These are razorfish (Aeoliscus punctulatus). There are currently 11 adults on display in the Oceans of Contrast: Indian Ocean Gallery at the Two Oceans Aquarium.
On 12 October 2015, something truly magical took place and some of the adults in the display spawned. In this display we also house a paperfish (Taenianotus triacanthus) and tiger cowries (Cypraea tigris). The cowries also spawned in the display and it seemed to coincide with the razorfish spawning. I am not 100% certain but I speculate that the razorfish spawned due to the cowries spawning, thus providing a potential food source for the razorfish larvae. It may have been an environmental cue to trigger spawning. It would make sense to allow hatching of your young when there is an abundance of food in the surrounding water!
Our aquarist (this is what someone who looks after animals at an aquarium is called) taking care of the display informed me of the event and I came down to investigate. There were eggs in the water column and also larvae in the process of hatching. We collected as much as we could and I moved them up to quarantine.
I set up a conical tank with a heater to match the temperature (23°C) from the display. Here the eggs would stay until they hatch. In a separate 100 litre cylindrical tank (black bin) I set up the rotifer culture.
Rotifers can be kept in culture by growing algae and feeding them algae paste or live algae (green water). They will happily multiply and reproduce in a tank and can then be used for feeding filter-feeding animals or, in my case, small razorfish larvae. They are very useful for larval fish as most larval fish are extremely small and need very small phytoplankton (algae) and zooplankton (animal plankton like amphipods and copepods) to eat.
Rotifers can be too big for some larvae and then you would have to look to copepods or an alternate food source that is smaller in size.
It is worth mentioning at this point that the majority of warm tropical marine fish have a larval stage. Most freshwater fish do not have a larval stage but rather the young hatch out as fry or juvenile fish at a much larger size. Because of this, rearing of most marine fish is quite an art.
Once the rotifer culture is started it is best to keep them well fed so they can reproduce, and the rotifer count (number of rotifers per millilitre of water) should be as high as possible. When hatching out larvae, you would want to make it as easy as possible for the larvae to find food as this is when they are most vulnerable and when most mortalities occur. You want to provide an environment where your larvae are surrounded by food and do not have to spend any energy finding or hunting food.
Two days later, the majority of the eggs hatched and the larvae were moved to the black bin that contains the rotifers. Larvae have a yolk sac when they hatch, which provides food for them during the first one to two days after hatching. Once the yolk sac is used up they will begin feeding on the rotifers.
The rotifers will keep on reproducing as long as green water and algae paste are added to the tank. The rotifers feed on the algae and in turn the larvae feed on the rotifers. This will also allow for smaller rotifers to be present. All larval fish have different mouth sizes and are able to eat different sizes of food post-hatching. In the case of the razorfish it turned out the rotifer species we are culturing in the Aquarium is small enough for the larvae to eat.
Below are pictures taken under a microscope of the larval growth at different stages, from day two to day 22 post-hatching. (Note: 500um is 0,5mm.)
At 22 days old, you can see the fins starting to develop and they are starting to look more like the adults. The next morphological change will probably be the development of the flute (the long snout-like mouth).
All the photographs were taken of larval fish that have since unfortunately died. When rearing larval fish you would expect quite a high mortality rate. These fish are extremely small and in the wild would form part of the planktonic food chain, and not many would make it to adulthood. This is the reason why so many fish spawn thousands of eggs: to increase the chances that some of them hatch out and make it to maturity.
I currently have nine larval fish and I won’t be taking any out for photographs, so that they have the best chance of making it through to juvenile stage which is when it will be easier to care for them. Maybe one day they could go down into the exhibit and be ambassadors for their cousins in the wild.
I’ve learnt with some luck that keeping things as constant as possible is the key to success. You need to check temperature, water quality and check aeration in your rearing tank twice a day. Keeping your cultures (food) in the rearing tank at high levels without compromising water quality will increase your chances of rearing larval fish successfully. It pays off. The more time you spend on looking after these larval fish the better your chances of getting them through to adult stage.
It seems spring has really sprung these last few weeks at the Two Oceans Aquarium. On the 30 October 2015, we had the Cape Sole (Heteromycteris capensis) spawn on exhibit too! I am currently in the process of rearing these little guys.