Tunicate vs. Coral Ultimate Battle

UPDATE: Replaced stock photo with my own photo for maximum grossness.

Living on a coral reef is like living in New York. Space is at a premium, you get crammed into all kinds of peculiar nooks, and you’ve got to be tough to maintain your personal space. (As a short person on the rush hour subway, I learned that my elbows are very conveniently placed for making tall people move.) So diving on the reef in Curacao feels oddly familiar. I’m here to investigate the mysterious explosion of a colonial tunicate on the reefs of Curacao.

Trididemnum solidum has increased by 900% over the past 30 years (Bak 1996). On Curacao’s central leeward coast, these tunicates are everywhere, taking over the reef and smothering coral heads. Trididemnum is native to the Caribbean, so why is its populations exploding? And is the tunicate domination related to human disturbance?

Trididemnum is like the armored tank of benthic organisms – it photosynthesizes, eats bacteria, is very toxic to anything that eats it, and even has pointy spines in its tissue for extra protection (Bak 1981, 1996, 1998 ). However, Trididemnum’s kryptonite has been its dependence on its symbiotic algae – if the tunicate is shaded, it dies. Trididemnum cannot live on filter-feeding alone. (Citation? Randy Olson, 1986! He was a tunicate god in his past life!)

Since corals (this is the belated Coral Week connection) are supposed to be the dominant competitors in lovely nutrient-poor water, what has changed? Potential changes (with credit to Dr. Bak and various people around Curacao) are:

1) Something used to eat Trididemnum, but doesn’t anymore. This is not very likely since Trididemnum is incredibly inedible, what with the toxins and the pointy spicules. But perhaps the tiny little baby colonies used to get bulldozed by the urchins, and the urchins are now gone.

2) Trididemnum is getting a brand new food source, such as bacterial runoff from the land. Tunicates have a mucous-y feeding basket that can capture very small particles, while corals have to grab them with their tentacles. If there are lots of very small tasty bits floating about, maybe Trididemnum is gorging itself.

3) Corals are weaker than they used to be, and can no longer fend Trididemnum off. Trididemnum will not settle on live coral as a larva (Van Duyl 1981), but it can overgrow it. So, for example, when the corals are stressed by too-warm water, the tunicate has a coral-killing party.

So here I am in Curacao, working on methods to test these hypotheses. Since this is not really my trip (it’s all funded by my partner for her fish trap work) I don’t have enough time to do a full-fledged study. However, I’m hoping that my nuggets of data will be enough to provide direction for a full-fledged study, perhaps this winter.

While I am extremely fond of tunicates, even I must admit that they are not as attractive as corals. In fact, it’s rather depressing to see all these fetching corals smothered by something that seems to be more a part of Jeremy Jackson’s Rise of Slime than of Pretty Tropical Coral Reef Land. I really hope that this work will eventually lead to a bit more insight as to why these tunicates are winning the battle for space, and why the corals are losing.


Bak, R. P. M., M. Joenje, I. de Jong, D. Y. M. Lambrechts, and G. Nieuwland. 1998. Bacterial suspension feeding by coral reef benthic organisms. Marine Ecology-Progress Series 175:285-288.
Bak, R. P. M., D. Y. M. Lambrechts, M. Joenje, G. Nieuwland, and M. L. J. Van Veghel. 1996. Long-term changes on coral reefs in booming populations of a competitive colonial ascidian. Marine Ecology Progress Series 133:303-306.
Bak, R. P. M., J. Sybesma, and F. C. Vanduyl. 1981. The ecology of the tropical compound ascidian Trididemnum solidum. Part 2/3. Abundance, growth, and survival. Marine Ecology-Progress Series 6:43-52.
Olson, R. R. 1986. Photoadaptations of the Caribbean colonial ascidian-cyanophyte symbiosis Trididemnum solidum. Biological Bulletin 170:62-74.
Van Duyl, F. C., R. P. M. Bak, and J. Sybesma. 1981. The ecology of the tropical compound ascidian Trididemnum solidum. Part 1/3. Reproductive strategy and larval behavior. . Marine Ecology-Progress Series 6:35-42.


9 Responses to Tunicate vs. Coral Ultimate Battle

  1. jebyrnes says:

    Hrm – here are two ideas that come out of my filter feeding work with Didemnum vexillum
    1) Examine that bacterial content in invaded v. uninvaded areas. I used cytometry, but I’d imagine there are other assays to get the relative proportion of phytoplankton v. bacteria. FYI, I have some preservation methods for water samples if you have the ability to get some glutaldehyde and liquid N2 and then transport the frozen samples home for cytometric analysis.

    2) Alternately, incubate Trididemnum with water from invaded v. uninvaded areas to look at what it’s feeding on – but indirectly (this is more low tech) – more bacterial consumption should mean more NH4 excretion (I can email you some citations on this). It’s worked for me – Didemnum shoots out massive amounts of NH4 compared to everything else it lives with on docks, and I think that’s due to more usage of bacteria. I think… I’ve been using the idophenol blue method for this, although Turner Designs also has a handheld ammonium sensor – similar to their in vivo chlorophyll meter that I’ve been using with great success.

  2. I am filtering water for direct bacterial counts using FSH, but they won’t give me the PP:bacteria ratios. I definitely want to incubate on my next visit – but collecting will be a problem since Trididemnum seem to only grow on large, partially living coral heads. When I get back, it would be great to chat about this more over the phone. Thanks for your suggestions!

  3. Gila says:

    Oh yes, the coral is much prettier. Save it!

  4. Hee! Thanks, Gila! If you need a coral fix, the Red Sea has really some really cool & unique corals, especially gorgonians (soft corals). Spring break in Eilat?

  5. KJ says:

    Interesting study.

    As I read the possible reasons that may have been the root cause for the population explosion of Trididemnum. Has any work been done to explain potential changes in the Trididemnum itself. Could it have evolved become more efficent in its reproduction.

    You also mentioned that you find the Trididemnum “seem to only grow on large, partically living coral”; any chance the Trididemnum have become the master squatter of the region. As soon as new land become avaiable it has so many lavae on the loose that it is able to be first on site?

    Just a couple of highly unscientific thoughts.

  6. KJ – Thanks for your thoughts! I don’t think evolution in 30-40 years would be very likely without very strong selection (for example, all tunicates that didn’t reproduce x amount didn’t reproduce at all). But maybe there is strong selection that we don’t know about, or maybe there was a cryptic invasion of another population of Trididemnum that is more invasive (this happened with the salt marsh grass Phragmites on the east coast of the US).

    As for being a master squatter, that has been Trididemnum’s role in the past – it does have extremely high reproduction, and preferentially settles on dead coral heads. There is certainly more dead coral than in the past, but the really big change is in Trididemnum’s ability to overgrow & kill living coral.

  7. Pipi says:

    Hi, I´m studying ascidians from a remote Island on Indonesia and there is an ascidian also overgrowing corals crazy but I have not identifyied it yet, s my question is: has Trididemnum solidum a shiny surface??
    thanks in advance!

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