Sea daisies (Infraclass Concentricycloidea; order Peripoda) make up an unusual group of deep-sea taxa belonging to the phylum Echinodermata, with three species described in the genus Xyloplax.
Sea daisies (or concentricycloids) have a disk-shaped flattened body and are generally less than 3 cm in diameter. They have a water vascular system
, typically with two concentric circumoral canals
and a single peripheral ring of tube feet
around the edge of the disk. They do not have arms, but five ambulacral areas
. A gut is either reduced or absent, depending on the species. An oral opening is present, but it is unclear how these organisms derive their nutrition.
Sea daisies have been discovered in three localities, including deep-sea habitats off New Zealand, the Bahamas and most recently from the northern central Pacific. They have been collected primarily from sunken, deep-sea (1000+ meters) wood. Although known from only a handful of specimens initially, nearly 100 specimens have been collected from the Bahamas with decent numbers collected from the north central Pacific.
Since they were discovered in 1986
, their position within the echinoderms has been debated. At first they were placed in a new class, Concentricycloidea, since it was unclear whether they might have affinities with asteroids or ophiuroids. The former view gained acceptance, and since 2006
, they are currently considered a sister group to the Infraclass Neoasteroidea, which represents all post-Paleozoic asteroids within the Class Asteroidea
Order Peripoda (or Peripodida),
- Genus Xyloplax
- Xyloplax medusiformis
- Xyloplax turneae
- Xyloplax janetae