Dose of Disease: The vector sector

The ocean is a web of interactions – who eats whom, who is friends with whom, who acquires infection from whom (WAIFW – a real live acronym). It’s a whole community of relationships that’s almost as complicated as a single middle school.

A disease ecologist wants to consider the importance of all the relationships between organisms. But you must start at the very beginning to figure out where the infection is coming from. That includes:

Step #1)    What is causing the infection? Is it a bacterium, a virus, a fungus…?

Step #2)    What is the mode of transmission?

Transmission! In some ways it’s the holy grail of disease ecology. Research on coral diseases has yielded some information on transmission, but in most cases coral biologists are still on step #1. We do know the etiology of some diseases, such as the bacterium that causes bleaching in Oculina patagonica and the fungus associated with sea fan Aspergillosis.  

As for step #2, there are several ways for diseases to be transmitted from a sick host to a healthy host and more than one can be happening at the same time. Here are the possible modes of transmission put in terms of of human disease:

1)    Contact – Ex) You come in direct contact with someone with a virus or their possessions

2)    Airborne/ waterborne – Ex) Flu particles float through the air and you inhale them

3)    Vectored – Ex) A mosquito bites you and transmits the malaria parasite from its salivary glands

We think about all these possibilities when we’re doing our disease surveys of sea fans in Puerto Rico. Sea fans may be in direct contact with each other, but pathogens may also be drifting through the seawater or traveling in biting organisms. Today I’ll focus on vectored diseases.

Disease vectors play an important role in both terrestrial and aquatic outbreaks. Some of the classic examples are mosquito-borne diseases like West Nile virus, malaria, and dengue fever. Some vectored diseases infect multiple host species, like Lyme disease. One of the reasons that disease ecologists find Lyme disease so fascinating is because it involves so many members of the biological community – the ticks, the bacterium (Borrelia burgdorferi), the humans, the mice, the deer… many organisms influence the disease dynamics!

In corals, research on disease vectors has linked sea snails to brown band disease and white band disease. Fish have also been raised as suspects in some disease mysteries. In sea fans specifically, research by past Harvell lab members suggests that flamingo tongue snails contribute to transmission of a fungal pathogen. Of course, organisms found on corals aren’t necessarily transmitting diseases to the corals, and in fact they may be lessening the impact of disease.

Knowing this, how could we not look for other organisms in our disease surveys of sea fans? No matter what they’re doing, our goal is to closely monitor each colony for signs of interactions, such as predation. 

In some cases we’re lucky enough to catch the culprits in the act! I excitedly snapped each of the photos above in just such a moment. The first photo is of a group of crinoids nestled in a pocket they’ve formed out of the flexible sea fan. Are they passing through or making a more permanent home? Are they eating sea fan tissue, or eating organisms off the surface that might otherwise be detrimental to these corals? The second photo captures two flamingo tongue snails that are most certainly in the midst of a sea fan feast.

We’re currently more than half way done with the field surveys. In the past two weeks, we’ve recorded data on 11 different sites! Each of the sites requires a long dive full of methodical planning and rapid underwater writing. But the reason for all this scribbling – even in the midst of being tossed into stinging fire coral by waves – is that we want to be able to ask questions about the role of environmental factors and the other organisms on the reef. We want to collect data on many different puzzle pieces to further our understanding of how coral diseases play out in natural populations.

With that perspective, the crinoids, fireworms, and snails are not simply interesting behavioral observations, but key contributors to the reality of coral diseases. No sea fan is an island.


With special thanks to my funding sources for this project:

National Geographic Young Explorers, Young Explorers Grant
American Academy of Underwater Sciences, Kathy Johnston
Betty Miller Francis ’47 Fund
Cornell Graduate School, Research Travel Grant 
Atkinson Center for a Sustainable Future, Sustainable Biodiversity
Paul P. Feeny Graduate Research Fund      
Sigma Xi, Cornell chapter