critical low salinity at which hyperosmotic regulation begins to break down (about 1-3% sea water). There is little difference in this pattern between fresh-water and estuarine populations of N. limnicola.3. After a transfer from one salinity to another, both salinities within the range of osmotic conformity, there is an immediate change in the total body chloride of the worms, due both to a movement of water and to a movement of chloride, in the directions of the imposed concentration gradients. When worms are adapted to low salinities in the range of hyperosmotic regulation, rates of chloride movements are much reduced after a transfer.4. Complete adaptation to a new salinity after a transfer may take more than two days.5. Rates of chloride and water movement are lower in N. limnicola than in several less euryhaline species.6. The results suggest that N. limnicola is less permeable to salts, and perhaps also to water, than the less euryhaline species, and that permeability may be further reduced after adaptation to low salinities. These features are considered to be of significance in the physiological adaptation of N.limnicola to life in low salinities and fresh water, in that diffusional salt loss is much reduced.7. The results do not support the hypothesis that active transport of salts from the medium across the body surface is important in the maintenance of hyperosmotic body fluids.8. The fresh-water population of N. limnicola previously studied may have: (a) A slightly higher level of hyperosmotic regulation. (b) A slightly lower critical low salinity. (c) A somewhat lower permeability of the body surface to salts. However, differences in osmotic responses among the several brackish-and fresh-water populations of N. limnicola are slight when compared with the osmotic responses of less euryhaline species.

RESPONSES OF THE FALSE LIMPET, SIPHONARIA PECTINATA LINNAEUS (GASTROPODA, PULMONATA) TO OSMOTIC STRESS

Previous articleNext article No AccessJournal ArticlesRESPONSES OF THE FALSE LIMPET, SIPHONARIA PECTINATA LINNAEUS (GASTROPODA, PULMONATA) TO OSMOTIC STRESSROBERT O. McALISTER and FRANK M. FISHERROBERT O. McALISTER Search for more articles by this author and FRANK M. FISHER Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Biological Bulletin Volume 134, Number 1February 1968 Published in association with the Marine Biological Laboratory Article DOIhttps://doi.org/10.2307/1539970 Views: 13Total views on this site © 1968 Marine Biological LaboratoryPDF download Crossref reports no articles citing this article.

Some Effects of Salinity on the Survival, Moulting, and Growth ofCorophium Volutator[Amphipoda

An experimental study of the effects of salinity on the mud-dwelling amphipod,Corophium volutator(Pallas), indicates that, if supplied with mud, it will survive the salinity range of 2.50 %0, and without mud the range 7.5-47–5 %0. Moulting occurred in salinities of 2.6–46 %0, but most frequently in the range 5–20 %0. Growth occurred at a maximum rate in 15.4 %0, and only slightly slower at 4.4 and 30.6 %0; but below 4.4 %0the growth rate was progressively reduced. Freezing point studies showC, volutatorto be a hyperosmotic regulator, having a tissue tolerance range of 13–50 %0. The importance of a supply of mud, and the significance of hyperosmotic regulation are briefly discussed.INTRODUCTIONThe amphipodCorophium volutator(Pallas) is an inhabitant of littoral muds, the populations often reaching high densities. It has been most commonly reported from shores of estuaries, although Zenkevitch (1963) has stated that it may also occur in muds submerged to a depth of 10 m. Segerstråle (1959) has summarized the data available on the occurrence and distribution ofC. volutator. Since that date, work has been done on substrate selection (Meadows 1964a–c), on burrowing behaviour (Meadows &amp; Reid, 1966) and on rhythmical swimming activity under tidal control (Morgan, 1965). Many authors (Nicol, 1935; Thamdrup, 1935; Beanland, 1940; Spooner &amp; Moore, 1940; Rees, 1940; Goodhart, 1941; Stopford, 1951; Rullier, 1959; Gee, 1961) have described the substrate in whichC. volutatoroccurs. They have agreed thatC. volutatoris found in mud or muddy sand, containing approximately 37 % silt or clay.

TRENDS IN WATER AND SALT REGULATION AMONG AQUATIC AND AMPHIBIOUS CRABS

1. In order of their increasing terrestrialness the following seven species of crabs were studied with respect to their osmotic regulatory ability in aqueous media: the aquatic Cancer antennarius, Hemigrapsus oregonensis, the semi-terrestrial Pachygrapsus crassipes, Grapsus grapsus, Ocypode ceratophthalma, Uca crenulata and the terrestrial crab, Gecarcinus lateralis. Cation regulation was studied in all of the above except Ocypode and Grapsus.2. All crabs examined except Cancer showed some degree of hypo- and hyperosmotic regulation. No correlation was found between the strength of hypoosmotic regulation and hyperosmotic regulation.3. There is some correlation between degree of terrestrialness and the ability to hyporegulate. That is, the more terrestrial crabs are stronger hyporegulators than the more aquatic crabs.4. The ratio, urine osmotic concentration/blood osmotic concentration (U/B), for Hemigrapsus, Pachygrapsus, Grapsus, Uca and Ocypode was essentially unity for all treatments. On the basis of cation concentrations in blood and urine, this seem to be true also for Gecarcinus. Therefore, there is no evidence that the antennary glands of any of these species are osmoregulatory in function.5. There were no apparent trends with increasing terrestrialness with regard to the regulation of K.6. Higher blood Ca concentrations were found in the more terrestrial crabs. Thus, when immersed in 100% sea water, the blood Ca of Cancer was 27.3 meq./l.; Hemigrapsus, 29.5 meq./l.; Pachygrapsus, 29.6 meq./l.; Uca, 39.3 meq./ l. and for Gecarcinus, 44.2 meq./l.7. Blood Ca remained relatively constant in all species even though the other cations were forced away from normal by osmotic stress.8. The U/B values for K and Ca showed inconsistent patterns with respect to species and/or treatment. The significance of these values awaits information concerning K and Ca fluxes, as well as rates of urine flow.9. The U/B values for Na decreased and those for Mg increased with increasing salinity of the external medium in Cancer, Hemigrapsus, Pachygrapsus and Uca, but there was no trend with respect to Na or Mg in Gecarcinus.10. With the exception of Gecarcinus, the more terrestrial crabs concentrated Mg higher in the urine than did the more aquatic crabs.11. Mg concentrations in the urine of Gecarcinus were the lowest of all crabs examined, suggesting the inability of the antennary glands in this species to regulate Mg. The low blood Mg of Gecarcinus immersed in sea water (27.8 meq./l.) indicates that low urine Mg does not preclude maintenance of low Mg in the blood.12. The antennary glands of Gecarcinus showed little tendency to concentrate Mg even when the crab was injected with Mg.13. The ability to concentrate Mg in the urine does not assure low Mg in the blood. Thus, the mean urine concentration for Mg in Uca immersed in sea water was 347 meq./l., yet its mean blood Mg was 61 meq./l., which is the same as that found for Cancer in which the urine Mg was only 102 meq./l. Since Cancer is aquatic and Uca quite terrestrial, there seems to be no correlation between Mg regulation in the blood and the land habit.14. The response of Gecarcinus with respect to Na and Mg, which differs from other species studied, suggests a fundamentally different mechanism in the antennary glands and a pathway to terrestrialness which differs from that of the other crabs.15. Three different types of response to Mg by the antennary glands of crabs are described.16. The evolution of terrestrial crabs is discussed. It is suggested that osmotic regulation and terrestrialness may have arisen independently but simultaneously in regions of varying salinity and high water temperatures.

OSMOREGULATORY ROLE OF THE ANTENNARY GLAND IN TWO SPECIES OF ESTUARINE CRABS

1. Total osmotic pressure measurements of urine were determined on two species of crabs, Hemigrapsus nudus and H. oregonensis, over a salinity range, 6% to 175% sea water, three temperatures, 5°, 15° and 25° C., and at two seasons summer and winter. Blood data are included from Dehnel (1962) for comparison.2. Urine and blood concentrations fall in dilute, and rise in concentrated media at rates directly related to the gradients between media and equilibrated body fluid concentrations, and are influenced by the seasonal adaptation of the animals and the experimental temperature. Major changes in body fluids occurred within 48 hours.3. Hyper-osmotic regulation in summer-adapted animals resulted in isosmoticity of blood and urine, implicating extra-renal mechanisms. The production of hypo-osmotic urine in winter-adapted animals indicated the participation of the antennary glands.4. In both species, summer and winter adaptation tended to favor stronger hyper-osmotic regulation at the respective seasonal temperatures than at temperatures foreign to the seasons.5. Changes in experimental temperature revealed seasonal and interspecific differences in 48-hour blood and urine concentrations. Blood concentrations of H. oregonensis, when measured at a series of temperatures and salinities, showed a general trend, particularly for winter animals. As the concentration of the experimental media decreased (from 75% to 12%) blood concentrations increased significantly with decreasing temperature. Blood concentrations of summer animals showed no real differences, but when compared with winter crabs, at lower salinities, blood concentrations of summer crabs were significantly lower. The same general trend was shown for H. nudus. With respect to urine concentrations, summer-adapted H. oregonensis, in dilute media, showed significantly higher urine concentrations at higher temperatures. H. nudus showed no temperature effects in any salinities. Winter-adapted animals of both species showed significant decreases in urine concentration in low and intermediate, but not high, salinities, when the experimental temperature was increased.6. Seasonal adaptation of osmoregulatory mechanisms in Hemigrapsus is shown to alter the balance of active processes so that for a given range of experimental conditions, urine is lower in winter animals than in summer, both in absolute concentration and relative to the blood.

ASPECTS OF OSMOREGULATION IN TWO SPECIES OF INTERTIDAL CRABS

1. Total osmotic pressure measurements of blood were made on two species of intertidal crabs, Hemigrapsus oregonensis and H. nudus, over a salinity range, 6% to 175% sea water, a temperature range, 5° to 25° C., and at two seasons, summer and winter.2. Major changes in blood concentration occurred at 48 hours. Both species at either season were hypertonic to all experimental salinities. Below 100% sea water, osmotic regulation was accelerated with decreasing salinity, and in hypotonic media both species showed strong hyper-osmotic regulation.3. The osmoregulatory abilities of these two species changed seasonally, and these responses resulted from the effect of temperature. Winter crabs maintained greater gradients at sea water concentrations below 100-125% sea water, and were better regulators in hypotonic media, whereas summer crabs were better regulators in hypertonic media, and consequently maintained the greater gradient. Interspecifically, winter H. nudus maintained the greater gradient over the major portion of the salinity range. Summer crabs of both species were similar from 6% to 125% sea water.4. Blood concentrations of H. oregonensis, when measured at a series of temperatures and at each salinity, showed a general trend, particularly for winter animals. As external sea water concentration decreased (from 75% to 12% sea water) blood concentrations increased significantly with decreasing temperature. Blood concentrations of summer animals measured at the three temperatures and each salinity showed no real difference, but at the lower salinities, blood concentrations of summer animals were significantly lower than those of winter crabs. The same general trend was shown for H. nudus. Temperature had no effect on blood concentrations of summer animals, but the low temperature (5° C.) had a highly significant effect on the blood concentrations of winter animals.5. No detectable weight changes resulted when animals were subjected to the extreme experimental sea water concentrations. Further, there was no evidence to suggest that size had any effect on osmotic response.6. Respiratory and osmoregulatory data for both species have been compared at the same temperatures and salinities to determine whether increased osmotic work can be resolved by measuring oxygen consumption. Comparison of these data does not permit such a conclusion.

OSMOTIC TOLERANCE AND REGULATION IN CRABS FROM A HYPERSALINE LAGOON

1. The crabs Pachygrapsus crassipes and Hemigrapsus oregonsis were found thriving in an hypersaline lagoon which had been isolated from the sea for months and had attained salinities in excess of 175% of normal. 2. Contrary to previous reports Hemigrapsus was found to be a strong hypo-osmotic regulator in concentrations as high as 175% sea water. The discrepancy between previous laboratory findings and the field results of the present investigation are attributed to the prolonged acclimatization period in lagoon water of gradually increasing salinity and to severe selection permitting only hypo-regulators to be sampled. 3. No living Hemigrapsus were found in the lagoon when the salinities had exceeded 180% sea water. 4. Pachygrapsus, as was expected from previous studies, was regulating osmotically in the lagoon when the concentrations were above 185% sea water. 5. Both Pachygrapsus and Hemigrapsus can tolerate blood concentrations in excess of 160% sea water and remain active. 6. Pachygrapsus is a stronger hypo-osmotic regulator than Hemigrapsus. Since it maintains its blood Mg considerably lower than does Hemigrapsus in concentrated sea water, the stronger ability to hypo-regulate may be made possible by the greater capacity on the part of Pachygrapsus to excrete Mg. 7. The urine Mg for Hemigrapsus is about equal in concentration to that of Pachygrapsus when the crabs are immersed in 175% sea water, even though the blood Mg concentration is more than three times higher than that of Pachygrapsus. 8. Hemigrapsus is a stronger regulator of Na in hypersaline water than is Pachygrapsus. 9. The antennary glands in both Pachygrapsus and Hemigrapsus are ineffective as organs of hypo-osmotic regulation. 10. When Pachygrapsus is transferred from normal to concentrated sea water the volume of its muscle tissue, as indicated by water content, becomes reduced. This reduction in volume persists in hypersaline waters even after months of immersion. Thus, there is no evidence of volume control for muscle tissue. 11. After months of isolation in a hypersaline lagoon Pachygrapsus shows a preference for normal sea water when offered a choice of salinities and will avoid the hypersaline water from which it has been captured. 12. Evidence is produced showing that coastal lagoons present selective pressures favoring: (1) hyper-osmotic regulation, (2) hypo-osmotic regulation, and (3) aerial respiration, three physiological characteristics common to terrestrial and semi-terrestrial crabs. Coastal lagoons are suggested as ideal sites for the evolution of land crabs.

IONIC AND OSMOTIC CONCENTRATIONS IN BLOOD AND URINE OF PACHYGRAPSUS CRASSIPES ACCLIMATED TO DIFFERENT SALINITIES

1. When specimens of Pachygrapsus crassipes were acclimated to 50%, 100% and 170% sea water the average blood osmotic concentrations were equivalent to 0.46, 0.57 and 0.89 normal NaCl, as compared with the medium of 0.29, 0.58 and 0.99, respectively. Urine osmotic concentrations in the same series were equivalent to 0.39, 0.53 and 0.90 normal NaCl.2. Thus the crabs are hyper-osmotic in a dilute medium and hypo-osmotic in a concentrated medium; the antennary glands may function slightly in hyper-osmotic regulation but not at all in hypo-osmotic regulation.3. Serum sodium concentrations in 50, 100 and 170% S.W. were 313, 465 and 668 mM, urine sodium 356, 318 and 264 mM corresponding to environmental concentrations of 229, 459 and 780 mM. Thus as the blood sodium increases the urine sodium decreases.4. The osmotic deficit in the urine is accounted for in part by magnesium which in 50, 100 and 170% S.W. was in blood 9, 29 and 33 mM, in urine 32, 144 and 345 mM while the medium was 26, 52 and 88 mM, respective...

Aspects of Osmotic Regulation in Crabs Showing the Terrestrial Habit

1. Terrestrial and semi-terrestrial crabs generally demonstrate the ability to regulate osmotically in dilute and concentrated sea water. 2. Osmotic regulation in terrestrial and semi-terrestrial crabs is not adaptive for aquatic life since osmotic stresses such as dilute and concentrated sea water are rarely experienced by these animals. Besides many such animals drown when completely immersed. 3. The branchial chamber of Pachygrapsus contains a volume of fluid equal to a maximum of about 3 per cent of the body volume. Thus, regulation against branchial fluid made concentrated by evaporation would not be adaptive as proposed by Jones because the total salt therein could not elevate the blood concentration above concentrations normally experienced in nature. 4. The respiratory rate of Pachygrapsus decreases when the crab is desiccated. This explains why the terrestrial behavior of Pachygrapsus is limited to brief periods. 5. Pachygrapsus can absorb water against a gradient when it has been desiccated. 6. The ability to regulate in concentrated sea water (hypo-osmotic regulation) is believed to be a manifestation of a mechanism for the conservation of water, namely, elimination of salts. 7. The mechanism involved in hyper-osmotic regulation seems to be necessary to the mechanism of hypo-osmotic regulation but apparently has no adaptive consequence such as salt elimination. 8. The coconut crab, Birgus latro, shows special adaptations to terrestrial life. a. It can drink water from small puddles. b. It can keep its respiratory membranes moist by placing water into the gill chamber without becoming immersed. c. It can control the concentration of its body fluids by selecting water of the appropriate salinity.