Everybody’s favourite: Nemo

In the last blog I described the importance of microscopic symbiotic algae as the primary producers of energy, and hence the engine that drives reef ecology. But there are a multitude of fascinating symbioses among reef organisms, and in the next few blogs, I will try to describe some of these.
Children from the UNBC Daycare Centre often come by to have a look at the UNBC reef tank. One of the primary attractions is of course “Nemo”, represented in the tank by three Clown Anemonefish, Amphriprion ocellaris, also known as Common Clownfish, Ocellaris Clownfish, or False Percula Clownfish. The kids obviously know this fish from the popular animated Disney movie “Finding Nemo”, first released in 2003, but equally popular now leading to the re-release of a 3D version in 2012. The movie made these fish so sought after that wild populations were actually threatened, and a substantial captive breeding program has now been established that can cover much of the demand, at least for this species. The story has been documented in the excellent documentary “Filmstar fish – struggle for survival” http://www.oasishd.ca/index.php?option=com_content&view=article&id=378&Itemid=2 .
Before I describe the role of symbiosis for this type of fish, I will describe some other fascinating aspects. Anemonefish belong to the large family of damselfish, Pomacentridae, which include many different types of fish. The family is further subdivided into four subfamilies, three of which are represented in the UNBC reef tank. These are the green chromis (Chrominae), yellowtailed damsels (Pomacentrinae), and of course the anemonefish (Amphiprioninae). In “Finding Nemo”, the young Nemo lived in an anemone with his father Marlin as the only two survivors of a barracuda attack. Making the only surviving young a male was correct, but that is pretty much where accuracy ends. In reality, Anemonefish tend to live in small groups (a breeding pair and up to 4 additional non-breeding fish) hosted by a sea anemone (see below), and within each group there is a strict hierarchy of dominance, reflected by a fairly predictable size distribution (Buston 2003; http://www.nature.com/nature/journal/v424/n6945/fig_tab/424145a_F2.html). The largest fish is the only female, and the next largest fish is her reproductive partner. The other members are non-reproductive males that help in defending the anemone. The fascinating part is that if the female is removed, the reproductive male will change to become female, and the largest non-reproductive fish will become the breeding male. This process takes about 4 months (Madhu et al. 2010). This is called protandrous (=first male) sequential hermaphroditism.
Also unlike “Finding Nemo”, the eggs of Anemonefish are protected until they hatch, but the fry then spend time as pelagic plankton, drifting in the water column until they are large enough to find their way to a suitable reef, which they do in part by smell (Dixson et al. 2008). In fact, the Anemonefish in the UNBC reef tank have spawned regularly, but because of the pelagic stage, none of the fry survive without special rearing facilities, which include feeding them tiny animals called rotifers (Phylum Rotifera).

The current female nestles in among the protective tentacles of her host anemone.

What about symbiosis then? The name “anemonefish” immediately gives away that these fish live with sea anemones. In nature, each species of anemonefish will live only in specific anemones, but in captivity, they will pretty much occupy whatever is available to them, including some soft corals. Sea anemones have stinging cells, however, so how can these fish live among tentacles that can capture and kill some other fish? The fish are protected by special mucous, but in addition the acquire compounds from their host anemone by constantly rubbing against the antennae. Consequently, the stinging cells of the anemone do not fire as they would in response to most other stimuli. Why do they live in anemones? Well, in nature they depend on the anemones to protect them from predation. The anemone in turn gets protection from its predators by the aggressive defense of their host by the anemonefish. When cleaning in the UNBC reef tank, it is not unusual to get bitten, often by the smallest member. In fact, when anemonefish are removed from its host, angelfish move in and attack the anemone within hours, and anemones lacking anemonefish experience negative growth and eventually disappear (Porat and Chadwick-Furman 2004). Thus, this type of symbiosis is a good example of mutualism.
In captivity anemonefish may live for up to 18 years if conditions are excellent, and because of the lack of predators, they don’t need anemones as they do in the wild. Nevertheless, they lay their eggs on a cleaned section of rock under the anemone as they would in nature. In breeding programs, they are deprived of this, and may use any suitable substrate for breeding. The fish in the UNBC reef tank are now 12-13 years old, and have survived a number of trauma that killed a lot of other fish. The original female died in 2012, and based on her behavior recently, the current female may not last long. There is no question, that once the last of these wonderful fish have passed, I will repopulate the tank with a new group.
References
Buston, P. 2003. Social hierarchies: Size and growth modification in clownfish. Nature 424: 145-146
Dixson, DL, GP Jones, PL Munday, S Planes, MS Pratchett, M Srinivasan, C Syms, and SR Thorrold. 2008. Coral reef fish smell leaves to find island homes. Proc Biol Sci. 275: 2831–2839.
Elliott, JK, RN Mariscal, and KH Roux. 1994. Do anemonefishes use molecular mimicry to avoid being stung by host anemones? J.Exp. Marine Biol. Ecol. 179: 99–113
Madhu,R, K Madhu and KM Venugopalan. 2010. Sex change of hatchery produced Amphiprion ocellaris: Influence of mating system removal on gonad maturation and nesting success. J. Mar. Biol. Ass. India, 52: 62 – 69.
Porat, D, and NE Chadwick-Furman. 2004. Effects of anemonefish on giant sea anemones: expansion behavior, growth, and survival. Hydrobiologia 530-531: 513-520.

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