10) Damselfish

Dascyllus melanurus

One of the most popular families of fish to be seen in hobbyist tanks are the anemonefishes and damselfishes, the Pomacentridae. Closely related to the almost entirely freshwater cichlids, they have very similar breeding behaviour, with one or both of the breeding pair guarding a nest of eggs laid on the substrate until the eggs hatch. The main difference from cichlids is that damselfish larvae are much smaller and disperse immediately into the plankton on hatching, whereas most cichlids engage in long term care of the fry.

Pomacentrids tend to be fairly small, and for marine fish “tough”, which means that they are more tolerant of variation in water conditions than most reef fish. The main drawback as aquarium fish is that they tend to be fairly aggressive in a confined space and many are territorial, which leads to problems in the average sized tank.

There are two species in the large marine tank at Bristol. Most showy is the single Black Tailed Damselfish, Dascyllus melanurus which is shown at the head of this post. There are numerous species of Dascyllus, all from the Indian and Pacific oceans. They almost all have a monochrome colour pattern, being variably marked with black and white spots and stripes. Many species become more uniform as they grow to adult size. D.melanurus is from Indonesian waters, where it lives in sheltered inshore waters and is often associated with small heads of Acropora coral. They grow to a maximum of around 10cm. In this species the eggs are guarded by the male until they hatch. They feed mainly on a variety of planktonic organisms.

Acanthochromis polyacanthus

  The other species is frankly rather dull in colour, being a uniform grey. Acanthochromis polyacanthus, the Spiny Chromis, is also among the larger species, reaching nearly 15cm. For this reason it is hardly ever seen in hobbyist tanks. They are found in Indonesia and the Philippines, where they are found in a variety of habitats from inshore lagoons to outer reef slopes. They form schools when young and split up into pairs once adult. As with Dascyllus spp., they feed on a variety of plankton.

A.polyacanthus

With this species, although the appearance is rather dull, the reproductive behaviour is slightly different from the normal pattern. They have lost the planktonic stage, and like freshwater cichlids the larvae remain with the parents. They have even been send to feed from the parents’ mucous coat, in the same way as Discus and some other species. As a result of this reduced capacity for long distance dispersal, there is considerable variation in the appearance across their range, as the other images on this page show.

A.polyacanthus

At present, all the damselfish for sale in the aquarium trade are wild caught. Unfortunately, as they are common and easily caught, there is little incentive to produce them from aquaculture, especially as the larvae are extremely small and consequently difficult to raise. If this changes in the future, it would be very advisable for hobbyists to select captive raised specimens, especially since many of the wild caught ones are obtained with cyanide in the Philippines, with obvious risks to the fishermen and invariably shortened lives for the fish.

Next week, the remaining species in the main marine tank, the Epaulette Shark.

 Images from wikipedia, fishes of Australia websites.