Introduction   

In recent years, coral diseases have increased in frequency (Peters 1997, Harvell 1999, Green and Bruckner 2000, Harvell 2001, McClanahan 2002, Sutherland 2004). Many diseases which affect corals have been documented (Black Band Disease (BBD), Antonius 1981a; Shut-Down-Reaction, Antonius 1977; microbial infection, Ducklow & Mitchell 1979; White Band Disease (WBD), Antonius 1981a, Gladfelter 1982), but for most of the diseases, their etiology is not fully understood.

Figure 1: Aquarium corals such as this Hydnopora sp. are regularly affected by a condition commonly referred to as Brown Jelly disease. Removal of decaying tissue, in combination with iodine dips, may halt the progression of this fatal condition (photograph: archive). 

The first protozoan coral-killer was identified in the Indo-Pacific (Antonius and Lipscomb 2000).  The causative agent was a folliculinid ciliate known as Halofolliculina corallasia. The effect of this disease is similar to other "band" diseases, such as BBD or WBD (Antonius 1985b). This disease is known as Skeleton Eroding Band (SEB) caused by an advancing mass of ciliates resulting in tissue loss and damage of the coral skeleton. Winkler et al. (2004) also reported this disease caused by the same ciliate species from the Gulf of Aqaba and later by Croquer et al. (2006) from the Caribbean. In all the above cases, the infection in the corals was caused by the ciliate Hallofolliculina sp.

"There have been reports of Brown Jelly disease in the aquarium, and its causative agent is believed to be a protozoan belonging to the Helicostoma genus."

Ciliates in aquaria

A common problem aquarium hobbyists face in their aquarium tanks is “Brown Jelly disease” which results in sloughing off of the coral tissues and eventually death of the coral (fig.1). This phenomenon develops randomly in coral pieces that have brown slime or brown stringy matter floating over them, while being attached to the corals. The slimy material seems to float above the coral, and often this disease and its result are hidden in between the coral’s tentacles. This continues for a period of time, until polyp loss and other damage to the polyp(s) has, or may have occurred. This disease has been observed in many corals including Montastraea sp., Acropora sp., Goniopora sp. and Mushroom corals. This disease is hard to avoid unless the hobbyist inspects the corals in the tank daily, to ensure the tank is clean and nothing is growing or deteriorating between the tentacles or on the coral. This disease is often followed by the onset of bacterial infectionS and the ultimate demise of the coral.

There have been reports of ciliate infections in corals (Montastraea cavernosa) kept in the aquarium and its causative agent is believed to be a protozoan belonging to the Helicostoma genus. The infection results in the formation of a jelly-like tissue mass on the infected corals (Borneman 2001, Borneman and Peters 2003) and is termed brown jelly infection.

Figure 2: Micrograph of three ciliates (Class Oligohymenophora, Subclass Scuticociliatia) containing ingested zooxanthellae. These remain photosynthetically competent until they are degraded by the ciliates (photograph: Dr. Shashank Keshavmurthy).

Ciliates in the field

In 2004 (Bourne et al. 2004, Willis et al. 2004) reported a new syndrome affecting Acropora sp. on the Great Barrier Reef and coined it Brown band syndrome. During this disease loss of tissue from the coral skeleton occurs, which is preceded by a narrow brown band. The disease was found to be restricted to Acropora sp. in the Great Barrier Reef. Colonization of ciliates (Class Oligohymenophora, Subclass Scuticociliatia) in/on coral colonies often resulted in the death of the coral host (Willis et al. 2004; Boyett 2006). These ciliates were seen engulfing zooxanthellae (figs. 2 and 3), causing the typical brown band appearance.

Ulstrup et al. (2007) have noted that zooxanthellae which are engulfed by ciliates during the infection process remain photosynthetically competent, and do not become compromised during the progression of the brown band zone. Thus, the question remains what happens to the zooxanthellae. Observation by our group has shown that the zooxanthellae present inside the ciliate cell are eventually degraded (fig. 4). There have been reports of similar infections caused by ciliates in branching corals from Okinawa (pres. comm. Dr. Hidaka, Ryukus University, Japan). In addition, tissue sloughing and coral death during the summer months has been reported from Akajima, Japan in corals belonging to Acropora sp. (pers. comm. Dr Omori, Akajima Marine Laboratory, Okinawa, Japan).

 Figure 3: Micrographs of several ciliates having engulfed zooxanthellae. Recent research has shown that these symbiotic algae are eventually degraded by the ciliate protozoa (indicated by black circles). Scale bars in middle and right photos are in micrometers (photograph: Dr. Shashank Keshavmurthy).

Fresh water, a source of ciliates?

A study has shown Helicostoma sp. to be of freshwater origin (Ishida and Ishibashi, 2006). Till date, brown band caused by ciliate infection has only been reported in the Great Barrier Reef and from some sites in Japan. Most of these reported locations have fresh water input into the sea. Also, most of the aquarium hobbyists use artificial seawater prepared from fresh water for their aquarium tanks. Hence, it is possible that the source of these ciliates is fresh water. Although ciliates which were isolated from different locations in Japan have not been identified yet (fig. 5), it is possible that they are different species than those found on the Great Barrier Reef, as indicated by their morphological features.

"It is possible that the source of ciliates is fresh water."

Conclusions    

Coral ciliate infection is mostly observed in aquarium tanks, resulting in coral losses, and is a major problem as there is currently no means to prevent it. However, in the field this infection is not so common and there still remain questions to be answered, such as what causes the corals to become susceptible to the ciliate attack, how the ciliates are able to dissolve the tissue layer and what happens to zooxanthellae after they are engulfed by the ciliates. It could very well be that fresh water is the main source of these illusive microorganisms..

References

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