Mick's marine biology: Sunfish in Oceanário

I’ve seen it for real: a sunfish. Not in real nature, but while visiting Oceanário in Lisbon, Portugal. A strange and grotesque creature. Huge, leathery skin, small eye and small beak. It looks like an animal that has been on earth for millions of years.

The scientific name of the sunfish is Mola mola. Mola means millstone. Does it resemble a millstone? The ‘Zeevissengids’ (see literature) quotes that it makes a grinding sound. Grinding -> millstone -> scientific name?
The name sunfish (also ocean sunfish and trunkfish) may have something to do with a habit (see later). In Dutch it is called maanvis, i.e. moonfish, the opposite of the sun. What’s in a name? It must be a lack of fantasy on my part.

The colour of the upper part and fins is usually greyish brown, the lower half has a lighter colour, sometimes silvery. The green colour in the first photograph of the lower part is due to reflection of the water and the ground.

The sunfish can reach 4 meter and can weigh 2 tonnes. This specimen was about 3 meter long. How did they catch and transport this sunfish to the aquarium? As a juvenile?

It is generally epipelagic (the zone from the surface to around 200 m), occasionally to depths of 600 meter. It is rarely seen near shores. Sunfish tracked with electronic tags dived frequently (up to 20 dives per day) to depths of 600 m to feed. They are often seen drifting on their side at the surface as if basking in the sun. Hence its name? This behaviour may help the sunfish to keep their muscle temperature and metabolism at a higher level. Wheeler says (see literature) ‘To what extent this basking behaviour is normal and whether these fish are sick or disabled is not known, but the latter seems more probable’.
The sunfish is found in all oceans except polar seas. Nowhere is it common, but they seem to prefer warm temperate areas.

The fecundity of this giant fish is prodiguous: the ovary of a 1,24 meter female was estimated to contain 300 million eggs.

In the books I consulted it is mentioned that they eat jellyfish, comb-jellies, crustaceans and small fish. In inshore waters specimens have been found with seaweeds, brittle-stars, bagpipes and bigger fish in their gut.
They are not good to eat though Japanese fishermen are reported to relish the thick chewy blubber-like tissue under the skin, which they eat raw. You have to eat something if whale-meat is not available…

© Oceanário

If you are going to Lisbon you should visit the Oceanário. It exhibits aquaria with 500 different species in 7 million litres of sea water. Build in the form of a cross, two storeys high, there are several aquaria, looking like just one. The main aquarium ‘Global Ocean’, hosting the sunfish, is the most impressive one. The panels are made of acryl and about 30 cm thick.

For more information about Oceanário, click here.