Nutrition
 
 
Herbivory hot buttons for nutrition part of BCCR hot button for primary productivity part of BCCR hot button for carnivory part of BCCR hot button for Corals: a case study part of BCCR hot button for herbivory part of BCCR hot button for detrivory/bacterivory part of BCCR
There are 4 major trophic levels on a reef, with herbivory being considered here. Information on the others, including "Corals: a case study" (corals use all trophic modes), can be accessed via the icons.
 
 

Seagrass-eaters

hot buttons for herbivory part of BCCR hot button for algivores section of BCCR hot button for phytoplanktivores section of BCCR hot button for seagrass-eaters section of BCCR

This section of herbivory deals with seagrass-eaters; information on other types of reef herbivores can be accessed via the icons.

There are at least 7 species of seagrasses in the Caribbean region, of which the most abundant and most commonly encountered is Thalassia testudinum. This is the species focussed on here. Seagrasses may be eaten outright by a herbivore green (turtles, manatees, sea urchins, parrotfishes), or the outer epidermis may be scraped off and eaten (snails), or algae that grows epiphytically on the seagrass blades may be stripped off and consumed (fishes). In areas of intense grazing, "halos" may be created by fishes and sea urchins, generally extending out from the bases of coral reefs where the herbivores reside when they are not feeding.

 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a batfish taken from a video

"Oh, here's a batfish on the edge of a seagrass bed. I'd like to follow it but I should really stay with the dive plan. Bummer! Divers often ignore sandflats and seagrass beds, and both areas can be quite interesting." - Turneffe Island 2001. Video courtesy Andy Stockbridge, Belize

NOTE shortnose batfish Ogcocephalus nasutus

 
 

Barbados sea-egg Tripneustes ventricosus
Seagrass-inhabiting sea urchinsTripneustes ventricosus, known as "sea-eggs" in Barbados and other islands, are prized food items. Only the gonads are eaten.

 

 

 

 

Sea urchin Tripneustes ventricosus 0.6X

 

  histogram showing growth of sea urchins Tripneustes ventricosus on several species of marine algae and seagrassGiven that Tripneustes is often found within beds of seagrass Thalassia testudinum and is known to eat it, we would predict that its growth on this plant would be superior to that on other plants. However, in experiments to compare growth of the sea urchin on several plants found commonly in its habitat in Barbados, including seagrass, we find that growth is actually poorest on the seagrass. Data from Lilly 1975 PhD Thesis, University of British Columbia.
 
 

So, growth of sea urchins on a diet of seagrass Thalassia is poor, suggesting that essential nutrients may be lacking. Or does it? What else might be going on here? Consider the possibilities below, then CLICK HERE to see explanations.

The sea urchins might be eating much less of the seagrass than of the other plants.

Absorption of nutrients in the gut may be relatively low on a seagrass diet.

Use of the absorbed seagrass-derived nutrients for growth may be relatively low.

Cost of eating and/or processing the seagrass food might be relatively high.

 
 

drawing of bucktooth parrotfish courtesy of Human 1994 Reef fish identification. New World PublSo, overall, seagrass seems to be a poor food for sea urchins. But what about for fishes? In St. Croix the dominant plant item in the diet of bucktooth parrotfishes Sparisoma radians is seagrass, especially blade-tips overgrown with epiphytes. Preference by these parrotfishes for different plants appears to be directly correlated with the amount of energy obtained per bite. The order of preference is given below, along with explanatory notes.

NOTE lit. "surface plant", referring to small plants that attach and grow on surfaces, such as other plants, rather than in the soil

 
number 1 of series of 5 photos showing different foods of bucktooth parrotfishes number 1 of series of 5 photos showing different foods of bucktooth parrotfishes number 1 of series of 5 photos showing different foods of bucktooth parrotfishes number 1 of series of 5 photos showing different foods of bucktooth parrotfishes number 1 of series of 5 photos showing different foods of bucktooth parrotfishes
No. 1 The normal seagrass with epiphytes is eaten 5X more than the next preferred plant food No. 2 Although eaten, Caulerpa has chemicals that parrotfishes don't like. Still, it's second preferred No. 3 Thalassia is not as tasty to the parrotfishes without epiphytes as it is with epiphytes No. 4 This alga has a high content of calcium carbonate, but it is still eaten 10X more than the next alga No. 5 Halimeda has both calcium salts and other defensive chemicals, and is least-preferred
 
 
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of seagrass Thalassia testudinum taken from a video

"Oh, look at this. Sometimes in a seagrass bed you can find spots where green turtles rest. I don't know whether this is one, but it's got the right general shape and appearance." - Bonaire 2004

NOTE Chelonia midas

 
 

photo collage of green turtle Chelonia midas and seagrass bed Thalassia testudinum
Caribbean green turtles sometimes maintain grazing plots in seagrass beds. Their ability to digest and to extract nutrients from the tough fibres of the seagrass is aided by microbial fermentation in areas of their hinguts. The advantage to the turtle in maintaining such plots is that regrowth of the grazed seagrass is nutritionally superior. It is richer in nitrogen and has a lower lignin content than old growth. Bjorndal 1985 Copeia 3: 736.

NOTE few if any animals have enzymes to digest the cellulose component of plants. Many herbivores, including rabbits and other mammals, house resident populations of special cellulose-digesting bacteria. Guts of termites contain symbiotic protists for the same purpose

NOTE lignin is a structural polymer found in supporting walls of cells in plants. Along with cellulose, lignin represents a major component of wood. It is essentially indigestible by herbivores

 

photograph of a patch reef in the Bahamas with a bare "halo"  in the seagrass bedsurrounding itIn St. John and other Virgin Islands there may be a conspicuous band of bare sand up to about 10m in width separating reefs and beds of sea-grasses Thalassia testudinum (see photograph on Left).  An experiment involving creation of an artificial reef out of building blocks within a Thalassia bed in St. John and leaving it for 8mo, suggests that the band is caused by parrotfishes and surgeonfishes venturing from the protection of the reef and consuming the vegetation out to a distance at which the risk of being eaten by bigger predatory fishes counterbalances their hunger (see photogaphs below).  Randall 1965 Ecology 46 (3): 255.

 

 

View from above of a small patch reef
in the Bahamas distinguished by a
seagrass-free "halo" surrounding it

 
photograph of artificial reef at Time 0

Artificial reef at Time 0
with an abundant growth
ofThalassia testudinum

photograph of artificial reef at Time 8mo
Artificial reef at Time 8mo
with the seagrass grazed
away to form a halo
around the blocks
 
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