Abstract
Calcareous sponges (Phylum Porifera, Class Calcarea) are known to be taxonomically difficult. Previous molecular studies have revealed many discrepancies between classically recognized taxa and the observed relationships at the order, family and genus levels; these inconsistencies question underlying hypotheses regarding the evolution of certain morphological characters. Therefore, we extended the available taxa and character set by sequencing the complete small subunit (SSU) rDNA and the almost complete large subunit (LSU) rDNA of additional key species and complemented this dataset by substantially increasing the length of available LSU sequences. Phylogenetic analyses provided new hypotheses about the relationships of Calcarea and about the evolution of certain morphological characters. We tested our phylogeny against competing phylogenetic hypotheses presented by previous classification systems. Our data reject the current order-level classification by again finding non-monophyletic Leucosolenida, Clathrinida and Murrayonida. In the subclass Calcinea, we recovered a clade that includes all species with a cortex, which is largely consistent with the previously proposed order Leucettida. Other orders that had been rejected in the current system were not found, but could not be rejected in our tests either. We found several additional families and genera polyphyletic: the families Leucascidae and Leucaltidae and the genus Leucetta in Calcinea, and in Calcaronea the family Amphoriscidae and the genus Ute. Our phylogeny also provided support for the vaguely suspected close relationship of several members of Grantiidae with giantortical diactines to members of Heteropiidae. Similarly, our analyses revealed several unexpected affinities, such as a sister group relationship between Leucettusa (Leucaltidae) and Leucettidae and between Leucascandra (Jenkinidae) and Sycon carteri (Sycettidae). According to our results, the taxonomy of Calcarea is in desperate need of a thorough revision, which cannot be achieved by considering morphology alone or relying on a taxon sampling based on the current classification below the subclass level.
Highlights
Calcarea Bowerbank, 1864 is one of the four currently recognized classes of Porifera [1,2]
Testing phylogenetic hypotheses To test whether the tree topologies obtained with our data were significantly better than other phylogenetic hypotheses, we reanalyzed the dataset with RAxML and the 7A-model of nucleotide evolution under specific topology constraints of the tested taxa
When standard GTR models were applied to the dataset partitioned in stem and loop, we observed a different position for Leucosolenia, which, in the case of the Bayesian analysis, results in a phylogeny with a weakly supported (PP: 55) sister group relationship of Baerida and Leucosolenida (Fig. S6)
Summary
Calcarea Bowerbank, 1864 is one of the four currently recognized classes of Porifera [1,2]. Its relationship to the other main sponge classes, i.e., Demospongiae Sollas, 1885, Hexactinellida Schmidt, 1870 and Homoscleromorpha Bergquist, 1978, has long been unclear, especially because molecular analyses have questioned the monophyly of Porifera. Phylogenomic studies found high support for sponge monophyly and a sister group relationship between Calcarea and Homoscleromorpha [3,4]. Calcareous sponges comprise approximately 675 accepted extant species [2]; this class is considerably less diverse than for example, the most species-rich class, Demospongiae, which contains approximately 7.000 recognized species [2]. Within Calcarea, the relationships are little understood This small group of sponges has long been of interest to zoologists because of the variety of simple and more complex organization forms found in extant species, and because of their apparent beauty caused by the occasionally geometrical arrangement of their skeletons. Since the days of Haeckel, the most important characters used in the taxonomy of Calcarea are the organization of the aquiferous system and skeletal features
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