Lophotrochozoa (, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence.
Groups
Lophotrochozoa was defined in 1995 as the "last common ancestor of the three traditional
lophophorata taxa (
Brachiopoda,
, and
phoronida worms), the
Mollusca and the
, and all of the descendants of that common ancestor".
It is a
cladistics (a node-based name), so the affiliation to Lophotrochozoa of spiralian groups not mentioned directly in the definition depends on the topology of the
tree of life, and in some phylogenetic hypotheses, Lophotrochozoa may even be synonymous to Spiralia.
Nemertea and
Orthonectida (if not directly considered as part of
Annelida) are probably lophotrochozoan phyla;
Dicyemida,
Gastrotricha, and
Platyhelminthes may be lophotrochozoans or placed in the
Rouphozoa clade outside Lophotrochozoa;
Chaetognatha,
Gnathostomulida,
Micrognathozoa, and
Syndermata are probably gnathiferans and so placed as a basal spiralian clade outside Lophotrochozoa;
Cycliophora could be a gnathiferan or a lophotrochozoan phylum. One of the candidate hypotheses is presented below.
The Lophotrochozoa has basal Cycliophora and Mollusca groups, and more derived Lophophore, Nemertea and groups.
With the introduction of Platytrochozoa and Rouphozoa, one candidate phylogeny is pictured below – though other studies recover a range of alternative possibilities:
In the most recent research, the three phyla Cycliophora, Entoprocta and Bryozoa makes up a single clade and are the first to branch off from the other lophotrochozoans. The second split is the molluscs, and the third consists of two sister phyla, annelids and nemerteans. Lastly remains the clade that consist of the phoronids and the brachiopods.[ Polyzoa is back: The effect of complete gene sets on the placement of Ectoprocta and Entoprocta][ Phyla of Tiny Filter Feeders Find a New Spot on the Tree of Life]
Another study recovers Lophotrochozoa as equivalent to Platytrochozoa, forming a sister group with Gnathifera at the base of Spiralia.
A number of fossil taxa can be identified as early Lophotrochozoans, even if their precise affinity remains contested. However, relevant Cambrian fossils are debated.
Characteristics
The clade Lophotrochozoa is named after the two distinct characteristics of its members; the
lophophore, a feeding structure consisting of a ciliated crown of tentacles surrounding a mouth, and the developmental stage of the
trochophore larva.
Lophophorata such as
Brachiozoa and
Bryozoa have lophophores, while members of
Trochozoa such as
molluscs and
annelids have trochophore larvae, although some may have none.
Further reading
