Physiological ecology
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  Osmotic regulation & salinity tolerance
  The topic of physiological ecology is divided into a section on osmotic regulation & salinity tolerance considered here, and other sections on CHEMORECEPTION, GAS EXCHANGE & METABOLISM, LOCOMOTION & TENACITY, DIEL SEASONAL & TIDAL RHYTHMS, and THERMAL STRESSES considered elsewhere. This section is separated taxonomically into genera: Cancer, Hemigrapsus, Pachygrapsus, Pugettia, and Rhithropanopeus.
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Cancer spp.

Research study 1

photograph of Dungeness crab Cancer magistergraph of hemolymph and urine sodium concentrations over a range of external salinities in the Dungeness crab Cancer magisterDungeness crabs commonly inhabit shallow waters in bays and estuaries where salinities tend to be generally low and to fluctuate seasonally.  Studies on ion regulation in Cancer magister collected from Coos Bay, Oregon, where salinities fluctuate from 12-301, indicate an ability to hyperosmoregulate in dilute seawater.  The regulation, not strong to begin with (see graph), starts to fail at salinities below about 30% full-strength seawater, or about 140mequiv . l-1 of sodium2.  The authors find no significant difference between Na+ levels in hemolymph and urine (i.e., the 2 fluids are isotonic), and conclude that the antennary glands (kidneys) are not involved in regulation of this ion (or of potassium, chloride, or calcium - data not shown here).  The authors conclude that the antennary glands3 are ineffective as salt-conserving organs, similar to what has been found for other euryhaline brachyurans. Regulation of these ions is likely done in the epithelium of the gills. Hunter & Rudy 1975 Comp Biochem Physiol 51A: 439.

NOTE1  parts per thousand; equivalent to g . l-1

NOTE2  in addition to sodium, the authors present data for other cations, as well as for the anion chloride

NOTE3  an exception is magnesium ion, which is shown by the authors to be regulated at low levels, and the antennary glands are involved in this (see Research Study 2 to follow)

Research study 2

graph of hemolymph and urine potassium concentrations over a range of external salinities in the Dungeness crab Cancer magistergraph of hemolymph and urine concentrations of magnesium ion over a range of external salinities in the Dungeness crab Cancer magister in summerIn an earlier study on osmoregulation in Cancer magister using specimens collected from Fraser River Estuary, British Columbia, similar evidence for hypertonic regulation of sodium, potassium, and calcium in dilute salinities is obtained.  These authors suggest some involvement of the antennary glands in both potassium (see graph on Left) and calcium regulation, but the variability in their data, especially for calcium (not shown here), seems too great to support this notion.

Magnesium ion is strongly regulated hypotonically in C. magister, as it is in other crustaceans, and here the antennary glands do play a major role (see graph on Right).  Note the high urinary levels of magnesium, especially in crabs kept in high-salinity seawater, showing that this substance is being excreted.  Maintenance of low hemolymph levels of magnesium is thought to be associated with its inhibitory effects on nerve-impulse transmission.  In support of this idea, fast-moving crabs such as terrestrial or swimming ones tend to have significantly lower hemolymph levels of magnesium than slower-moving species, such as stone crabs and other lithodids.  Engelhardt & Dehnel 1973 Can J Zool 51: 735. 

NOTE 100% salinity is equivalent to 32; salinities above 100% are created by addition of sea salts

Research study 3
  Studies in Sequim, Washington show that Dungeness crabs Cancer magister are able to detect and respond to salinity changes of relatively slight magnitude.  For example, crabs held in seawater of ambient salinity 31‰ could detect low and high salinity changes to levels of 30 and 33‰, respectively.  Experimental changes in salinity beyond these limits elicit opening and closing of the outer or 3rd maxillipeds, rapid beating of maxillipedal flagellae and, ultimately, tight closure of the buccal cavity by pulling in and clamping the maxillipeds, retraction of all appendages, and cessation of activity. When the salinity is increased experimentally, the first of these responses occurs at 35%o or 113% of ambient.  When salinity is decreased experimentally, the first of these responses occur at 23‰or 75% of ambient.  The authors suggest that anntenular flicking may enhance the perception of salinity.  Sugarman et al. 1983 Estuaries 6: 380.
Research study 4

graph showing osmoregulatory abilities of different life stages of crabs Cancer magister, including the magalopa stageWhat are the osmotic challenges for the developing stages of a weakly regulating species like Cancer magister and how are they met?  This is investigated by researchers at the Oregon Institute of Marine Biology, Charleston using 4 life stages, including the megalopa settling stage.  The researchers expose each stage to 3 salinities for 8h, then measure osmotic and ionic concentrations of their hemolymph.  Results show that all 4 life stages have at least some ability to hyperosmoregulate, with megalopae being more capable in this regard than the 1st-instar juveniles, which are least capable (see graph, data for 10oC only).  The authors attempt to explain their results on the basis of ontogenetic changes correlated with changes in lifestyles of the different stages, but find their data not as convincing as for other crustaceans.  For example, the megalopae are good hyperosmotic regulators, yet spend much of their lives offshore in high-salinity waters.  Also, the stage least able to osmoregulate, the 1st instar juvenile, inhabits shallow mudflats where extremes in salinity and temperature are common.  More convincing are ontogenetic patterns relating to regulation of specific ions.  For example, ion regulation in 5th-instar juveniles more resembles that in adults than in 1st-instar juveniles.  Also, while the 5th instars are beginning to show the adult pattern of ion regulation, their overall capability to osmoregulate appears to be not as good as the adult.  An interesting finding is that magnesium is regulated hypoosmotically equally well in all life stages, thus underscoring the importance of this ion in the physiogical well-being of decapod crustaceans.  Brown & Terwilliger 1992 Biol Bull 182: 270.

NOTE sodium, chloride, potassium, calcium, and magnesium ions.  The first 3 ions parallel total osmolality in all 4 stages

NOTE  sample sizes are small, ranging from only 1-3 in megalopae and the 2 instar-stages, and one wonders how much this may have contributed to non-significance in the data.  Experiments are done at both 10 and 20oC but with mostly non-significant effects being shown

Research study 5

photograph of the crab Cancer gracilis courtesy Iain McGaw, U Nevada
graph of hemolymph osmolality over a range of external saliniities in the crab Cancer gracilisThe graceful or slender crab Cancer gracilis lives mostly subtidally on sand/mud and is generally considered to be an osmoconformer (see graph on Right). However, because it inhabits shallow bays, it may be exposed to periods of low salinity that could be lethal1.  Given an intolerance to low salinity, can C. gracilis detect changes in salinity and move away from low-salinity areas?  

histograms showing salinity preferences of crabs Cancer gracilisExperiments at the Bamfield Marine Sciences Centre, British Columbia show that when offered a range of salinities2 (60-100% and 30-70% seawater) in a salinity-gradient tank3, test individuals spend more time at the high end of the ranges (see histograms on Left). Thus, although an osmoconformer, C. gracilis can mitigate potentially deleterious effects of low salinities through behavioral means.  In low salinities (<65%) the crabs become quiescent and close off their branchial chambers.  During closure, which can last many minutes, salinities within the branchial chamber are maintained at levels about 30% higher than ambient, but at the cost of rapid depletion of oxygen.  On resumption of activity the oxygen content quickly returns to normal levels. 

During closure the authors record a decrease in heart-rate (bradycardia), which lasts graph showing heart rate of Cancer gracilis over time in different salinity seawaterlonger than the closure and may help to reduce ion diffusion across the gill surfaces during exposure to low-salinity water (see graph on Right).  Curtis et al. 2007 Biol Bull 212: 222. Photo courtesy Iain McGaw, University Nevada.

the authors describe mass mortality of C. gracilis in one shallow creek area following periods of heavy rain

NOTE2 100% = 32‰

NOTE3 the tank is rectangular and is divided into 5 chambers with seawater and freshwater outlets at the bottom of each chamber.  A passageway allows free access of crabs to each chamber.  Tests are conducted with 5 different salinities over a 3-h period at 12oC

Research study 6

graphs showing effect of food presence on scaphognathite ventilation and oxygen uptake over time in the crab Cancer gracilisgraphs showing combined effects of feeding and salinity stress on ventilation and oxygen-uptake rates in the crab Cancer gracilisDoes an osmoconforming crab like Cancer gracilis deal with osmotic stress differently when fed, or starved? What about the energy cost of feeding and food processing?  If osmotic stress occurs at the same time as feeding, does the crab "prioritise" its energy allocations?  Studies at the Bamfield Marine Sciences Centre, British Columbia on C. gracilis, which is commonly exposed to low-salinity conditions in its shallow-water habitats, show that feeding significantly increases an individual’s ventilatory rate (frequency of scaphognathite beating) by 13% and doubles its oxygen uptake (see graphs on Left). The effect lastst for at least 2d after the meal, with full recovery not for another 7-12h after that (not shown on the graph). 

What is the effect of salinity stress? When placed in simulated low-salinity conditions (65% seawater), ventilation rate declines significantly by 18% and oxygen consumption by 80% (graphs on Right).  After return to 100% seawater there is an over-shoot in uptake suggesting that an oxygen debt is being met, perhaps that incurred by extra demand of intracellular digestion and protein synthesis (SDA). 

What happens if crabs are fed and, at the same time, subjected to salinity stress? If they are fed and then 3h later subjected to salinity stress, there is a slight, but significant, increase in ventilatory rate, but the depressing effect of low-salinity exposure, in relative terms, is partially ameliorated (data not shown here).  Again, there is an oxygen debt that must be repaid.  However, if crabs are fed 21h previously and then, while oxygen-uptake levels are still comparatively high, are exposed to low-salinity conditions, the overall effect on oxygen-consumption rate is lessened and recovery is swift.  Under these circumstances only a relatively small oxygen debt has to be repaid.  Energy costs for feeding/digestion and for withstanding salinity stress in C. gracilis are clearly competing, and there may be sparing effects of one on the other.  In view of the fact that osmotic-stress and osmoregulatory studies are commonly done on unfed crabs, the author notes the importance of studying an organism’s physiological responses under conditions most closely approximating those of its natural environment.  McGaw 2006 J Exp Biol 209: 3766; see also McGaw 2006 Physiol Biochem Zool 79: 169.

NOTE  feeding in animals requires that energy be expended.  This can include direct costs of handling/dismembering a prey, costs of “excitement”, swallowing, and digestion, and also “indirect” costs of Specific Dynamic Action.  SDA is a postprandialand post-digestive metabolic cost that can represent 10-30% of the energy value of a meal.  It is highest on protein diets, suggesting a strong component of amino-acid deamination and transamination costs, and is generally considered to represent the broad range of metabolic-conversion costs involved with growth. More information on SDA in crabs can be found at LEARNABOUT CRABS & RELATIVES/FOODS, FEEDING, & DIGESTION/SPECIFIC DYNAMIC ACTION

Research study 7

histograms showing times to process food in various regions of the gut of crabs Cancer gracilis
Related studies at the Bamfield Marine Sciences Centre, British Columbia on food processing during exposure to low salinities in Cancer gracilis show that exposure to low salinity increases the time for passage of food through the gut. Note the “dosage-dependent” appearance of the data, with significantly more time being required for emptying of each portion of the gut with decreasing salinity.  In the lowest salinity of 60% seawater, which is just above the critical survival level for the species (55%), crabs actually regurgitate the food from the foregut after about 6h.  This could be a way of avoiding subsequent increases in metabolism associated with digestion, or perhaps a stress-response to low-salinity exposure.  The slowing of digestion during exposure to low salinities may therefore spare energy resources for other more essential physiological needs, such as withstanding osmotic stresses.  McGaw 2006 J Exp Biol 209: 3777;  see also McGaw 2008 Comp Biochem Physiol 150A: 458 for effects of hypoxia on gastric processing in Cancer crabs.

Research study 8

histogram showing gut-clearance times in the crab Cancer gracilishistogram showing effect of feeding on activity in crabs Cancer gracilisCrabs are highly motile. They exhibit short bursts of activity during escape from predators, intraspecific aggression, and burying, and more prolonged activity during tidal and other migrations.  Studies on Cancer gracilis at the Bamfield Marine Sciences Centre, British Columbia show that activity also slows digestion, as exhibited by significantly increased time for emptying of the foregut and midgut during enforced walking (18h vs. 34h in resting and active individuals, respectively; see histogram on Left).

Spontaneous activity also decreases in fed crabs, an effect that lasts for a day or so (see histogram on Right). During forced exercise (such as repeated flipping onto their backs and righting) oxygen uptake increases 3- to 4-fold.  Interestingly, there is no significant effect on ability to right themselves repeatedly between animals fed or unfed (3-4 days without food). This study also suggests a “prioritisation” of metabolic allocation in C. gracilis, where energy is diverted to activity at the expense of digestion. This, of course, is of selective advantage for escape from predators or avoidance of unfavourable circumstances, such as being stranded by a receding tide.  McGaw 2007 Physiol Biochem Zool 80: 335.

NOTE interestingly, similar work on “prioritising” of energy in Cancer magister by the same author suggests that individuals can slow the processing of food in the gut and may even be able to suspend specific dynamic action if they encounter low-salinity conditions immediately after a meal

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Research study 9

photograph showing a crab Cancer magister outfitted with data-sensing and -storage packagesgraph showing data collected from sensors attached to a Dungeness crab as it wanders around the sea bottomAn interesting application of remote-sensing technology is used to monitor the movements of Dungeness crabs Cancer magister in the Sarita River estuary near the Bamfield Marine Sciences Centre, British Columbia.  Researchers attach not just ultrasonic transmitters to crabs to track their movements, but also data-storage tags to monitor conditions of temperature, salinity, and depth through which the crabs move (see photograph on Left).  Locations of the crabs are recorded by hydrophones. Data-storage tags are recovered from concentrated trapping or from recreational fisherfolk provided a reward for their effors.  Overall return rate is 50% from 54 crabs released and monitored for periods of 1-34wk.  Three other tag packages are set up at fixed locations within the estuary, positioned at crab-heights of about 10cm above the substratum.

Results show that conditions at the fixed stations are not reflective of those experienced by the free-ranging crabs.  In particular, the crabs tend to avoid areas of low salinity, and the authors think that when the crabs do encounter low salinities, it is when they enter shallow areas to forage.  An example of this is shown in the graph on the Right for a single crab over a 3-wk period. The authors note a correspondence between depth and salinity during 8 forays into the intertidal area at high-tide periods, although this can't be readily seen in the graph.  The data are interesting, as all behavioural data are, and the study represents a novel application of remote-sensing technology used to answer some important biological questions. Curtis & McGaw 2008 J Zool 274: 375.

NOTE  the field of remote-sensing technology has been described as biologging

NOTE  one crab, deployed twice and caught twice, provides data for almost a full year

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Research study 10

histogram showing effect of salinity on food consumption in Dungeness crabs Cancer magisterhistogram showing effect of starvation and salinity on food consumption in Dungeness crabs Cancer magisterDungeness crabs Cancer magister are common in estuarine embayments around the Bamfield Marine Sciences Centre, British Columbia. However, they are weak osmoregulators and the question arises as to what effect lower-salinity waters may have on day-to-day activities such as feeding and food-processing.  Results show that when exposed to salinities of 50 and 75% , feeding activities are significantly depressed (see histogram on Left). Note in the histogram on upper Right the trend for longer starvation periods to progressively over-ride the depressing effect of exposure to low salinity. With only weak ability to osmoregulate, C. magister appears not able to handle the concurrent demands of feeding and digestion.  The authors suggest that C. magister may employ an “eat-and-run” strategy in the field, moving into lower salinities in the estuary to eat a meal, then retreating to areas of higher salinities to digest it.  Curtis et al. 2010 Mar Biol 157: 603.

NOTE  100% seawater is considered equivalent to 32‰ by the authors

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Hemigrapsus spp.

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Research study 1

graph comparing osmoregulatory abilities of grapsid crabs Hemigrapsus oregonensis and H. nudus over a range of external salinitiesAn early study at University of California Berkeley on osmotic regulation in grapsid and other crabs provides baseline information for many later studies.  Results, expressed as freezing-point depression (delta), show good hyperosmotic regulation in the 2 Hemigrapsus species down to about 0.4delta (about 20% that of ocean seawater), but both species are isosmotic in salinities above that of ocean water.  Neither of these species lives habitually in salinities much higher than that of the ocean, although exposure to dilute salinities down to the levels tested here (about 0.1delta) is common in estuarine areas.  Jones 1941 J Cell Comp Physiol 18: 79.

NOTE  a third grapsid species Pachygrapsus crassipes is included in the study and data for it can be found in the Pachygrapsus section below this one

NOTE  measured as –oC, with oceanic seawater of 35‰ being equal to about 2delta. Seawater concentrations greater than this are created by addition of sea salts

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Research study 2

graph comparing osmoregulatory abilities of shore crabs Hemigrapsus nudus and H. oregonensis over a range of external salinitiesShore crabs Hemigrapsus nudus and H. oregonensis live sympatrically on beaches around Vancouver, British Columbia and are subject to low-salinity conditions year-round owing to outflow from the Fraser River (salinities on local beaches range from 25%1 in summer to 75% in winter).  To test the ability of each species to osmoregulate, the author holds specimens for 48h in 8 different salinities ranging from 6-175% seawater and measures the osmotic concentration of the hemolymph in each treatment2.  Results show a good ability in both species to hyperosmoregulate over a “physiological3” range of salinities from 25-75% seawater and a poor ability to hypo-osmoregulate in salinities over 100% seawater. Statistical analyses of summer data reveal little difference between the species in osmoregulatory ability but, when winter data are included, H. nudus appears to be a somewhat better regulator than H. oregonensisDehnel 1962 Biol Bull 122: 208.

NOTE1  100% salinity is equivalent to 32‰ in this publication

NOTE2  treatments actually include 3 experimental temperatures in each of summer and winter seasons. For a simpler presentation, a common coloured line to all data has been eye-fitted for H. nudus. A similar eye-fitted coloured line has also been "imported" for H. oregonensis. Note the similarily in the 2 species

NOTE3  these are salinities that would normally be encountered in the habitat.  It should be noted that in this geographical region, seawater in excess of 100% would rarely, if ever, be encountered

Research study 3

graph showing sodium-ion concentrations in hemolymph and urine of shore crabs Hemigrapsus nudus over a range of external salinities in summer





Studies on osmoregulatory capability of the intertidal grapsoid crabs Hemigrapsus nudus and H. oregonensis in British Columbia similarly show a capability for hypertonic regulation of cations (but not including magnesium) in dilute seawater (see graph on Right). Note in the graph that the regulatory ability breaks down in seawater sodium concentrations of 100% (= 32‰) and above. The geographical area of the study is moderately estuarine through inflow from the Fraser River, and field salinities range seasonally from about 35-75%. 

graph showing hemolymph and urine magnesium-ion concentrtions in shore crabs Hemigrapsus oregonensis over a range of external salinities in summerAs evidenced by urine/blood concentration ratios approximating unity for sodium, potassium, and calcium, principal regulation of these ions seems not to occur in the antennary glands (kidneys). Rather, the authors suggest that secretory cells in the gill epithelia are likely involved.  In contrast, magnesium is strongly hypotonically regulated even in experimental salinities up to 175% (see graph on Left). Here, based on high urine/blood ratios for this ion, it seems likely that the antennary glands play the major, if not only,  role in regulating levels. As evidenced for other decapod crustaceans, low hemolymph magnesium concentrations seem necessary to facilitate neuromuscular impulse transmission. Dehnel & Carefoot 1965 Comp Biochem Physiol 15: 377.

NOTE  data are given for 4 cations (including magnesium) in hemolymph and urine, at 3 experimental temperatures and 2 seasons for both species, but are presented here only for sodium ion for H. nudus in summer at 15oC over 8 experimental salinities.  No significant seasonal or temperature differences are apparent in the data. Selected data, one set for each species, are shown here

NOTE  inclusion of test salinities higher than 100% is a bit unusual, given that open-coast field salinites never exceed about 32‰

Research study 4

graph showing mortality of shore crabs Hemigrapsus nudus in differnt salinities over timeStudies at the Bamfield Marine Sciences Centre, British Columbia on salinity tolerance and behaviour of purple shore crabs Hemigrapsus nudus indicate that they can survive indefinitely in salinities as low as 8‰ (25%; see graph on Left), prefer salinities of 22-32‰, and can discriminate in choice experiments between pairs of salinities separated by a difference of 2 parts (with preference for the higher of the pair in each case).  The species is strongly thigmotactic.  If shelter is available, their salinity preference expands to 10-32‰ and their tolerance to freshwater immersion (as in creeks) increases to 11h (i.e., almost a complete tidal cycle).  The author suggests that in their usual estuarine habitats availability of shelter may be the principal delineator of distribution.  McGaw 2001 Estuarine Coast Shelf Sci 53: 865.

NOTE note in the graph that time to 50% survival (LT50) is actually about 20d.  Researchers in this area of study consider a lethal time greater than 500h (21d) to be an “indefinite” survival period for a marine  crustacean.  In practical terms, in intertidal areas subject to heavy rainfall or river/stream influence, tolerance to any salinity beyond the duration of a complete tidal cycle would likely ensure survival

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Pachygrapsus crassipes

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Research study 1

graph showing osmoregulatory ability of the shore crab Pachygrapsus crassipes over a range of salinitiesAn early study at University California Berkeley on osmotic regulation in grapsid and other crabs provides information on Pachygrapsus crassipes.  Results, expressed as freezing-point depression (delta) of hemolymph samples taken from crabs exposed to water of different salinities, reveal excellent hyperosmotic regulation by Pachygrapsus from 2delta (equivalent to about 100% ocean seawater) down to 0.2delta (about 10% ocean seawater), and good hypo-osmotic regulation up to 3.4delta (about 170% seawater). Pachygrapsus is often exposed to dilute salinities in tidepools and freshwater rivulets, but only rarely encounters salinities much higher than that of the ocean; however, see the results of Research Study 4 below.  Jones 1941 J Cell Comp Physiol 18: 79.

NOTE  measured as –oC, with oceanic seawater of 35‰ being equal to about 2delta

NOTE  the author uses the term blood for hemolymph

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Research study 2

histograph comparing exoskeleton permeability in several species of intertidal and subtidal crabsShore crabs Pachygrapsus crassipes, Hemigrapsus nudus, and H. oregonensis have much less permeable exoskeletons than subtidal crabs Cancer spp. and Pugettia producta (see histogram).  Of the 3 intertidal crabs represented in the graph, P. crassipes is the better osmoregulator, while none of the 3 subtidal crabs shown is able to regulate to any great extent.  Osmoregulation in the intertidal species is believed to take place mainly in the branchial chamber, and involves active transport of cations across the gills.  The antennary glands (green glands or kidneys) appear to play no role in ionic regulation for the cations sodium, potassium, and calcium, as hemolymph and urine are generally isotonic for these ions, but magnesium ion is strongly regulated to low levels by production of a urine hypertonic in magnesium. In 50% seawater, for example, the hemolymph/urine ratio for magnesium may be as low as 6, but this may rise to over 20 in 150% seawater. Pachygrapsus crassipes is also able to osmoregulate through behavioural means.  Thus, after being subjected to osmotic stress and then given a choice of different salinities ranging from 50-150% seawater, Pachygrapsus crassipes prefers 100% seawater and is able to maintain normal sodium and potassium hemolymph concentrations by selecting fluid of appropriate salinity. Gross 1955 Amer Nat 89: 205; Gross 1957 Biol Bull 112: 43; Gross 1957 Biol Bull 113: 268; Gross 1959 Biol Bull 116: 248; Gross & Marshall 1960 Biol Bull 119 (3): 440.

NOTE  the author notes that before the effectiveness of the  antennary glands can be determined, the volume of urine produced must be known for each experimental situation

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Research study 3

Laboratory studies in southern California show that Pachygrapsus crassipes, if given a choice of trays containing seawaters of different salinities, will select the 100% tray over the others.  While making their choices, the experimental animals spend about half their time out of water, suggesting a high degree of aerial adaptation.  Through this behaviour and when given free choice of salinities, including 100% seawater, Pachygrapsus is able to maintain normal hemolymph sodium and potassium levels.  The author comments that selection of “normal” seawater in this way creates a behavioural mechanism tending to restrict crabs to the intertidal and subtidal zones of the sea.  Gross 1957 Biol Bull 113: 268.

NOTE  salinities tested are 50, 75, 100, 125, and 150%, based on an unspecified 100% (likely in the range 32-35‰)

Research study 4

graph of hemolymph concentrations in shore crabs Hemigrapsus oregonensis and Pachygrapsus crassipes at different times of the year over a range of seawater concentrationsphotograph of shore crab Pachygrapsus crassipes courtesy Jackie Soanes, Bodega Marine Laboratory, California

A natural large-scale field experiment involving suvival of 2 species of crabs Pachygrapsus crassipes and Hemigrapsus oregonensis in a land-locked hypersaline lagoon reveals the potential of these crabs for hyposmotic regulation.  The lagoon, near San Diego, California, became closed off from the open sea by a sand bar in January, 1959.  Over the course of the next 10mo the researchers measure survival of the crabs, and periodically determine osmotic concentrations of hemolymph and of the lagoon water.  Both species are good hyperosmotic regulators and both tend to avoid conditions of osmotic stress.  It is surprising, then, to see good hyposmotic regulation in both species, but especially so in the less terrestrial H. oregonensis (note that any point below the isosmoticity line indicates hyposmoregulation).  By September, the lagoon has reached 180% seawater and regulatory mechanisms are beginning to fail, especially in Hemigrapsus. By October (190% seawater) all Hemigrapsus are dead and only a few Pachygrapsus survive. Hemigrapsus oregonensis is not known from other laboratory experiments to hyposmoregulate, and the author suggests that its ability to do so here may be explained by the long, slow period of salinity increase, permitting physiological acclimatisation. Gross 1961 Biol Bull 121: 290; see also Prosser et al. 1955 Biol Bull 109: 99. Photo of P. crassipes courtesy Jackie Soanes, Bodega Marine Laboratory, California.

NOTE  refers to maintaining an internal osmotic concentration below that of the external medium (as opposed to hyperosmotic regulation).  Mostly, intertidal open-coast regions are bathed by “100%” seawater (32-35‰) and the only exposure to more dilute water is by rainfall.  Thus, a capability for hyposmotic regulation would not be expected to evolve.  However, in estuaries, especially ones with a marked halocline at the surface, where freshwater forms a layer on the seawater, the intertidal region can be washed with highly dilute seawater.  It is in these regions that the ability to hyperosmoregulate is selected for

NOTE  at this time hemolymph concentrations are around 180% (it is not mentioned in the paper, but this percentage is likely based on a salinity of around 35‰) and are close to being isosmotic with the external seawater.  That some individuals are higher than this is explained by the fact that several individuals tested had crawled out of the water into the sun, thus leading to some additional evaporative water loss

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Pugettia producta

Research study 1

photograph of kelp crab Pugettia productaStudies on kelp crabs Pugettia producta at the Bodega Marine Laboratory, California show that transfer from 100 to 80% seawater leads after about a day in normal crabs to a reduction in hemolymph sodium concentration to 80% equivalent seawater. If the graph showing sodium concentrations in hemolymph of kelp crabs Pugettia producta over several-hour period in 80% seawaternephridiopores are occluded to block the release of urine (which would be more dilute than the hemolymph), the hemolymph concentration drops even further. These and other data suggest that Pugettia is an osmoconformer.  The author does not discuss the relevance of the results to the natural biology of kelp crabs.  Cornell 1980 Biol Bull 158: 16.

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Rhithropanopeus harrisii

Research study 1

photograph of mud crab Rhithropanopeus harrisii courtesy D. Keith and US Geological SurveyThe mud crab Rhithropanopeus harrisii is a small (2cm carapace width), omnivorous brackish-water species introduced to San Francisco Bay from the Atlantic in the late 1930s.  It has since colonised estuarine areas in California and Oregon, and also some freshwater sites, although reproduction may be constrained in strictly freshwater habitats.  Prior to the present study it was not known whether the species’ success in brackish water owed to enhanced exoskeleton impermeability or to osmoregulatory processes of kidneys or gills, or  to some other feature(s).  Results show that like other brachyurans the hemolymph of Rhithropanopeus is hyperosmotic in salinities less than about 60% seawater and basically isosmotic in salinities above this.  Chloride determinations show that the urine is isotonic with graph showing osmoregulatory capability of crab Rhithropanopeus harrisiithe hemolymph and is produced in copious amounts (up to 20% of body mass per day in 50% seawater) to compensate for the excess amount of water diffusing inwards.  About one-third of total salt loss is via the urine.  Measurements of influx of heavy water (deuterium oxide, D2O) suggest that an individual may be capable of decreasing overall permeability in lower salinities, although how this occurs is not known.  The author concludes that, with the exception of the adaptive response just mentioned of being able to lower its water-permeability,  Rhithropanopeus shows osmoregulatory features consistent with those of other brackish-water crabs. Smith 1967 Biol Bull 133 (3): 643. Photograph courtesy D. Keith, Tarleton State University, Texas and US Geological Survey, Nonindigenous Aquatic Species Database, Florida.

NOTE  on the graph this corresponds to about 350mM (seawater of 35‰ is about 600mM)

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