Beadlet anemone Actinia equina, copyright Pitschuni.

Belongs within: Actiniaria.

With fronds like these
Published 31 July 2018

I’m sure pretty much anyone who’s spent time looking into rock pools along the coast will be familiar with sea anemones. These sessile animals with their squidgy bodies and crown of tentacles can be seen almost anywhere there’s a rock for them to stand on and a tide to cover them. As a kid, I used to amuse myself by poking them with a finger, noting the slight velcro-ish feel as the harassed anemone would vainly attempt to sting its attaker as it withdrew for protection. In hindsight, I was perhaps just fortunate that New Zealand anemones lacked the strength of venom to affect a human.

Waratah anemones Actinia tenebrosa, copyright John Turnbull.

Many of the anemones I was encountering as a child probably belong to a particular clade known as the Actinioidea. As recognised by Rodríguez et al. (2014), familiar members of this group include the beadlet anemone Actinia equina* from the Atlantic coasts of Europe and Africa, the red sea anemone Actinia tenebrosa of eastern Australia and New Zealand, and the aggregating anemone Anthopleura elegantissima and giant green anemone Anthopleura xanthogrammica of the Pacific coast of North America. Wikipedia informs me that another actinioid, the snakelocks anemone Anemonia viridis, is eaten after being marinated in vinegar and fried in parts of the Mediterranean. Rodríguez et al. recognised their Actinioidea primarily on the basis of molecular phylogenetic analysis but most members of this group had previously been recognised as relatives due to their possession of a sphincter muscle around the edge of the gastric cavity near the top of the column. This muscle allows the body cavity to be pulled tightly closed, providing protection and, for intertidal species, holding water inside the body to protect against desiccation.

*Actinia equina, offhand, was given its species name by Carl Linnaeus who described it under the name Priapus equinus. ‘Equinus‘ means ‘of a horse’ whereas ‘priapus‘ means… exactly what you think it means. Yes, the name of this species literally means ‘hung like a donkey’.

Pompom anemone Liponema brevicornis, copyright Ocean Networks Australia.

Other common features of actinioids include well-developed muscles around the base of the column and an adhesive basal disc for clinging to rocks. However, both the upper sphincter muscle and the basal muscles have been lost in various subgroups of the actinioids, often at the same time. Anemones lacking these muscles, such as the ghost anemones Haloclava, are generally deeper water forms that do not cling to rocks but instead live burrowed into sand with their tentacles extended above the surface. One such anemone, the twelve-tentacled parasitic anemone Peachia qinquecapitata, develops as a larva as a parasite on the hydrozoan medusa Clytia gregaria. The larvae gain entry to their host by being eaten as food particles but proceed to themselves feed on the contents of the host’s gastric cavity and eventually on the host itself. Another group of deep-sea actinioids, including such species as the deeplet anemone Bolocera tuediae and the pompom anemone Liponema brevicornis, are able to shed their tentacles as a defence thanks to small sphincter muscles at the base of each tentacle. Bolocera tuediae, found in the North Sea, is a particularly large anemone reaching up to a foot in diameter.

Aggregating anemones Anthopleura elegantissima fighting over space, copyright Brocken Inaglory. The white ‘tentacles’ the anemones are extending towards each other are inflated acrorhagi (see below).

Many actinioids form symbiotic associations with microscopic algae such as zooxanthellae, containing them within their body and supplementing their own nutrition through the algae’s photosynthesis. A number of species reproduce by brooding larvae within the body cavity, only releasing them when they are more developed and better equipped to survive the outside world. Finally, many species of actinioid have the column ornamented by various protuberances such as vesicles or verrucae. These structures may serve environmental protective functions, such as increasing desiccation resistance or functioning in camouflage. Members of Anthopleura and related genera often have specialised bulbous protuberances called acrorhagi around the distal part of the column (Daly et al. 2017). These acrorhagi are packed with stinging cells and are used not so much to protect against predators as against other sea anemones. The acrorhagi-equipped anemone flails its column about, pressing the acrorhagi against any competitor that gets too close and stinging it until it is forced to back off. Its a tough world out there and any anemone worth its salt has got to be willing to defend its position.

Systematics of Actinioidea
<==Actinioidea [Endomyaria]RB14
    |--Andresia [Andresiidae]RB14
    |    |--Cryptodendrum adhaesivum Klunzinger 1877BS17
    |    `--Heterodactyla hemprichii Ehrenberg 1834BS17
    `--Actiniidae [Haloclavidae, Preactiidae]RB14
         |  i. s.: Entacmaea quadricolor (Leuckart in Rüppell & Leuckart 1828)BS17
         |--EpiactisRB14 [incl. Cnidopus Carlgren 1934EF91]
         |    |--*E. prolifera Verrill 1869EF91
         |    |--E. japonicaC-SC03 [=Cnidopus japonicusEF91]
         |    |--E. lisbethaeRB14
         |    `--E. ritteri Torrey 1902 [=*Cnidopus ritteri]EF91
         `--+--+--Urticina coriaceaRB14
            |  `--+--Harenactis argentinaRB14
            |     `--Stephanthus antarcticusRB14
            `--+--+--Haloclava producta (Stimpson 1856)RB14, D02
               |  `--Peachia cylindricaRB14
                  |  |  |    |--B. kerguelensisRB14
                  |  |  |    `--B. tuediaePP15
                  |  |  `--Liponema [Liponematidae]RB14
                  |  |       |--L. brevicornisRB14
                  |  |       `--L. multiporumRB14
                  |  `--+--IsosicyonisRB14
                  |     |    |--I. albaRB14
                  |     |    `--+--I. striataRB14
                  |     |       `--+--Isotealia antarcticaRB14
                  |     |          `--Glyphoperidium bursaRB14
                  |     `--+--Actinostephanus [Actinodendridae]RB14
                  |        |    `--A. haeckeliRB14
                  |        `--+--Macrodactyla doreensis (Quoy & Gaimard 1833)RB14, BS17
                  |           `--Phymanthus [Phymanthidae]RB14
                  |                |--P. loligoRB14
                  |                `--P. muscosus Haddon & Shackleton 1893BS17
                     |    |--StichodactylaRL04
                     |    |    |--S. gigantea (Forskål 1775)BS17
                     |    |    |--S. haddoni (Saville-Kent 1893)BS17
                     |    |    |--S. helianthus (Ellis 1768)RL04
                     |    |    |--S. mertensii Brandt 1835BS17
                     |    |    `--S. tapetum (Hemprich & Ehrenberg in Ehrenberg 1834)BS17
                     |    `--HeteractisRB14
                     |         |--H. crispa (Hemprich & Ehrenberg in Ehrenberg 1834)BS17
                     |         |--H. magnificaRB14
                     |         `--H. malu (Haddon & Shackleton 1893)BS17
                        |  |    |--A. aureoradiataHS01
                        |  |    |--A. elegantissimaS79
                        |  |    |--A. kuroganeC-SC03
                        |  |    |--A. midori Uchida & Muramatsu 1958K03
                        |  |    `--A. xanthogrammicaMH96
                        |  `--+--Dactylanthus antarcticusRB14
                        |     `--+--+--Bunodactis verrucosaRB14
                        |        |  `--Bunodosoma grandisRB14
                        |        `--+--Anthostella stephensoniRB14
                        |           `--+--Korsaranthus nataliensisRB14
                        |              `--Preactis milliardaeRB14
                           |    |--A. contareniiPP64
                           |    |--A. edulisR26
                           |    |--A. sulcata (Pennant 1777)LB12
                           |    |--A. vagans Risso 1826R26
                           |    `--A. viridisRB14
                                |--*A. equina (Linnaeus 1758) [=Priapus equinus]DB07
                                |--A. alba Risso 1826R26
                                |--A. brevicirrhata Risso 1826R26
                                |--A. cernua Bosc 1797B97
                                |--A. concentrica Risso 1826R26
                                |--A. corallinaR26
                                |--A. effetaR26
                                |--A. fragaceaRB14
                                |--A. glandulosa Risso 1826R26
                                |--A. helianthusG20
                                |--A. pedunculataG20
                                |--A. pictaR26
                                |--A. rosea Risso 1826R26
                                |--A. rufa Risso 1826R26
                                |--A. striata Risso 1826R26
                                |--A. tenebrosaHS01
                                `--A. violacea Risso 1826R26

*Type species of generic name indicated


[B97] Bosc, C. 1797. Description des objets nouveaux d’histoire naturelle, trouvés dans une traversée de Bordeaux à Charles-Town. Bulletin des Sciences, par la Societé Philomathique de Paris 1 (2): 9–10.

[BS17] Bryce, C., & A. Sampey. 2017. Kimberley marine biota. Historical data: additional phyla (Brachiopoda, Bryozoa, Annelida, Platyhelminthes, Sipuncula, Cnidaria and Chordata). Records of the Western Australian Museum Supplement 84: 345–361.

[C-SC03] Cavalier-Smith, T., & E. E.-Y. Chao. 2003. Phylogeny of Choanozoa, Apusozoa, and other Protozoa and early eukaryote megaevolution. Journal of Molecular Evolution 56: 540–563.

[D02] Daly, M. 2002. A systematic revision of Edwardsiidae (Cnidaria, Anthozoa). Invertebrate Biology 121 (3): 212–225.

[DB07] Daly, M., M. R. Brugler, P. Cartwright, A. G. Collins, M. N. Dawson, D. G. Fautin, S. C. France, C. S. McFadden, D. M. Opresko, E. Rodriguez, S. L. Romano & J. L. Stake. 2007. The phylum Cnidaria: a review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa 1668: 127–182.

Daly, M., L. M. Crowley, P. Larson, E. Rodríguez, E. H. Saucier & D. G. Fautin. 2017. Anthopleura and the phylogeny of Actinioidea (Cnidaria: Anthozoa: Actiniaria). Organisms, Diversity & Evolution 17: 545–564.

[EF91] Edmands, S., & D. G. Fautin. 1991. Redescription of Aulactinia veratra n. comb. (=Cnidopus veratra) (Coelenterata: Actiniaria) from Australia. Records of the Western Australian Museum 15 (1): 59–68.

[G20] Goldfuss, G. A. 1820. Handbuch der Naturgeschichte vol. 3.Handbuch der Zoologie pt 1. Johann Leonhard Schrag: Nürnberg.

[HS01] Hayward, B. W., A. B. Stephenson, M. S. Morley, W. M. Blom, H. R. Grenfell, F. J. Brook, J. L. Riley, F. Thompson & J. J. Hayward. 2001. Marine biota of Parengarenga Harbour, Northland, New Zealand. Records of the Auckland Museum 37: 45–80.

[K03] Kim, I.-H. 2003. Copepodid stages of Critomolgus anthopleurus (Copepoda, Poecilostomatoida, Rhynchomolgidae). Journal of Crustacean Biology 23 (3): 558–567.

[LB12] Luque, Á. A., A. Barrajón, J. M. Remón, D. Moreno & L. Moro. 2012. Marginella glabella (Mollusca: Gastropoda: Marginellidae): a new alien species from tropical West Africa established in southern Mediterranean Spain through a new introduction pathway. Marine Biodiversity Records 5: e17.

[MH96] Miller, S. A., & J. P. Harley. 1996. Zoology 3rd ed. Wm. C. Brown Publishers: Dubuque (Iowa).

[PP64] Peres, J. M., & J. Picard. 1964. Nouveau manuel de bionomie benthique de la mer Mediterranee. Recueil des Travaux de la Station Marine d’Endoume, Bulletin 31 (27): 5–137.

[PP15] Pisani, D., W. Pett, M. Dohrmann, R. Feuda, O. Rota-Stabelli, H. Philippe, N. Lartillot & G. Wörheide. 2015. Genomic data do not support comb jellies as the sister group to all other animals. Proceedings of the National Academy of Sciences of the USA 112 (50): 15402–15407.

[RL04] Richter, A., & Á. A. Luque. 2004. Epitonium dendrophylliae (Gastropoda: Epitoniidae) feeding on Astroides calycularis (Anthozoa, Scleractinia). Journal of Molluscan Studies 70: 99–101.

[R26] Risso, A. 1826. Histoire naturelle des principales productions de l’Europe méridionale et particulièrement de celles des environs de Nice et des Alpes maritimes vol. 5. F.-G. Levrault: Paris.

[RB14] Rodríguez, E., M. S. Barbeitos, M. R. Brugler, L. M. Crowley, A. Grajales, L. Gusmão, V. Häussermann, A. Reft & M. Daly. 2014. Hidden among sea anemones: the first comprehensive phylogenetic reconstruction of the order Actiniaria (Cnidaria, Anthozoa, Hexacorallia) reveals a novel group of hexacorals. PLoS One 9 (5): e96998.

[S79] Shelton, G. A. B. 1979. Co-ordination of behaviour in cnidarian colonies. In: Larwood, G., & B. R. Rosen (eds) Biology and Systematics of Colonial Organisms pp. 141–154. Academic Press: London.

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