I feel like sea anemones are often overlooked, seen as the pretty but uninteresting sidekick for the clownfish that waft around in the currents not doing a lot. But in reality, sea anemones are not to be underestimated. From willingly tearing themselves in half repeatedly to form a clone army, to detaching from the sea floor and swimming around and forming clever mutualistic partnerships, sea anemones have a lot more to them than initially meets the eye.
Like their cousins, the corals, sea anemones are often thought to be plants, but they are in fact predatory animals in the Phylum Cnidaria. Some species host a symbiotic relationship with dinoflagellates, like corals, for additional nourishment (a double edged sword that also makes them vulnerable to stress bleaching), others are their own self without a symbiotic partner, while others have very unusual mutualistic partners that we will discuss later in the post.
To understand the complexities of anemones it is best to start with the basics – body structure. Anemones don’t possess a rigid bony skeleton, so to keep their shape and remain stable their gastrovascular cavity is kept at a constant volume. When their mouth is closed their muscle fibres, which are both longitudinal and circular, can pull against the fluid in the cavity to flex and move their body, this is the principle of a hydrostatic skeleton. They only possess a single opening to the outside which is their mouth, however this means they both consume food and excrete waste through their mouths, known as an incomplete gut. The gastrovascular cavity functions like a stomach, with a lining on both sides and filaments of tissue that secretes digestive enzymes.
Although some burrow the majority of their body into sediment leaving only their tentacles and mouth exposed (the burrowing anemone can be seen around the UK and the North of France), others attach themselves to hard surfaces using their basal disc. Although they are attached to the floor, they are hardly stuck in one place – a wave of contraction moves one side of the foot towards the other side, which detaches the foot, stretches forward, then reattaches, similar to how a snails foot moves. If they are not moving fast enough then can fully let go of the surface and drift to a new spot, some can even swim! Stomphia’s can swim by flexing their columns, watch one escape a predatory sea star HERE. I fell down a hole watching these videos – enjoy!
The tentacles that surround their mouths are equipped with thousands of tiny explosive cells called cnidocytes (hence being in the Phylum Cnidaria!). Cnidocytes contain organelles called nematocysts that are like coiled hollow threads that are also sometimes barbed. When the cnidocytes are activated, they explode outwards, firing the coiled threads into the flesh of the prey/predators/competing sea anemone and releasing toxins. These toxins can paralyse their prey while the other tentacles use their cnidocytes to harpoon their prey and move it into their mouth. Watch a microscopic view of the nematocysts being ejected from the surface of the sea anemone Aiptasia pallida HERE.
In my post about the immortal jellyfish I explained general Cnidarian lifecycle, from a fertilised egg, to planula, to polyp to medusa and around again. Like the immortal jellyfish, sea anemones diverge from this life cycle but in a different fashon – the anemones never reach the medusa phase (the bell-shaped form we identify as a jellyfish). Adult Anemones produce eggs and sperm which become the fertilised egg, then a free-swimming planula larva before it settles on the sea floor to become a polyp. When this polyp has matured enough it too can then produce eggs/sperm and continue the cycle.
In some species the sexes are separate, while in others they are sequential hermaphrodites (meaning they change sex). Others are hermaphroditic and self-fertilise, while others can even reproduce asexually.
The brooding anemone, Epiactis prolifera, begins its life as a female, and becomes hermaphroditic when she reaches a certain size. As the name implies, and is seen unusually often in the Epiactis genus, this anemone broods its young at least until juvenile stage. Brooding in sea anemones is relatively rare and can be produced from sexual or asexual reproduction. The offspring can be brooded internally in the gastrovascular cavity, or externally on the surface of the adult, as you can see in the picture.
In asexual reproduction anemones they can form a new individual by budding like yeast, or by longitudinal (A) or transverse binary fission (B). In longitudinal reproduction they quite literally pull themselves apart dividing down the middle producing a clone of itself. While in transverse fission (which is only know to occur in two species), a rudimentary band of new tentacles begin to grow on the column, then the upper and lower parts pull themselves apart. The upper part grows a new pedal disk while the lower one grows a new oral disk. It has even been seen that another fission event is happening before the previous one has been completed.
If tearing themselves in half sideways isn’t strange enough, sea anemones also have the incredible ability to regenerate from pedal laceration (C). When the basal pedal disk is fragmented, the small piece can regenerate into a whole new anemone. Exaiptasia pallida has been used in regenerative research studies because of its aptitude to regenerate whole animals from even the smallest amputated parts. This comes in very handy for the pom pom crab.
The pom pom crab, or boxer crab, holds two anemones in each claw, use the small stinging animals to fend off predators and to immobilise prey. In return the anemones get ferried around and exposed to more food sources that they would usually have to wait to drift past them. Although they can survive independently, pom pom crabs that are pom pom-less will take on another crab to try to steal one of their anemones. If they are successful then each crab will split their anemone in half so they have one for each claw, utilising their pedal laceration regeneration knowledge, smart crabs. Some crabs have also been known to altruistically share their anenomes if they are in a tank with a pom pom-less crab, so it depends on the crabs involved whether they willingly share or whether they fight to steal one. The crab splitting their anemones is the first known case of one animal causing another to reproduce asexually.
Another group of interesting underwater organisms are the nudibranchs, which are beautifully vibrant underwater sea slugs. If you haven’t hear of these organisms before or seen any pictures other than the one featured here I highly recommend having a quick google after finishing this post – they are beautiful! These slugs are hardly boring pests, and the Aeolids, a suborder of the Nudibranchs, have the incredible ability of stealing the defences of the stinging animals they feed on – including sea anemones. Chemicals in the aeolids slime prevents the cnidocytes from firing, allowing them to feed on the sea anemone without being under attack. The Aeolids then steal the anemones stinging cells to use for themselves, in a process called kleptocnidae. There was some debate whether the slugs consumed only the immature cnidocytes, or whether the mucus produced during feeding allows them to remain dormant until it had reached the cnidosacs where they are then stored. A study in 2012 found that the nematocysts that were eaten were not yet mature and therefore not fully functional at the time of eating, so do not discharge while being moved to the cnidosacs. The study also found that when the cnidosac was fully inhabited, the nudibranch lowered the pH of the sac. Acidification is at least one of the processes required to mature cnidocytes, so the nudibranch can then mature the stinging cells at will to use at a later date, wow.
Some of the many papers used as research for this post
– A Study of the Muscular Anatomy and Swimming Behaviour of the Sea Anemone, Stomphia coccinea, Paul N Sund, University of Washington
– Evolution of Brooding in Sea Anemones: Patterns Structures and taxonomy, Paul G Larson, Ohio State University
– The rapid regenerative response of a model sea anemone species Exaiptasia pallida is characterised by tissue plasticity and highly coordinated cell communication, Chloe Van der Burg et al – SO interesting!
– The Cnidaria, past present and future (book), chapter: Reproduction of Sea Anemones and Other Hexacorals, Ekaterina Bocharova
– Incorporated nematocysts in Aeolidiella stephanieae (Gastropoda, Opisthobranchia, Aeolidoidea) mature by acidification shown by the pH sensitive fluorescing alkaloid Ageladine A, Dana Obermann, Ulf Bickmeyer and Heike Wagel