title for learn-about section of A SNAIL'S ODYSSEY
  Life in the intertidal zone
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Salinity effects

  Topics dealing with life in the intertidal zone include salinity effects considered here, and TEMPERATURE EFFECTS, HEAT-SHOCK PROTEINS, GAS EXCHANGE, WAVE EFFECTS, and TRANSLOCATION STUDIES, WATER-CHEMISTRY EFFECTS, considered in other sections.
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Research study 1

An investigation of the effects of low salinity on survival of gametes and larvae of Mytilus californianus at Scripps Institution of Oceanography, La Jolla, California shows that susceptibility begins at salinities below about 30‰.  Moreover, even though fertilisation still occurs at salinities as low as 21‰, larval survival is poor.  The author suggests that gamete and/or larval mortality may at least partially explain the absence of adults in brackish bays. Young 1941 Ecology 22: 379.

NOTE  the author lists 33-34‰ as typical salinity for the La Jolla region

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Research study 2

graph comparing heart rates in mussels Mytilus trossulus and M. galloprovincialis at different acclimation temperatures and salinitiesThe Mediterranean mussel Mytilus galloprovincialis1 has a demonstrated ability to invade novel locations around the world, for example, Africa and Japan, and has been highly successful at displacing the resident by mussel M. trossulus along much of the California coast after its introduction during the first half of the 1900s to southern California, likely via ships.  Does its invasive prowess owe to better tolerance of temperature and salinity?  This is tested at Hopkins Marine Station, Pacific Grove, California by monitoring the effects of both parameters, both acute and following acclimation, on heart rates2 of the 2 species3

Results show that at different acclimation temperatures and salinites the heart rate of M. trossulus is significantly higher by about 1.5-fold than that of M. galloprovincialis, consistent with the notion of evolutionary adaptation to a higher habitat temperature in the latter species (see graph). Interestingly, heart rates of hybrids are intermediate between those of the 2 species (data not shown).  At acclimation temperatures of 14oC and 21oC both species show partial compensation.  At increasing experimental temperature, heart rates of both species rise until a critical high temperature is reached with, as predicted, M. galloprovincialis being more heat tolerant than M. trossulus.  Conversely, the evolutionarily cold-adapted M. trossulus is the more cold-tolerant of the 2 species.  Exposure to low salinity seawater is tolerated more by M. galloprovincialis than its congenor.  The authors discuss their findings in relation to the potential for further range expansion along the coast by M. galloprovincialis.  Braby & Somero 2006 J Exp Biol 209: 2554; for a review of physiological differences between M. galloprovincialis and M. trossulus see Lockwood & Somero 2011 J Exp Mar Biol Ecol 400: 167.

NOTE1  current thought on the evolutionary history of bay-mussel species is that edulis evolved from a trossulus ancestor (via trans-Arctic migration), invaded the Mediterranean, and later gave rise to galloprovincialis

NOTE2  measured via an impedance-pneumograph converter employing copper electrodes implanted into the pericardial space through small holes drilled into the shell

NOTE3  a third species, the Atlantic M. edulis, is also included in the study but, as it is not naturally present on the west coast it is not included here. The authors routinely identify the experimental specimens using multiple DNA isolation protocols

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Research study 3

In a later, complementary study on the same subject of salinity effects on distribution of mussels, researchers at California Polytechnic State University compare proteomic responses to low salinity in invasive Mediterranean mussels Mytilus galloprovincialis with those of native mussels M. trossulus.  The basis for the study is that northward expansion of the warm-adapted species M. galloprovincialis may be limited by extreme preciptiation events and freshwater runoff that are predicted to accompany impending climate change.  The researchers expose both species to 4h laboratory treatments of 24.5, 29.8, and 35.0psu followed by a 24h recovery period in ambient seawater (35psu).  Various proteomic responses in ctenidial tissues of each treatment set are measured at the start and end of the recovery period.  Results show significant responses in levels of molecular chaperones and cytoskeleton elements such as actin, tubulin, and associated binding proteins, and in certain energy-metabolism and oxidative-stress proteins.  Overall, M. trossulus is more tolerant of lower salinity than M. galloprovincialis (24.5 versus 29.8psu, respectively), suggesting that the latter’s northward expansion through out-competing the former for habitat may be limited by its lesser tolerance of hyposaline conditions.  Tomanek 2012 J Exp Biol 215: 3905.

NOTE  members of the Environmental Proteomics Laboratory of this university are setting new standards of research excellence in the relatively new field of environmental proteomics.  However, the similarity of this paper with one on proteomic responses of these same 3 Mytilus spp. to temperature, done by members of this research group and published in the same volume of the Journal of Experimental Biology, where the format, methodology, and conclusions are basically identical with the present paper on salinity, carries the risk of dulling a reader’s interest.  See Fields et al. 2012 J Exp Biol 215: 1106 at LEARNABOUT/MUSSEL/LIFE IN THE INTERTIDAL ZONE/TEMPERATURE EFFECTS

NOTE  Practical Salinity Units: a more accurate expression of salinity units based on properties of seawater conductivity

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