Sphaeromid¹ isopods, such as Gnorimosphaeroma (oregonensis²) oregonense and Sphaeroma pentodon, inhabit intertidal regions often characterised by a wide range of salinities.  Experiments in California to test their ability to osmoregulate over a range of salinities from 0-125%³ seawater show that there is some hyperosmoregulation in dilute salinities and some hypo-osmoregulation in concentrated salinities (48h test periods).  Riegel 1959 Biol Bull 116: 272.

NOTE¹  the first species listed, G. (oregonensis) oregonense is widely distributed along the west coast; S. pentodon is more restricted, initially only in San Francisco Bay, but spreading to other areas

NOTE²  in a companion paper to this one the author discusses the taxonomic status at that time of 2 subspecies of G. oregonensis (oregonensis and lutea), but it appears now that the species currently recognised are G. oregonense  and G. lutea.  Riegel 1959 Biol Bull 117: 154.

NOTE³  seawater (34‰) from Marin County, California = 100%. This is diluted with distilled water to produce salinities of 25-75%, or boiled off to produce 125%

Several isopods Gnorimosphaeroma oregonense 1X

A study on morphological adaptations, and salinity and humidity tolerances, of 5 species of isopods collected around the Pacific Marine Station, Dillon Beach, California and representing a transitional series from marine to terrestrial life, reveals features correlative with the habitats occupied.  In general, the more terrestrial isopods are more heavily chitinised than marine forms, are less tolerant of seawater immersion, and resist drying better. Brusca 1966 Bull S Cal Acad Sci 65: 146.

NOTE  the species include (from marine to terrestrial) Cirolana harfordi, Idotea wosnesenskii, Ligia occidentalis, Alloniscus perconvexus,  and Porcellio scaber

Two ligiid species inhabit west coast shores.  Both live close to the shore, most within a few meters above the high-tide mark.  Ligia pallasii has a more northerly distribution, inhabits cool moist areas, tends to be more nocturnal in habit, and is relatively slow-moving.  The second species Ligia occidentalis has a more southerly distribution, lives in warmer drier habitats, tends to forage more in daytime than L. pallasii, and is much faster-moving.  Studies in southern California show that osmoregulatory capabilities of both species correlate well with overall behaviour and ecology.  Both species are able to hypo-osmoregulate in salinities more concentrated than seawater (i.e., in desiccating conditions), but L. pallasii appears to be the superior regulator in dilute salinities (see graphs).  In fact, L. occidentalis dies within a few days in salinities of 50% seawater or less.  Some differences in temperature-effects related to osmotic regulation are found for the species. Wilson 1970 Biol Bull 138: 96.

NOTE  the species overlap in parts of northern California.  Prior to a decade ago, L. occidentalis occurred no further north than southern Oregon.  Recently, however, it has been found in Howe Sound and on Savary Island, British Columbia

NOTE  although both species live close to the shore, neither seems inclined to enter seawater for more than brief periods.  So it may actually have been more appropriate to measure osmoregulatory abilities of these semiterrestrial forms in their normal medium of air, rather than in water

Sphaeromid isopods Gnorimosphaeroma oregonense are often air exposed but seek out microhabitats with high relative humidities.  In experiments using isopods in humidity gradients of 76-100%, researchers in Edmonton, Alberta find that the animals aggregate in high-humidity areas by a “reversal turn” reaction occurring in lower humidity areas.  In this behaviour the test animals simply reverse their direction of locomotion in sharply executed 180^o angle turns.  A klinokinesis, that is, a change in frequency of turning in response to the intensity of a stimulus (in this case, humidity), is not seen, although the authors do not rule out the presence of an hygro-orthokinesis, or change in rate of movement of an individual in response to the degree of humidity.  The authors provide preliminary evidence that humidity information may be received indirectly by Gnorimosphaeroma via receptor cells that are sensitive to internal salt concentrations. Standing & Beatty 1977 Can J Zool 56: 2004.

NOTE  when this article was written hygroreceptors had not been identified in isopods, and may still not be.  The preliminary evidence noted above is that behaviour in a humidity gradient varies in relation to internal osmotic concentrations, suggesting a link between internal osmotic or ionic status and hygroperception

Several Gnorimosphaeroma oregonense in
typical underrock habitat. The larger isopod
at lower Right is an Idotea sp. 2X

A study on osmoregulatory ability in Idotea wosnesenskii collected at Cape Arago, Oregon reveals some ability to hyperosmoregulate in low salinities but with osmoconformity at high salinities (12-14d immersion in each salinity treatment; see graph). By ablating different combinations of pleopods, then testing hemolymph osmotic concentration, the author determines that the endopodites of the posterior 3 pairs of pleopods are responsible for osmoregulation in 50% seawater (500 mosmol ^. Kg ^-1), while ablation of the exopodites of the same pleopods has no effect. These 3 sets of endopodites also have significantly higher Na⁺/K⁺-ATPase enzyme-specific activities than the corresponding exopodites (indicated by silver staining; see drawing below), suggesting that they are most important in osmoregulatory ion transport in Idotea.  Holliday 1988 J Exp Biol 136: 259.

NOTE the exoskeletons on these are notable thin and these appendages are also responsible for most of the gas exchange

NOTE 100% salinity is set at 1000mosmol ^. kg^-1

Of the 5 pleopods only the endopodites of Nos.
3- 5 show silver staining indicative of enzyme-
specific activity related to osmoregulation

Somewhat different results to the above (Research Study 3) are presented in a study on osmotic concentrations in hemolymph and marsupial fluids in Ligia occidentalis¹ by researchers at Claremont College, California. A similar protocol is used, with isopods being immersed in seawater for a period of time, then hemolymph (and in this case) marsupial fluid being analysed for osmotic concentration. However, rather than simultaneously immersing groups of isopods in seawater of different salinities and then sampling, in the present study seawater of 600mOsmol ^. L^-1is allowed to evaporate over about 3wk to an osmolality² of 1700mOsmol ^. L^-1, with sampling of individuals occurring within this time frame³. Results show that hemolymph and marsupial fluid are isosmotic, which is surprising considering that this species has an “open” marsupium, that is, open at both ends to the external environment. This means that marsupial fluid is being regulated in tandem with the hemolymph. Note in the graph that in experimental salinities below about 100% (1000mOsmol ^. L^-1) and different from what is shown for L. occidentalis in Research Study 3 above, hemolymph and marsupial fluid are hyperosmotic with the external environment. Hemolymph and marsupial fluid are similarly isosmotic in the fully terrestrial species Armadillidium vulgare. This is perhaps to be expected owing to its closed marsupium and intimate connection of the parental hemolymph and marsupial fluid via cotyledons that extend from the parent into the egg/juvenile mass. These transfer ions, moistening fluid, and possibly nutrients from the parent to the marsupium. In the researchers’ view the closed marsupium of this species is part of the overall hemolymph space. Yoshizawa & Wright 2011 Crustaceana 84 (11): 1307.

NOTE¹ 3 other terrestrial species are included in the study, Ligidium lapetum, Alloniscus perconvexus, and Armadillidium vulgare, but only the last with a closed marsupium is included here

NOTE² the osmolarity of ocean seawater is about 1000mOsmol ^. L^-1

NOTE³ this protocol is time-saving in that individual seawater batches of different salinity need not be made up, but it risks unknown chemical changes in seawater over time through food, fecal, and urinary contamination, and perhaps more seriously, through acclimation changes modifying physiological responses over the 3wk time frame