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.