The aggregating anemone (Anthopleura elegantissima), a sea animal closely related to corals and jellyfish, gets its name from the sprawling clusters it creates along the shores of the Pacific Coast. These aggregations may seem harmless enough to us, but they’re more than a colorful mass of tentacles. These are war-stricken replication factories. Each factory is made up of castes, and their many pieces collectively fight all-out territorial battles against neighboring aggregations. Together, these anemones act as a giant carnivorous net that can trap and viciously consume any clumsy invertebrate. Each individual also has a gentle side, capturing and cultivating plankton communities that help it survive in the uncompromising intertidal. Thanks to their collective hunting, defense, and plankton caretaking, the aggregating anemone is the most abundant anemone on California’s coast and can even be found even in areas with high levels of industrial pollution or sewage [1, pp. 51-54].
An aggregating anemone spends its early life in a larval stage, called a planula, freely swimming and scouring the ocean for a home. It fuels its journey by occasionally stopping for hours at a time to eat small food particles. These planulae don’t just want food, however; they want a whole garden. After feeding, they trail a mucus net behind them that captures single-celled algae like dinoflagellates and green algae. The anemone’s new friends are then ingested and incorporated into their outer skin [2] where they photosynthesize just like plants. The anemone benefits from getting these algae’s leftovers, namely carbon compounds fixed from the water (and waste from the anemone!) such as sugars and fats, which add to the anemone’s supply to invest in survival and reproduction [3]. Since both the algae and anemone are benefitting, we call the algae “symbionts.”
Incredibly, aggregating anemones will change their behavior to take care of their symbionts. Anemones with dinoflagellate symbionts will deliberately move themselves toward sunlight [4]. They also stretch their tentacles like house cats in the sun, inflating their bodies for maximum photosynthesis, and shrinking back down if the sun gets unhealthily bright [5]. This is not merely a story of coexistence between two very dissimilar organisms; it is a faithful, nurturing partnership.
After some months, the lone aggregating anemone next enters its matured polyp life stage. Aggregating anemones have two modes of reproduction: 1) sexually, broadcasting their gametes in hopes of producing another larval settler, or 2) stretching and splitting into two halves, each regenerating until it is a full adult individual [6]. This splitting in two, termed “binary fission,” is what forms dense aggregations consisting entirely of clones of the original settler. Why could it be advantageous for anemones to live in such close quarters? Anemones are squishy and largely water, so when the tide is low and the sun high, they face a very real possibility of drying out which often prevents them from spreading too high upshore in tidepools [7]. Some anemone species hold rocks on their oral disc for shade which has been shown to resist heat [8]. By forming dense aggregations, aggregating anemones further reduce their sun-exposed surface area while dampening stress from water currents and allowing for collaborative capture of larger prey [8, 9, p. 39].
Researchers first uncovered anemone aggression by observing empty channels between clonal aggregations [10]. This led to the idea that these channels were in fact borders of territorial disputes. A follow-up study on one-on-one anemone fights led to confusion. Some clones appeared much better at fighting than others. How then do clones coexist without one wiping one another out [11]? The confusion washes away when we consider teamwork, as despite all sharing the same genetic code, the individuals in an aggregation divide into castes with different jobs as the tides of war rise.
The caste coordination and tactics of these anemone aggregations is reminiscent of a classic fantasy setting, with warriors and scouts on the front lines protecting the citizens inside. The act of warfare looks something like this:
Soon after tidal waters wash over a colony, scouts (smaller than warriors, but equally armed) at the border begin a stereotypical searching behavior: they lengthen their bodies, inflate specialized tentacle-like fighting structures called acrorhagi, and twist and rotate in search of a genetically different individual [12]. After contact, the attacking anemone will discharge some of the outermost layer of their acrorhagi. Exactly like jellyfish, these acrorhagi contain stinging cells called nematocysts that discharge regardless of if they are still attached to their original body [13]. Injured anemones may retreat back to their clone-mates, or they may die and get swept out [12]. Away from the front lines, the center of the aggregations remain hubs for reproduction with two additional castes. The first are the reproductives, which have invested in being larger with more capacity to reproduce rather than having weaponry. In addition, there is a caste of smaller polyps in the center that might be recent clones, injured warriors, or reserves waiting to grow into what the aggregation needs, and it is not yet known exactly which [12].
It may seem odd that the warriors on the front lines are so willing to sacrifice themselves for the rest of their clone-mates. Evolution, which at the end of the day drives these behaviors, encourages organisms not to act for the good of their species, but in a way that spreads their own genes. Since the reproductives in the middle contain the exact same genes as the warriors, the warriors are willing to fight because the reproduction of their clone-mates is as if they themselves are reproducing. The aggregating anemone shines as a study organism for many open questions about the evolution of social groups, because despite each aggregation only containing one set of genes and the fact that anemones are one of the most simple multicellular animals, having split from the rest of the animal kingdom 741 million years ago [14], these aggregations parallel drastically more complicated eusocial insect species’ societies.
From my first encounter with them in Friday Harbor, Washington, to researching for this piece, learning about this complexity coming from what seemed at first like a pretty simple organism reminded me of how easy it is to underestimate nature. I hope this has served as a similar reminder to you, and that you will keep your eyes peeled next time you end up near a tide pool in search for these anemones or one of the many other wonders of the intertidal. From our perspective they may be indistinguishable from squishy, mossy rocks, but look from their level and you will see elegant green and pink gradients, a net of petrifying tentacles, a diverse army of warriors, and a green-thumbed giant, all aggregating into one.
Written by: Brady Nichols is a first-year animal behavior PhD student at UC Davis studying biomechanics and insect ecology/evolution in the Combes lab. He hopes to touch on flight, predator-prey interactions, weather, and paleobiology. Outside of work, he enjoys doing no more than two of the following at a time: birding, biking, cooking, rock climbing, and reading.
References:
[1] Ricketts, E. F., Calvin, J., Hedgpeth, J. W., & Phillips, D. W. (1985). Between Pacific Tides: Fifth Edition. Stanford University Press.
[2] Schwarz, J., Weis, V., & Potts, D. (2002). Feeding behavior and acquisition of zooxanthellae by planula larvae of the sea anemone Anthopleura elegantissima. Marine Biology, 140(3), 471–478. https://doi.org/10.1007/s00227-001-0736-y
[3] Muller‐Parker, G., & Davy, S. K. (2001). Temperate and tropical algal‐sea anemone symbioses. Invertebrate Biology, 120(2), 104–123. https://doi.org/10.1111/j.1744-7410.2001.tb00115.x
[4] Pearse, V. B. (1974). Modification of Sea Anemone Behavior by Symbiotic Zooxanthellae: Phototaxis. Biological Bulletin, 147(3), 630–640. https://doi.org/10.2307/1540746
[5] Pearse, V. B. (1974). Modification of Sea Anemone Behavior by Symbiotic Zooxanthellae: Expansion And Contraction. The Biological Bulletin, 147(3), 641–651. https://doi.org/10.2307/1540747
[6] Sebens, K. P. (1983). Morphological Variability during Longitudinal Fission of the Intertidal Sea Anemone, Anthopleura elegantissima (Brandt). http://hdl.handle.net/10125/652
[7] Dayton, P. K. (1971). Competition, Disturbance, and Community Organization: The Provision and Subsequent Utilization of Space in a Rocky Intertidal Community. Ecological Monographs, 41(4), 351–389. https://doi.org/10.2307/1948498
[8] Hart, C. E., & Crowe, J. H. (1977). The Effect of Attached Gravel on Survival of Intertidal Anemones. Transactions of the American Microscopical Society, 96(1), 28. https://doi.org/10.2307/3225960
[9] Shick, J. M. (1991). A Functional Biology of Sea Anemones. Springer Netherlands. https://doi.org/10.1007/978-94-011-3080-6
[10] Francis, L. (1973). Clone specific segregation in the sea anemone Anthopleura elegantissima. The Biological Bulletin, 144(1), 64–72. https://doi.org/10.2307/1540147
[11] Ayre, D. J., & Grosberg, R. K. (1995). Aggression, Habituation, and Clonal Coexistence in the Sea Anemone Anthopleura elegantissima. The American Naturalist, 146(3), 427–453. https://doi.org/10.1086/285808
[12] Ayre, D. J., & Grosberg, R. K. (2005). Behind anemone lines: Factors affecting division of labour in the social cnidarian Anthopleura elegantissima. Animal Behaviour, 70(1), 97–110. https://doi.org/10.1016/j.anbehav.2004.08.022
[13] Williams, R. B. (1991). Acrorhagi, catch tentacles and sweeper tentacles: A synopsis of ‘aggression’ of actiniarian and scleractinian Cnidaria. Hydrobiologia, 216(1), 539–545. https://doi.org/10.1007/BF00026511[14] Park, E., Hwang, D.-S., Lee, J.-S., Song, J.-I., Seo, T.-K., & Won, Y.-J. (2012). Estimation of divergence times in cnidarian evolution based on mitochondrial protein-coding genes and the fossil record. Molecular Phylogenetics and Evolution, 62(1), 329–345. https://doi.org/10.1016/j.ympev.2011.10.008
[Edited by Jacob Johnson]