Competition in motile reef organisms is usually for space/territory, food, or mates. Competition among motile organisms such as fishes is considered in this section, while competition among SESSILE ORGANISMS is found in its own section.

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of bluehead wrasses taken form a video

"A busy reef is full of competitive encounters, commonly food-stealing and tail-biting in fishes. Most occur in your peripheral vision, so you don't really see them. Not this one. A porkfish crosses through a damselfish's territory, and gets nipped." - Turneffe Island 2000. Video courtesy Andy Stockbridge, Belize.

NOTE Anisotremus virginicus

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 space/territory is considered in this section, while other types of competition in motile organisms can be accessed via the icons.

Motile organisms: space/territory competition

Research on competition for space by Caribbean reef fishes mostly focuses on parrotfishes and damselfishes. It involves aggressive defense of territory that may include food, mates, and egg nests. Caribbean parrotfishes defend territories of 100-300 square meters in area, depending upon species. Mumby & Wabnitz 2002 Envir Biol Fish 63: 265.


Motile organisms: space/territory competition: parrotfishes/ damselfishes

The topic of space/territory competition is divided into a larger part on parrotfishes/damselfishes, considered here, and a smaller one on BUTTERFLYFISHES/GRUNTS/STINGRAYS/TILEFISHES/HERMIT CRABS considered in its own section.

photograph of terminal-phase stoplight parrotfishphotograph of 3-spot damselfish

Terminal-phase stoplight
parrotfish Sparisoma

A 3-spot damselfish Stegastes
aggressively defends
its garden-plot territory 0.4X

NOTE the fish is Left of
centre facing the camera

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of 3-spot damselfish taken from a video

"What a pretty pair of bicolour damselfishes, and what a nice spot to live...just at the top of the reef. Pretty brave in chasing off smaller fishes, but not so brave with the bigger one!" - Cayman Islands 2001

NOTE Stegastes partitus

photographs of food components in a damselfish's garden

Damselfishes are strictly herbivorous and their territories are actually garden-plots that include not only turf algae, but also diatoms that grow interspersed in the turf and that are also eaten as food. Photograph of diatoms courtesy Max Taylor, University of British Columbia.

NOTE a type of single-cell photosynthesising plant. Some species are planktonic, while others live on the sea bottom

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

"Parrotfishes are herbivores and directly compete with damselfishes for food. Let's watch this juvenile stoplight parrotfish for a while. Oops, lost it in the coral. But here's another. Oh, it just got nipped as it passed through a yellowtail damselfish's territory." - Cayman Islands 2001

NOTE Sparisoma viride

NOTE Microspathodon chrysurus

photograph of beaugregory damselfish
Damselfishes appear to have territorial heirarchies, with a dominant male and its mate, and other recessive males. The dominant male is usually the territorial aggressor, but in some species females may also be involved. Studies in Jamaica, for example, show that female beaugregory damselfishes feed, patrol, and chase intruders along with the male. Horne & Itzkowitz 1995 J Fish Biol 46: 457.




Male beaugregory damselfish
Stegastes leucostictus 0.5X

seahorse dive leader for Biology of Caribbean Coral Reefs website photograph of a terminal-phase stoplight parrotfish 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 Islands 2001

NOTE Sparisoma viride

NOTE Stegastes partitus

photograph of a stoplight parrotfish biting at a bicolor damselfish
Aggression between damselfishes and parrotfishes is not always on the side of the damselfish. Occasionally, a parrotfish may be the aggressor, but this is usually in anticipation of an impending nip, or in retaliation for one, and not to chase the damselfish out of the parrotfish's territory.




Terminal-phase stoplight parrotfish
Sparisoma viride snaps at a bicolour
damselfish Stegastes partitus 0.2X

photograph of an aggressive 3-spot damselfishThe benefits for a male damselfish in having its own territory are reasonably obvious, and include handy access to females, a place for the female to spawn eggs easily fertilised by the male, a refuge from predators, and a night-time resting place.

There are disadvantages, though, including time and energy expended in patrolling the territory, and risk of injury or sometimes death during aggressive encounters. Another, less obvious detriment is that novel foods may be located outside of the territory. Although damselfish territories have rich algal growths and, hence, good nutritional diversity, access to foods with possibly other nutritional benefits must still be limited for the territory-holder.



A 3-spot damselfish Stegastes planifrons in a
"garden" not particularly rich in algae. Sometimes
the only way a diver or snorkeler is aware of
being in a damselfish's territory is the fish
aggressively tapping on the facemask

cartoon of yellowtail damselfish in its garden
Damselfishes cannot completely protect their gardens against other grazers. Parrotfishes and surgeonfishes may get in for a bite, and small herbivorous invertebrates take their fill. In fact, studies indicate that the plot-holder may only eat a small portion of the yearly algal production, with most being eaten by other grazers. For example allocation of garden yield for damselfishes in the Great Barrier Reef is as follows:

25%: resident damselfish

31%: various amphipods, copepods, and polychaete worms

14%: other herbivorous fishes, mainly parrotfishes

remaining %: mostly lost to erosion and storm damage

Data from Russ 1987 J Exp Mar Biol ECol 110: 1; see also Klumpp & Polunin 1989 J Exp Mar Biol Ecol 125: 145.

photograph of a yellowtail damselfish in its garden
What effect does damselfish territorial protection have on algal diversity? That is, would the number of different algal species be expected to go up, go down, or stay the same? Let's do an experiment to test this using 3 experimental treatments set out in an area of the reef where damselfishes and other herbivorous fishes live. Ideas and data for experiment from Hixon & Brostoff 1983 Science 220: 511.



Yellowtail damselfish Micro-
spathodon chrysurus

We first set up 3 test cages in an area of the reef populated by damelfishes:
drawing of experimental cage 1 in diversity test drawing of experimental cage 2 in diversity test drawing of experimental cage 3 in diversity test
The 1st treatment is a cage open at the bottom. It is designed to exclude all herbivorous fishes. It is placed close to but not in a damselfish's territory The 2nd treatment is an open plot, the same size as the cage, and demarcated by small pegs. It is placed within a damselfish's territory. The 3rd treatment is also an open plot demarcated by small pegs. It is placed outside of, but near to, a damselfish's territory.
The experiment is run for 1yr, with measurements being taken of algal diversity after 1wk, 6mo, and 1yr. The results are as shown here, with "high algal diversity" signifying that there are lots of different algal species present.
graph of results for damselfish algal-diversity experiement Within a damselfish's territory and with the owner present, algal diversity increases over time because the damselfish preferentially eats competitively dominant algae that might otherwise displace weaker species (thus reducing diversity).

In comparison, within the cage and in the absence of all herbivorous fishes, diversity at first increases, then decreases over time as a few dominant algal species take over.
Finally, in the open plot outside of a damselfish's territory, algal diversity remains low because of the unrestricted feeding activities of roving herbivorous fishes, such as tangs, parrotfishes, and triggerfishes.

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

"Check out these hungry rabbits. A school of blue tangs...doesn't care who's in the way or whose territory it is. They're like vacuum cleaners. And any more than a couple oftangs will easily overwhelm a damselfish's defenses.." - Turks & Caicos 2003

NOTE Acanthurus coeruleus, behaving more like a shoal than a school


graph showing per capita feeding rates of blue tangs in damselfish garden-plots

Studies in Panama show that food intake of blue tangs Acanthurus coeruleuswithin damselfish territories increases disproportionately with group size because each member of the group can be attacked that much less by the resident damselfish (see graph). Foster 1985 Diss Abstr Int Part B: Sci & Engineer 46: 155; Foster 1985 Anim Behav 33: 782.

NOTE this would only occur during the initial stages of the tang "attack". Later on, when the resources are mostly or entirely consumed, "per capita" intake by the tangs would fall off, eventually to zero. By this time the group would have moved on. On the graph the line corresonding to this would curve the other way, eventually dropping to zero or near zero. The third alternative, that of a straight line on the graph signifying steady intake regardless of group size, would only occur in feeding grounds of unlimited size and without interference from any plot-holder

photograph of longfin damseslfish
Longfin damelfish Stegastes dienceus in its territory 0.25X

photograph of shoal of blue tangs
Shoal of blue tangs Acanthurus coeruleus feeding 0.07X

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

"Damselfishes often have adjacent territories, as witnessed by the gauntlet traversed by this terminal-phase queen parrotfish. First comes two attentive bicolour a nipping by a dusky, now a yellowtail. Brutal, huh?!" - Cayman Brac 2001

NOTE Scarus vetula

NOTE Stegastes partitus

NOTE Stegastes fuscus

NOTE Microspathodon chrysurus

view of reef showing disposition of dusky and 3-spot damselfishes in PanamaIn Panama, 3-spot damselfishes outcompete dusky damselfishes for preferred deeper areas of the reef slope (2.5m depth) where diatom food-resources are richer, thus relegating the dusky damselfishes to less-preferred surge-dominated shallow reef-flat areas (0-2m) where food resources are poorer. Cleveland 1998 Diss Abstr Int Part B: Sci Engineer 59.


Quiz for damselfishophiles1: with this information in mind, try to answer these questions, then CLICK HERE for help.

1. Which species is aggressively dominant over the other?

2. Which species has a longer relative gut length2?

3. Which species has a higher absorption efficiency3?

4. Which species grows faster?

5. Which species defends a larger territory?

6. Which species has a higher cost of territorial defense?

NOTE1 "lovers of damselfishes"

NOTE2 refers to length of gut relative to body length. A value of 3, for example, means that the gut is 3 times longer than the body. The author of the study notes that damselfishes, in general, have values of 1-3 - much lower than values of 1-10 recorded for other herbivorous fishes, and owing in all liklihood to the nutritionally richer diatoms eaten by damselfishes in preference to other less-nutritious algal foods in their gardens

NOTE3 the proportion of food matter eaten that is retained in the body; in other words, not defecated