Marine anemones are often admired for their colourful beauty (although some are, admittedly, rather drab), but people are often surprised to hear that they are actually animals. They are in fact members of the cnidarians, a group of animals which all possess a cell type known as cnidocytes – cells which can fire barbed stingers into any prey which brush against them – hence the stings of jellyfish, who are also members of this group (you might think of an anemone as being something like an upside-down jellyfish, attached to a solid substrate). Like jellyfish, they are muscular, and can move about to an extent, depending on the species. Their behaviour can be fascinating. Some species even engage in vicious (if slow-motion) combat with other members of their species when they come into contact.
Many anemones are omnivorous suspension feeders, meaning that they obtain their food from suspended particles in the water column as they are washed over their tentacles, but they will take all manner of organic detritus when they can get it. Some will also consume any small organisms unfortunate to blunder into their stinging tentacles, such as amphipods and other small crustaceans. Two such anemone species are Cereus pedunculatus, which lives partly buried in sediment with its tentacles projecting from the surface, and Calliactis parasitica, which lives attached to the shells of hermit crabs and gastropods. Under normal conditions, these two species are typical mild-mannered suspension feeders, but once the level of dissolved oxygen in the water begins to drop, things start to change……..
The snakelocks anemone, Anemonia viridis.
It’s not part of this research, but it’s pretty, don’t you think?
Whilst conducting experiments into the effects of reduced oxygen levels on marine fauna, study authors Riedel et al. observed something curious about the behaviour of these anemones in relation to another species, the brittle star Ophiothrix quinquemaculata. The experiments involved artificially creating small oxygen deficient zones on the seabed, by placing a plexiglass chamber over a benthic assemblage, such that oxygen levels gradually decreased as the organisms consumed the available oxygen. The chamber also contained an oxygen monitor and a camera, to record what happened.
Under normal circumstances, anemones would occasionally come into contact with brittle stars, but would not attempt to capture them. Indeed, brittle stars have never been observed to be a feature of the diet of these two species. Still, brittle stars themselves tended to give the anemones a wide berth, and would move away if they came into contact with them. Perhaps they knew what was coming.
Once the chambers were closed, oxygen levels began to decline, and behaviours began to alter. Brittle stars responded by trying to elevate their central ‘discs’ above the substrate – standing on the tips of their arms (a response to reduced oxygen, presumably intended to lift them up into more oxygenated water), and they no longer fled from the anemones. The anemones themselves took no action, or at least not until oxygen levels approached near-anoxia (anoxia is the total absence of dissolved oxygen from the water). At this point the anemones turned killer. Now they took advantage of the brittle stars behavioural changes (and apparent lethargy) – simply put, they pulled the brittle stars in with their tentacles and ate them.
The anemones were able to survive the anoxic conditions (which eventually killed any brittle stars not consumed by the anemones), and were thus able to take advantage of the much lower oxygen stress tolerance of the brittle stars. Anoxic conditions altered the usual interactions of these two species.
So called ‘oxygen crises’ in marine environments are a concern, and can result from any human activities which cause nutrient enrichment and eutrophication. This research has shown that not only does oxygen depletion result in mortality of marine organisms (as expected), but also that it can cause fundamental shifts in the ways that predator and prey species interact, due to differential tolerances of anoxic conditions. These shifts are not readily anticipated, and might alter ecosystems in unexpected ways.
Riedel, B., Stachowitsch, M., Zuschin, M. (2008). Sea anemones and brittle stars: unexpected predatory interactions during induced in situ oxygen crises. Marine Biology, 153(6), 1075-1085. DOI: 10.1007/s00227-007-0880-0