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  Life in the intertidal zone
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  Salinity & osmotic regulation

The topic of life in the intertidal zone includes a section on salinity & osmotic regulation considered here, and sections on TEMPERATURE & DESICCATION, WAVES & CURRENTS, SEASTAR WASTING DISEASE, OCEAN ACIDIFICATION, OTHER PHYSIOLOGICAL STRESSES, COLOUR MORPHS OF PISASTER, and SYMBIONTS presented elsewhere.

Research study 1

photograph of sea star Leptasterias hexactisBay and fjord areas of British Columbia and Alaska in summer are often characterised by seawater overlain by freshwater lenses caused by rainfall and glacial melt.  These lenses may be several meters deep and effectively wash the intertidal area with water of reduced salinity with each tidal change.  The effect that this has on behaviour and survival of sea stars Leptasterias hexactis is examined in laboratory studies on specimens collected along the Lynn Canal in southeastern Alaska.  Generally, survival, activity, feeding, and growth all decrease proportional to decrease in salinity.  However, the fact that Leptasterias can maintain growth for at least 3wk at 20‰ (at 13oC) suggests to the authors that is is a euryhaline species and not stenohaline, and that brief periods of exposure to low salinity should not limit its distribution.  Shirley & Stickle 1982 Mar Biol 69: 147; see also Shirley & Stickle 1982 Mar Biol 69: 155.


Oral view of the sea star Leptasterias hexactis 1.7X

Research study 2

map showing collecting sites for sea stars Pisaster ochraceus in osmoregulatory studySea stars, and echinoderms in general, are notoriously intolerant of low salinities, and they mostly lack any ability to osmoregulate.  Does acclimatisation to low salinities occur?  This is tested with 2 populations of ochre stars Pisaster ochraceus, one from a high-salinity site at Bamfield, British Columbia and the other from low-salinity sites at Vancouver, British Columbia (see map). Salinities at the first site range around 30psu, while at the second site they range from 15-25psu, with a strong seasonal component.  The authors first test both populations for osmoregulatory ability in salinities of 15-30psu over 24-h exposure.  They then test performance, measured as feeding rate and righting ability, in salinities of 15, 20, and 30psu. 

graph comparing osmotic concentrations in sea stars Pisaster ochraceus over a range of external salinitiesResults show, as expected, isosmoticity between coelomic fluid and external seawater over the full range of salinities tested, with remarkably similar values for the 2 populations (see graph on Right). 

histograms comparing feeding by sea stars Pisaster ochraceus from different locations, over a range of test salinitiesRates of feeding by the sea stars on mussels are significantly lower at low salinities, but a location effect is lacking (see histograms on Left). Note for each salinity that there is a non-significant trend for adaptation over 24-72h of testing. Righting activity is significantly depressed in both populations at the lowest salinity tested, but the overall pattern does not differ between the 2 populations (data not shown here).  The Bamfield sea stars exhibit high mortality at 15psu, but no mortality is seen in the Vancouver population.  The authors conclude that there is evidence of at least some acclimatisation to low salinity in P. ochraceus.  Held & Harley 2009 Invert Biol 128: 381.

NOTE  echinoderms are generally considered to be stenohaline osmoconformers; that is, their body fluids match the osmolalities of the external environment, but only over a narrow range of salnities

NOTE  psu, or practical salinity units, is now commonly used in oceanographic studies in place of the more familiar ppt, or parts per thousand (‰).  The two give similar values for salinity of seawater, but psu is more accurate, being based on conductivity in a KCl standard solution of 32.4356g . kg-1 (at 15oC)

Research study 3

histogram comparing heights of day versus night incursions by ochre stars Pisaster ochraceus into the upper shore areasOchre stars Pisaster ochraceus make daily incursions into the upper parts of the intertidal zone to prey on mussels and other organisms.  Since it is known that asteroids, and echinoderms in general, are highly intolerant of low salinities, the question arises as to what effect low-salinity surface waters might have on these migrations.  Light also may play a role, and its effect may vary depending on the timing of high-tide periods (higher high tides of the day may come during daytime or nighttime, depending upon season and phase of the tide).  These issues are investigated at the Bamfield Marine Sciences Centre, British Columbia using field and laboratory observations.  histogram showing effect of salinity on righting ability in sea stars Pisaster ochraceus

SCUBA-diver observations show, firstly, that Pisaster crawls upwards in the intertidal zone significantly less during daytime high tides than during nighttime ones (see histogram on Left). In tall cylinders in the laboratory, the sea stars remain near the bottom during daylight and move upwards at night.  Addition of a overlying freshwater layer in the cylinders significantly restricts the vertical extent of these migrations.  A similar effect was observed in field seastars before and after a severe summer rainstorm, which created a 3-m halocline event that lasted for 10d.  Before the storm, sea stars move up to 3m above MLLW, while after the storm they mostly stay below the lower boundary of the freshwater lens.  The researchers again show (see Research Study 2 above), a significant negative effect on righting ability in Pisaster when exposed to seawater of salinities lower than 30‰.  An important ecological consequence of reduced vertical excursions by ochre stars will be reduced predation on mussels Mytilus spp.  Garza & Robles 2010 Mar Biol 157: 673.

NOTE  the researchers take care to ensure that temperature and dissolved oxygen levels do not differ between the overlying layer of freshwater and the underlying layer of seawater 

NOTE  a useful research project might be to observe sea-star movements in inner-coast fjord habitats where predictable seasonal haloclines occur.  These haloclines may be several meters in depth and would provide another type of natural testing.  One problem that would need to be worked out, though, is that because they are created during warm-weather seasonal runoffs, the haloclines may be stable, and thus have the added factors of being warmer, with less dissolved oxygen, than the deeper more saline waters