title for learn-about sections for chitons in A SNAIL'S ODYSSEY
  Adaptations to intertidal life
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Tolerance of freshwater & osmoregulation

  This hodge-podge selection of topics relating to intertidal life in chitons includes tolerance of freshwater & osmoregulation, considered here, and WAVE EXPOSURE & TENACITY, GAS EXCHANGE, pH & OCEAN ACIDIFICATION, and LIGHT, DESICCATION, & TEMPERATURE STRESS, considered in other sections.
Research study 1

graph showing volume changes in a chiton Cyanoplax hartwegii after 24h in different salinitiesphotograph of a chiton Cyanoplax hartwegii courtesy James Watanabe, CaliforniaLike all intertidal invertebrates, chitons are subject to potential hemolymph-diluting rainfall, and survival will depend upon their ability to resist or tolerate the effects.  Laboratory tests at Hopkins Marine Station, Pacific Grove, California involving immersion of chitons Cyanoplax hartwegii in dilute and concentrated seawaters for a 24-h period show osmoconformation over the range of 75-125% seawater.  Note that after an initial “over-shoot” response in live mass in both dilute and concentrated salinities, there is some regulation, possibly involving volume regulation by muscles.  There are no marked behavioural changes during the test period, nor do any animals die.  Survival over 24h in these test salinities is longer than necessary to ensure survival over a low-tide period in the field.  McGill 1976 Veliger 18 (Suppl): 109. Photograph of Cyanoplax courtesy James Watanabe, California and seanet.stanford.edu.

NOTE  salinities greater than 100% (=34‰) are created by adding sea salts.  Experiments are run at 13oC

NOTE  this is easily tested by measuring osmotic concentration of the body fluids

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Research study 2

All intertidal-inhabiting animals must resist, or be tolerant of, the diluting effects of freshwater.  This could be in the form of rainfall, flow from nearby streams and rivers, or as a lens of dilute water overlying the seawater that washes up and down the shore with the tide.  A comparison of chitons Katharina tunicata and Mopalia muscosa at 2 open-coast and brackish-water locations in Oregon, show that both species can tolerate immersion in 40% seawater for at least 36h and both are osmoconformers, that is, their internal hemolymph concentrations match that of the outside seawater.  Although both species exhibit symptoms of stress in seawater less than 60% salinity, such as a reduction in oxygen uptake, the authors conclude that neither species is constrained in its distribution by salinity.  Rostal & Simpson 1988 Veliger 31: 120.

NOTE  this commonly occurs in estuarine regions where less dense freshwater floats on top of more dense seawater.  The 2 layers mix to varying extent, thus forming a zone of disjuction known as the halocline

Research study 3

graph showing change in body mass of chitons Nuttallina californica after 24- exposure to different salinitiesA study on osmotic-stress effects in Nuttallina californica at Hopkins Marine Station, Pacific Grove, California compares body-mass changes in high- and low-intertidal individuals during exposure to hyposmotic and hyperosmotic conditions over a 24-h period.  Results show that low-intertidal individuals gain body mass faster and to a greater degree than high-intertidal ones, and that low-intertidal animals lose more body mass than high-intertidal ones in hyperosmotic conditions (although the differences are probably not significant; see graph).  Thus, although chitons are generally thought to be isosmotic with the external seawater, there seems to be at least some adaptation by high-level individuals – either some osmoregulation at low salinities as the author suggests. or perhaps an increased ability to restrict physically the entrance of water.  This topic might repay further investigation.  Simonsen 1976 Veliger 18(Suppl): 113.

NOTE  “high”-level individuals live at 1.2-1.6m above mean tide level, while “low”-level ones live at 0.6-1.1m.  The high-level individuals are exposed to almost 2 times greater air exposure than the low-level ones.  Mean sizes are 2.1g live mass for “high” and 1.9g for “low”.  Number of individuals tested at each salinity = 6-18.  The author uses more test salinities than are shown here

NOTE  salinities of >100% are unlikely to be encountered by Nuttallina in this area