Yesterdays post was about sea anemones, and today is going to cover briefly how these creatures feed, and the relationships they have with other species of animal (symbiosis, from Greek sym together, and biosis living, so simply meaning things that live together…see, those scary sounding terms which put people off are fairly straight forward usually)
So, yesterday we covered the structure of the sea anemone, where the tentacles at the top of the animal direct food into the top of the pharynx (like we do with our hands), but, anemones do not have tongues, or a digestive system as advanced as ours, so how do they actually eat, and digest their food? And, having a simple gut as mentioned yesterday, it only has one entrance/exit, so how exactly does that work, and is it as gross as it sounds?
As with us, when food enters the throat (pharynx is from the Greek for throat), the tissues within the throat expand by a wave of contractions, which work up from the base to the top, so the part of your throat at the back of your mouth expands to allow the food to enter, then the wave reverses, and the food is pushed downwards, and out of the throat into the stomach. (Try putting your hand on your throat as you swallow to feel this).
Once dinner has been digested, contractions within the tissues of the stomach (peristaltic contractions, from Greek peri and stallien meaning to wrap around), push the waste back up so they can be excreted. This type of movement of tissue is the same as we have in our small intestine, and our throat after swallowing.
The diagram below shows the different stages in the feeding cycle of sea anemones, and is followed by some other pictures from the same site, mainly because they are awesome pictures, but also because they show anemones in different stages of contraction and expansion.
So, is the feeding as gross as it seems? I do not think so, although when watching it, it can make your stomach feel a bit queasy, but this is purely because we have two exits from our stomach, and so associate exiting of material through our mouth as a sign that something is wrong.
Now, onto the symbiosis I mentioned at the start of the post. There are several forms of symbiosis, with varying degrees of benefit to the host. Parasitic symbiosis is the one most people are aware of, where one species lives in or on another, and is harmful to the host, examples of this in humans are malaria and tape-worms.
Mutualism is when both species gain a benefit from the symbiosis. The bacteria in our stomachs, bees and flowers are both examples of mutualism. In sea anemones, they have mutualistic relationships with clown fish, as shown in the picture below. The clown fish is protected from predators by the tentacles of the sea anemone, and in return, the clown fish fights off fish which would otherwise feed on parts of the anemone. Also, the clownfish also excretes ammonia rich waste, which is used by the bacteria in the stomach of the anemone.
Commensalism is when one species benefits without harming the other. Anemones are often used as examples of this, and with good reason! The picture below shows a Boxer Crab. This animal carries anemones in its claws for protection! They are the white objects in the picture below. If anything threatens the crab, it waves around the anemones, with the tentacles towards the attacker… if the attacker gets too close, the nematocysts will fire from the tentacles. This relationship may be more mutualistic than commensalist, as the crab excretes nitrogen rich waste in the same way the clownfish does, and so may provide nutrients for the anemone.
Another really good example from the anemone is of the anemone crab (a porcelain crab species), which lives in the tentacles of the anemone, and filter feeds particles passing through the tentacles. As with the other species which have symbiosis with anemones, the crab has had to evolve an immunity to the toxins of the tentacles.
I find it fascinating trying to work out reasons for how these relationships evolved, and why they arose. For a crab or fish to begin living on what is an aggressive toxic animal means that the benefit of the protection gained must outweigh the danger of being accidentally eaten, or stung by the host. In the case of the boxer crab, I think that originally, anemones may have settled onto a crabs claws, and then over time, the crab began to utilise the anemones in the local area. In the case of clownfish, or porcelain crabs, which came first, the immunity, or the behaviour, is something which keeps me busy for hours when I start thinking about it. I am sure there are evolutionary biologists out there who know the answer, but, I prefer for now to try and work it out for myself, maybe over time I will study more ethology (the study of animal behaviour), and be able to better understand it for myself. Presently, I think that it was a mix of immunity and behaviour. Some ancestral fish would have had a slight immunity, which made it able to utilise the anemone for a short period of time, and over time, this was selected for because of the protection gained from the behaviour….being able to hop into an anemone when a predator came past, even for a few moments, is an advantage, and if while there, food is available, that is a double advantage.
Next time will be starting out on corals…I have no idea at the moment how many posts that will be, as the topic is huge, and amazingly interesting!