Physiology & physiological ecology
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Gas exchange & metabolism


The topic of physiology & physiological ecology is divided into a section on gas exchange & metabolism considered here, and sections on pH & OCEAN ACIDIFICATION, CHEMORECEPTION, LOCOMOTION & TENACITY, DIEL SEASONAL & TIDAL RHYTHMS, OSMOTIC REGULATION & SALINITY TOLERANCE, and THERMAL STRESSES presented elsewhere.

Research study 1

photograph of the opening into the left branchial chamber in a Dungeness crab Cancer magisterGas exchange in crabs occurs across the surfaces of gills that hang into a space, the branchial chamber on either side of the body.  Seawater is pumped into and through the chamber by the beating of special paired appendages, the gill bailers or scaphognathites. Seawater enters the chamber at the back and over each of the walking legs.  A fringe of bristles hangs down from the lower edge of the carapace (its balloon-shape forms the branchial chamber) for protection against entry by unwanted organisms and for screening out particulate matter.  There are few adaptations within the branchial chamber of temperate species associated with gas exchange in air, such as increased structural support for the gills, but in tropical and subtropical regions where evolution to semiterrestrial life is more prevalent such adaptations are common.  These include reduction in gill number and size, increased structural support for gills, and vascularisation of gill chambers for aerial exchange of gases.

NOTE  lit. “hollowed out + jaw”  G., referring to the sculpted shape of the bailers and/or to their location on either side of the mouth

NOTE  these are ramified blood vessels in close contact with the skin surface lining the branchial chambers.  Oxygen and carbon dioxide diffuse across these surfaces


View across mouthparts of a Metacarcinus magister to show the opening
to the left branchial-chamber. A viewing angle like this of a live crab
would allow the flapping of the scaphognathite (gill bailer) to be seen

Research study 2

graph showing oxygen consumption of starved Pachygrapsus crassipes over timeAs in all animals, many environmental factors affect VO2, including temperature, salinity, and nutritive and reproductive status.  Studies on Pachygrapsus crassipes in Palos Verdes, California indicate that VO2 goes up about 2wk prior to moulting.  Starvation, as expected, causes a decline in oxygen consumption and also has an inhibitory effect on moulting (see graph). Roberts 1957 Physiol Zool 30: 232.  Photo courtesy Jackie Soanes, Bodega Marine Laboratory, California.

NOTE  rate of oxygen uptake.  PO2 refers to concentration of oxygen

NOTE  experiments are run at 16oC and the data are converted to a "standard" animal size of 25 live g

Research study 3

photograph of box crab Lopholithodes foraminatus showing view of opening or "foramen" into branchial area
It must be detrimental to the health of a brachyuran to have other animals living in the branchial chamber.  This is noted for 2 specimens of box crabs Lopholithodes foraminatus in Monterey Bay, California which, on collection, are discoved to have several larval blacktail liparid fishes Careproctus melanurus inhabiting the branchial chamber.  Parrish 1972 Cal Fish Game 58: 239.

NOTE this species uniquely has symmetrical openings for water flow for gas exhange



It is not known whether the fishes described above are
accidental "visitors" to the crab's branchial chamber,
or whether they are parasites 0.5X

Research study 3.1

Researchers at the University of San Diego, California compare activity levels of two intertidal crab species Pachygrapsus crassipes (southern California) and Eurytium albidigitum (Gulf of California) with respect to branchial-water stores and oxygen uptake, when immersed and emersed by the tides. During low-tide periods the former species is quite active, while the latter is more sluggish. The explanation appears to lie in relatively less water being carried in the branchial cavity by P. crassipes (by a factor >5) and a smaller decrease in oxygen uptake when in air than in water shown by P. crassipes as compared with E. albidigitum (55 vs. 95%, respectively). This is a sort of “apples and oranges”-type study, and would have been better received had the authors been more selective in their choice of species. Burnett & McMahon 1987 Physiol Zool 60 (1): 27.

NOTE the authors actually state the opposite for this species in their abstract

NOTE the comparison becomes even less interesting when it is learned that the species live in different habitats in different areas and come from different families. A third species Hemigrapsus nudus is actually included in the study, but it lives even further away (British Columbia), and some comparative data for it are missing in the presentation

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Research study 4

graph showing changes in scaphognathite beating during air-emersion and water-immersion in crabs Hemigrapsus nudusLaboratory studies on “air-breathing” in shore crabs Hemigrapsus nudus show that, when given a choice, individuals spend more time out of water than in.  Adaptations for gas-exchange directly from air include reduction in surface area of the gills and a well-vascularised “lung” area within the branchial chamber.  Eight pairs of gills provide gas exchange when immersed.  The 2 pairs on each of maxillipeds 2 & 3 provide almost 30% of the total surface area, while the 2 pairs on Walking Leg 1 provide 43%, and 1 pair on each of WL 2 & 3 provide the remainder.  As shown in the accompanying graph, on emersion and again on immersion, a crab hyperventilates and its heart rate increases (tachycardia).  Greenaway et al. 1996 Physiol Zool 69: 785; for details on acid-base balance in relation photograph of shore crab Hemigrapsus nudus in airto emersion and haemocyanin function in gas transport in H. nudus, see Morris et al. 1996 Physiol Zool 69: 806 and Morris et al. 1966 Physiol Zool 69: 839.


NOTE  “ventilation” refers to scaphognathite beating, monitored using an impedence electrode inserted through the carapace

Shore crab Hemigrapsus nudus in air 1.7X

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Research study 5

By equipping crabs Metacarcinus magister with flowmeters and various pressure transducers, researchers at the Bamfield Marine Sciences Centre, British Columbia are able to monitor changes in cardiac performance and ventilation during walking activity.  Results show cardiovascular shunting during activity leading to decreased flow to the digestive system, and increased flow to muscles of the walking legs and respiratory system.  Ventilation rate also doubles during walking. Wachter & McMahon 1996 J Exp Biol 199: 627.

NOTE  done in a treadmill apparatus in the laboratory

Research study 6

schematic of photos and graph illustrating the functioning of the gas-exchange membranes on the leg of a porcelain crab Petrolisthes Petrolisthes llustrating Porcelain crabs Petrolisthes cinctipes and P. eriomerus often live sympatrically, but with the former species living higher in the intertidal zone than the latter. Are there any special gas-exchange modifications for life in the high zone where access to seawater is restricted? Studies at Hatfield Marine Sciences Center, Oregon show that the porcelain crab P. cinctipes has unique uncalcified membranes on the merus segment of each walking leg.  By measuring whole-animal oxygen uptake before and after coating the membranes with nail polish, these membranes can be shown to function in augmenting oxygen uptake in air. Half-coating the membranes does not affect their function (see graph on Right).  Although the membranes are present in juveniles, they appear not to function in gas-exchange (data not shown).   The membranes are also augmentative only at high temperatures, but this is when body metabolism is high and when drying of gill surfaces is likely to be maximal.  Additional experiments show that lactate accumulation is high after 5h air-exposure in adults whose membranes are coated, but not in control animals whose membranes are untreated.  The authors provide convincing evidence that Petrolisthes cinctipes is living at or near its limits of physiological tolerance. Stillman & Somero 1996 J Exper Biol 199: 1845.

NOTE  the authors note that similar membranes are found in 16 of 79 world species of Petrolisthes, but do not know whether their presence correlates positively with high-intertidal life. The lower-dwelling P. eriomerus appears not to have gas-exchange membranes

Research study7

photograph of shore crab Hemigrapsus nudus courtesy Iain McGaw, U NevadaIt seems unusual to think of an ectothermic shore crab regulating its internal body temperature, but studies at Bamfield Marine Sciences Center, British Columbia show that Hemigrapsus nudus is able to do this through behavioral means.  First, crabs acclimated to 16oC (summer seawater temperature) for 2wk, when given a choice, prefer water of about 15oC and avoid water warmer than 25oC.  If, however, crabs are acclimated to water of 10oC (winter temperature), they prefer water of 17oC and avoid water warmer than 27oC.  As expected, H. nudus is better able to survive prolonged exposure to cold temperatures than to warm temperatures.  During simulated intertidal exposure in the laboratory, crabs are able to control their body temperatures by shuttling between air and water.  In this regard, the time they spend in either air or water is influenced more strongly by temperature than by the medium they are in. For example, a crab with a choice of water at 5oC and air at 34oC in a laboratory-choice apparatus spends the first 2h moving between the 2 until finally reaching a body temperature of 8-13oC.  It does this by immersing and emersing its body as needs be.  In the reverse circumstance, with a choice between water at 34oC and air at 5oC, the crab is still able to hold a body temperature between 8-13oC for most of the 12-h experimental period. 

When 5 specimens are released into the field on a date in summer when air temperatures are lower than water temperatures (in the night-time), the crabs maintain their body temperatures at around 15-17oC by behavioral means just described (upper graph).  On another date 2wk later, when air and water temperatures are reversed (in the daytime), the crabs still maintain their body temperatures at about 13-17oC for most of the day (lower graph).  Thus, despite wide temperature variation in their habitat of up to 20oC, H. nudus is able to utilise thermal microhabitats underneath rocks to maintain its body temperature within narrow limits. McGaw 2003 Biol Bull 204: 38. Photo courtesy Iain McGaw, U Nevada.

NOTE  the author discusses this apparent contradiction in acclimation and preferred temperatures

NOTE  temperatures are monitored by thermocouple implants with up to 2m leads feeding to a data-acquisiton instrument

Research study 8

photograph showing frontal view of Dungeness crab Cancer magister buried but with provision for entry of clean water into its branchial chambersAreas of low oxygen content (hypoxia) are not uncommon in the marine environment, even in shallow, apparently well-mixed systems.  In Barkley Sound, British Columbia, for example, inshore habitats of Dungeness crabs Metacarcinus magister can vary in PO2 from 28kPa at the sea surface to less that 1kPa just above the sediment-water interface (see graph).  What effect does low oxygen tension have on the behaviour of Cancer?  Lab studies using crabs in an oxygen gradient show that feeding ceases at low oxygen levels and that crabs generally tend to move out of areas with low oxygen content.  Crabs are less likely to enter hypoxic waters, even to obtain food and, when they do, they move the food to an area of higher oxygen level to feed and digest.  Both field and lab parts of the study suggest that the preferred range of oxgen tension for M. magister is 8-17 kPa (about 40-80% saturation). The authors note an ability in M. magister to detect oxygen gradients and to orientate in response to oxygen tension.  However, whether this ability would permit orientation on a larger spatial scale in the field is, in the opinion of the authors, questionable.  Bernatis et al. 2007 Mar Biol 150: 941.

NOTE  this is a higher than normal oxygen concentration and likely a result of oxygen-enrichment from phytoplankton or vascular plant growth; “normoxic” levels range from 15-21 KPa and “hypoxic” levels are <13 kPa

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Research study 9

photograph of arterial system of a lithode crab Lopholithodes mandtii created by a novel method of "corrosion casting, courtesy McGaw & Duff 2008 J Morph 269: 1295 )photograph of ventral view of Puget Sound king crab Lopholithodes mandtii courtesy Iain McGaw, Memorial UniversityAlthough outside the intended scope of the ODYSSEY, recent studies at the Bamfield Marine Sciences Centre, British Columbia on circulatory systems in west-coast brachyurans cast new light on the degree of “open-ness” of crustacean cardiovascular systems.  The technique of used to map the systems, known as corrosion casting, yields stunning views of a crab's circulatory system, as seen on the Right for the crab Lopholithodes mandtii. Although the return hemolymph system can still be considered “open”, consisting as it does of tissue lacunae and larger sinuses, in view of the extensive arterial system consisting of sub-branching arterioles terminating in fine capillary-like vessels revealed in these studies , the authors propose to classify the circulatory systems of these and other brachyurans as “incompletely closed”.  McGaw & Duff 2008 J Morph 269: 1295; see also McGaw 2005 Microsc Microanal 11: 18. Photographs courtesy the authors.

NOTE  the technique involves drilling a hole through the carapace above the pericardial sinus, inserting a catheter, and infusing thinned resin solution.  Several leg-tips are cut off to allow the hemolymph displaced by the resin to escape.  After a 1-d curing of the resin, the soft tissues of the crab are digested away over 7-d treatment with potassium-hydroxide solution.  After drying of the preparation, the carapace and remaining hard tissues are dissolved away in concentrated hydrochloric acid over a 24-h period

NOTE  however, other research shows that rather than being completely “open”, these sinuses may form precise networks around the internal organs

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Research study 10

photograph of red rock-crab Cancer productus in airgraph showing effects of emersion and food on hemolymph-oxygen concentration in red rock-crabs Cancer productusCrabs vary greatly in their ability to tolerate exposure to air (emersion).  Intertidal-inhabiting species like Hemigrapsus spp. and Petrolisthes spp. are able to maintain oxygen uptake in air for long periods, as long as the gills are kept moist.  Cancer spp., however, are generally obligate “water breathers” and will eventually die if emersed.  Several things happen to a red rock-crab C. productus when emersed, as could happen by being stranded in the intertidal zone on an ebbing tide.  First, as shown in investigations at the Bamfield Marine Sciences Centre, British Columbia, collapse of the gills in air results in a rapid decrease in oxygen uptake.  Oxygen levels in the hemolymph decrease (see graph showing effect of nutritional status on survival of red rock-crabs Cancer productus during air exposuregraph upper Right), anaerobic respiration kicks in, and lactic acid concentration in the tissues rises (see graph lower Right). Mechanical digestion is actively suppressed during emersion.  Nonetheless, if the crab has recently fed, that is, post-prandial, ammonia concentration in the hemolymph will build to even higher levels than it would if unfed, indicating that protein catabolism proceeds, at least for a time, anaerobically. If the emersion period is short, recovery occurs quickly when re-immersed.

Survival of crabs during air exposure relates in large part to whether they are recently fed or not, with unfed individuals surviving longer than fed ones (see graph lower Left).  McGaw et al. 2009 Can J Zool 87: 1158.

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Research study 11

graphs showing effects of pH and temperature on metabolic rate and thermal tolerance in porcelain crabs Petrolithes cinctipesesClimate-change models predict increasing extremes of tidal-associated thermal stress and decreasing pH in coastal ecosystems.  A recent investigation by California university researchers on how these might act alone and together on oxygen uptake/metabolism of adult porcelain crabs Petrolisthes cinctipes uses a laboratory tide-tank with warming lights and CO2-conditioned seawater to create various combinations of temperature and pH.  After 2.5wk acclimation to each set of treatments, results show that at the most extreme conditions of high temperature (30oC) and low pH (7.1), metabolic rate is lowest, but thermal tolerance is highest (see graphs).  Note in the  upper graph that low pH (blue line) and high temperature (red line) on their own do not significantly affect oxygen consumption, but together they lower the rate by 25% (green line).  At the same time, thermal tolerance, that is, the capability of Petrolisthes to tolerate higher temperature stress, is significantly raised under the combined effects of severe temperature and pH stresses (see green line on lower graph). The authors interpret this to mean that at these extreme conditions, a bigger slice of energy is being diverted from maintenance and growth to costs of stress responses, the latter increasing thermal tolerance.  Paganini et al. 2014 J Exp Biol 217: 3974.

NOTE  although couched in terms of climate change and ocean acidification, the study really shows only acute effects of temperature and pH on metablic rate in the test species. What is needed in such studies is a longer-term approach involving several successive generations, preferably with a species with a shorter generation time. Transcriptomics is also a useful approach, as has been used in similar studies with sea urchins Strongylocentrotus purpuratus LEARN ABOUT SEA URCHINS: PHYSIOLOGY/PH & OCEAN ACIDIFICATION

NOTE  these include expression of heat-shock proteins, known to occur in porcelain crabs

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