In my last blog I wrote about clown fish, the lovable Nemo in most children’s mind. Clown or anemone fish belong to a large family of fish called the Pomacentridae, the damselfish. While I promised to write about symbioses, in this blog I will cover the other members of this family that inhabit the UNBC reef tank. Damselfish are relatively small, with the largest species reaching just over 35 cm in length. Many of them have stunning colours, while others may be relatively plain. A genus familiar to most people who have visited a tropical reef for some snorkeling is the sergeant majors, adorned with the delightful Latin genus name Abudefdaf . What damselfish lack in size, they often make up for in temperament becoming of much larger fish, however. One of the most commonly kept damsels, Chrysiptera cyanea, bears the telling common name “blue devil”, for example, even though it isn’t necessarily the worst offender. Even the most timid species generally come with a warning that they can take over and terrorize an aquarium.
The UNBC reef tank is home to two types of damselfish,neither of which fall in the terror category. One is the so called green chromis, Chromis viridis. The green (or blue green) chromis is relatively plain as damselfish go, but in certain light it displays a gorgeous blue-green sheen that gives the species its name. The colour is apparently produced by the
presence of chromatophores, allowing some colour change (Fujii et al. 1989). This species is as close to a schooling fish that you can come, and it is also readily available in the pet trade, relatively peaceful, completely reef safe (i.e., they do not feed on coral polyps) active and hardy, and therefore one of the most popular fish to keep.. The oldest member in UNBC reef tank is approximately 12 years old. The other two were added about 8 years ago, and it is only due to the size of the tank that they are still with us. Even the peaceful green chromis does not take kindly to newcomers in their territory it turns out. I added them because someone had traded them in to the LFS (=live fish store), they needed a new home, and the UNBC tank could use a few more. The minute the newcomers were released into the tank, however, the residents turned from peaceful bystanders to vicious bullies, attacking the new fish mercilessly. This went on for hours, but fortunately the victims managed to hide in the rock crevices and hence get some relief from their pursuers. Eventually they were accepted, and now live with the remaining elder. In agonistic encounters, green chromis apparently produce sounds (Amorim 1996), like many other fish so this may have been a noisy time in the tank! The lesson learned is that it is a bad idea to add fish of a species, or even one similar in appearance, to a tank with established residents of that or similar species.
The other species of damselfish is a relative newcomer to the UNBC reef tank. It is the yellowtail damselfish, Chrysiptera parasema. Considered one of the less aggressive damselfish, this species can still take over a tank under the right circumstances. In nature it lives in small groups over branching coral in protected waters, and you can see the natural behavior of hanging out above the coral, only to dive in among the coral branches and rocks when danger is perceived.
The yellowtail damsel is a jewel of a fish, with iridescent dark blue patterns over its blue body, and a yellow tail fin that gives it its common name. It is very hardy, and is sometimes used to help cycle a newly established tank. This is a practice that I (and most responsible reef tank keepers I should think) discourage, as it is cruel to subject fish to the often very high nitrate levels that build up before the microbial fauna has established itself in a new tank, even if the fish are capable of tolerating it and may survive. All it takes to cycle a tank properly is some patience. Nevertheless, both of these damselfish are quite hardy, so they are considered among the most suitable species for beginner reef tank enthusiasts. It helps that they are relatively inexpensive, fetching $4-8 depending on the retailer, and they accept most foods. In the wild, they forage on algae, thereby performing a valuable service on the coral reef, but they can occasionally nip coral polyps, and are therefore not completely reef safe. Different individuals may vary in their propensity to damage corals. Interestingly, some species of damselfish actively cultivate their preferred food source (Hata and Kato 2006) by defending a patch and pruning out unpalatable algae! The yellowtail damsel has interesting reproductive behavior, where males prepare a spawning nest where a female will lay eggs. The male will then aerate the eggs, and he aggressively defends the nest until the eggs hatch.
Amorim, M.C.P.D. (1996). Sound production in the blue-green damselfish, Chromis viridis (Cuvier, 1830) (Pomacentridae). Bioacoustics 6: 265–272.
Fujii, R., Kasukawa, H. and Miyaji, K. (1989). Mechanisms of skin coloration and its changes in the blue-green damselfish, Chromis viridis. Zool. Sci. 6: 477-486.
Hata H, and Kato M (2006). A novel obligate cultivation mutualism between damselfish and Polysiphonia algae. Biol Lett 2: 593–596.