Study reveals new killifish egg structures

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Researchers in Brazil have discovered structures on the surface of the eggs of killifish never documented before.

D. Fava and M. Toldeo-Piza from the University of So Paolo in Brazil, studied 19 species of Simpsonichthys killifish gathered from areas throughout South America including Brazil, Venezuela, Uruguay and Argentina.

Using scanning electron microscopy they discovered that each species of fish features unique features on the surface of the eggs.

The structures varied from palm-like structures through to conical projections, round bumps across the surface and concentric rows of spines around the micropylar region (a pore in the membrane of the egg through which sperm can enter - other studies on rosy barbs have shown that this region means that the chance of fertilization is a huge 99.7% once sperm are within this region.)

Rivulid killifish are found throughout the neoptropics (Caribbean, Florida, Mexico, South and Central America). The majority are known as annual fish in that they live in seasonal freshwater pools formed during rainy season.

During the dry periods all the individuals die leaving the eggs which have been laid in the mud surviving in a kind of stasis, allowing the eggs to survive for periods of up to 10 years known as diapause. The following rainy season as the freshwater pools form again, these eggs hatch out and the cycle begins again.

It is thought that the structures found on the egg surface may assist in the survival of eggs when affected by severe drought. These features have previously been used as a key to the identification of species. In turn, each of these characters has developed from another and they can be extremely helpful in analysing relationships between species.

Killifish are small fish 2.5- 5cm found throughout the world in the fresh and brackish waters of the Americas, southern Europe, most of Africa, the Middle East and Asia. (the only places they are not found is northern Europe, Antarctica and Australia).

The Simpsonichthys genus of killifish was revised recently by Wilson Costa, who recognised 50 species across five subgenera. (See Simpsonichthys killifish revised, News, January 2, 2008).

For more information see the paper: Fava D and M Toledo-Piza (2007) " Egg surface structure in the annual killifishes Simpsonichthys (subgenera Ophthalmolebias and Xenurolebias) and Nematolebias (Teleostei: Cyprinodontiformes: Rivulidae): variability and phylogenetic significance. Journal of Biology (2007) 71, 889-907.