Algae Eating Damselfish

The threespot damsel (Stegastes planifrons) is a species of farmer damsel that selectively attacks other fish that have a similar taste for detritus and algae. Photo by Scott W. Michael

The damselfishes (family Pomacentridae) comprise one of the largest families of coral reef fish, with more than 330 species. Most of these fishes inhabit tropical reef environments, and a number of them make their way into local fish stores. There are good reasons for their omnipresence in the aquarium trade. One of their best selling points is their extreme hardiness.

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They also tend to stay small, with many never exceeding 4 inches in total length. As a result, most can be housed in small to moderate-sized aquariums. And some of the damsels are unrivaled in chromatic splendor.

But not all damsels are highly desirable for the smaller home aquarium. One group of pomacentrids that has a dubious reputation among experienced fishkeepers are the algae-farming damsels. That said, they may have a place in a specialized aquarium system. I’ll focus on the amazing behavior of the farming damselfishes, how they impact coral reefs, and some of the pros and cons that go along with keeping them in our captive oceans.

Algae-eating and Farming

The majority of pomacentrids are diurnal omnivores, with the amount of plant material in their diets varying greatly from one species to the next. A number of species feed heavily on algae (including members of the genera Dischistodus, Microspathodon and Stegastes). These fishes often have villiform or incisiform teeth that are suited for scraping or biting algae from the substrate. Some species (e.g., Microspathodon spp.) have flexible comblike teeth that are used to brush detritus and algae from the algal mat. These species feed on a wide variety of cyanobacteria and algae species, including green, red and brown algae. They concentrate on microalgae, especially filamentous forms, while macroalgae usually constitutes a minor part of their diet. This is in part due to the fact that damsels have difficulty grinding up the cell walls by mechanical means (e.g., they lack the pharyngeal “mill” like the one found in parrotfishes) and instead rely on chemical mechanisms to break down plant material.

The herbivorous damsels have acidic stomach secretions. The pH of the stomach is among the lowest found in any reef fish, ranging from 1.9 to 3.0 in some of the Stegastes species. Although these acidic stomach conditions are able to lyse the cell walls of blue-green algae and more delicate unicellular algae, it has little impact on tougher macroalgae species.

Many of the damsels (e.g., members of the genera Dischistodus, Microspathodon and Stegastes) are well-known for their algae-farming habits. Herbivorous damsels usually defend feeding territories — if a plant-eating fish nears or enters its territory, the damselfish will attack it. The pomacentrids are so pugnacious that they will drive off fish many times their size and will attack large schools of roving herbivores. By preventing competitors from entering their territories and obliterating their food sources, the damselfishes maintain an algal garden or turf. One study found that the coverage of algae in noncoralline damselfish territories was between 91 to 96 percent, while in territories outside pomacentrid zones, 59 to 73 percent of the substrate was covered with algae.

The more scientists study damsels and coral reef productivity, the more they have found that amorphic detritus (fish feces, coral mucus, other organic particulates and microbes) is an important source of nutrition for pomacentrids, as well as other coral reef fish. The algal mat that is created by damsel farming activity acts as a matrix that collects detritus. Studies have shown that 10 to almost 80 percent of the organic content of the algal mat is comprised of detritus. In fact, farming damsels may benefit as much from the detritus that accumulates in their algal garden as the plant material itself. Members of the genera Dischistodus, Hemiglyphidodon and Stegastes are known to take advantage of amorphic detritus.

Herbivorous damsels that maintain territories are usually classified as either “nonselective farmers” or “selective farmers.” The nonselective farmers feed on a variety of algae species present in their territory rather than actively target specific types of algae. By feeding this way, these damsels “weed out” the slower-growing algae species. This results in fast-growing algae species (e.g., red filamentous forms) dominating their gardens. (In some damsel species, the mature garden will consist almost entirely of a single algae species.) In at least some damsels, these faster-growing algae species are also maintained in the more rapid growth phase of their life cycle (i.e., the early phase when the “shoots” are finer and more delicate, and are also easier for these fishes to handle and digest). While nonselective “weeding” activities result in the damsel garden having a lower standing crop than the areas surrounding their territories, they are much more productive. For example, the garden of the giant blue damsel (Microspathodon dorsalis) can be 34 to 47 times more productive than the adjacent algal mat.

The selective “farmers” eat cyanobacteria and algae species (including diatoms) that grow on the surface of the algal mat and leave the basal layer intact. In some cases, this basal layer is comprised of red macroalgae, which pomacentrids cannot digest and thus rarely ingest. This macroalgae mat traps detritus and becomes a sanctuary for small benthic invertebrates; the invertebrates provides a valuable supplemental food source for these omnivorous fish.
Juvenile farming damsels are less efficient than adults at assimilating the nitrogen from plants. Therefore, the juveniles feed more on animal matter (e.g., foraminiferans, small crustaceans and other benthic invertebrates) to meet the requirements needed for basal metabolism and growth. Adults are better equipped to absorb nutrients from plant material. The juveniles ingest about twice as much food for their body weight as the adults.

Damselfishes and Corals

Pomacentrids can have both a positive and negative effect on the health and distribution of stony corals. Studies have shown that grazing by nonfarming herbivores promotes the settling and growth of coral larvae and the development of coralline algae. By removing algal growth, these herbivores provide substrate for corals and coralline algae to grow on. But this is usually not the case in pomacentrid territories. The farming behavior of these fishes leads to an increase in filamentous algal growth, which chokes out most sessile invertebrates and coralline algae. Because they suppress the growth of coralline algae — which is an important cementing agent that creates a more solid reef structure — pomacentrids are often detrimental to the physical development of the reef framework.

The coral-killing habits of some damsels are also well-known. These species kill the polyps of stony corals in order to provide more growing substrate for the algae that they eat. As a result, the presence of damselfish territories can greatly impact coral communities. On Panamanian reefs, the Acapulco gregory (Stegastes acapulcoensis) inhabits heads of Pocillopora corals. The damsels will bite at and kill the ends of the coral branches to encourage algae growth on the tips. Even so, they do little damage to the entire colony. These damsels attack herbivores and corallivores that move into their territory. As a result, coral larvae are able to settle and grow within damsel territories without being eaten by coral-feeding fishes. While the presence of these damsels is favorable for Pocillopora species, they have a deleterious effect on Pavona corals. This is a nonbranching species, and unlike Pocillopora, the nipping behavior by damselfish kills much of the coral colony. Therefore, where damsels are present, Pocillopora colonies are common; at greater depths, where Stegastes acapulcoensis is less common or absent, Pavona corals are more abundant. A study conducted on the white damsel (Dischistodus perspicillatus) has shown that when this species establishes a territory over a coral colony, the stony coral is usually overtaken by a layer of fleshy algae.

Farming Damsels in the Home Aquarium

After reading the preceding information, it should be obvious why keeping these fishes in the home aquarium can be problematic. First, most are highly territorial, excluding close relatives and other herbivores from their algal gardens. In the aquarium, they often are not as discriminating, attacking any fish that enter their territory, which may encompass the entire aquarium. For example, the dusky damsel (Stegastes dorsopunicans) defends a minimum area of approximately 6.5 square feet (that is more than the surface area of a 75-gallon tank). While that may be the size of the territory of this species in the wild, it will often defend a larger area in an aquarium where predators are in short supply.

Also, many farming damsels actually defend a larger territory than S. dorsopunicans. So, if you throw a dusky damsel in a 135-gallon tank, it is likely to claim the entire aquarium as its “kingdom,” making the addition of most other fish difficult. These fishes are not going to work with more peaceful tankmates and may even chastise larger, more aggressive fish if they are added after the damsel has claimed the entire tank.

The second problem is that farming damsels may kill corals to encourage microalgae growth. They are farmers, and they want to grow algae. Last time I checked, most aquarists are trying to get rid of algae, not grow it. This is especially true for coralkeepers. The farming damsels are not suitable for reef aquariums.

 Pomacentrid Territories & Coral Reefs

Territorial pomacentrids greatly affect the ecology of coral reef communities. Here are some ways they have an impact:

  • The farming behavior of damselfishes often results in the number of algae species in the pomacentrid’s territory being depressed, as they often eliminate the slower-growing species.
  • By defending a specific area and encouraging the growth of filamentous algae, territorial pomacentrids affect the distribution and abundance of small mobile invertebrates that utilize the algal mat as a refuge, and the distribution of amorphic detritus on the reef.
  • Because their farming behavior often excludes coralline algae and coral growth, damsel territories often consist of unconsolidated rubble, and as a result they are more likely to erode.
  • Pomacentrid territories are also important sites for nitrogen fixation because they contain a significant amount of the blue-green algae biomass found on coral reefs. (Blue-green algae and certain bacteria take dissolved nitrogen and “fix” it so that plants can utilize it.)
  • Damsel territories increase the primary productivity of the reef overall. The territories of one damsel, Stegastes planifrons, contributed 70 to 80 percent of the overall productivity of coral reefs off St. Croix in the Virgin Islands.

The third problem is a chromatic metamorphosis that occurs in some species. For example, many of the Atlantic members of the genus Stegastes exhibit various yellow and blue color combinations and schemes. They are lovely when little, and neophyte fishkeepers may be lured into acquiring a juvenile for their community aquarium because of their striking colors. But many of these fish undergo a color change, turning from beautiful to outright boring (they turn brown). Microspathodon and Dischistodus also change from dramatic to dull as they grow larger, though members of the former genus do retain some brilliant colors as adults. If you want a striking adult fish, you may want to look at different damsels.

So, there are three strikes against the farming damsels, but these fishes can make entertaining, fascinating aquarium fish. On several occasions, I ended up adopting a Stegastes from an aquarist who acquired one of these bellicose species for a community tank. These fishes become real pets when housed on their own in a 55-gallon tank. You can watch them farm, weed out certain algae types, or pick up and remove competing hermit crabs or sea urchins from their algae gardens (they will grab a sea urchin spine in their mouth, lift the urchin and move it to the other side of the aquarium). Another upside of owning a farming damsel is that they are some of the most durable fish on the coral reef. Be prepared, though, as they may bite your hands as you do maintenance.

Species Available to Aquarists

There are a number of algae-farmers that show up in the aquarium trade. Most of these are members of the genus Stegastes, which are often referred to collectively as “gregories” or “farmerfishes,” and most available in the hobby are collected on tropical Atlantic reefs. Some species occasionally offered by Atlantic fish collectors include the dusky damsel (S. fuscus), threespot damsel (S. planifrons) and the beaugregory (S. leucostictus).

Members of the genus Microspathodon are also farmers, and at least two of these show up in the aquarium trade. Hobbyists regularly encounter the yellowtail damsel (Microspathodon chrysurus). This species should not be confused with the yellowtail demoiselle (Chrysiptera parasema), which is diminutive. Juveniles of this species are striking, exhibiting bright blue spots on a dark background, but the color changes as the fish grows.

The genus Dischistodus (often called farmer damsels collectively) contains six species that occur in the Indo-West Pacific. These are aggressive farming species. In fact, I have been bitten more by these damsels than any other marine animal (they will attack divers if they rest in their territories). They do bite, and it can hurt if they nail you in the right place. More often than not, you just feel them make abrupt contact without injury or pain. Dischistodus gardens are often found on dead coral branches and tend to occur on more silty reefs. Juveniles of this genus have distinct ocelli on the middle of the dorsal fin, which in all but one species disappears as the fish grows. The species most often encountered by aquarists is the white damsel (Dischistodus perspicillatus). This species is often sold under the generic label “assorted damsel.”

While they may not be suitable for your aquarium, farmerfish greatly impact coral reef health and productivity (see “Pomacentrid Territories & Coral Reefs” sidebar). All lovers of nature should be aware of the vast impact that these amazing animals have on the reefs. For those who are willing to dedicate a tank to a farmer damsel or who have larger fish-only systems housing piscine residents that can fend for themselves, this intriguing guild of pomacentrids can make for interesting pets. Until next time, happy fishwatching!

Scott W. Michael is the author of Reef Sharks and Rays of the World, as well as Reef Fishes: A Guide to Their Identification, Behavior and Captive Care and more. His photos have appeared in publications around the world.

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