Umbellula

Umbellula is a genus of deep-sea cnidarians in the monotypic family Umbellulidae. Sea pens of this genus are known to be bioluminescent and hermaphroditic.

Species of Umbellula form colonial structures, which is common amongst pennatulacean octocorals (sea pens). Around the primary polyp distal region, secondary polyps form which are either larger autozooids (feeding polyps) or smaller siphonozooids (water circulation polyps).^[2] The morphological adaptations of Umbellula species reflect adaptations to unique deep-sea environments, including differences in polyp arrangement and sclerite presence or absence. Species such as Umbellula encrinus having a typical cluster of autozooids,^[3] while Umbellula pomona only possessing a terminal cluster of three autozooids.^[4] Umbellula autozooids are found clustered at the very end of their stalk, unlike most other sea pens in which autozooids are spread along the rachis.^[5]

Most species of Umbellula have a strong, muscular peduncle which is used to anchor them in soft sediment, allowing them to inhabit areas of uniform benthic environments such as abyssal plains.^[6]

Similarly to many deep-sea animals, some species of Umbellula can shift the color of their bioluminescence from blue to green using fluorescent proteins.^[7][8]

The reproductive biology of sea pens in general remains poorly studied. Unlike in other sea pens in which oocytes develop in the autozooid column, Umbellula oocytes develop at the base of the polyps and in the cluster tissue. It appears that the potential fecundity of Umbellula species is high, with individuals containing up to 2,000 large ooyctes.^[5] This is thought to be advantageous to overcome long inter-colony distances seen in Umbellula species. It was previously believed that Umbellula exhibited gonochorism until a 2025 study confirmed the presence of hermaphroditism in five out of six Umbellula species. This study examined 16 specimens across six distinct species of the genus Umbellula, and found gametogenic tissue in all specimens studied. Two of the six species were shown to be trioecious, another first for Umbellula species^[9]

While there is a lack of conclusive study for Umbellula's distribution patterns, they can be found ranging from bathyal to abyssal depths, primarily in the Arctic Ocean, and also the northeast and southern Atlantic Ocean.^[10][11]

Umbellula are known to be one of the only genus of sea pen found at depths of 4000m or greater.^[12]

Despite the unique morphology of this sea pen genus, species identification is made difficult due to limited access to these organisms, and a traditional reliance on a small number of morphological characteristics, which has resulted in limited knowledge of intraspecific variation.^[13]

The following species are considered valid by the World Register of Marine Species:^[1]

The following species are of uncertain validity, and many are nomen dubium:^[1]

Only an estimated ten of the forty-three described Umbellula species are considered valid members of the genus.^[14]

Species of Umbellula were traditionally classified as a single group based on their morphology, but a 2013 molecular analysis that used the mitochondrial genes mtMutS and ND2 has revealed that the genus Umbellula is polyphyletic, and the species currently in the genus did not form a single clade (natural group).^[15] These two distinct evolutionary lineages managed to have evolved similar appearances through evolutionary convergence. The following cladogram is based on that 2013 study:

Umbellula monocephalus is now placed in Solumbellula.^[16][17][18]

• Williams, G.C.; van der Land, J. (2001). Octocorallia - Pennatulacea. in: Costello, M.J. et al. (Ed.) (2001). European register of marine species: a check-list of the marine species in Europe and a bibliography of guides to their identification. Collection Patrimoines Naturels 50: pp. 105–106.
• Williams, G.C. (1992). Biogeography of the octocorallian coelenterate fauna of southern Africa. Biol. J. Linn. Soc. 46: 351–401.
• Williams, G.C. (1999). Index Pennatulacea: annotated bibliography and indexes of the Sea Pens (Coelenterata: Octocorallia) of the World 1469–1999. Proc. Calif. Acad. Sci. 51(2): 19–103, 1 fig., 14 plates.
• Liu, J.Y. [Ruiyu] (ed.). (2008). Checklist of marine biota of China seas. China Science Press. 1267 pp.
• Cairns, S.D.; Gershwin, L.; Brook, F.J.; Pugh, P.; Dawson, E.W.; Ocaña O.V.; Vervoort, W.; Williams, G.; Watson, J.E.; Opresko, D.M.; Schuchert, P.; Hine, P.M.; Gordon, D.P.; Campbell, H.J.; Wright, A.J.; Sánchez, J.A.; Fautin, D.G. (2009). Phylum Cnidaria: corals, medusae, hydroids, myxozoans. in: Gordon, D.P. (Ed.) (2009). New Zealand inventory of biodiversity: 1. Kingdom Animalia: Radiata, Lophotrochozoa, Deuterostomia. pp. 59–101., available online at http://si-pddr.si.edu/handle/10088/8431 Archived 2014-02-18 at the Wayback Machine