«The historic biogeography of the aquatic fauna of the Levant is reconstructed upon evidence of the gastropod groups with a well-established taxonomy and a rich fossil record. Faunal origins include ancient northern elements coming from an Anatolian–Iberian faunal province that, in the early Miocene, bypassed mainland Italy. This scenario is more plausible than the current ‘Lago Mare’ biogeographical theory. Palaeotropical elements represent either an Oriental or an African source; in either case these elements could have first reached Anatolia, from where they eventually spread southwards into the Levant, perhaps broadly coinciding in time with the invasion of the northern elements. Within the Levant, the faunas of the Euphrates, Orontes, and Jordan formed at first one common pool, but by 2 Mya the fauna of the Jordan had diverged from that of the Orontes; later, the Orontes fauna diverged from that of the Euphrates. By 1.4 Mya the fauna of the Levant consisted of both ancient survivors and new species. Some 780 000 years ago there was an exceptionally diversified fauna with many new, northern elements. Many of these elements did not survive to recent times. The fossil record suggests that the fauna of the Jordan Valley was separate from that of the Orontes throughout the Pliocene–Pleistocene. Taxa common today to the Jordan Valley and coastal sites may reflect an ancient system of rivers that drained westwards to the Mediterranean Sea, across the Jordan Valley of today.»

http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.2007.00850.x/full

I wondered if the rivers once flowed westwards. Any number of theories could explain it, but this seems quite likely. Also, under a wetter climate the species would have had many more routes to navigate the terrain.

One more source on the occurrence of wetter periods in Dead Sea:

«Part of the most recent, detailed case studies conducted in the Southern Levant, a series of investigations in the Dead Sea region, reconstruct the vegetation history (dominated by cultivation) and climatic fluctuations for the last 3500 years. This reconstruction draws on pollen and sedimentological observations from sections on the Dead Sea west shore (Neumann et al. 2010) and a core from the Dead Sea itself (Leroy 2010). These recent studies indicate the presence of wetter periods in the Dead Sea region during the Middle Bronze Age, part of the Iron Age and particularly during Roman–Byzantine times, which allowed intensive cultivation to take place, as well as during the end of the nineteenth to early twentieth century. In addition to climatic fluctuations, two earthquakes, in 31 BC and AD 363, affected agricultural practices in the area, inducing a clear drop in cultivated species in the pollen records (Leroy et al. 2010). These events have been shown to disrupt agriculture in the Dead Sea rift for periods of a few (4–5) years.»

http://rsta.royalsocietypublishing.org/content/368/1931/5225.full

It seems that Potamon potamios, one of, or the only freshwater crab around the dead sea is able to survive during at least two weeks in sea-water (Warburg & Goldenberg, 1984) . Hence, freshwater crab migration could have happened as recently as Dead sea salt concentration was around open sea salt concentration. Besides, an intense rain episode in 1979/80 caused a three year long meromictic phase in the Dead sea meaning that surface water was far less concentrated than usual concentrations (Anati et al. 1987).

http://www.sciencedirect.com/science/article/pii/0012821X87901816

Reference about resistance to salinity by Potamon potamios:

http://www.sciencedirect.com/science/article/pii/0300962984905449

In moderate latitudes many artificial ponds, e.g. Waterbodies in quarries, fishes and plants are transferred by ducks, bearing eggs on feather. Usually every pool in a quarry has some fishes in several years.

Same experience as Igor. Any new and isolated pond eventually gets occupied by fish which possess problems for amphibian conservation.

I doubt if eggs would be carried on birds, because they're carried on the abdomen of female crabs, but early juveniles stages of the crabs might be able to survive short transits on wet feathers.

No one seems to have noticed that Potamon potamios is a semi-terrestrial crab. Its areal is whole of Eastern Mediterranean and it requires neither continuous water paths, nor birds for its dispersal.

Molecular phylogeny of these isolated populations would be very interesting & may shed light on the competing possibilities of recent dispersal vs ancient river channels.