Fascinating Symbiosis

Published May 25, 2013 in MARINE LIFE

There are nine species of Anemonefishes that can be found in the Wakatobi region. In this special Wakatobi feature article we’ll take a detailed look at the anemonefishes and their relationship with their host sea anemones. Are there benefits for both the fishes and their host?

There are many animals that live together in the coral reef ecosystem, but none is better known than the anemonefish (clownfish)-sea anemone partnership. The term symbiosis is used to describe a special relationship between two unrelated species. A symbiotic relationship can be good for both (mutualism), good for one and neutral for the other (commensalism), or even good for one and bad for the other (parasitism). But what about the anemonefish and its invertebrate host? Is it good for one or both partners? Let’s take a closer look.

False Clownfish

Benefits for the Anemonefish

Most divers know that sea anemones have stinging cells (known as nematocysts) in their tentacles that they use to capture small prey animals. It would seem that this characteristic would preclude any animal from using them as a home – but not the anemonefishes. They have the ability to acclimate to the stinging cells of the anemone (more on this in a future news letter). When threatened by a predator, many anemonefishes dive among the tentacles of their host, often temporarily vanishing from sight among the anemone’s appendages. When the threat is gone, the fish will reemerge.

The anemonefishes are always found with sea anemones. Without their host, most anemonefishes would be in great peril. Although little information exists on fishes that prey on them, anemonefishes are probably eaten by morays, trumpetfishes, scorpionfishes, large hawkfishes, and snappers. In a study conducted by Dr. Gerald Allen, individual anemonefishes removed a short distance from their sea anemone hosts and released were often fed on by groupers, particularly the Whitelined Grouper (Anyperodon leucogrammicus).

That said, the mortality rate of anemonefishes is lower than other reef fishes, probably because of the protection afforded by their invertebrate host. While all anemonefishes are always found living with sea anemones, they vary in their dependency on their host. Those species that are relatively poor swimmers, like the Pink Skunk Anemonefish (Amphiprion perideraion), the Orange Skunk Anemonefish (A. akallopisos), False Clownfish (A. ocellaris), and Percula Clownfish (A. percula), are found in close association with their anemones. When they swim, these less agile species use their pectoral fins more and tend to move in a more sinuous or exaggerated fashion than their relatives. They also spend more time among the anemone’s tentacles, rarely straying far from their host. When approached by a potential threat, these species are quick to seek shelter.

In contrast, species like the Orangefin (Amphiprion chrysopterus) and Clark’s Anemonefishes (A. clarkii) are more loosely associated with sea anemones. These fishes are efficient swimmers, regularly employing the caudal fin in locomotion. They regularly feed in the water column, moving one meter or more away from their hosts when foraging (they may move over much great distances when immigrating from one anemone to another). These anemonefishes are more likely to come out and confront a threat, whether egg or anemone predators. For example, if a diver moves close to the anemone of a Bluestriped Anemonefish, it is likely that the fish will charge at you and may even bite you! In the same situation, the Pink Skunk Anemonefish would dive into its anemone and attempt to hide deep in among the tentacles. One indicator of anemonefish host dependency is the morphology (shape) of the caudal (tail) fin. Generally speaking, those species that are more strongly dependent on their host have a rounded tail, while those that are less anemone-dependent have more emarginate (notched) caudal fins. Caudal fin shape is also indicative of swimming ability.

Spine Cheek Anenomefish (Premnas biaculeatus)

Benefits for the Sea Anemone

It is widely accepted that anemonefishes benefit their hosts, although sea anemones can survive without their being present (at least in some areas). But what advantages do the sea anemones gain from this relationship? One anemone benefit is that the presence of anemonefish increases the rate of host growth and the rate of asexual reproduction. For example, in one study it was reported that Magnificent Sea Anemones (Heteractis magnifica) that harbored Orangefin Anemonefish grew three times faster than those sea anemones that did not have anemonefish present. Sea anemones with two or more anemonefish associates also had the highest fission rates (this is a form of asexual reproduction where the anemone basically tears in half to produce two sea anemones), while those with no fish present had the lowest fission rates.

Anemonefishes are not the only organisms that call sea anemones home. The anemones cells harbor brown algae, known as zooxanthellae. The anemone provides a place for these plant cells to live and in turn the algae produce food (by photosynthesizing) for the anemone. It turns out that the zooxanthellae also benefit from the presence of the anemonefishes. The algae utilize ammonium excreted by the resident fish as a nutrient source. Studies have shown that sea anemones kept with anemonefish regenerate tentacles more rapidly than those without a fish symbiont and that their tentacles contained more zooxanthellae than those kept in anemonefish-free aquariums.

Not only is the inorganic nitrogen source an important zooxanthellae fertilizer, the movement of the anemonefish among the tentacles stimulates the host to open, facilitates the circulation of oxygen-rich water among the tentacles and skin folds and wafts debris from the sea anemone’s oral disc. One study found that Bubbletip Sea Anemones (Entacmaea quadricolor) that had resident Twobanded Anemonefish (A. bicinctus) expanded their tentacles significantly more than those that did not. By expanding their tentacles, they expose more of their surface area to the sun and as a result their zooxanthellae can engage in more photosynthesis (that is, produce more food for the anemone). Researchers have also found that when anemonefish associates are too small or the anemone had no resident fish at all, the sea anemone would shrink and sometimes die.

Another important benefit provided by the larger anemonefishes is protection from anemone predators. In some areas, anemones are quickly consumed if the anemonefishes are removed – the primary predators in these areas are large butterflyfishes. For example, on the Great Barrier Bubbletip Anemones that were of cleared of anemonefish by researchers were gone (apparently eaten) within 24 hours. When Twobanded Anemonefish were removed from E. quadricolor in the Red Sea, Red Sea Raccoon Butterflyfish (Chaetodon fasciatus) attacked the anemones, causing them to retract into holes in the reef. In certain regions, like subtropical Japan, it is

possible to find healthy symbiotic sea anemones that lack anemonefish associates. But these are areas where major sea anemone predators are uncommon. In more tropical climes (like Indonesia), the majority of host anemone species will contain anemonefishes.

It was once thought that anemonefishes regularly fed their hosts. However, reports of such behavior appear to be an artifact of divers or aquarists giving anemonefishes oversized pieces of food. The anemonefish will place the food item in the anemone’s tentacles and try to rip it into smaller pieces. In the wild, anemonefishes rarely feed on prey items that they cannot swallow whole.

In conclusion, there is no doubt that both the anemonefish and sea anemone benefit from this fascinating partnership – that is, it is a mutualistic relationship. Next time you are diving on a Wakatobi reef, make sure you take some time to watch the anemonefish as it snuggles within the tentacles of its host and remember how important this association is to both the fish and the invertebrate.