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. Bleaching is included here because it is such an important factor in the health of corals.

This section deals with photosynthesis, while other topics can be accessed via the icons.

Corals: a case study: photosynthesis

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of coral reef taken from a video "Surprisingly, catching food on the polyps is not the primary mode of nutrition in corals. Most of their nutritional and energy needs are met from photosynthetic activities of symbiotic cells living in their tissues. These cells often contribute a brownish colour to the coral polyps." - Turneffe Island, Belize 2000. Video courtesy Andy Stockbridge, Belize.

photograph of coral Agaricia lamarcki showing layered structure to catch light
Corals contain photosynthesising symbionts in their gut tissues. The need for the sun's energy by these symbionts, called zooxanthellae, explains by corals compete for sunlight. On a bright day, a shallow-dwelling coral is provided with much of its energy and nutrient needs from the symbionts.

NOTE lit. "animal flower" Gr. The first part refers to the fact that the cell is motile as part of its life cycle; the second part refers to its photosynthetic activity. For more on zooxanthella symbionts in corals see ZOOXANTHELLA-TYPE MUTUALISMS


Layered colonies of sheet-coral Agaricia lamarcki
compete for light for their symbionts 0.33X


photograph of shallow reef in Belize showing high diversity of coralsphotograph of a deep coral reef in Belize showing low coral diversityThe need for sunlight explains, in part, why the maximum number of coral species occurs in shallow water; by 30m depth, diversity may only be a quarter of this.


Shallow reef in Belize
showing high coral diversity



Deep reef in Belize
showing low coral diversity

  graph comparing efficiencies of light utilisation by shallow and deep boulder coralsStudies in Jamaica on boulder corals Montastrea annularis reveal that efficiency of light utilisation by the coral's zooxanthellae increases with depth of colony. Dustan 1982 Mar Biol 68: 253; Battey & Porter 1988 Proc 6th Int Coral Reef Symp Vol. 3: 79.

photograph of zooxanthella of a coral, courtesy Max Taylor, UBC
Now, why should this be? Here are 4 possible explanations, 2 of which are true and applicable, one of which is logical but does not occur, and one of which makes no sense. Study them and, as a start, try to pick out the nonsense entry from the others, then CLICK HERE.

Zooxanthellae numbers increase with depth.

Chlorophyll content per zooxanthella is greater in deep colonies.

There is more red wavelength-utilising chlorophyll in zooxanthellae in the deeper colonies.

There is more blue wavelength-utilising chlorophyll in zooxanthellae in the deeper colonies.


illustration showing performance of boulder corals after transfer to deep and shallow depths
Let's do an experiment. We'll translocate each of the boulder-coral colonies Montastrea to the other's habitat and observe how they perform.

Several months after the translocation we see that neither colony is doing well.

The explanation may relate less to fewer zooplanktonic food items being available at depth (which is true, but we know that prey capture accounts for only a small portion of the coral's food needs) than to the fact that each translocated colony has the wrong type of chlorophyll for its new habitat.

Good thinking if this was also your conclusion!


photograph of lettuce coral Agaricia grahamae from deep reef in the BahamasHow deep can zooxanthellate corals live and still benefit from photosynthesis? One would guess that deeper-dwelling corals would rely to progressively lesser extents on their symbionts for energy and nutrition. The answer varies with species and area, but for lettuce corals Agaricia grahamae in the Bahamas it is 119m. What evolutionary modifications in zooxanthellae physiology in lettuce corals that permit this are presently unknown. Reed 1985 Proc 6th Int Coral Reef Symp 6: 249.

NOTE Pacific representatives of lettuce coral Leptoseris sp. live at depths of about 155m

photograph of 70m-deep Pulley Reef, FloridaNOTE a group of 10 U.S. scientists notes that part of Pulley Reef, Florida at 70m depth is "the deepest known photosynthetic coral reef on the North American continental shelf" (see photo on Right). This reef includes lettuce corals A. lamarcki and A. fragilis, and several other zooxanthellate species, as well as numerous species of photosynthesising coralline algae and other macroalgae. Shall we let the authors of the 2 studies fight it out? Culter et al. 2006 Coral Reefs 25: 228; photograph on Right courtesy the authors.

Lettuce corals Agaricia spp., coralline algae, green
algae Anadyomene menziesii, and a SCUBA diver (for
scale) at Pulley Reef, Florida at a depth of 70m.
At this depth there would be little ambient light and
the reef would hardly be visible to the naked eye


photograph of deep zoozanthellate reef in Puerto Rico/US Virgin IslandsAnother candidate for “deep-reef status” is a set of reefs off the cloast of Puerto Rico and US Virgin Islands at depths of 30-47m that boasts over 40% cover. Photographic surveys of these reefs by a Seabed Autonomous Underwater Vehicle show at least 10 species of corals (dominated by boulder corals Montastraea franksi), several sponges, and different macroalgae and coralline algae species (all, of course, photosynthetic and relying on these photosynthates for their nutrition). Armstrong 2007 Coral Reefs 26: 945; photograph courtesy the author.

Boulder corals Montastraea and
several pink and red sponges at
depth in Puerto Rico/US Virgin Islands


photograph of deep zooxanthellate corals in BermudaSome of the most northerly coral reefs in the world are in Bermuda (32oN). Deep reefs there (50-70m), consisting of zooxanthellate boulder, lettuce, and pencil corals must face the double physiological challenges of low light and low temperature. Interestingly, the presence of large colonies of the finger/pencil coral Madracis carmabi at 50m+ depth is the first record of this species in the Bermuda reef system, although it occurs fairly commonly elsewhere in the Caribbean basin. The researchers that discovered and explored these deep corals did so using technical deep-diving gas mixtures with SCUBA to 60m depths. The authors stress the special value of studying these and other deep reefs, notably whether they may provide potential refugia from global threats of warming with associated disease and bleaching. Venn et al. 2009 Coral Reefs 28: 135; photograph courtesy the authors.


Deep reef in Bermuda showing small colonies of
boulder coral Montastraea cavernosa and lettuce
coral Agaricia fragilis. Tape length for scale = 50cm


hot button for parasitism part of Biology of Caribbean Coral Reefs website hot button for mutualism part of Biology of Caribbean Coral Reefs website hot button for commensalism topic in BCCR hot button for symbiosis quizzes section of BCCR hot button for commensalism part of BCCR hot button for symbiosis quizzes part of BCCR hot button for mutualism part of BCCR hot button for parasitism part of BCCR hot button for corals: a case study part of BCCR hot button for photosynthesis part of BCCR hot button for herbivory part of BCCR hot button for carnivory part of BCCR hot button for detritivory/bacterivory part of BCCR