By Dr Deborah Robertson-Andersson.
This week both myself and The Two Oceans Aquarium received a number of calls from concerned citizens regarding fish with missing tails washing up near Velddrif on the west coast. Sightings of these animals, both dead and alive, have also been reported on beaches and in shallow water off Blouberg, Melkbos and Noordhoek earlier this year.
The fish that are washing up is called a slender sunfish (Ranzania laevis). They normally occur in the Agulhas current and get caught in warm water rings that move into the Benguela current. As slender sunfish are a tropical species and prefer temperate conditions, when they get into the cold Benguela water they get hypothermic and stop functioning and then wash ashore.
According to Ocean Sunfish.org, “The Polynesians called these sunfish the King of the Mackerels. It was seen as bad luck to catch and kill Ranzania, as the act would render the mackerel incapable of finding their way to the islands.
Presently, three distinct species are recognized within the family Molidae including: the common sunfish, Mola mola Linnaeus 1758, the sharp-tailed sunfish, Masturus lanceolatus Lienard 1840, and the slender sunfish, Ranzania laevis Pennant 1776. While ocean sunfish and sharptail sunfish are relatively common in our waters at certain times of the year, slender sunfish are rare and little is known about them. They belong to the order Tetraodoniformes which includes other fishes like trigger fish, boxfish, porcupine fish, and puffer fish. The name Tetraodontiformes refers to the four fused teeth that comprise their characteristic beak.
At 80 – 90 cm in length, the slender sunfish is a small animal compared to the giant ocean sunfish (Mola mola), which attains a size of 3 metres and holds the record for the world’s heaviest bony fish – a 3.1 meter long specimen weighed in at 2235 kilograms from snout tip to clavus “tail fin” and 4.26 meters from dorsal fin to anal fin tip. This animal was struck by a boat off Sydney, New South Wales, Australia in September, 1908. The report goes as follows: “On 18 September 1908, the Steamer Fiona, was 65 kilometers from Sydney when it suffered a ‘violent concussion’. A boat was lowered over the side and the men onboard saw a Sunfish jammed in the framework of the port propeller.”
Another sunfish, stuck on the bulbous bow of the cement carrier, MV Goliath on 13th October 1998. The fish became stuck on the bow off Jervis Bay, New South Wales and weighed approximately 1400 kilograms. It caused the speed of the ship to slow from 14 to 11 knots. The skin of the Ocean Sunfish was so rough it wore the ship’s paint work back to the bare metal. The fish measured 3.1 metres from the tip of the dorsal fin to the tip of the anal fin, and 2.5 metres from the tip of the snout to the end of the clavus. What is really impressive for me is that these animals reach this size eating a diet of jelly in the form of jellyfish, salps, ctenophores and occasionally small crustaceans and fishes.
The common sunfish, Mola mola, are infamous for their impressive parasite load. Some 40 different genera of parasites have been recorded on this species alone. In fact, even their parasites have parasites. One of my best days at sea was spent sunfish wrangling. We caught and rode 18 sunfish in one day and the objective of all of that work was to get the parasites off the sunfish. This would usually entail us rubbing our wetsuits on the sunfish during the wrangling and then the parasitologist picking them off us with a pair of forceps. Since parasites often sport multiple hosts, they can offer valuable insight into sunfish interspecies associations. For instance, one sunfish parasite is the larval stage of a shark tapeworm so at some point the sunfish most likely falls prey to shark enabling this parasite to complete its lifecycle. Other known predators of sunfish are orcas and sea lions. Slender sunfish predators are little known, but animals have been found in the stomach of marlin off Hawaii and numerous youngsters have also been found in the guts of common dolphinfish (Coryphaena hippurus).
At sea you normally only catch a fleeting glimpse of a fin or a large flat disk before the fish dives into the ocean depths – as a result many people mistake sunfish for sharks. It is often thought that the sunfish are basking in the sun and doing some form of thermoregulation, however sunfish spend just over 30 % of their time in the top 10 metres of the water column, and more than 80% of time in the top 200 metres. The maximum depth recorded by any fish is 844 m.
Sunfish produce an impressive number of eggs and hold another world record for the largest number of eggs ever recorded in a single vertebrate at any one time. A 1.4 metre female was estimated to be carrying 300 million eggs in her single ovary. (Larger sunfish would most likely carry even more.) Needless to say, the eggs are tiny and would fit into the size of this “o”.
After hatching, the larvae expose their similarity to their spiky puffer fish relatives by looking more like, swimming pincushions than small sunfish. As they grow the spines disappear, as do their tails.
Members of the public who encounter slender sunfish on the beach or in shallow water are encouraged to contact the Two Oceans Aquarium immediately on telephone 021 418 3823.