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A Review of “Diseases in coral aquaculture: causes, implications and preventions” (Sheridan et al. 2013)
Written by: Keith P. Kovatch
R2R name:Keko721
The following is a review of the article written by Sheridan et al. (2013) which takes an in depth look at the various types of aquaculturing that is taking place for the research and commercial distribution of corals, with an emphasis on the occurrence and treatment of disease within these cultures. Whether you are a simply an aquarium hobbyist or a full on researcher in the field of marine biology and coral propagation the review of literature in this article is very useful and provide insight for keeping corals in our own tanks healthy.
The demand for corals is steadily increasing along their economic value. In addition to the commercial value for hobbyists and reef keepers, corals are cultivated for the purpose of researching their application in natural products (Leal et al, 2012; Shafir et al.2006), general research for understanding basic biology and lifecycles, and of course restoration of wild reef systems, which not only have a natural value as an integral part of our planetary ecosystem, but also a monetary value with respect to tourism (Sheridan et al., 2013). By obtaining a better understanding of corals in cultured samples, we are better able to understand the external factors leading to their mortality, how to mitigate or treat them, and grow corals that may even be hardier when reintroduced to the wild.
Aquaculturing, or the growth and propagation of corals in controlled environments or captivity serves the purpose of decreasing stress on the natural reefs where many of these species are harvested. For my own part, I was intrigued to learn from this article that there are various methods for aquaculturing coral. Corals are typically cultured in situ (open) or ex situ (closed) systems. Open systems are actually a form of mariculture in which corals are grown in their natural environment near local reef systems. Although this nearly replicates the natural ecosystem where corals grow, the negative external factors such as predation, pollution, storms etc. can make research more difficult. Thus closed systems- cultures outside the corals natural environment- are implemented. Sheridan et al. (2013) mention two types of closed cultures: Flow-through aquaculture systems (FTAS) in which nearby seawater is actually pumped through the system, and recirculating aquaculture systems (RAS) which are ostensibly coral aquariums much like we create in the reefing hobby. Despite water filtration and sterilization that occurs in both FTAS and RAS cultures, diseases that afflict corals may occur in both.
“A disease is defined as any impairment to cells or tissues of an organism that results in its dysfunction (Stedman, 1976). A diseased state involves an interaction between an organism, its environment and a disease agent (biotic or abiotic) (Stedman, 1976). Diseases may be either communicative, spreading the agent from one individual to another (e.g. tuberculosis) or not (e.g. cancer). In corals, a number of diseases have been described and classified morphologically, though for most of them the infectious agent is still unknown” (Sheridan et al., 2013).
Prior to delving into the actual diseases that can afflict corals Sheridan et al. (2013) discuss the natural defenses corals have against the same. Defense and immunity corals possess is an area of research that has gained considerable interest. Unlike other animals, corals do not have a traditional adaptive immune system, although they do have a number of defense mechanisms. In your own tank, you may have noticed corals responding to stress by production of mucus in an attempt to “trap” and discard unwanted microorganisms. Additional defense mechanisms include melanin syntheses, production of antioxidants, amoesbocytes, and antibacterials (Palmer et al., 2008; Mydlarz et al. 2008; Doens et al., 2009a). It also appears that corals with faster growth and reproductive rates (such as acropora) are more susceptible to disease than their slower growing counterparts (Palmer et al. 2010). This may be in part due to the faster growing corals expending more energy on growth and reproduction than growing antibacterials or bolstering their immune systems (Palmer et al., 2008).
Due to the ambiguity of infectious agents, and the similar morphology and symptoms of these diseases, it can be very difficult, if not impossible in some cases, to differentiate the cause of sickness in coral colonies. We do know that the majority of coral diseases are causal of symptoms such as changes in coloration such as bleaching or spotting, tissue loss, and necrosis (Sheridan et al., 2013). The best hypothesis as to how coral disease is introduced into a closed system is that the relevant pathogens have hitchhiked into the system, and/or undue stress in the transport and handling of the corals may have been a contributing factor of the affliction. Furthermore, the identification of the origin of such pathogens is also a difficult. In many cases, there may be no original source, as it appears many diseases have been found in cultures of corals with seemingly healthy tissue. The manifestation of pathogens in healthy coral seem to be associated with environmental and anthropogenic factors, biological factors, or both. The primary stressors associated with coral disease are often correlated with change in water temperature, water quality degradation, coral eating predators, and genetic disposition (Sheridan et al., 2013).
“Thermal stress (higher than normal seawater temperature) is by far the most referred to of all environmental stressors and has often been described to be a determining factor in facilitating coral disease development and infection by pathogens (e.g. Bourne et al., 2009; Harvell et al., 1999; Kuta and Richardson, 2002; Ward et al., 2007). The effect of thermal stress as a catalyst for coral disease development is particularly likely when coral cover is high (Bruno et al., 2007), as is expected in coral mariculture facilities. Temperature stress can increase chances of coral disease development in two ways: coral olobiont stress (and decreased coral resistance to infection) and/or increased growth and virulence of opportunistic coral pathogens. For example, it was established that the virulence factors associated with infections of Oculina patagonica by V. shiloi (in the Mediterranean) and of Pocillopora damicornis by V. coralliilyticus (in the Red Sea and Indo-Pacific) are triggered by elevated seawater temperatures (Ben-Haim et al., 2003; Kushmaro et al., 1998; Toren et al., 1998). Furthermore, increased temperatures have been shown to decrease the production of antimicrobials by symbiotic bacteria in the coral mucus, thereby facilitating the growth of opportunistic and potentially pathogenic bacteria (Ritchie, 2006). However, Ritchie (2006) could not resolve whether a temperature-dependent growth of the population of opportunistic bacteria reduced the population of symbiotic bacteria and consequently the production of antimicrobials, or a temperature-dependent shutdown of antimicrobial production by symbiotic bacteria allowed the development of opportunists. Later work by Shnit-Orland and Kushmaro (2009) supports the theory of temperature-sensitive antimicrobial compounds suggesting that both the quantity produced and their stability decrease past a specific threshold.” (Sheridan et al. 2013)
The take away here is that temperature fluctuation in our own tanks does not merely stress our coral inhabitants, but in fact makes them more prone to disease as they are unable to employ their natural defense mechanisms in these conditions.
Water quality, as we all know, plays a significant role in the health of corals within our system. While some corals may be more sensitive to water chemistry fluctuations than others, all have their own particular threshold. Sheridan et al. (2013) go on to explain that the true culprit with respect to water degradation is in fact the amount of dissolved and particulate organic carbon nutrients (phosphate, nitrates and ammonia). These levels are often increased in nature by terrestrial run off, and heavy rainfall leading to increased sedimentation in the water column. Additionally, pollution including human sewage has been directly linked to pathogens causal of a variety of bacterial coral diseases.
“Finally, it has been argued that many coral diseases are the result of an increase in abundance of specific opportunistic coral pathogens following changes in environmental parameters (Lesser et al., 2007). Though not subjected to natural or anthropogenic modifications of the environment, closed systems could nevertheless be affected by changes in environmental conditions (e.g. temperature and nutrient loadings) through certain unplanned events such as equipment dysfunction. Such environmental changes have been shown to cause shifts of the coral microbial population from mutualistic/commensal to potentially pathogenic and opportunistic (Vega Thurber et al., 2009).” (Sheridan et al. 2013).
An additional source of coral disease are predators, which feed on coral tissue. Corallivorous snails, worms, starfish, other invertebrates, and even some species of fish can be carriers of diseases such as Brown Band Syndrome, Black Band Disease, White Syndrome and White Band Disease (Sheridan et al. 2013).
One surprising source of the spread of potential disease may in fact be rooted in out attempts to culture colonies and reintroduce them into the wild. The process of harvesting corals for the aquarium trade involves several steps from the time the coral is actually harvested, packaged, shipped, released for wholesale, and finally sold to the consumer. While governmental regulations are in place to monitor coral trade practices, Sheridan et al. (2013) go on to say that, there is room for improvement regarding these policies so as to avoid disastrous incidents such as the examples below:
“It has been argued that poor management both in exportation, importation and regulation in the animal trade has led to some widespread ecological problems. This is particularly the case in the aquarium industry. Two prominent cases include the release of two species of lionfish (Pterois volitans and Pterois miles) from aquaria in the Caribbean and the spread of the amphibian fungus Batrachochytrium dendrobatidis, which causes Chytridiomycosis (Gahl et al., 2012; Gründler et al., 2012; Rasconi et al., 2012). Zoos, public and private aquaria and hobbyists around the world have been partially implicated for the significant spread in these particular cases, either accidently by releasing captive animals by good natured people or simply by unwittingly transporting diseased animals from site to site (as is thought to be the case with the Chytrid fungus) (Der Sluijs et al., 2011; Lannoo et al., 2011; Puschendorf et al., 2011). Although no coral species are yet officially considered exotic/invasive species, they may also arise several concerns. The corals Tubastraea coccinea and Tubastraea tagusensis, both originally from the Pacific Ocean, were introduced in Brazil in the 1980s and are threatening local benthic biodiversity (Lages et al., 2011). Coral diseases associated with these introduced species have currently not been reported, however the possibility remains that pathogenic organisms (responsible for certain coral diseases) may have also been transported around the world in a similar manner. Indeed the white diseases (white plague and White Band) found throughout the Caribbean are a relatively recently phenomenon, first being reported in the early 1970s (Aronson et al., 2005; Bythell et al., 2004; Gladfelter, 1982) and spreading rapidly with very high mortality rates following their introduction. (Sheridan et al. 2013).”
The author goes on to state that within the trade more information and educational material should be provided with respect to keeping and breeding corals that are particularly prone to certain parasites and pathogens that could potential spread. Quarantine procedures, which should in theory, be implemented for a minimum of 30 days after the coral is harvested, are often ignored, as they are not mandated by government agencies, and comes at a cost for wholesalers who wish to export their product as expeditiously as possible. Implementation of better coral trade practices may facilitate in rooting our diseased organisms from the start before they reach the hands of consumers.
Although quarantine of corals can reduce the likelihood of the spread of pathogens to closed systems, the process is not a guarantee. Chemical prophylactic treatments are often used in the quarantine or corals, but may have issues of their own. The presence of pathogens or disease may go unnoticed until stress of the coral induces and environment for that pathogen to grow and replicate. Thus, corals that are quarantined with prophylactic treatment may be assumed to be healthy, but in fact are carriers of pathogens that will only manifest when conditions are right. For this reason it is recommend that these treatments only be used once the presence of a pathogen has been determined (Sheridan et al. 2003). As mentioned previously, changes in environmental conditions such as temperature or water quality may act as the catalyst that is necessary for disease to become present.
Keeping a culture or aquarium free of disease and pathogens is simply not realistic. These organisms are found both within the natural environment of corals, as well as closed systems. However, their opportunistic nature combined with the right conditions is what will cause them to thrive- much in the same way that dinoflagellates are present in every aquarium, we only see the catastrophic outbreak when nutrients have been depleted along with any competition and biodiversity. Sheridan et al. (2013) note that within aquacultures, best practice may be to separate specific species into isolated systems to prevent disease transmission from one coral type to another. Unfortunately, this practice assumes higher costs of maintenance. At the hobbyist level, it is unlikely that anyone would employ this practice. However, the recommendations by Sheridan et al. (2013) coincide with what many of us are taught upon entering this hobby: good filtration including mechanical filters, biological filters (live rock and bacteria), as well as chemical filtration (activated carbon); sterilization of incoming water via ultraviolet (UV) light or ozonation; good quarantine practices; no using prophylactic treatments until pathogens or disease is confirmed (this does not include dipping your corals which is considered antiseptic rather than prophylactic); and routine testing and maintenance of water parameters with particular emphasis on nutrient concentrations.
The research on treatment of diseased corals is still relatively new, although many studies are currently being carried out. Many of the remedies for sick corals seem to be first implemented and tested by hobbyists, zoos and public aquariums- examples of which include iodine supplementation, fresh water dips, and antibiotic treatments. However, several remedies appear to have the greatest efficacy: fragging, use of pro-biotic bacteria, and phage therapy.
The diseased portion of the coral may be cut off from the rest of the colony and removed from the tank. While this may work in some cases, it may not alleviate the system as whole of the pathogens that contributed to the disease in the first place (i.e bacterial infections). I have personally found this method beneficial with respect to zoanthid colonies that have begun to melt- by removing the melting portion from the colony and only leaving healthy, open polyps the colony usually survives and no further melting occurs.
Ongoing testing is taking place with the use of pro-biotic bacteria, which may aid corals by blocking pathogens and assisting in the production of antimicrobials. Research conducted by Teplitski and Ritchie (2009) suggests that treating corals with beneficial bacteria in an isolated setting could protect them against such pathogens (Sheridan et al., 2013).
Finally, phage therapy has become an area of interest with respect to treating diseased colonies. This process involves using lytic bacteriophage viruses to target the pathogens causing the disease. While these viral treatments are highly specific in targeting the desired pathogens with little risk to beneficial bacteria, it may only be attacking part of the problem if the disease is caused by more than one pathogen.
“… phage therapy has been applied successfully in the case of two coral diseases, 1) bacterial bleaching of P. damicornis by V. coralliilyticus (Efrony et al., 2007) and 2) white plague disease of Favia favus caused by Thalassomonas loyana (Atad et al., 2012; Efrony et al., 2009). These studies provide the only strong evidence linking specific causal agents to certain diseases/syndromes to date. Furthermore, Atad et al. (2012) took this process to the next level and were the first to successfully prevent both progression and transmission of a coral pathogen (T. loyana in this case) within the natural environment, a result showing high promise for the future treatment of coral diseases in both mariculture and ex situ aquaculture (Sheridan et. Al, 2013).
While this article is keenly focused on prevention and treatment of diseases in aquaculture, one can appreciate a better understanding of the defense mechanisms corals possess in fighting pathogens, as well as the importance of how the environments we create for these animals directly affects them. Furthermore, the review of this literature points to the importance of regulation in the steps taken in harvesting and aquaculturing corals, such that we can prevent the spread of further disease not only on the hobbyist level, but in our restoration of wild reef systems.
Citation
C. Sheridan et al. / Aquaculture 396–399 (2013) 124–135
Cover image from the Australian Institute of Marine Science
