Living together: an insight on anemonefish cohabitations

Living together: an insight on anemonefish cohabitations

A famous example of mutualistic symbiosis: anemonefish and sea anemones

Fig 1

Many aspects of the anemonefish’s life and social structure are already known, since these little fish have been studied since years as examples of mutualistic symbiosis and hierarchical –and matriarchal- typical social structure. Yes, the big boss of the anemone is the older female; all the other fish are just subordinate. Anemonefish live in symbiosis with sea anemones, for some species very selectively; on the contrary other species are more generalist, so can occupy more than one single species. Anyway, many anemonefish species change from generalist to specialist depending on locations.

It is something similar of what happens when different species of monkeys live in the same area. Some of them may like to live only in a specific tree. Other monkeys maybe prefer to stay on the ground, while for other it doesn’t really matter, whatever tree is fine. (For the science purists: yes, I know that it doesn’t exactly fit the situation, but it gives the idea)

Anyway, every single species of anemonefish has a preferred sea anemone to live within, called preferred host. In areas with a very high anemonefish biodiversity, as the Coral Triangle (Indonesia, Philippines, Malaysia, Papua New Guinea) different species of anemonefish can compete for the occupancy of a single sea anemone, especially if this specific anemone is someone’s preferred host. As it’s almost impossible to find an “empty” anemone (both because it can be easily eaten by predators, if left alone, both because of the abundance of larvae ready to settle), it is easily understandable how the main limiting factor for the anemonefish population is the availability of anemones, and how the occupancy of a specific host can be precious for any anemonefish species.

Fig 2

In some areas of Indonesia, as the Bunaken National Park, in North Sulawesi, it is possible to observe up to 7 different species of anemonefish. The maximum biodiversity recorded so far was in Papua New Guinea, where up to 8 species have been recorded: in these areas, the competition between different species of anemonefish can be really though.

 

One or Many? Preferred host and “niche-overlap”

In the Indo-Pacific sea, one of the most common species is the Clarki Anemonefish (Amphiprion clarki) (Fig. 1). It’s a very generalist and competitive species, actively defending his anemone host against predators, even very big “aggressors” including divers.  Usually his preferred host is the Sebae Anemone (Heteractis crispa), but can also occupy the large Merten’s Carpet sea anemone (Stichodactyla mertensii) and many others, even less frequently.  Quite often a Clarki anemonefish can occupy several anemones at the same time, even if the effort to protect a large territory can be exhausting. Many other anemonefish, like the smaller Orange Skunk (Amphirpion sandaracinos, Fig 2) and the Pink Anemonefish (A. perideraion, Fig 3) have less potential hosts., usually one or two. Orange anemonefish only inhabits Merten’s Carpet anemones, while the Pink anemonefish usually prefer Sebae anemones or Magnificent Sea Anemones (Heteractis magnifica), with a marked preference for the first one.

Fig 3

So what happens if two anemonefish species have the same preferred host, or even they share a secondary host? This situation is what scientists call “niche-overlap”, and in the case of anemonefish it happens only in regions with a very high biodiversity, where many different species share their habitat.

In these areas the competition for the same host leads to a very unusual situation: the cohabitation of two anemonefish in the same sea anemone.

Of course every species’ desire is to be the only “owner” of the anemone (would you like to share your apartment with a monkey?), so cohabitations drive to quite long and strong conflicts. It’s not yet sure what is the mechanism involved in this competition: in the case of Clarki and Orange Skunk Anemonefish cohabitation (inside the Marten’s anemone, Fig. 4), the bigger Clarki seems to be very stressed by the “invader” presence, and the dominant female often leaves to younger member of the hierarchy the duty of eliminate the undesired neighbor (Fig. 5). The same happens in another cohabitation, with Clarki and Pink anemonefish inside Sebae anemones (Fig. 6). It should be nice to monitor cohabitations for a long period to see what really happens and their evolution during the time: I’ve tried personally for some months, but nothing changed, probably the cohabitation lasts for a quite long period.

 

Conflict or peace?

Fig 4

As always happens in Nature, there not a fixed rule, even about different species cohabitations. For example, the Spinecheek (Premnas biaculeatus, Fig. 7) and the Red&Black Anemonefish (Amphiprion melanopus, Fig 8) live –at least in the Indonesian Seas- preferably in the Bubble Anemone (Entacmaea quadricolor), but there was never observed any kind of cohabitation of the two species. Probably, the different life-style (only a pair of fish in each anemone for the Spinecheek, while enough large colonies of also hundreds of fish and “anemone-cities” are the common situation for the Red&Black) makes their niche different enough to avoid this phenomenon.

A high potential probability of cohabitation exists as well between the Pink and the Ocellaris Anemonefish (Amphiprion ocellaris, Fig. 9) inside the Magnificent Sea Anemone, but cohabitation has never been observed, even if these two species share habitat, host and has a similar social structure. Don’t ask me why, maybe they are simply more peaceful species.

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