Aim We used molecular data to answer the following
questions: (1) Is morphology-based (and to some extent, geography-based)
classification of the freshwater crab family Potamidae congruent with a
molecular phylogeny? (2) What historical biogeographical event could have shaped
this phylogeny?
Location Material from the entire geographical
range of the family Potamidae was analysed, including specimens from
East Asia (China, Taiwan, the Ryukyus),
Southeast Asia, South Asia (northern India, the Middle East and Near East),
North Africa, and southern Europe.
Methods Mitochondrial DNA sequences encoding 503
base pairs (excluding the variable regions) of the large subunit rRNA (16S rRNA)
gene were obtained from 72 species belonging to 49 potamid genera, representing
51% of all known genera in this species-rich family. Sequences were compared by
means of phylogenetic analyses (minimum evolution, Bayesian inference, maximum
likelihood and maximum parsimony) and Bayesian relaxed molecular clock
estimates.
Results The family Potamidae was found to be
monophyletic with two major lineages, and there was support for the recognition
of two mostly allopatric subfamilies, Potaminae and Potamiscinae. This is
largely consistent with the current classification proposed. The 'Potamiscinae'
clade comprised three subclades: (1) a well-supported 'eastern Asia' subclade
that included species from the eastern part of the range (China, Taiwan, the
Ryukyus, the Philippines, Indochina, Malay Peninsula, northern India and
Myanmar/Burma); (2) a weakly supported 'Sunda Shelf islands' subclade that
included species from the larger Southeast Asian islands on the Sunda Shelf
(Borneo, Sumatra and Java); and (3) a 'Socotra' subclade that comprised only
Socotrapotamon from Socotra Island, off the north-east coast of Africa.
Main conclusions The discrete distribution of the
two subfamilies in Europe/Asia is hypothesized to be the result of vicariance
due to the collision of the Indian tectonic plate with the Asian continent, and
the orogeny that caused the separation of the two freshwater crab lineages
around 22.8 Ma. Within the Potamiscinae, the 'Sunda Shelf islands' subclade
separated from other potamiscines around 21.1 Ma; and the endemic fauna of the
East Asian islands (Taiwan, the Ryukyus and mainland Japan) was isolated from
the Asian continent c. 8.4 Ma, following the opening of the Okinawa
Trough. The 'Socotra' subclade diverged from the 'eastern Asia' subclade at 19.1
Ma during the Miocene. Its taxonomic position, however, remains unclear as the
members of this clade possess the key potamine character of a transverse ridge
on thoracic sternite 8, suggesting that this may in fact be a relict potamid
group.
Keywords
Biogeography, East Asian islands, freshwater crabs, mtDNA, Okinawa Trough, plate
tectonics, Potaminae, Potamiscinae, Sunda Shelf islands, vicariance.
Figure 1 Distribution of the family
Potamidae, with the ranges of three subclades of the subfamily Potamiscinae:
'eastern Asia' (pale dotted line), 'Sunda Shelf islands' (pale solid line) (both
iconized without 'ridge') and 'Socotra' subclades (iconized with 'ridge'), and
the subfamily Potaminae (pale dashed line) (iconized with 'ridge'). The grey
dots represent the collection sites, with the numbers representing the
collection sites in Table 1. The thick dark dotted line is the supposed division
between Potaminae and Potamiscinae; the dark solid line is Wallace's line. The
icons used show the thoracic sternites; the transverse ridge being represented
by the double horizontal lines.
