Welcome to the Damselfish Family
Together with the anemonefish the damselfishes belong to one of the largest families of reef fishes, the family Pomacentridae. There are approximately 340 species of Pomacentrids, of which the anemonefishes occupies just 30 species. The vast majority of this family is constituted by damselfishes, which are again divided into various genera like the many species of Chromis, Dascyllus, Demoiselles and Sergeants. While the genus Chromis constitutes with 75 documented species the biggest genus of damselfishes, the genus Dascyllus is probably the most studied and best understood so far. Especially concerning their sexual and social behavior many damselfishes are still poorly understood (probably many scientists were still shocked by the weird life of the clownfish ladyboys).
Where they live?
Like their sisters the anemonefishes, damselfishes are relatively small and depend on an environment providing various hiding places. When threatened they shoot into holes, cracks and crevices. Coral reefs with lots of branching corals are thus ideal places for damselfishes. The vast majority lives
at tropical coral reefs at a depth above 20 meters. Only a few species live in brackish or fresh water. A few species of the genus Chromis are also found in deep-water, down to almost 300 meters (Chromis Struhsakeri). Concerning their habitats we can distinguish two groups of damselfishes: The one staying close to the reef and feeding mainly on algae, and the one higher above the reef in the water column feeding mainly on zooplankton. The zooplankton-eaters have usually more cylindrical bodies and deeply incised caudal fins, which allows them to swim in stronger currents and flee from predators into the farer hiding places. The other groups are a bit less aerodynamic (which in the water should actually be called hydrodynamic, but who is using this term?).
The vegetarian damselfishes (scientifically be called herbivores) feed on a variety of algae, including green, red and brown algae. As they are too weak to break down the algae’s cell walls with their teeth, they possess one of the most acid stomachs of the reef-dwelling life forms (pH between 1.9 to 3.0) allowing them to digest softer multicellular algae species. However, the main form of their nutrition is constituted by unicellular algae (microalgae) anyway, probably easier to digest. The vegetarian damselfishes are famous for their “algae-farming” methods. They defend their algae gardens from other algae-eating fishes, as well as weed out certain slow growing algae and let fast growing algae flourish. Algae which are indigestible for damselfishes are indirectly protected by them too. Scientists showed that damsel’s territories contained much higher densities of algae and a much higher overall productivity than unoccupied places. Interestingly those damselfish algae gardens are a growing spots for bacteria and small invertebrates such as demersal zooplankton organism (amphipods and copepods), as well as tiny crabs, worms and brittle stars. Those animals are ingested together with the algae and some scientists even believe that those tiny animals constitute the most important part of the algae-farming damselfish’s diet. Thus algae-eating damselfishes are in fact omnivores (fish that eat animals and plants), or harshly said “fake vegetarians”.
Straight and not so straight Carnivores
The straight carnivores (mainly those in the genera Chromis, Dascyllus and Demoiselle) feed mainly on zooplankton (mainly copepods, to a lesser degree also amphipods, isopods, crabs and shrimps). Beside zooplankton they were occasionally observed to eat sponges, small snails, nudibranchs, small insects and hydroids. During coral spawning season many damselfish change their diet to coral spawn. A minority of carnivore damselfishes feed also on stony coral polyps (Biglip Damsel), soft corals and anemones (Black Damsel). Other damselfish eat occasionally fish eggs and fish larvae. But even those straight carnivores aren’t that straight and often switch to a more algae-rich diet if circumstances require it. Like the “fake vegetarian” they are better thought of as omnivores then straight carnivores.
Damaging and supporting Coral Reefs
How about the interaction of damselfishes and corals? Non-farming damsels, who occasionally feed on algae, may remove the detrimental green organisms, which surely support the growth of coral polyps. But how about the farmers? We know that there algae gardens flourish and that the increased growth of algae is negatively correlated with the growth rate of coral polyps. Some farmers even attack and kill coral polyps directly to make space for a new patch of algae to feed on. On the other hand they protect their coral patches from any intruders. Damselfishes have been shown to be real heroes when it comes to protect their territories from other herbivore and carnivore damselfishes. But they defend their spots vigorously from any intruders,
even if they’re many times their size, they aren’t scared. Damsel often mob or harass even their own predators. It was observed that some species harassed scorpionfishes, moray eels and lizardfishes. Even barracudas, triggerfishes and octopuses were attacked by brave damselfishes, real heroes. Because of their protection coral larvae are able to settle within their territories without being eaten by coral-eaters. Beside the farmers many damselfishes, especially the zooplankton-eaters have been shown to have a supportive effect on coral growth rates. Their movement among the coral branches helps to circulate fresh, oxygenated water to the inner parts of a coral colony. Also their excrements are rich in ammonium, nitrogen and phosphorus, important nutrient supplements for corals. Studies showed that coral colonies that contained Dascyllus damsels grew considerably faster than those without any.
Sexual life: The Tomboy Dascyllus
What about their sexual life? As mentioned, many species are still poorly understood, but there is plenty of information about the Dascyllus available. Most species of the Dascyllus genera change from female to male, which means they do exactly the opposite of their clownfish sisters. The most dominant fish is always a male and all other fishes in the group are either female or asexual juveniles. When the male dies or escapes to another place, a big female will change into a male. The mating system of the Dascyllus depends on their habitat. If they live among scattered coral heads, a male usually occupies an isolated coral head as its territory. Females and juveniles will join his territory and other males will be attacked and driven away. But on continuous coral reefs the Dascyllus follows a more promiscuous mating system. Males will occupy their territories again, but the female usually form feeding groups which roam through several territories of several males. When it’s time to reproduce, the female will travel from one male’s territory to another and spawn several times (real b…busy fishes). Like the anemonefishes Dascyllus deposit their eggs on a hard substrate like a piece of dead coral, a dead shell or a man-made object. It’s the duty of the male to protect its nest till the eggs hatch. The larvae will swim into the water column and join the plankton for 17 to 47 days, considerably longer than anemonefish larvae. When the larvae transformed itself into a fully developed tiny Dascyllus, the baby fish will descend to locale a coral colony in which to settle and start a new life cycle.