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Talitrids are the only family within the order Amphipoda to have colonised supralittoral and terrestrial environments. They live in a variety of settings, from sandy to rocky and pebble beaches, to river and lake banks, and to leaf litter and caves. A common feature is the absence of a planktonic larval stage to facilitate passive dispersal over long-distances. However, some species have broad distributions. Genetic studies over the past 25 years have tried to explain this apparent contradiction by assessing patterns of species genetic structuring on different geographical scales. Here, we review the molecular studies available to date and focus on the population genetics of talitrids. Most of these studies considered populations in the Mediterranean area, but also along the Atlantic coast and in Canary Island caves. From this review, the group emerges as a potential model to understand processes of dispersal and divergence in non-highly-vagile supralittoral organisms. At the same time, studies on these issues are still too restricted geographically: a worldwide scale including different regions would provide us with a better perspective on these problems.
Background: The Visayan Tarictic Hornbill (Penelopides panini) and the Walden's Hornbill (Aceros waldeni) are two threatened hornbill species endemic to the western islands of the Visayas that constitute - between Luzon and Mindanao - the central island group of the Philippine archipelago. In order to evaluate their genetic diversity and to support efforts towards their conservation, we analyzed genetic variation in similar to 600 base pairs (bp) of the mitochondrial control region I and at 12-19 nuclear microsatellite loci. The sampling covered extant populations, still occurring only on two islands (P. panini: Panay and Negros, A. waldeni: only Panay), and it was augmented with museum specimens of extinct populations from neighboring islands. For comparison, their less endangered (= more abundant) sister taxa, the Luzon Tarictic Hornbill (P. manillae) from the Luzon and Polillo Islands and the Writhed Hornbill (A. leucocephalus) from Mindanao Island, were also included in the study. We reconstructed the population history of the two Penelopides species and assessed the genetic population structure of the remaining wild populations in all four species.
Results: Mitochondrial and nuclear data concordantly show a clear genetic separation according to the island of origin in both Penelopides species, but also unravel sporadic over-water movements between islands. We found evidence that deforestation in the last century influenced these migratory events. Both classes of markers and the comparison to museum specimens reveal a genetic diversity loss in both Visayan hornbill species, P. panini and A. waldeni, as compared to their more abundant relatives. This might have been caused by local extinction of genetically differentiated populations together with the dramatic decline in the abundance of the extant populations.
Conclusions: We demonstrated a loss in genetic diversity of P. panini and A. waldeni as compared to their sister taxa P. manillae and A. leucocephalus. Because of the low potential for gene flow and population exchange across islands, saving of the remaining birds of almost extinct local populations - be it in the wild or in captivity - is particularly important to preserve the species' genetic potential.