Pathogens & Opportunists
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While I certainly don't enjoy seeing bad things happen in my tank, I hope to be able to show different parasites and pathogens sooner or later (hopefully sampled from another tank and not my own
).
A hard truth of this hobby is that we don't always know what happened to a polyp, or whole coral, that caused it to suffer tissue loss or death. All we get to observe is what is left
after the fact. This distinction matters, because opportunistic organisms move fast and are often already present by the time we manage to take a sample. In other words: whatever you see on a dying fish or coral, you can't be sure it's the primary cause. It's entirely possible it's simply helping break down tissue, or feeding on other organisms that are doing so.
Below are some ciliates that I find on dying tissue whenever two corals start stinging each other. In this case, I found them on a zoa polyp:
(Ciliates with previously consumed zooxanthellae of a zoanthid in their body)
Some ciliates are known to be associated with coral diseases, such as
Philaster lucinda and
Helicostoma notatum, both found in brown band disease samples from the Great Barrier Reef. The ciliates pictured above share a similar shape to
P. lucinda, though shape alone isn't enough to identify them.
There were, of course, other ciliates present as well. They resemble
Cohnilembus sp. but I'm not sure either. Interestingly, both
Philaster and
Cohnilembus belong to the same subclass,
Scuticociliata. I've never seen these ciliates carry any
Symbiodinium sp. zooxanthellae in their bodies, so I can only assume they have another reason for being there.
(Ciliates near the decaying tissue)
Bacteria are present in abundance too, but they are too small for light microscopy to identify them with any confidence. Even 1000x magnification wouldn't change this as we are hitting the physical limit of what visual light can resolve.
(Bacteria around zooxanthellae)
None of this tells us what started the problem, and that's kind of the point I'm trying to make. Finding ciliates or bacteria on damaged tissue is normal and doesn't automatically mean you've found the primary pathogen.
). Bright-field microscopy alone is often not sufficient and you need access to phase contrast or better differential interference contrast (DIC).
I knew I saw it somewhere before. Interestingly the two gaps in the shell were present as well and I'm reasonably sure that I didn't squish the specimen. That means this might be part of its shell.
