The pelvic-brooding ricefish Oryzias eversi – hardly known and already lost?

Ricefishes are a family of small freshwater fishes from South, East and South-East Asia, which are known in particular for the medaka (Oryzias latipes complex), which is well-known as a model organism in biology and also from its attractive color breeds. However, the greatest diversity of the family can be found on Sulawesi, where around half of all described species occur.

Some of these species show remarkable peculiarities. A good example for this is Oryzias eversi. First, there is its peculiar reproductive biology: O. eversi is one of the so-called pelvic-brooding ricefish species, which only occur on Sulawesi. The majority of ricefishes are so-called transfer brooders, meaning that shortly after mating, the females stripe off fertilized eggs, which stay attached to the genital pore of the female via filaments, on substrates like plant matter or fine roots. However, a small selection of ricefishes from Sulawesi has evolved a different way. Instead of striping them off, their eggs stay attached to the body via filaments until the point of hatching. This carrying period can take two to three weeks. The evolutionary reasons for the emergence of this strategy are still being researched. For other pelvic brooding species, it was first believed to be an adaptation to open water habitats in big lakes, allowing the fish to reproduce despite a lack of available spawning substrates. But O. eversi clearly does not inhabit such a habitat.

The distribution and habitat are the second peculiar thing about this species. The Tilanga karst pool, located in the Tana Toraja region of Southern Sulawesi (Sulawesi Selatan), in which Oryzias eversi was discovered is tiny – only about 40 meters long and 10 meters wide. Despite some efforts to find similar habitats in the proximity of the Tilanga Pool, the species has so far not been found anywhere else. Possibly, the pool really is the only existing place where this ricefish can be found, making it a so-called micro-endemic species. It is only a few meters deep, strangely enough with plenty of substrates like plant roots available to potentially transfer eggs to.

Unfortunately, the Tilanga Pool is not exactly a safe haven for Oryzias eversi. It is a popular destination for both locals and tourists as a natural swimming pool with crystal-clear, cool water. People have discovered the economic potential of this oasis, and in order to enter the pool, you have to pay an entrance fee at a small yellow cashier’s office. The pool is used for diverse activities. Some people wash themselves there, and inevitably, soaps and other detergents enter the water. Moreover, the pool is also inhabited by non-native fish species introduced by humans. Carps roam in the clear water, and swarms of guppies can be seen everywhere. The only other native fish species are a freshwater halfbeak (Nomorhamphus rex), as well as the Giant mottled eel (Anguilla marmorata), which grows up to 1,5 m in length and is fed in the pool by locals with raw egg yolks, another reason for which the pool is known there.

In 2010, when the species was discovered, plenty of ricefish could be seen in the pool. Males defended small territories, whereas females were seen staying together in small schools. In 2019 however, when researchers of the Zoological Research Museum Koenig in Bonn, Germany revisited the pool, they only spotted around ten individual ricefish underwater, all of them in poor condition, covered in parasites and fungal infections. Some of the parasites were anchorworms, parasitic crustaceans which attach to the skin and fins of fish. Most likely, these parasites have been introduced to the pool by other, non-native fish species. Further, fungal infections are often caused by stress, for example due to pollution with chemicals. Clearly, human interaction with this peculiar fish did have a very negative impact on this fish so far. Today, two years after the pool was visited by ichthyologists the last time, it appears possible that the species has already gone extinct there, which – due to its possible micro-endemism – means that it may also have disappeared in the wild completely now.

Fortunately, the Tana Toraja ricefish had been discovered by aquarium enthusiasts Hans Evers and his friends, who managed to collect some fish for the aquarium hobby when the population was still intact and ricefish were plentiful in the Tilanga pool. Not only because of the parental care provided by the egg-carrying females, it turned out to be an interesting aquarium fish and quite easy to breed, with offspring that could be raised with little to normal effort. Unfortunately to its own disadvantage, the species did not show pretty colors like some of the other Sulawesi ricefish species that were discovered at a similar point in time. This prevented it from gaining a comparable popularity among aquarists as the blue and red Neon ricefishes (Oryzias woworae group) from South-Eastern Sulawesi (Sulawesi Tenggara). Nonetheless, its brooding biology is unique, and due to its smaller size and non-pelagic ecology, it is much easier to keep than the other pelvic-brooding ricefish found in the aquarium hobby, the related Oryzias sarasinorum from Lake Lindu, which grows larger than other ricefish and needs much more space for swimming. Several zoological gardens have already adopted Oryzias eversi for their aquarium exhibitions, and participate in maintaining a livestock in captivity, so that its final extinction seems less certain to happen. With more aquarium hobbyists and zoological institutions keeping this species, new ways may open for a possible reintroduction project in the future.

Jan Möhring