I ended a blog some time ago with “I will in my next blog entry shortly discuss the relationship within an anemone fish “colony”. Be prepared for a grizzly world, combining forced starvation, castration due to stress, a willingness to let conspecific meet a certain death outside the anemone and cannibalism in one and the same anemone. Quite a far way from the cutie, cuddly feeling that Nemo left most of us in!”
So, now is the time to expose the myth of the cutest fish of all. In order to understand the finer points of anemone fish violence, I think it is important to define the ecological setting and the life history of anemone fish.
First of all, anemone fish belong to the Pomacentrids together with the damsels and a bunch of other fish, most of which score pretty high on the aggressiveness scale. Anemone fish all form what is called an obligate symbiotic relationship with sea anemones, that is the anemone fish need the anemone to survive and will die without the presence of an anemone. Some anemone fish can use several different species of anemones as host, but most species use only one or a few species. As the anemones used by anemone fish are pretty rare, and anemone fish can produce quite a large number of offspring, anemone fish densities will with very few exceptions be limited by anemone availability. Furthermore, anemone fish feed primarily on algae and zooplankton, which are small animals drifting by with currents. Algae are grazed from nearby hard surfaces, while zooplankton are taken from currents running very close to the host anemone. Thus, not only are anemones in short supply, but once an anemone fish has settled in an anemone, food availability in the vicinity will be very limited. Therefore we can expect a fierce competition for residency in an anemone, and, once in an anemone, we can expect high competitive levels between resident fish. We can also expect resident fish to be pretty mean to new fish wanting to enter the anemone.
So, what does research say about it. Anemone fish, share the general life history of marine animals with a period spent drifting in currents followed by an effort to settle in a benign environment. For anemone fish this benign environment is very limited in numbers and size, and will more or less never be free of prior occupants. Given that anemone fish seem to be unable to survive outside anemones, this sets the scene for some quite interesting and quite gruesome interactions between resident fish and fish wanting to join an anemone.
Studies have shown that small anemone fish wanting to settle in an anemone will only be allowed to do so if the anemone has few resident fish. If resident fish experience the anemone as full, they will without mercy bully the small juvenile trying to enter the anemone and keep the young fish outside of the anemone, where it quite soon will die. So why doesn´t the settler find another anemone? Well, it seems more or less impossible for an anemone fish to move from one anemone to another. So, being a small juvenile fish and wishing to enter an anemone is probably a though deal, and it is highly likely that the vast majority of settlers are driving to an early death by resident fish in the anemone.
How about the interactions in the anemone? Shouldn´t fish living in a specific anemone be friends? Well, a ladder of terror rules interactions between resident fish. The largest fish being female, bullies all other fish, including its mate. The next largest, being male, bullies the rest of the fish resident in the anemone. The bullying reaches levels enough to stunt the growth of smaller fish and to a certain extent in practical terms temporarily castrate them. Only when one of the larger fish dies, will one of the smaller fish experience less bullying and mature into a male, while the surviving larger fish continues its female life or, if male, changes sex and becomes a female.
How about the small fish in the anemone? Are they just victims of a ruthless adult pair? Observations show that this is not entirely the case. When the adult pair have eggs, I have several times seen small fish steal eggs as soon as the breeding pair is occupied by something else. So in some sense there is some level of revenge from the small fish. But why would the small fish consider eating eggs of the adults? The small fish in an anemone is not related to the breeding pair. Thus, it will incur no loss for the small fish to eat the eggs of the breeding pair. On the contrary, eggs are probably the most nutritious food item ever available to them. Together with competition for available food, this egg cannibalism can also to some extent explain why the breeding pair will be so reluctant to let several small fish reside in the anemone. Smaller, nonbreeding fish will also want to keep potential competitors from the anemone in order to increase their own chances to reach the prime spot as a breeder in the anemone.
So, in conclusion, cuddly and nice Nemos are the stuff of cartoons and children’s toys! The real life of anemone fish includes sex change, murder, bullying, cannibalism and shear terror which to me is way more interesting!. The really cool thing is that after reading this blog post, I promise you that you will easily be able to see examples of those interactions whenever you spend a little time watching a group of anemone fish.