Annual fish inhabit isolated bodies of freshwater in Africa and South America that dry up seasonally. The species survive extinction by laying drought-resistant embryos which undergo various phases of reversible embryonic developmental arrests or diapauses. A hypothesis regarding their survival strategy in nature is presented here based on the current state of knowledge regarding its biology and the environmental factors which regulate the onset and the termination of diapause.
Annual fish and the Continental Drift
We now know that the supercontinent called Pangaea existed approximately 225 million years ago during the Permian Age. Pangaea began to split into two new continents—Gondwana (comprising Africa, South America and Asia) and Laurentia (comprising North America, Europe and Siberia). Gondwana was originally named by Eduard Suess and was named after the Gondwana region of central northern India (from Sanskrit gondavana “forest of Gond”). Gondwana began to drift apart during the mid-Jurassic Period (167 million years ago). During the Cretaceous Period (130 million years ago) South America began drifting from Africa and finally linked up with North America at the Isthmus of Panama creating the Atlantic Ocean.
The early evidences for continental drift came in the form of plant and animal fossils of the same age found around different continental shores, the fossils of the freshwater animals, such as crocodiles in Brazil and South Africa and the discovery of fossils of the aquatic reptile Lystrosaurus from rocks of the same age from locations in South America, Africa, and Antarctica. The living evidence comes from similar animals being found in both continents, such as the insects called mite harvestman that had existed for millions of years yet lacked the mobility to move great distances and annual fish that can live only in small seasonal pools that dry up during the summer. Both living life forms came along for the ride over the millennia.
Current reconstruction of the paleoclimatology of Gondwana showed that an arid, seasonally dry climate existed. The ability to postpone development to survive environmental extremes favored specialized species, such as annual fish. Thus, annualism began during the Cretaceous period and has continued long after the break up of Gondwana into Africa and South America. Annual fish, thus, is among the many notable examples that supported the concept of Continental Drift long before knowledge of how such a process can possibly occur.
During World War II, Vanderplank initiated the introduction of Nothobranchius taeniopygus to combat malaria in seasonal swamps in Tanganyika and Kenya ( letter to R. A Jubb, 1977). There were no field data available then to validate the performance against malarial vectors In 1952, the noted ichthyologist, George Myers, made the field observation that annual fish may have potential in biological control of mosquitoes since mosquito bites were diminished in areas normally populated by these fishes. It was not until two decades later in 1978 when Jonathan Matias and Jules Markofsky, then with the Orentreich Foundation in New York, that this idea was again re-introduced during the Columbia University Seminars on Pollution and Water Resources. It took yet again another two decades when Jonathan Matias at Nova Pacific Research Institute (Philippines) received a grant from Conservation, Food and Health Foundation (Boston, Massachusetts) to undertake laboratory studies on the predatory activity of annual fish against mosquito larvae. The results of this research are highlighted here to demonstrate the practical use of annual fish as a biological control method.
Annual Fish: Hobbyists and Conservationists
Most of our knowledge about annual fish comes from avid collectors, ichthyologists and hobbyists who go to Africa and South America in search of exotic fishes. Even today, the vast wealth of information on these unusual fishes comes from hobbyists who collect these fishes from the wild and breed them in captivity. Although the existence of these fishes was known even before the 20th century, it was in 1942 when George Myers described these unusual fishes that live in temporary freshwater pools and coined the term ‘annual fish.’ In the 1960’s Jorgen Scheel of Denmark began writing about killifish, which was later circulated letters among hobbyists. This period started the passion for the hobby. Since then, the interest has grown into a worldwide scope with over a thousand hobbyists in various societies who collect, breed and characterize the local environment, anatomy, behavior and location of various species in nature.
The destruction of natural habitats due to human encroachment, property development and man-made changes in terrain features are major threats to the survival of various annual species. By maintaining the various species of annual fish in captive breeding and through exchange of fishes among society members, hobbyists are ensuring the continued survival of many species. As new species are being discovered in the wild, it is also a distinct possibility that many more species may have already been lost to extinction. Thus, the conservation of annual fish comes about naturally as a result of the enthusiasm of hobbyists in collecting and breeding these fishes. Unlike game fishing, annual fish collection is not an extractive process that decimates the local population. Only a few breeding pairs are taken, brought out of the wild and bred in captivity.
For more information about annual fish:
The website of the American Killifish Association (www.aka.org) offers greater details about various species than can be covered here and provides convenient links to other societies.
On Jorgen Scheel letters: