How the oceans get sickMarine Infectious Diseases
Marine diseases in Thailand are a major focus for marine biologists and infectious diseases have been recognised as a threat of increasing significance to the marine world, with infections reported at every trophic level of marine ecosystems, from producers such as seagrasses and corals (Rosenberg and Ben‐Haim, 2002), to apex predators like cetaceans (Pasamontes et al., 2017). Since coral diseases first being reported in 1973 (Pollock et al., 2011) a large number of outbreaks of coral disease have been reported over the past decades with nearly 8% of all literature reporting disease published between 1999-2001 being on the subject of coral diseases and bleaching (Ward and Lafferty, 2004). Despite this, little is known of the causal agents of infectious marine diseases and increasing evidence shows climate change to be a driving factor.
Coral is a fundamental part of the marine ecosystem here on Koh Tao and is comparatively more affected by marine disease than other parts of the community. Some diseases we witness when corals are under stress are the pink spot syndrome on the surface of Porites corals, which can be caused by a number of burrowing trematode larvae which cause physical damage and facilitate predation by other species. White band syndrome is also seen to affect acroporid corals in the Gulf of Thailand, which manifests itself in a white band that can be as large as 10 centimetres wide and work its way from the base of the coral to the tips, causing tissue loss as it travels across the colony.
Big Blue Conservation actively monitors our local waters, dive sites, and coral reefs for signs of infectious disease and works with local marine authorities to mitigate and stop disease outbreaks.
These are the infectious marine diseases we face here on Koh Tao:
White Syndrome (WS), not to be confused with White Patch Syndrome (WPS), is a general term for scleractinian coral diseases that present themselves as advancing tissue lesions normally from the base of the coral, which slowly affects the whole colony resulting in vast mortality. As these lesions spread, the tissue turns white as the symbiotes are expelled, hence the name: White Syndrome.
For a time, the scientific community didn’t know what the causal agent of these diseases was, and even if one pathogen or many were responsible. In 2003, Vibrio coralliilyticus was identified as a bacterial pathogen that bleaches coral and in 2008 that a study based in Australia by Sussman et al revealed a further group of vibrio bacterial pathogens responsible for the outbreaks. Bacterial isolates were collected from corals displaying signs of white patch syndrome and then cultivated in the lab on living corals. Both studies found that in higher temperatures, the probability of successful infection was higher – showing how climate change plays a vital role in driving these infections and ultimately widespread mortality in coral reef systems,
It is important to note that the presence of the bacterial pathogen on diseased individuals does not correlate with causation, however, such an association between signs of disease and pathogen presence meets the requirements of Evan’s rule A: ‘‘prevalence of the disease should be significantly higher in those exposed to the putative cause than in cases controls not so exposed’’.
If white syndrome, along with other common coral and marine diseases, were to, with the driving force of global climate change, facilitate the destruction of coral reefs globally the knock-on effects would be catastrophic. Despite reefs making up only an estimated 0.1% of the Earth’s surface, they are thought to be home to as much as 25% of all marine species. Without coral reefs, ecosystems would be shaken, and we’d be sure to feel the effects in our own lives.
Although the signs of disease may appear to be similar, it is important to try not to confuse white syndrome (WS) and white patch syndrome (WPS). White patch syndrome differs in that this disease usually only affects Poritid corals and manifests as discrete patches of white tissue or tissue discolouration as opposed to a large wave of discolouration across the whole colony.
White patch syndrome has been affecting Poritid corals for decades, however, it is in recent years where we have seen an increase in sea temperatures that we have seen a greater prevalence of this disease. It is now thought that WPS is driven by a viral pathogen, with some studies finding that tissues affected by WPS have a greater load of virus-like particles.
Further investigation of these affected tissues has also revealed a much lower chlorophyll A content, but notably, polyps and tissues remained intact. This indicates that WPS is potentially driven by this viral agent only affects the symbiotes within coral tissues, rather than the polyps themselves (Lawrence et al 2014).
This is still highly dangerous, as, without their photosynthetic symbiotes, Poritid corals will undoubtedly perish over time. Like other coral diseases, WPS is correlated with stressors such as increased water temperature and it is predicted that the prevalence of WPS will increase with global climate change.
Ulcerative skin disease is a term that encompasses any skin diseases that manifest as patches of necrotizing tissue or ulcers on the surface of the skin and can be caused by a variety of different causal agents. Like lots of other diseases, they usually emerge when stress is put on the host species and this can be through increasing sea temperatures, temperature fluctuations, reduced prey availability or pollution. Ulcerative skin disease has reportedly broken out in the Galapagos Islands as well as Mexico when sustained periods of high sea temperatures were experienced, and this is hypothesized to be due to a cumulative effect of immunosuppression of the host as well as hotter temperatures enhancing the virulence of pathogens such as bacteria already present in the environment. This immunosuppressive effect of stressors allows bacteria to take hold, and infect the skin and tissues of the host fish, Ulcerative skin diseases in some species of reef fish in the Galapagos have caused population declines of as much as 86%, and hence the predicted knock-on effects on reef ecology are immense.
Ulcerative skin diseases in teleost fish are yet to be investigated in the Gulf of Thailand however we are keeping a close eye on the fish communities on the reefs in Koh Tao to identify and monitor any potential outbreaks.
Pink spot syndrome (PSS), Podocotyloides stenometra, is a bit different from other coral diseases in that it is linked to a multicellular pathogen rather than a bacterial or viral agent. PSS manifests as small, discrete pink lesions on the coral, or swollen pink polyps which are both signs of the coral reacting to the burrowing trematode Podocotyloides stenometra and is most likely to affect Poritid corals. Podocotyloides stenometra is a flatworm that has a rather complex life cycle with three life stages. In the first life stage, Podocotyloides stenometra has an intermediate mollusc host and then the second metacercarial stage encysts in the tentacles of the coral polyps. The lifecycle is complete when a fish host, usually the butterflyfish, ingests the infected polyp and the trematode can live out its life within the fish.
During its second life stage, this parasite can do considerable damage to poritid corals and even stunt growth by up to 50%. Any factor that puts a host under stress will increase their susceptibility to infection, as well as enhance the parasite’s ability to launch an infection, so it is likely that with global climate change acidifying and heating the oceans we will see more pink spot disease, and other marine diseases, on tropical reefs.
At Big Blue Conservation, we are keeping a close eye on signs of marine diseases at every one of our dive sites through our reef monitoring protocols; read more about how we survey the reefs here.