Bacteria and Viruses
Very little research has been done on the types of bacteria and viruses that can afflict corals. This is unfortunate as it is becoming o increasingly clear that in areas such as the Caribbean these organisms are potentially more devastating to coral reefs than the mechanical effects of storms and humans (Shinn, 1989). The research on bacteria and viruses affecting bivalves, especially mussels and oysters, is much better. This is solely due to the fact
that they represent billions of dollars in cash crops. Until recently, very few studies had been done on pathogens affecting tridacnid
clams (see Norton et al., 1993a; Norton et al., 1993b and; Sutton and Garrick, 1993). Hopefully, more research funds will be forthcoming for work on both coral and tridacnid clam diseases as their potential impact on local and national economies becomes better a p p re ciated.
Certain coral diseases such as White Paste, White Band and Black Band Disease (see trouble-shooting section) may be caused by pathogens that are opportunistic and only become plentiful when some other stress has occurred. For example, physical damage caused by mechanical injury or predation may expose the underlying tissue and skeleton to infectious agents. Increased water temperature, poor water quality, excessive LTV light, coral stings and lack of trace elements can also weaken corals, making them more susceptible to infections.
In tridacnid clams, a large number of bacteria, both pathogenic and non-pathogenic, have been isolated (see Humphrey et al., 1987). Among these, Vibrio algi no lytic us and V. a ngi t ilia ru m ha ve been associated with mortalities in larval and adult oysters (Humphrey, 1988). However, these organisms have commonly been isolated from healthy clams too. Bacterial infections have also been implicated in mass mortalities of larval cultured tridacnid clams in Australia (see Humphrey, 1988). Antibiotic treatments with streptomycin, neomycin, penicillin, and/or rifampin have been tried, and combinations of streptomycin and neomycin proved most effective for increasing survival of tridacna clam larvae (Fitt et al., 1992). (This combination may also prove helpful for treating corals.) At the present time no viral agents have been isolated from tridacnid clams (Humphrey, 1988).
The biggest problem with bacterial and viral diseases is that they are very difficult to identify and treat. Many treatments can severely stress the other inhabitants and are therefore of limited use in the reef aquarium. In the next section we will offer some treatments that have proven effective in many cases of bacterial coral infection. The study of bacterial and viral diseases is definitely an area where the captive maintenance of corals and clams can play a useful role.
"Brown jelly" Helicostoma infection on a Euphyllia glabrescens. J.C. Delbeek.
Helicostoma sp. protozoan. Note the ingested zooxanthellae. J. Yaiullo.
Phylum: Protozoa
Common Names: Ciliates, Protozoans
Like bacteria and viruses, many of the protozoans found in corals and clams occur naturally. In aquariums their numbers only increase in response to a decrease in health of the specimen. It is therefore possible that many of these protozoans are opportunistic feeders, not true parasites.
In corals, protozoal infections often take on the appearance of a "brown jelly". The most commonly encountered protozoa in these infections belong to the family Philasteridae such as Helicostoma nonatum, but others such as Euplotes spp. may be involved too.
Some of these protozoa may feed directly on the coral tissue, while others may feed on bacteria associated with the open wounds. This is explained more thoroughly in the troubleshooting section of this chapter.
In tridacnid clams, protozoa from the genus Perkinsus have been found in specimens collected throughout the Great Barrier Reef. However, their role in clam mortalities remains unknown. Additionally, an unidentified ciliated protozoan has been found that invades the mantle and ingests zooxanthellae (see Humphrey, 1988). We have no experience with protozoal infections in tridacnid clams and they do not appear to be very common in aquaria.
Most protozoan infections in corals are the result of some sort of physical injury. Once the tissue has been damaged, the protozoans can quickly appear on both hard and soft corals (see "brown jelly"
infection in the troubleshooting section). Protozoan infections can also occur as a result of the same adverse conditions mentioned above for bacterial infections. In the case of the Atlantic hard coral, Acropora cervicornis (Staghorn Coral), it was found initially that no matter how healthy a specimen was when collected, they always succumbed to protozoan infections in captivity. It was not until additions of a strontium chloride solution were begun that this coral survived for more than a few months ( J. Sprung, pers. obs.). With strontium additions this particular species is now easily kept in properly maintained closed systems, and grows very rapidly. This is an excellent example of the importance of an inorganic element to overall coral health.
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