Friends and Anemones
(On the resistance of clownfish to their host anemone’s stings)

(by Stratos Kotzabassi)

Ever so close and yet so inaccessible, the marine world has managed to keep many secrets from the scientific community. It is often remarked that scientists know more about the moon, 238,870 miles away, than the oceans at our doorstep. One of the many mysteries the sea offers is that of the anemonefish. This diminutive creature is almost entirely unremarkable in the ocean. It has no real size or physical defenses to deter predators, no special camouflage or coloration to hide it, and yet it remains a highly successful group of fish. This is because of their association with the sea anemone, a stinging, largely immobile and multi-tentacled creature. Within the anemone, the anemonefish remains protected from all but the most aggressive and well armored predators. However, how these small fish survive nestled in the deadly tentacles of certain sea anemones remains a mystery. While many studies have been done on the subject and many theories proposed, no single explanation has proven adequate. It is likely, however, that the truth lies in a combination of these theories.

The anemone fish belongs to the Pomacentridae family (aka damselfish) sub-family Amphiprioninae, genus Amphiprion. There exists another sub-genus under Amphiprion known as Premnas; however this sub-genus only contains a single fish. Overall, the sub-family Amphiprioninae contains 28 species of anemonefishes, most commonly known (and heretofore referred to as) clownfishes. The genus Amphiprion is further divided into 6 complexes, the details of which are outside the scope of this paper.

The clownfish itself is structured differently than most ocean fish. It is significantly more rounded and stockier, with rounded fins. Given that most clownfish never travel more than a meter from their host anemone, this body size would seem logical, as the shape of the fins and body would be more optimally suited to maintaining a stationary position, as opposed to the thinner fish with acutely angled fins and tails which are built for speed and escaping predators. The clownfish typically features bright vibrant coloration, ranging from yellow to maroon, punctuated by spots or stripes of white. Clown fish predominantly inhabit warm tropical waters of the Indo-Pacific, ranging from the Red Sea to Australia and the Solomon Islands. No species of clownfish are found in the Atlantic of Caribbean. Clownfish are omnivorous, and in the wild primarily consume zooplankton, surrounding algae and any others bits of meat which might float their way.

The reproductive behavior of the clownfish is particularly fascinating. They are considered protandrous hermaphrodites, meaning that they are born and mature as males, but can become females later on. This change in sex is precipitated by a dominance hierarchy established when the most dominant male in a clutch of clownfish becomes the female and grows larger than the males. The result is a matriarchal society in which a single female presides over a harem of submissive males. Constant attention is necessitated on the part of the female to assure that the other fish are kept male. If the dominant female goes absent for any reason, the next most dominant male becomes the female. Of the group, only the dominant female and one male will mate. Once fertilized, the female will lay her eggs on a hard surface near or beneath the base of the host anemone that the small clownfish community inhabits. The task of protecting the eggs is given exclusively to the male. The male frequently rubs the host anemone is such a way as to cause it’s tentacles to extend over the eggs, thus protecting them further from predation. Though small, these fish become extremely aggressive when eggs are present and have been known to nip at divers with extreme veracity.

Host sea anemones are of the class Anthozoa and of the phylum Cnidaria. The basic structure of the host anemone is superficially simple. It is composed of a base which allows it to attach to rock or substrate, a central disk (which contains the mouth) and tentacles surrounding it. These tentacles are covered by nematocysts. These nematocysts are small proteinaceous capsules which contain a wound “thread” at the tip of which is a sharp point. When triggered, the cell literally explodes, causing the thread to fire at whatever has come into contact with it. This force has been measured at a strength of up to 40,000G. This provides the nematocysts with enough explosive force to pierce through things as hard as mollusk shells and arthropod exoskeletons. The anemone itself is coated with mucus, which seems to prevent the nematocysts from discharging, regardless of what the mucus is located on, including food items (Pantin, 1942). On the other hand, mucus from an anemone of a different species will elicit a stinging reaction, regardless of the object (Ertman & Davenport, 1981). The anemone, however, does not derive all (or even most) of its energy by prey capture. Most anemones maintain a symbiotic relationship with photosynthetic algae called zooxanthellae, which they house and keep safe between the outermost and innermost layers of their tentacles and oral disk. These algae utilize available sunlight to photosynthesize and produce wastes, which in turn become nutrients for the host anemone to grow. Host anemones have a distribution similar to the of the clownfish, with non-host anemones found even further afield. Reproduction in most anemones is accomplished either by sexual reproduction (accomplished via spawning gametes en mass into the water) or by clonal budding, whereby a single anemone splits into two identical ones.

The relationship between the clownfish and anemone is of the of the hallmark images of the reef. Clownfish in the wild almost always find and utilize a host anemone. Those which do not often do not survive, as they are often preyed upon by predatory fish and they do not possess any significant defenses, such as speed or camouflage. The benefits of hosting in an anemone are numerous. The primary benefit is that of protection—the clownfish is protected from a wide variety of predatory fish by the lethal stinging tentacles of its host anemone. The anemone also provides a protective medium for reproduction, as it serves to protect the eggs which are deposited near or beneath its base. Studies have shown that the tentacles of the host anemone do not sting the clownfish’s eggs (Elliot & Mariscal, 1996). The relationship between symbiont and host is not one-way. The anemone receives benefits due to the presence of the clownfish as well. There has been some speculation that the anemone utilizes the small clownfish as a lure for larger prey-items (Saville-Kent, 1893), however later observations showed the completed opposite. The study showed that anemones (E. quadricolor specifically) which had their symbiont clownfish removed were often eaten in short order by butterfly fishes (Family Chaetodontidae). When present, the hosting clownfish was shown to selectively recognize and chase off common predatory butterflyfish (Moser, 1931). Furthermore, there has been some speculation that the presence of a host fish has lead to physiological adaptations in the anemone structure; namely the creation of an enlarged oral disk (above which the clownfish spends most of it’s time hovering). The resulting changes, it is argued, make the anemone more efficient at absorbing sunlight for it’s zooxanthellae and hosting clownfishes, but decreases it’s predatory efficiency. The anemone might therefore be dependant on the clownfish’s feces for certain nutrient inputs which it might normally have obtained via predatory means (Fautin, 1991). As mentioned previously, the clownfish also displays physical adaptations to living in an anemone, such as the rounded fins and body.

How the relationship between an individual clownfish and anemone begins has been studied extensively, and demonstrates a very important biological aspect—that of imprinting. Young and newly hatched clownfish spend their time around their parents’ host anemone, which contains a variety of chemicals in it’s mucus the fish can “smell”. Later on in life, when the fish disperse, they learn to seek out similar anemones by following the trail of a chemical known as amphikuemin. These fish show indications of natal imprinting, as they seek anemones of the same species near which they were raised. Clownfish raised in an artificial environment lacking any anemones showed some limited hosting instinct but no preference for anemone, indicating that the action of hosting itself was neurally hard-wired, but that the specific host was determined by imprinting (Arvedlund, McCormick et al, 1999). Further experiments comparing the role of both visual and chemicals cues showed that visual cues played almost no part in which anemone the young clownfish chose and that chemical senses were the dominant factor (Elliot, Elliot et al, 1995).

Given this lifelong association with these deadly animals, the question remains—how do these diminutive fish manage to live in direct contact with tentacles designed to kill and capture fish twice their size? A whole modicum of theories have been proposed to answer this question and, unfortunately, none of them seem to provide any total or definitive answer. However, a small group of theories seem to offer the most promising potential for unlocking the mystery.

Some of the earlier theories, while intriguing, were soundly proven incorrect. One early theory proposed the idea that the host anemone’s tentacles did not contain stinging nematocysts. However, this theory was proven wrong by a variety of studies which showed that the host’s tentacles not only contain active nematocysts (Gudger, 1946), but that the number and strength was typical to that of other sea anemones (Dunn, 1981). Based on the example of over 30 species of Caribbean fishes which associated with anemones, a hypothesis was formulated which held that the clownfish never actually touched the tentacles of the host anemone, thus avoiding it’s stings. However subsequent study and observation proved this theory to be incorrect, as the clownfish not only contacted the tentacles, but were shown to rub against them (Verwey, 1930). Yet another theory held that the skin of the clownfish were simply impenetrable to the sting of the anemone, however, as studies on the nematocysts themselves show, a strike with a force of 40,000G had the potential to pierce the hard calcareous shell of mollusks, let alone the relatively soft scales of the clownfish. A study done to address this theory used cotton swabs to gently remove the mucus from a localized area. The fish was then presented to it’s host anemone—and promptly stung severely, thus disproving the theory (Lubbock, 1980) An even earlier study even indicated that the skin of the clownfish was thinner than average (Caspers, 1939).

While largely unsupported, the aforementioned theories paved the way for research into some of the more likely scenarios for clownfish protection. These theories share one aspect in common—they regard the “immunity” of the clownfish to be a phenomena localized around the fish itself, and not related to any potential changes caused by the hosting clownfish. Another fairly common theme in the most prominent theories is the special attention they pay to the mucus coating of both clownfish and anemone.

One critical split in the prominent theories centers around the ideas of innate protection and acclimated protection. The acclimation theory held that when clownfish first begin the process of hosting an anemone, they were initially stung but were, after some time, able to swim unharmed in the tentacles. In contrast, the latter theory proposed that clownfish protection was innate, and that no acclimation period was necessary for the clownfish to host. In a study intended to evaluate these two theories, the results were mixed. Some anemonefish were innately protected from some species of anemone, while others were obligated to go through the “acclimation” process (Elliott & Mariscal, 1997). It should be noted that the specific causes of either innate or acclimated protection were not the focus of the Elliott & Mariscal study—later studies would examine the potential mechanisms for both innate and acclimated protection.

Perhaps the most promising and simplest theory is related to the thickness of the clownfish’s mucus coating. Using an optical technique known as Nomarski optics, it was shown that the average mucus thickness of anemone hosting fish (mostly Amphiprion) was significantly thicker than that of fish which did not host in anemones. Furthermore, no significant difference in mucus thickness was demonstrated between two fish of the species A. clarkii, one of which was currently in association with a host anemone and another which had not been in contact with an anemone for 5 months. This suggested that, “…in all instances the observed mucus layer had been largely if not wholly produced by the fish itself.” (Lubbock, 1980)

Lending to the “innate protection” idea, some researchers proposed the idea that clownfish employ a sort of “molecular mimicry” (Schlichter, 1976), in which the clownfish’s mucus contains chemicals which mask it’s presence from the host anemone. The alternative hypothesis, more in line with the “acclimated protection” theory, suggests that instead of the masking chemicals being already present in the clownfish’s mucus, the clownfish covers itself with the mucus of the anemone during the process of acclimation, thus effectively camouflaging itself from the anemone’s nematocysts. In a study conducted by Lubbock, a variety of interesting results were revealed. First, he found that “the mucus produced by clownfishes was chemically different from that of related species not found in sea anemone”. By treating samples of the anemone mucus, Lubbock attempted to cause denaturizing of any nematocysts inhibitors which might be present in the mucus. What he found was that the denaturized mucus elicited no reaction, just as before. This lead him to the conclusion that clownfish mucus, rather than containing inhibitors, actually lacked excitatory substances which most non-hosting fish did contain. Therefore the aforementioned chemical differences observed by Lubbock, may have been the lack of excitatory substances, rather than the presence of inhibitory substances (Lubbock, 1980). The possibility of acclimated protection via anemone mucus camouflage was studied in depth by Elliott et al. By measuring the abundance of antigens (these antigens are what allow the anemone’s mucus to distinguish “self” from “other”, thus stopping the nematocysts from firing on the anemone), Elliott et al were able to determine whether hosting and non-hosting clownfish contained a substantial layer of anemone mucus. What they found was that while the antigens reached very high levels in the surrounding sea-water, little was found in the mucus coat of the clownfish. Furthermore, Elliott remarked that it was unlikely that, “anemonefishes can incorporate large amounts of soluble antigens” (Elliott et al, 1994).

While these theories are by no means conclusive and still await further study, they do allow us to form a potential picture for clownfish “immunity”—namely that the clownfish is protected via its extremely thick non-excitatory mucus. The inconclusive results of the 1997 Elliott & Mariscal, (which found that some fish were innately protected from some anemones and that some had to acclimate) seems to imply that at least one of the variables responsible for the clownfish’s immunity varies across different clownfish species. New theories are still forthcoming, including one which theorizes that the anemone’s initial stinging actually creates a chemical change in the clownfish’s mucus, and thereby protects it (Lubbock, 1980). This and newer theories may eventually serve to close the gaps between the currently dissonant acclimation and innate protection theories.

References:

Arvedlund, M. & Nielsen, L.E. 1996. Do the anemonefish Amphiprion ocellaris (Pisces: Pomacentridae) imprint themselves to their host sea anemone Heteractis magnifica (Anthozoa: Actinidae)? Ethology. Vol. 102, pp. 197-211

Arvedlund, M., McCormick, M.I., Fautin, D.G., Bildsøe, M. 1999. Host recognition and possible imprinting in the anemonefish Amphiprion melanopus (Pisces: Pomacentridae). Marine Ecology Progress Series. Vol. 188, pp. 207-218

Caspers, H. 1939. Histologische Untersuchungen über die Symbiose zwischen Aktinien und Korallenfischen. Zool. Anz. Vol. 126. pp. 245-253.

Dunn, D.F. 1981. The clownfish sea anemones: Stichodactylidae (Coelenterata: Actiniaria) and other sea anemones symbiotic with pomacentrid fishses. Trans. Am. Phil. Soc. Vol. 71. pp. 1-115

Elliott, J.K., Mariscal, R.N., Roux, K.H. 1994. Do the anemonefishes use molecular mimicry to avoid being stung by host anemones? Journal of Experimental Marine Biology and Ecology. Vol. 179, pp. 99-113
Elliott, J.K., Elliott, J.M., Mariscal, R.N. 1995. Host selection, location and association behaviors of anemonefishes in field settlement experiements. Marine Biology. Vol. 122, pp. 377-389

Elliott, J.K., Mariscal, R.N., Roux, K.H. 1996. Ontogenetic and interspecific variation in the protection of anemonefishes from sea anemones. Journal of Experimental Marine Biology and Ecology. Vol. 208, pp. 57-72

Elliott, J.K., Mariscal, R.N. 1997. Acclimation or innate protection of anemonefishes from sea anemones. Copeia. Vol. 2, pp. 284-289.

Elliott, J.K. & Mariscal, R. N. 2001. Coexistence of nine anemonefish species: differential host and habitat utilization, size and recruitment. Marine Biology, Vol. 138, pp. 23-36

Ertman, S.C. & Davenport, 1981. Tentacular nematocyte discharge and “self recognition” in Anthopleura elegantissima Brandt. Biological Bulletin. Vol. 161, pp. 366-370

Fautin, D.G. 1986. Why do anemonefishes inhabit only some host actinians. Environmental Biology of Fishes. Vol. 15, No. 3, pp. 171-180

Fautin, D.G. 1991. The anemonefish symbiosis: what is known and what is not. Symbiosis. Vol. 10, pp. 23-46

Gudger, E.W. 1946. Pomacentrid fishes symbiotic with giant sea anemones in Indo Pacific waters. J. Roy. Asiatic Society of Bengal, Science Vol 12. pp 53-76

Miyagawa, K. 1989. Experimental analysis of the symbiosis between the anemonefish and sea anemones. Ethology. Vol 80, pp.19-46

Lubbock, R. 1980. Why are clownfishes not stung by sea anemones? Proceedings of the Royal Society of London. Vol. 207, pp. 35-61

Mebs, D. 1994. Anemonefishe symbiosis: vulnerability and resistance of fish to the toxin of the sea anemone. Toxicon. Vol. 32, No. 9, pp. 1059-1068

Moser, J. 1931. Beobachtungen über die Symbiose von Amphiprion percula (Lacepede) mit Aktinien. Sitz. Gesell. Naturforsch. Freunde Berlin Vol 2. pps. 160-167

Murata, M., Miyagawa-Kohshima, K., Nakanishi, K., Naya, Y. 1986. Characterization of compounds that induce symbiosis sea anemone and anemone fish. Science. Vol. 234, pp. 585-587

Pantin, C.F.A., 1942. The excitation of nematocysts. Journal of Experimental Biology. Vol. 19, pp. 294-310

Saville-Kent, W. 1893The Great Barrier Reef of Australia: its Products and Potentialities. W.H. Allen and Co., London. pp. 387

Schlichter, D. 1976. Macromolecular mimicry: substances released by sea anemones and their role in the protection of anemone fishes. In, Coelenterate ecology and behavior, edited by G.O. Mackie, Plenum Press, New York, pp. 433-441

Verwey, J. 1930. Coral reef studies. I. The symbiosis between damselfishes and sea anemones in Batavia Bay. Treubia. Vol. 12. pp. 305-366

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