Corals: a case study hot buttons for nutrition of corals part of BCCR hot button for prey-capture section of BCCR hot button for photosynthesis part of BCCR hot button for coral-bleaching part of BCCR hot button for mucus-net feeding part of BCCR
Corals gain nutrients and energy in 3 ways: prey capture, mucus-net feeding, and photosynthesis. Coral bleaching is included here because it is such an important factor in the health of corals.

This section deals with coral bleaching, while other topics can be accessed via the icons. More on bleaching as a threat to the future of coral reefs can be found at FILES/PERIL/CORAL BLEACHING.

Coral bleaching

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of bleached finger coral taken from a video

"See the white patches on both the finger and boulder corals? Those are bleached areas caused by expulsion of algal symbionts by the corals. Perhaps the bleaching explains why the polyps are not extended on some of the finger coral." - Little Cayman 2002

NOTE Porites porites

NOTE Montastrea sp.

  Bleaching is the disappearance of a coral's colour as a result of temperature and perhaps other stress(es). It can happen in 3 different but related ways. Before you think of what these may be, review below the structure of a coral-polyp's digestive cavity and the location of the photosynthesising symbionts (zooxanthellae) within the cells lining the digestive cavity:
illustration showing disposition of symbiotic zooxanthellae in the gastrodermal cell of a coral polyp

Here are 3 possible ways that the colour of a coral can be "bleached out". Since the colour relates in large part to the colour of the photosynthetic chemicals contained within the zooxanthellae, then we are really looking for mechanisms that reduce or eliminate the amount of these photosynthetic chemicals. Based on observations from Fitt et al. 2001 Coral Reefs 20: 51:

1. The first possibility is that some or all of the zooxanthellae may be lost (expelled) from the host's gastrodermal cells, with effective loss of photosynthetic chlorophyll. The mechanism by which this happens is not well understood. After being expelled, the zooxanthellae may develop flagellar tails and swim out of the polyp via the mouth, possibly to take up residence later in another host. drawing showing one possible mechanism of coral bleaching
2. A second possibility is that some or all photosynthetic activity of the zooxanthellae might be lost or inhibited, perhaps through loss of chlorophyll.
drawing showing one possible mechanism of coral bleaching
3. A third possibility is that some or all zooxanthellae could be killed by stress or other causes relating to temperature.
drawing showing one possible mechanism of coral bleaching

photograph of bleached boulder coral Montastrea sp.
Discovery that there are actually several species of zooxanthellae in a single boulder coral, rather than just the single species that had long been assumed, raises the idea that such a multi-species community might engage in competitive "turf wars" to the possible disadvantage of the host. Conversely, because the different symbiont species may have differing tolerances to temperature and other bleaching-related stresses, the host may actually benefit by having more than one species of symbiont. If one species were to die out for whatever reason, another may survive. This idea has implication for long-term survival of coral reefs in face of possible extinction through effects of impending climate change. Rowan et al. 1997 Nature 388: 265; Baker et al. 2004 Nature 450: 741.

Partially bleached boulder
coral Montastrea sp. 0.1X



An interesting observation by researchers in Curacao on brain corals Colpophyllia natans and boulder corals Montastraea faveolata suggests that bleached and, thus, nutrient-deprived corals may seek out alternative forms of nutrition. During nighttime over a 3-day period both types of corals are seen to extend mesenterial filaments from their colony edges into neighbouring growths of turf algae up to 4cm distance. The filaments are extended only into turf algae and not into other corals or bare substratum (see photographs below). The author suggests that the behaviour is a feeding strategy used by corals during extended periods of bleaching that enable s access to bacteria, diatom, and other microalgal foods, and possibly to photosynthates extruded from the tissues of the turf algae. Marhaver 2011 Coral Reefs 30: 651; photographs courtesy the author.

NOTE these filaments extend from the lower edges of mesenteries that subdivide the internal gastrovascular cavity of each polyp. The filaments bear nematocyst (stinging cells) and are involved in obtaining, tangling, and killing live organisms taken in as food, and in defense. Examples of the last include tangling and stinging of predatory worms, and warding off polyps of neighbouring colonies that make competitive encroachments

photographs of brain corals Colpophyllia natans and boulder corals Montastraea faveolata extending nutrient-seeking mesenterial filaments into neighbouring growths of turf algae during times of bleaching
  Partially bleached boulder coral Montastraea faveolata with filaments extended Boulder coral Colpophyllia natans possibly obtaining nutrients from neighbouring turf algae The filaments may contact macroalgae as well as turf algae, as shown in the 1st photo on the Left  
photograph of clionid sponge Cliona varians unbleachedphotograph of encrusting sponge Cliona varians bleached  

While not so common as seen in corals, sponges may also bleach. These organisms also rely on photosynthetic products produced by mutualistic plant cells (Symbiodinium sp.) for their nutrition. Bleaching in encrusting clionid sponges Cliona varians incrustans in the lower Florida Keys in 2015 is thought by scientists to owe to higher than normal seawater temperatures over a 10d period in September, with the first bleached specimens being noted in October (see photographs). Hill et al. 2016 Coral Reefs 35: 153.

Unbleached Cliona varians on Left;
bleached specimen on Right

  boulder coral Montastrea sp : bleached or scraped by parrotfishes?boulder coral Montastrea sp : bleached or scraped by parrotfishes?
So, you now know a bit about bleaching in corals. As a test of what you have learned, identify which of these boulder-coral colony is bleached, as opposed to being scraped by parrotfishes. Then CLICK HERE for the answer.