Nutrition
 
column spacer Nutrition
  This part of BCCR deals with how reef organisms obtain nutrients. As on land, flow of nutrient and energy in coral reefs begin with photosynthesis. Plants are eaten by herbivores, herbivores are eaten by carnivores, and decaying organic matter is consumed by detritivores and bacterivores. This is the food web of the coral reef. Each of the entries on the web is known as a trophic level.
 
 
seahorse dive leader for Biology of Caribbean Coral Reef website photograph of a coral reef taken from a video

"A reef like this one probably has thousands of trophic interactions happening every day; that is, one thing eating another. This part of BCCR deals with how reef organisms satisfy their nutrient and energy needs. So, just like the parrotfish, we can start smacking our lips in anticipation." - Turks & Caicos 2003

NOTE juvenile princess parrotfish Scarus taeniopterus

 
   
 

Nutrition of coral reefs begins with seaweeds and phytoplankton. Growth of these is known as primary productivity because it starts with sunlight and inorganic materials, and through the process of photosynthesis leads to the formation of living tissue. There are 4 major trophic, or feeding/nutrition, levels on a reef, and information on each can be accessed via the icons.

"Corals: a case study" is included as a 5th topic. Because corals utilise all trophic modes it will be a good place to begin our study of coral-reef nutrition.

hot buttons for nutrition part of BCCR hot button for Corals: a case study part of BCCR hot button for primary productivity 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
 
 
Corals: a case study hot buttons for nutrition of corals part of BCCR hot button for prey-capture by corals section of BCCR hot button for photosynthesis by corals part of BCCR hot button for coral-bleaching part of BCCR hot button for mucus-net feeding by corals 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 prey capture, while other topics can be accessed via the icons.
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a coral reef taken from a video

"Divers expect coral-reef waters to be crystal clear, and they're disappointed when they aren't. However, clear water usually means little food available in the form of plankton, which is a food source for corals. So, I wonder what corals live on?" - Turneffe Island, Belize 2000. Video courtesy Andy Stockbridge, Belize

 
 


Clear water around a reef usually means that nutrients required for growth of phytoplankton, such as nitrates and phosphates, are absent or in short supply. Someone once likened the situation to a "nutrient desert". It is explained by the presence of a thermocline to a depth of about 50m, caused by less dense warm water overlying more dense cold water. This condition is quite stable, for in the absence of distinct seasons the surface layers in tropical oceans are not regularly "turned-over". Nutrients required by phytoplankton are therefore continually used up in the surface layer. Also, large rivers that might replenish these nutrients from the land in the form of sediments, agricultural fertilisers, sewage, and so on, are generally absent from Caribbean islands.

NOTE an abrupt change in temperature

photograph of a coral reef in Barbados
At usual recreational SCUBA depths of 30m or less, water temperatures in the tropics are relatively warm. Below the 50m depth of the "photic zone" (the depth to which plants can live) nutrients are in greater relative supply, but are unavailable owing to the stability of the thermocline..
graph showing tropical thermocline
 


This satellite photo captures light that reflects chlorophyll content. Purple and blue colorations indicate areas of low chlorophyll and, hence, low primary productivity, while orange and yellow colorations indicate higher levels of chlorophyll, and thus higher productivity. The photograph is taken in the photograph of chlorophyll distribution around the world during northern springtimenorthern springtime during an intense phytoplankton bloom.

Temperate regions are more productive than tropical ones because of autumnal turnover of warm, nutrient-depleted summer-surface waters. The turnover occurs when surface-water temperature decreases in autumn with resultant increase in density. Autumn winds aid in the turnover, and the result is replacement of surface waters by deeper, nutrient-rich waters. This often leads to a small bloom of phytoplankton, but the larger bloom occurs in springtime, coincidental with increasing light and temperature. Where more-or-less continuous upwellings occur, as off the coast of California, a more steady renewal of nutrients leads to rich growth of phytoplankton and seaweeds.

 

 
 

Corals: a case study: prey capture

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of diver near a mound coral taken from a video

"Oh, this is a nice area of mound corals that we're swimming over. As with most corals, the polyps are closed during the day, but they open up later around dusk to catch small planktonic prey. Look closely at these pillar-coral polyps and you can see small white patches on the tentacles. These are batteries of stinging cells." - Turks & Caicos 2003. Close view of pillar coral courtesy Andy Stockbridge, Belize

NOTE Montastrea sp.

 
  photographs illustrating some of the prey items of a boulder coralcorals
A sampling of gut contents of polyps of Panamanian boulder corals discloses a large assortment of zooplanktonic prey including copepods, amphipods, polychaetes, nematodes, salps, fish eggs, and an assortment of invertebrate larvae. Porter 1974 Proc 2nd Int Coral Reef Sympos 1: 111.
 

Of the several modes of nutrition of a coral, what proportion of its energy requirements is met by prey capture? This is asessed in Panamanian boulder corals by isolating 92 polyps after 2h post-dusk feeding and checking their digestive cavities for numbers and types of prey. Porter 1974 Proc 2nd Int Coral Reef Sympos 1: 111.

The results are as follows:

 
drawing 1 of 5 in coral-polyp experiment on feeding drawing 2 of 5 in coral-polyp experiment on feeding drawing 3 of 5 in coral-polyp experiment on feeding drawing 4 of 5 in coral-polyp experiment on feeding drawing5 of 5 in coral-polyp experiment on feeding
  Overall, the polyps ate 188 prey items in 2h, or an average of 2 prey items per polyp. The polyps are all interconnected, so what is taken in by one polyp is shared by all polyps. The researcher reckons that 2h post-dusk feeding can satisfy no more than about 11% of a coral's daily energy requirements.
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of elkhorn coral taken from a video

"You know, a coral like an elkhorn stands up so tall, you would think that its polyps would be exposed to a much larger portion of the current than a flatter type of coral might be. I wonder why all corals aren't tall?." - St. Maarten 2005

NOTE Acropora palmata

 
 


With respect to efficiency of prey capture, would it would be better for a coral to grow higher off the substratum in order to present more surface area of polyps to the current? More surface area would translate to more polyps feeding on a greater portion of the zooplankton-bearing current. A Jamaican study relating to this photograph of deep-growing lettuce coralsfinds that lettuce corals actually adopt different growth forms depending upon photograph of lettuce coral Agaricia depth. In shallow areas (less than 10m) their growth form is more flat and plating (see photo on Left), while in deeper areas (greater than 20m) they tend to be more upright and facing at right angles to the current (photo on Right). Helmuth 1991 Am Zool 31: 48A.

Lettuce coral Agaricia
agaricites
at deeper
depths 0.5X

 

 

Growth form of Agaricia
agaricites
in more
shallow depths 0.5X

 

Our first response is that this observation supports our theory but, in fact, the author notes that the shallow, plating form can feed just as well as the deeper, upright form, and can do so over a broader range of current velocities. In fact, it seems that lettuce corals have a great deal of morphological plasticity, and thus their shapes may be responsive to factors other than ones relating to feeding, such as the need to capture light for their photosynthesising symbionts, to compete for space, or to reduce fragility in waves and currents. What these factors actually are and how they might benefit the coral in different areas at different times will require further study.

NOTE refers to the possession of genes whose expression varies widely under different environmental conditions

 
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