so, my reasoning is _ I have found papers on effects of salinity, with large differences in salinity causing differences in water potential, making eggs in saltier sand effectively experience drier conditions. In general, eggs from drier conditions produced smaller, lighter hatchlings.

***Could this be the reason the Med hatchlings are smaller than the NA ones? And that the Australian (Mon Repos) hatchlings have a higher mass than the NA ones (for the same SCL)? ****

(papers I read report salinity ranging from 0-70ppm, and water potential differences from -10 to 17000 kPa, and -150 to -750 kPa. I think salinity of 35ppm produces water potential of 13000 kPa )

Also, in this report (link #1) it says that the salinity might act as an barrier between NA and Med sea turtle population, and yet in this discussion (link#2), even though it some stress is caused by living in fresh water, it is evident they can survive in fresh water! (difference of 35ppm as opposed to difference of 1-3 ppm that exists between the seas)

>>> reports of tissue density / water content / dry-to-wet-mass ratios for juveniles or adults (even hatchlings) would really be helpful!

I found just one report for Australian hatchlings,  it's from 1981... has there really been no measurements of this sorts since? My search is failing so far..

http://www.sciencedaily.com/releases/2008/12/081203095425.htm

https://www.researchgate.net/post/Sea_Turtle_conservation-what_information_is_out_there_about_the_effects_of_freshwater_versus_salt_water_on_sea_turtles_for_long_periods_of_time

I am not aware of any studies on how salinity affects sea turtle development, although it is always possible that there are some I don't know about.  Offhand, it would not seem to me that the salinity of nearby seawater would necessarily have a huge impact on the salt concentration of sand up in the dryer parts of the beach where turtles normally nest.  I would think that the amount of rain, for example, would be more important in this regard, as large amounts of rain might tend to wash away residual salt.

     The paper claiming that different salinity concentrations act as a "barrier" between turtle populations in the Mediterranean is very interesting -- but we should keep in mind that this is based only on a correlation.  In other words, all we really know is that the two populations tend to occupy different parts of the Mediterranean, but we don't know why; it might have to do with temperature, or salinity, or food or habitat preferences, or a genetically encoded "magnetic map" that leads turtles to remain in a particular geographic area (reported in young salmon and in Atlantic loggerheads), etc.  It is worth thinking about salinity in the context of sea turtles, as not much is known ... but my guess would be that salinity does not matter greatly to turtles as long as it falls within a normal range representative of ocean basins.

The attached article (citation below) uses stable isotopes of the barnacles found on loggerhead shells to reconstruct their use of ocean and brackish water.  There is no indication as to how the relative use of one habitat or another may affect their growth and development.  But given that they can use brackish waters, I would agree with Ken Lohmann that salinity concentrations are probably not a barrier between populations in the Mediterranean, and may be more correlated with other factors.  I do not know much about salinity in relation to embryo development, and cannot comment on that aspect of your question.

Killingley, J. S., and M. Lutcavage. 1983. Loggerhead turtle movements reconstructed from 18O and 13C profiles from commensal barnacle shells. Estuarine, Coastal and Shelf Science 16:345–349.

Hey Kenneth and Hannah! Thank you for your answering and adding interesting points to the discussion! Also,thank you for the article Hannah! Yeah,correlation vs.causation is always an interesting one.. 

From some other papers I read in the meantime,seems that they really don't have a problem going to a lower salinity range (eg housing regulations in the us prescribe salinity to be between 20-35ppt), and also in the article about Mediterranean it seems that the ones from the Mediterranean go to the less salty part,but ones from the NA don't go to the saltier part. Also,since loggerheads can be found in estuaries ,obviously brackish is not such a problem for them, but maybe higher salinity has different effects.

Tissue densities would really be nice, I am surprised how hard it is to find these in the literature (protection probably has something to do with it haha)

Hi Nina,

Sea turtles are such excellent osmoregulators that the minor differences in salinity in different ocean basins are of no consequence to them.  There will be some slight energetic cost osmoregulating via the salt gland when salinity is higher, but they are capable of producing secretions from the salt gland that are twice the salinity of seawater, so are well within capacity at 35ppt.  See a couple of papers I wrote on this below.  I think that any differences in distribution are more likely to be because of local differences in productivity and/or food distribution, or the depth of the halocline and thermocline, rather than any biological consequence of the salinity itself to the turtle.  I agree with Ken that salt in the sand is much more impacted by local rainfall than salinity of the water.

Regards,

Richard

Reina, R.D., and Cooper, P.D. (2000). Control of salt gland activity in the hatchling green sea turtle, Chelonia mydas. J. Comp. Physiol. B 170, 27-35.

Reina, R.D., Jones, T.T., and Spotila, J.R. (2002). Salt and water regulation by the leatherback sea turtle, Dermochelys coriacea. J. Exp. Biol. 205, 1853-1860.

Hi Richard,

thank you very much for your comments and papers (and sorry for my late reply) :)

This discussion really helped with my thought process!