The Aggression of The Beadlet Anemone, Actinia equina

Actinia equina. Acrorhagi (blue) visable on left hand side

Actinia equina, the beadlet anemone, is the most common anemone to be found in Irish and European waters. In fact, its range extends all over the Mediterranean and even down the coast of Africa. Most conspicuous at low tides when the solitary, deep red polyps are visable clinging to rockfaces, A. equina can be an aggressive individual, with a full arsenal of toxins at its disposal.

Actinia equina exposed at low tide

It is named after the circle of blue, bead-like structures found at the base of the feeding tentacles. These are the acrorhagi, organs found only on certain species of the Actiniidae family. They are small sacs full of cilia-covered nematocysts that the beadlet anemone uses to subdue and kill prey. The acrorhagi are dilated by absorbing seawater and pressed against the victim. Outer pieces of the acrorhagi stick to the prey, into which the nematcysts are discharged (Shiomi 2009, Toxicon 54 pp. 1112–1118).
A. equina reproduces both sexually and asexually by parthogenesis, with reproductive modes varying at different sites. Non-clonal anemones can be quite aggressive to each other, and this has been shown to be linked to the colour of pedal disc colouration (Brace et al. 1979 Journal of Animal Behavour 27 pp. 553). Three morphs exist on Irish shores, U, M and L morphs (see table). This variation in reproductive modes, coluration and aggression suggest that A. equina may not be a single species at all but may be several cryptic species, subspecies or morphs (Chomsky et al. 2009 Journal of Experimental Marine Biology and Ecology 375 pp. 16–20).

Morph	Pedal Disc Colour	Location
U	Dark red	Upper shore
M	Light red	Mid-shore
U	Grey-green	Lowershore

As can be seen from the table, the U morphs are the most aggressive of the three. The reason for this lies in they fact that they populate the verticle sides of gullies, where wave action provides a large supply of food. Competition for this resource would therefore be fierce, promoting aggression. The weapons used by A. equina in this aggression are peptide toxins contained in the nematcysts which block sodium and potassium ion channels (e.g. acrorhagin I and II).