Molecular phylogeny of the Astrophorida (Porifera, Demospongiae(p)) reveals an unexpected high level of spicule homoplasy

Abstract

Background: The Astrophorida (Porifera, Demospongiae(p)) is geographically and bathymetrically widely distributed. Systema Porifera currently includes five families in this order: Ancorinidae, Calthropellidae, Geodiidae, Pachastrellidae and Thrombidae. To date, molecular phylogenetic studies including Astrophorida species are scarce and offer limited sampling. Phylogenetic relationships within this order are therefore for the most part unknown and hypotheses based on morphology largely untested. Astrophorida taxa have very diverse spicule sets that make them a model of choice to investigate spicule evolution.

Methodology/principal findings: With a sampling of 153 specimens (9 families, 29 genera, 89 species) covering the deep- and shallow-waters worldwide, this work presents the first comprehensive molecular phylogeny of the Astrophorida, using a cytochrome c oxidase subunit I (COI) gene partial sequence and the 5' end terminal part of the 28S rDNA gene (C1-D2 domains). The resulting tree suggested that i) the Astrophorida included some lithistid families and some Alectonidae species, ii) the sub-orders Euastrophorida and Streptosclerophorida were both polyphyletic, iii) the Geodiidae, the Ancorinidae and the Pachastrellidae were not monophyletic, iv) the Calthropellidae was part of the Geodiidae clade (Calthropella at least), and finally that v) many genera were polyphyletic (Ecionemia, Erylus, Poecillastra, Penares, Rhabdastrella, Stelletta and Vulcanella).

Conclusion: The Astrophorida is a larger order than previously considered, comprising ca. 820 species. Based on these results, we propose new classifications for the Astrophorida using both the classical rank-based nomenclature (i.e., Linnaean classification) and the phylogenetic nomenclature following the PhyloCode, independent of taxonomic rank. A key to the Astrophorida families, sub-families and genera incertae sedis is also included. Incongruences between our molecular tree and the current classification can be explained by the banality of convergent evolution and secondary loss in spicule evolution. These processes have taken place many times, in all the major clades, for megascleres and microscleres.

Figure 1. Presentation of the Astrophorida morphology.

(a–d) A few Astrophorida species. (a): Geodia phlegraei (Geodiidae) collected in the Denmark Strait. Uniporal oscules are on the top surface. (b): Cross-section of a Stelletta raphidiophora (Ancorinidae) collected on the ‘Schultz Massive’ seamount (Greenland Sea) (ZMBN 85223). The grayish thick cortex is clearly visible. Specimen is 13 cm in diameter. (c): Calthropella geodioides (Calthropellidae) collected South of the Azores (ZMAPOR 21659). (d): Thenea valdiviae (Pachastrellidae) collected on the Norwegian coast. (e): Characteristic Astrophorida microscleres. ox – oxyaster of Geodia papyracea (diameter: 23 µm); st – sterraster of Geodia barretti (diameter: 80 µm); as – aspidaster of Erylus expletus (length: 330 µm); mi – microrhabd of Pachymatisma normani (length: 20 µm); pl – plesiaster of Poecillastra compressa (diameter: 37 µm); sa – sanidaster of Stryphnus raratriaenus; am – amphiaster of Characella pachastrelloides (length: 18 µm); sp – spiraster of Thenea levis (length: 23 µm). (f): cross-section of the cortex of Geodia barretti showing the skeleton organization. ec – ectocortex made of a thin layer of strongylaster and microxeas. en – endocortex made of a thick layer of sterrasters. ch – choanosome. tr – triaene supporting the cortex. Scale: 1 mm. (g): Characteristic Astrophorida megascleres. cal – calthrop of Pachastrella sp. from Norway (actine length: 100 µm); tr – long-shafted triaene of Stelletta sp. from Panama (rhabdome length: 850 µm); dicho – dichotriaene of Characella pachastrelloides (rhabdome length: 500 µm); ana – cladome of anatriaene of Geodia tumulosa from Panama (clad length: 24 µm); disco – discotriaene of Discodermia polymorpha (disc diameter: 180 µm) (photo: A. Pisera); phyllo – phyllotriaene of Theonella sp. (cladome: 730 µm) (photo: A. Pisera).

Figure 2. Maximum-likelihood phylogeny of the Astrophorida ^p using 28S+COI partial sequences from 153 taxa (89 species).

Bootstrap nodal support values >50 are given at the nodes (2,000 replicates). Species names (according to the Linnaean classification) and sampling localities are given in Table S1. Names established under the PhyloCode are in italics and identified with the symbol ‘p’.

Figure 3. Presence and absence of megasclere spicules mapped on the Astrophorida ^p 28S+COI ML tree from Figure 2 .

The ML reconstructions of the ancestral conditions at the nodes were estimated using Mesquite 2.74. For the readers' convenience, species clades have been reduced to one sample (except in cases of para- or polyphyletic species). Species names in blue represent deep-water species. Species names in black represent shallow-water species. For the color-codes of the Astrophorida families sensu Systema Porifera, see Figure 2.

Figure 4. Presence and absence of microsclere spicules mapped on the Astrophorida ^p 28S+COI ML tree from Figure 2 .

The ML reconstructions of the ancestral conditions at the nodes were estimated using Mesquite 2.74. For the readers' convenience, species clades have been reduced to one sample (except in cases of para- or polyphyletic species). Species names in blue represent deep-water species. Species names in black represent shallow-water species. For the color-codes of the Astrophorida families sensu Systema Porifera, see Figure 2.

Figure 5. Phylogenetic classification of the Astrophorida ^p on the 28S+COI ML tree (cf. File S1 for definition of names).

Species names are given according to the PhyloCode (Article 21.5). Bootstrap nodal support values of clades defined by the PhyloCode are given (2,000 replicates). For the color-codes of the Astrophorida families sensu Systema Porifera, see Figure 2.