We’ve arrived at the last installment of my blog series on colors and coral reefs! I’ve talked about coral pigmentation in relation to coral bleaching and coral reef fish colors, also in relation to coral bleaching.
In this post, I’m going to talk about two more colorful reef organisms: sponges and Christmas tree worms (Spirobranchus giganteus). First, I’ll discuss a study published in 1995 that looked to understand, among other things, the ecological significance of sponge colors. Then, I’ll discuss a study that highlighted a positive relationship between Christmas tree worms and Porites coral colonies.
Sponges are important, and colorful, components of coral reef ecosystems. Sponges are filter feeders, sucking up organic matter release by corals and other planktonic life1. Sponges are thus responsible for recycling organic matter and nutrients on reefs, which is especially important considering that reefs thrive in nutrient poor waters2. Sponges are susceptible to predation, and scientists have investigated possible protections against or responses to the threat of predation.
A study by Joseph Pawlik et al. investigated sponges in the Caribbean and their deterrent responses to predatory reef fish. The researchers looked for and, surprisingly, did not find any relationship between sponge color and deterrency3. One might expect that the bright colors characteristic of some sponges serve as warning coloration, but results indicated otherwise.
In the genus Agelas, one species (Agelas clathrodes) that is brightly colored red or orange was observed in addition to five other species that are all brown or black and belong to the same genus. All six species were deterrent to the same extent3. These results are shown in Figure 1 below, highlighted in the yellow box. This result indicates that these sponges are not aposematic. Aposematic is just a fancy word for when coloration or markings serve to warn or repel predators. Other hypotheses for the ecological significance of these sponge’s coloration must be tested.
Fig 1. This graph shows the number of sponge-infused pellets eaten by the Bluehead wrasse (Thalassoma bifasciatum) for various species of sponge (along the x-axis). The pellets each contained organic extracts from the sponges at natural concentrations. Notably, in the Agelasidae genus (yellow box) there was no significant difference in predation despite variation in coloration. Graph modified from Pawlik et al.3
The Christmas tree worm (seen in the photo below), resembles a small, decorated spruce tree, and is a notable bio-eroder of coral reefs. It is a polychaete worm, which means it’s in a class of worms generally found in marine environments4.
Fig 2. A “forest” of colorful Christmas tree worms on a coral. They are part of the group polychaetes, which includes worms mostly found in the ocean4. There are many color morphs of Spirobranchus gigantea and not all are known or described. Photo by Nick Hobgood.
These colorful creatures might be more helpful to coral colonies than previously thought, especially coral in the Porites genus. A study done by DeVantier et al. showed that coral colonies in the Porites genus and Christmas tree worms engaged in a mutualistic symbiosis5. A symbiosis is just the interaction between two different organisms that live in close proximity to each other. Mutualistic symbiosis occurs when two organisms live together, depend on each other and both benefit from the interaction. In this case, Porites coral colonies provide protection for the worms and a location for the worms’ suspension feeding. The Christmas tree worms protect the coral from predation by the Crown of Thorns Starfish (Acanthaster planci)5.
In this study, the researchers first established the possibility of a mutualism by recording observational data. A table included below, shows visual census data of partially and totally consumed Porites colonies. There is a strong correlation between the presence of Spirobranchus gigantea, the Christmas tree worm, and living Porites colonies.
Fig 3. Visual census data of Porites colonies that are either partially consumed or totally consumed. The colonies that were totally consumed had no Christmas tree worms. The partially consumed colonies (or those that had living coral polyps) had lots of Christmas tree worms. This suggests that the worms protect the Porites to some extent from predation threats. Table from DeVantier et al.5.
The authors discuss how they believe the Christmas tree worm deters Crown of Thorn Starfish predation. They think that the worms irritate the starfish’s tube feet or stomach and this irritation drives the predator away from the Porites colonies5. So, Christmas tree worms aren’t just pretty decorations! The colorful little worms have a role in protecting reef species against pesky predators.
Thanks for reading my blog posts! I hope you’ve learned about coral reef ecosystems and their spectacularly colorful inhabitants. We all need to appreciate the complexity and beauty that characterize coral reef ecosystems and prioritize their continued existence and health. It is more important than ever to take action to protect reef ecosystems. Sponges and worms, although colorful and thus more noticeable, are not the only coral reef ecosystem inhabitants that are affected by coral bleaching, which can have cascading effects throughout an entire ecosystem. In order to protect coral reefs and coral reef organisms, we need to try to understand and then protect colorful and ‘bland’ species alike.
1 Morgan, James. “Sponges Help Coral Reefs Thrive in Ocean Deserts.” BBC News. BBC. 07 Oct. 2013. Web. 19 Apr. 2017. http://www.bbc.com/news/science-environment-24398394.
2 Sheppard, Charles R.C. et al. “The Biology of Coral Reefs.” Oxford University Press. 2012. Print.
3Pawlik, Joseph R., et al. “Defenses of Caribbean Sponges against Predatory Reef Fish. I. Chemical Deterrency.” Marine Ecology Progress Series, vol. 127, no. 1/3, 1995, pp. 183–194., www.jstor.org/stable/24855132.
4 Frost, Emily. “The Christmas Tree Worm, Decorating Coral Reefs Year-Round.” Smithsonian.com. Smithsonian Institution, 14 Dec. 2012. Web. 19 Apr. 2017. http://www.smithsonianmag.com/science-nature/the-christmas-tree-worm-decorating-coral-reefs-year-round-161933735/.
5 DeVantier, L.M. et al. “Does Spirobranchus giganteus protect host Porites from predation by Acanthaster planci: predator pressure as a mechanism of coevolution?” Marine Ecology Progress Series, vol. 32. 1986. Pp. 307-310. http://www.int-res.com/articles/meps/32/m032p307.pdf?pagewanted=all