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.

An aspect of the topic gas exchange & metabolism deals with OCCLUSION OF GAS EXCHANGE BY PARASITIC FISHES presented in its own sub-section below

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

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 using 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 PetrolisthesPorcelain 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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Possible occlusion of gas exchange by parasitic fishes
  It may be surprising to learn that fishes, most notably several species of liparids or snailfishes, parasitise species of lithodid crabs, including both Alaska king crabs and box crabs. They do so by laying their eggs within the branchial chambers of their hosts.
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Research study 1

An early1 account of this phenomenon on the west coast is is from a fisheries scientist from the Bureau of Commercial Fisheries, Seattle, who was alerted by a commercial sea captain, Mel Leonard, who noticed fish eggs issuing from male king crabs Paralithodes camtschatica caught in traps in the Aleutian Islands, Alaska and being processed shipboard. The eggs, tentatively identified as those of a liparid fish snailfish Careproctus (so named because they possess a ventral sucker that they use to attach to things), are later found in other male king crabs to fill most of one of the branchial chambers2. How do the fishes deposit their eggs in a crab? All Careproctus species have an extensible ovipositor3 that would enable this, but how the symbiosis may have evolved is another story. Hunter 1969 Pac Sci 23: 546. Photograph courtesy the author.photograph of posterior region of Alaska king crab Paralithodes camtschatica possibly showing egg mass in left branchial chamber

NOTE1 fish eggs in branchial chambers of king crabs are known from Russian publications dating from the 1950s, but this study appears to be the first for Alaska

NOTE2 in this account and in others to follow, only one of the branchial chambers appears to be affected. Given that gill function must be impaired by the compacted eggs, a double infestation could prove fatal to the host - not a happening of good selective value for the fish

NOTE3 a 40cm-long Careproctus sinensis (a northwest Pacific species) apparently has an ovipositor 8cm in length. The female fish’s ventral sucker could come in handy for the insertion of eggs, which is likely done via the space under the crab’s overhanging carapace, or possibly the foramen opening into the branchial chamber, the latter likely being a more dicey enterprise for the fish if it were to use this route

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

NOTE the researcher simply describes the relationship as a symbiosis (i.e., “living together”). Based on what we know now, parasitism is likely a better descriptor, but at the time the researcher was not clear on the relationship of the participants
photograph of larval fishes Careproctus melanurus removed from the branchial chamber of a box crab Lopholithodes foraminatu
Eggs and larvae of the fish outside of the branchial chamber 1.5X
The openings or "foramens" communicate directly with the branchial chambers, presumably allowing "used"respiratory water to escape. They may be useful for when the animal is closed tightly, either against predators or if buried in sediments  
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Research study 3

A later study by researchers at the Royal British Columbia Museum confirms the symbiotic relationship of box crabs Lopholithodes foraminatus and liparid fishes Careproctus melanurus. The right branchial chamber of a gravid female is seen to be filled with over 400 fish eggs. Despite noting that the gills are completely collapsed, thus presumably impeding gas exchange, the authors term the relationship a commensalism rather than parasitism. Peden & Corbett 1973 Can J Zool 51: 555.

NOTE the original use of the term symbiosis (“living together” G.) in Britain and Europe was in reference to organisms living in mutual benefit, what in North America at the time was termed mutualism. Nowadays, common worldwide use of symbiosis refers to organisms simply living together, without further caategorisation. Mutualism is defined as a symbiosis with both participants benefitting, commensalism as one partner benefitting without harm to the other, and parasitism as one partner benefitting at the expense of the other. This is clear enough, but the problem is the categorisation tends to “pidgeon-hole” relationships, especially in instances of putative commensalism where too little is known about exactly how one symbiont affects another. The present authors, in discussing previous authors' designations of the crab/fish drawings of eggs of liparid fish Careproctus malanurusrelationship simply as "symbioses" (RS1 & RS2 above), note that “the eggs must have impaired the crab’s respiration”, but then go on to say that “Until a benefit to the crab can be demonstrated, the relationship should be referred to as commensalism or, if oxygen is shown to be extracted from the crab’s respiratory current, as parasitism”. This initially sounds confusing, but perhaps they have interpreted the earlier authors' use of "symbiosis" in its British/European way as meaning "mutualism" (i.e., mutual benefit)

Side and front views of an embryonic fish Careproctus malanurus showing
the suction cup or adhesive disc that develops from modified pelvic
fins. Most or all liparid species have these for clinging onto things 10X

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Research study 4

photograph of golden king crab Lithodes aequispinus showing parasitic fish eggs in left branchial chamberA later publication by scientists in Alaska correctly describes the relationship as a parasitism, rather than a commensalism, owing to damage to a host’s gills by the egg mass. The authors remind us that in addition to gas exchange, gills of crustaceans serve as sites for ion regulation. The participants are golden king crabs Lithodes aequispinus and liparid snailfishes Careproctus sp. in southeastern Alaska. In this study, 2 of 14 ovigerous females, but zero of 824 males, all harvested from the same general area at roughly the same time, are parasitised. However, presence of a few hatched fish larvae scattered about the processing station on the day that this observation is made suggests that at least a few infected males have been present. Overall, though, it is clear that the incidence of parasitism in the Alaskan population is relatively small. The authors review the literature on branchial-chamber parasitism by fishes in 7 world lithode species and note that all are parasitised by members of genus Careproctus. As well as noting the damage and probable impairment of gill function incurred by parasitised crabs, the authors point out advantages to the snailfishes of clear water circulation and protection offered by their hosts. Love & Shirley 1993 Crustaceana 65 (1): 97. Photograph courtesy the authors.

NOTE apparently it is difficult to assign species names to the genus owing to absence of definitive markers in the larvae (see RS5 below). The authors comment that of 74 described species in SubFamily Liparidinae, only 2 have known larval histories

NOTE none of the authors of these papers on crab/fish parasitism speculate on how the relationship may have evolved. Such obvious questions as whether other types of crabs are used as hosts, whether size limitations exist for either participant, or whether rock-crevice habitats are additionally used as laying sites by the fishes, are not discussed. Furthermore, the expectation is that larvae incubated in crabs must enjoy significantly greater recruitment success than ones hatching from under a rock, but are other factors involved? As for the crabs, to what extent does egg presence actually interfere with gas exchange, and would a double load of eggs really be fatal? Apparently, so little is known about the natural biology of Careproctus and related snailfishes as to make such considerations, well, just specuations, but is it naive to think that a few simple laboratory experiments might not be informative?

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Research study 5

More details on the parasitic relationship of snailfishes and lithode crabs Lithodes aequispinus are provided by fisheries scientists in Alaska. The most interesting is that the spawning fishes seem able to sense the capacity of the branchial chamber, for more eggs are deposited into larger crabs. Additional findings are that egg masses may be present in both gill chambers, that male crabs are preferred, and that female crabs early in their moult cycle are preferred. The authors suggest that the last may be a strategy ensuring stability of the branchial chamber during the developmental period of the parasite’s eggs; in other words, decreased chance of the eggs being ejected during casting off of the exoskeleton. A gravid snailfish may be able to assess the moult status of its intended female host by the external presence of the crab’s own eggs, which are attached soon after moulting, or perhaps by perception of hormones emanating from the female. The authors do not discuss how this may relate to males, but they do speculate on whether spawning in parasite and host may occur synchronously (apparently, not enough is known about snailfishes to resolve this). Descriptions of damage to the host’s gills are more detailed than those of other researchers, and include physical compression, localised necrosis, and sometimes complete loss of gill tissue in an affected branchial chamber. Causes may relate to constriction of blood flow to the gills and/or interference of gill-cleaning activity of the 5th walking leg, which inserts into the branchial chamber under the carapace overhang. In instances of complete gill loss, regeneration may or may not occur at the next moult. Another interesting finding is that a single crab may bear multiple egg masses (up to 4 in one individual), and these may be deposited by different snailfish species. The researchers note in their summary that mortality in commercial-sized male crabs in the holds of fishing vessels may be increased by 35% over non-parasitised ones. However, absolute rates of infection in natural populations are sufficiently low (0-30% depending upon locality and depth) that the effect on commercial fisheries is considered small. Somerton & Donaldson 1998 Fish Bull 96: 871. Photograph courtesy Alaska Fisheries Science Center and NOAA.photograph of golden king crab Lithodes aequispinus

NOTE the researchers initially believe that 2 species of Careproctus are involved in infesting golden king crabs in the eastern Bering Sea, the pink snailfish C. furcellus and an undescribed species referred to as the red snailfish. This belief is based upon a positive correlation found between size of eggs in crabs and size of eggs in gravid females of the 2 predominant snailfish species captured in trawls coincidental with the crabs. However, the researchers are later able to confirm their preliminary identifications using genetic analysis

NOTE the values given for probability of parasitised males and non-parasitised males dying in the holding tanks are 0.035 and 0.25, indicating an actual increase of 40%


Golden king crab Lithodes aequispinus. This individual has a
missing Right 2nd walking leg, and the two 5th walking legs
are hidden under the carapace overhang at the back. The
walking leg will presumably regenerate at the next moult 0.1X

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Research study 6
photograph of liparid snailfishes

A helpful publication by fisheries researchers at several university and governmental facilities in Seattle, Washington compares DNA sequences from mitochondria in “eyed” embryos from 75 snailfish egg masses taken from golden king crabs Lithodes aequispinus and for the first time from scarlet king crabs L. couesi. Overall, 23 species of adult snailfishes are collected from locations in the Bering Sea and Gulf of Alaska, and their sequences compared with those of the egg masses. Their findings reveal parentages from 4 species of snailfishes (F. Liparidae, see photographs) as follows: Careproctus melanurus (38), C. colletti (29), C. furcelllus (7), and C. simus (1). Based upon sequencing data and clustering features of the eggs, the authors determine that each single egg mass is deposited by a single female of a single species. However, as we already know from RS5 above, more than one female of more than one species may deposit eggs into the same branchial chamber of a single crab. As to whether egg deposition in crabs is obligatory for snailfish species is unclear. However, based upon their own knowlege and other published observations of snailfish eggs being found on inanimate objects, the authors think that the behaviour is facultative or limited to only some snailfish species. The researchers also note that the nature of this parasitic symbiosis is unusual in that the pairings are not species-specific, as is most often the case. Gardner et al. 2016 Copeia 104 (3): 645. 2016 Copeia 104 (3): 645. Photographs courtesy the authors.

NOTE the region sequenced is the “barcoding” gene cytochrome c oxidase subunit I, so-named because of its widespread use in species identification

Four liparid snailfish species identified
as depositing eggs into the branchial
chambers of lithode crabs in the study.
Note the protruding ovipositors
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Research study 7

Gas exchange in sea spiders or pycnogonids takes place through the exoskeleton, most notably via the extensive surface area of the legs. A heart is present that moves hemolymph through the trunk of the body, but transport in the limbs is impeded by their narrow apertures. A novel finding by a group of American and Australian scientists studying 12 southern hemisphere pycnogonid species is that movement of oxygen-carrying hemolymph into and out of the limbs is actually aided by peristaltic contractions of the gut. The gut has diverticula that extend most of the length of each of the limbs, and these diverticula are observed to undergo rhythmical expansion and contraction. Through injections of fluorescein dye into the gut, the authors note that peristaltic outward movements of gut fluid in the leg diverticula displace hemolymph photograph of pycnogonid Dodecalopoda mawsoni showing relative cuticle thicknessesproximally towards the body trunk, and vice versa during peristaltic movement of gut fluids inward. The resulting tidal wash of hemolymph means that the legs are acting like gills, carrying oxygen-enriched hemolymph into the body and carbon dioxide-enriched hemolymph outward. Woods et al. 2017 Current Biol 27 R639. Photograph courtesy the authors.

NOTE pycnogonids are more closely related to spiders than to crabs, but are included here for convenience. Although fairly common in shallow waters of most world oceans, they are not well studied

Dorsal view of Dodecalopoda mawsoni showing relative cuticle
thicknesses: green being thicker than brown. Thicker leg tips
are advantageous for locomotion, while thinner parts aid in
gas diffusion. While most pycnogonids have 4 pairs of legs,
some have 5 and even 6 pairs (as shown with this species)


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