Role of Depth and Substrate in the Evolution of Sea Pens (Pennatulacea, Octocorallia): a Phylogenetic Study

Sea pens are a widely distributed group of colonial and clonal benthic cnidarians taxonomically classified as Order Pennatulacea (Octocorallia, Anthozoa), characterised by the presence of a central axial structure or rachis formed by modification of a single polyp. Other polyps arise either directly on the rachis or on secondary leaf-like structures extending from it. The colony is anchored by the peduncle, the muscular lower portion of the rachis. Pennatulaceans had been considered soft sediment specialists until the recent discovery of “rock pens”, which use a modified suction cup-like peduncle to attach to rock. Sea pen species inhabit both shallow and the deep sea and some have a vertical distribution that spans both, thus making them an interesting group to study evolution and diversification across depths.

The deep sea differs from shallow environments in several physico-chemical factors, the most prominent ones being absence of light and low availability of food, high pressure and low temperature. These impact the ecological and evolutionary dynamics of deep-sea taxa. This study used molecular phylogenetic methods to examine the role of depth in diversification of the sea pens and the evolutionary history of rock pens. I used the OBIS database to ascertain the depth ranges for taxa and built phylogenetic trees by Bayesian Inference using mtMutS gene sequences; depth distribution and presence of suction-cup like peduncle were mapped onto the consensus tree separately. The results indicate monophyly of Pennatulacea with respect to outgroups even though the suborders, several families, and genera, are either paraphyletic or polyphyletic. The ancestral sea pen appears to be a deep-sea taxon that underwent multiple migrations to shallower depths followed by diversification over the course of evolution. There also appears to multiple origins of the specialised rock pens among the Pennatulacea.