Echinodermata is formed by five classes:
- Crinoidea (feather stars)
- Asteroidea (sea stars)
- Ophiuroidea (brittle stars)
- Holothuroidea (sea cucumbers)
- Echinoidea (sea urchins and sand dollars)
It is clear that Holothuroidea and Echinoidea form a clade, Echinozoa, but four hypotheses for its relationships with the other classes compete. The predominant ones are the Asterozoan and the Cryptosyringid hypotheses:
- Asterozoan hypothesis, according to which Asteroidea and Ophiuroidea would form a clade, Asterozoa, which would be sister to Echinozoa.
- Cryptosyringind hypothesis, according to which Echinozoa form a clade with Ophiuroidea, Cryptosyringida, which then forms a clade with Asteroidea.
Two other hypotheses are based in mitochondrial gene alignments:
- (Echinozoa + Asteroidea) + Ophiuroidea, with basal Crinoidea
- (Echinozoa + Asteroidea) + Crinoidea, with basal Ophiuroidea
With 23 de novo sequenced transcriptomes from all five clades of echinoderms, and “multiple phylogenetic methods at a variety of sampling depths”, the “well-supported phylogenetic tree” of this work support the Asterozoan hypothesis.
Picture summary (adapted from the publication):
- Reich A, Dunn C, Akasaka K, Wessel G (2015) Phylogenomic Analyses of Echinodermata Support the Sister Groups of Asterozoa and Echinozoa. PLoS ONE 10(3): e0119627. doi: 10.1371/journal.pone.0119627