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Motile organisms

hot button for motile competition part of Biology of Caribbean Coral Reefs hot button for motile-organism space/territory competition hot button for motile-organism food competition hot button for motile-organism mate competition

Competition for food is considered in this section, while other types of competition in motile organisms can be accessed via the icons.

This section deals with competition for food among sea urchins, hermit crabs, and parrotfishs, and between hawksbill turtles and angelfishes.


Motile organisms: food competition

Food competition in motile animals can be of the preemptive type, where one animal deprives another of a certain food resource, or of the interference or encounter type, where one animal physically interferes with another's ability to feed on a certain food.
seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of spinyhead blenny feeding, taken from a video

"Competition for food may be direct, like stealing, or preemptive. The crustacean eaten by this spinyhead blenny is deprived from all other reef organisms relying on such food." - Turneffe Island, Belize 2000. Video courtesy Andy Stockbridge, Belize.

NOTE Acanthemblemaria spinosa


Motile organisms: food competition: sea urchins

photograph of sea urchins amongst seagrass
Studies in Panama on encounter-competition between sea urchins show that pushing and biting at spines occur both within and between species. Usually the larger individual, or the resident individual if within a crevice, wins out. Because the long-spined sea urchin is larger the other species, it generally forces the smaller species into crevices, away from choice areas of turf-algal food. Shulman 1990 J Exp Mar Biol Ecol 140: 197.




Sea urchins in a seagrass bed: the long-spined Diadema antillarum (Left)
and Echinometra lucunter (several in crevices in the coral rock) 0.33X

  An interview with an interested SCUBA-diver:
number 1 in a series of diver/sea urchins cartoons number 2 in a series of diver/sea urchins cartoons number 3 in a series of diver/sea urchins cartoons
number 4 in a series of diver/sea urchins cartoons number 5 in a series of diver/sea urchins cartoons number 6 in a series of diver/sea urchins cartoons
number 7 in a series of diver/sea urchins cartoons number 8 in a series of diver/sea urchins cartoons number 9 in a series of diver/sea urchins cartoons
  NOTE pedicellariae: 4 types of small biting jaws found on the skin of sea urchins, 3 of which are for crushing or slicing and one of which is toxic

Motile organisms: food competition: hermit crabs

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of terrestrial hermit crabs eating an apple

"This isn't part of the dive, but it's fun to watch terrestrial hermit crabs squabble over food. Somewhere under this pile is a piece of apple. The hermits come from many meters distance once they pick up the smell of the food." - Cayman Brac 2004.

NOTE Coenobita sp.


photograph of two terrestrial hermit crabs fighting over a food scrap
Encounter competition between hermit crabs involves not only disputes over shells, but also fighting over food scraps.




Terrestrial hermit crabs Coenobita sp. are omnivorous. Here,
two individuals fight over a scrap of picnic food 0.75X


Motile organisms: food competition: parrotfishes

Defense of territories by stoplight parrotfishes involves competition for space and, thus, for algal foods. In Bonaire, studies show that non-territorial males spend more time feeding than territorial males... histogram showng food intake by parrotfishes ...but take in about 30% less organic matter. Bruggermann & Breeman 1992 Proc 7th Int Coral Reef Sympos 2: 951. histogram showing organic-matter intake by stoplightparrotfishes

But, how can this be? If a fish spends more time eating, it should take in more food, shouldn't it? The explanation is that dominant male parrotfishes select their territories based principally on the amount and quality of food contained within. In Bonaire, territorial stoplight parrotfishes feed mainly on nutrient-rich turf algae...
photograph of turf algae

...while non-territorial males feed primarily on nutrient-poor encrusting red and crustose-coralline algae, and on endolithic algae.

NOTE lit. "within rock": algae growing in porous dead coral. The fish has to rasp this away to access the algae within. "Crustose corallines" are collectively a group of heavily calcified red algae

photograph of various encrusting red algae
Turf algae 1X   Encrusting red algae (dark) and crustose-coralline algae (pink) 0.3X

photograph of d
While parrotfishes and damselfishes are both territorial, the areas defended are hugely disparate in size; in fact, there may be many damselfish territores within a single parrotfish's territory. Parrotfishes are constantly being chased and nipped by damselfishes as they pass through the latter's gardens looking for algae to eat.






The garden may not look like much, but its
owner, a 3-spot damselfish Stegastes planifrons,
defends it energetically against incursion by
a queen parrotfish Scarus vetula 0.2X

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a stoplight parrotfish about to get bitten by a damselfish taken from a video

"Now let's watch this terminal-phase stoplight parrotfish...see if he gets nipped. Yes, right there, a bicolour damselfish and right on the mouth. Probably didn't hurt much, but he decides that it's time to see another part of the reef." - Cayman Brac 2004.

NOTE Sparisoma viride

NOTE Stegastes partitus



Motile organisms: food competition: hawksbill turtles & angelfishes

  photograph of a hawksbill turtle and a French angelfish sharing a sponge meal
Hawksbill turtles and angelfishes both eat sponges, often preferring the same species. Neither species appears to exhibit aggressive behaviour to the other and the interaction is quite amicable. Ttheir competition is therefore a preemptive one for a common resource. Because the outer cortex of a sponge is usually the toughest part, the hawksbill often starts things off by taking the first bite with its strong beak. When the turtle takes a break for air at the surface, the angelfish has free license to eat.