Clathrina blanca (Miklucho-Maclay, 1868)

Voigt, Oliver & Wörheide, Gert, 2016, A short LSU rRNA fragment as a standard marker for integrative taxonomy in calcareous sponges (Porifera: Calcarea), Organisms Diversity & Evolution (New York, N. Y.) 16 (1), pp. 53-64 : 62

Clathrina blanca

Imešek and co-workers ( Imešek et al. 2013) found Clathrina blanca from the Mediterranean to be closely related to Clathrina ramosa , a result that we corroborate here too. In contrast, a specimen similar to Clathrina blanca , ( Clathrina aff. blanca ) from Norway was most closely related to C. conifera based upon an ITS-phylogeny ( Klautau et al. 2013). In our phylogeny, C. conifera is not closely related and genetically quite distinct from a C. blanca from the Mediterranean (p distance of 10.7 %). Therefore, the Norwegian specimen despite its similarity most likely represents a different species than C. blanca from the Mediterranean.

LSU C-region— a DNA-barcoding marker for species identification?

DNA barcoding aims at species unambiguous determination based upon DNA sequences. For this, a DNA barcoding marker should possess a ‘barcoding’ gap, i.e. that the intra-specific variation can be clearly separated from the interspecific variation ( Meyer and Paulay 2005). Certain sequences in our phylogeny from different species however posses identical LSU sequences: Clathrina helveola and C. wistariensis share identical sequences, as also do all specimens of the genus Grantiopsis , with at least two different species. Clathrina helveola and C lathrina wistariensis , which both have close type localities, are morphological distinct and differ for example by their colour (pale yellow and white, respectively) and slightly different spicule sizes ( Wörheide and Hooper 1999). In Grantiopsis , the specimen analysed here differ for example by the relative diameter of the osculum, their aquiferous system (syconoid and sylleibid) and also sizes of spicules (e.g. size of microdiactines). In both cases, only few specimens were included in measurements of the spicules and observation of the other diagnostic characters in the species descriptions ( Wörheide and Hooper 1999, 2003). From our results, it remains unclear, whether the identical sequences result from insufficient variation of the applied DNA marker in these cases or from an over interpretation of morphological intra-specific variation that may have been mistakenly interpreted as species diagnostic characters. The latter possibility would be corroborated, if studies with additional specimens demonstrate that the proposed morphological differences are indeed within a continuum of intra-specific variation. In contrast to these cases, the LSU fragment allows the separation of different phylogeographic clades in the Leucetta chagosensis species complex ( Wörheide et al. 2002, 2008). Additionally, species delimitation is possible in other cases. For example, Burton (1963) criticised that many species of Leucettusa , including L. imperfecta and L. haeckeliana , were only described from one or very few specimens, and that intra-specific variation was so far neglected. He concluded that many Leucettusa species should be synonymized and suggested that L. haeckeliana and L. imperfecta were conspecific ( Burton 1963). The phylogeny with LSU C-region data could clearly show genetic differences of the two species. Using this DNA marker it should therefore also be possible to evaluate additional, morphological more similar species, like Leucettusa lancifer , which shares many similarities (e.g., subcortical tetractines, v-shaped choanosomal triactines) with Leucettusa imperfecta . In summary, the potential for species delimitation with this marker remains to be evaluated in more detail with additional specimens for each taxon, to check if a ‘barcoding gap’ ( Meyer and Paulay 2005) exists between most species. But at least in some very closely related species the C-region of LSU may not suffice as a species-level marker. This, however, also applies to the commonly used ‘universal’ DNA barcode marker—COI—in some demosponge species ( Pöppe et al. 2010). In cases where morphologically different species have identical LSU sequence, additional markers like the ITS-region ( Wörheide et al. 2002, 2008; Bentlage and Wörheide 2007; Rossi et al. 2011; Klautau et al. 2013; Imešek et al. 2013; Azevedo et al. 2015) or mitochondrial markers such as cytochrome oxidase subunit 3 ( Voigt et al. 2012a) can be applied to refine the resolution for molecular species delimitation/determination. But even then the Cregion of LSU can provide valuable information about the phylogenetic position of specimens and will be helpful to identify reliable outgroup taxa.