Oniscidea, Latreille, 1802

Lins, Luana S. F., Ho, Simon Y. W. & Lo, Nathan, 2017, An evolutionary timescale for terrestrial isopods and a lack of molecular support for the monophyly of Oniscidea (Crustacea: Isopoda), Organisms Diversity & Evolution (New York, N. Y.) 17 (4), pp. 813-820 : 816-817

Monophyly of Oniscidea

Our phylogenetic analyses of multiple data sets failed to support the monophyly of Oniscidea . The polyphyly of oniscids was strongly supported by our analyses of mitochondrial genomes. Analyses of the mito-nuclear and 18S data sets revealed the presence of three divergent oniscid lineages, although support values uniting oniscids with non-oniscid taxa were not strong.

These results go against the generally accepted hypothesis of oniscid monophyly, which is based on morphological data ( Wägele 1989; Brusca and Wilson 1991; Erhard 1995; Tabacaru and Danielopol 1996) or a combined analysis of molecular and morphological data ( Wilson 2009). Instead,

equal to or greater than 0.5 (posterior probability) and 50% (bootstrap); and white denotes values below 0.5 (posterior probability) and 50% (bootstrap). Species names are colored by suborder, with species of the order Oniscidea indicated by red circles at the tips the oniscid families Ligiidae and Tylidae were placed outside the main oniscid clade. Most species in these two families live in near-shore environments, such that they have been regarded as representing a transitional stage in the conquest of terrestrial habitats by isopods ( Edney 1968; Schmalfuss 1989; Schmidt 2008). This idea is often linked to the early divergence of these families within Oniscidea . As noted by Carefoot and Taylor (1995), however, the fact that these species are well-adapted to this amphibious lifestyle does not necessarily imply that they represent an early-branching group within Oniscidea .

On its own, the strength of our evidence is not sufficient to reject conclusively the monophyly of the suborder Oniscidea . However, our results draw attention to a possible scenario that should be further explored, whereby this diverse group could have colonized the land on more than one occasion. With the diversification of plants on land providing a suitable environment to detritivores and herbivores to thrive in this environment and escape the predatory pressures in the ocean, it is possible that lineages of isopods, as with other groups, have convergently adapted to land. In this scenario, the families Tylidae and Ligiidae would have evolved from lineages other than the main group of Oniscidea (Crinocheta, Mesoniscidae , and Synocheta). The monophyly of Oniscidea is very wellsupported by morphological characters such as the complex water-conducting system and reduced first antenna with only three articles. Although the reduction of the first antenna has been regarded as a prominent synapomorphy for Oniscidea ( Wägele 1989; Schmidt 2008), this reduction has been discussed as different and possibly convergent in Tylos Audouin, 1826 , and Ligia ( Schmidt 2008) . Similarly, the water-conducting systems within Oniscidea are not all the same. Hornung (2011) found two structurally different types of water-conducting systems, the B Ligia type^ and the B Porcellio type,^ with the former being considered an ancient open system. Thus, the two main characters that have been used to support the monophyly of Oniscidea might actually be convergent responses to the challenges posed by the terrestrial environment.

Previous studies also presented evidence of Oniscidea not being a monophyletic group. Vandel (1943, 1964, 1965) proposed that the order is polyphyletic, with the oniscid family Tylidae being closely related to Valvifera. Other molecular phylogenetic studies have produced varied estimates of the relationships of Tylidae and Ligiidae , depending on the meth- od of analysis ( Michel-Salzat and Bouchon 2000; Wetzer 2002; Lins et al. 2012). Vandel (1943, 1964, 1965) and Michel-Salzat and Bouchon (2000) did not use a phylogenetic approach in their studies ( Schmidt 2008), whereas the study by Wetzer (2002) was based on only mitochondrial markers and included only a few isopod taxa. Collectively, the variation in phylogenetic estimates reflects uncertainty over the monophyly of Oniscidea . Further analyses of the phylogenetic relationships of this suborder with larger numbers of genetic markers will help to resolve these relationships with confidence.