Killifish have a broad range of geographical distribution, though the principal location of their natural habitats are the fresh waters of rivers, brooks, lakes, and swamps of tropical countries where an abundance of mosquito live. Species of Killifish and Culicidae are present in the ephemeral ponds of tropical Africa and South America, regions that experience pronounced dry seasons.
During these periods the water source evaporates, leading to drastic environmental changes that have important effects on the life history of some species of fish such as Nothobranchiidae and Rivulidae, also named annual or seasonal killifish for which the embryonic stage is punctuated by several arrest periods called diapauses, also present in the mosquito.
In killifish, the chorion is a thick, acellular, and multilayered coat which surrounds and protects the mature oocyte or embryo. Comparative studies of chorion have suggested that differences in the external morphology may reflect adaptation to extreme ecological conditions or phylogenetic history.
As killifishes live in a variety of locations, a study to compare and contrast the architecture of the chorion using eggs of the seasonal and non-seasonal species from two different continents, Africa and South America, was undertaken by scanning electron microscopy (SEM) in order to consider how the observed morphological differences might be representative of adaptations to variable environments (Fig. 1). Equally interesting, in countries with endemic diseases related to the mosquito and in view of certain species of seasonal killifish (eg. Fundulopanchax walkeri living in a tropical region of the Ivory Coast) and mosquitoes presenting similar annual cycles of reproduction was an analysis of the effective predatory capacity of some killifish to control the mosquito population.
Observed by SEM, the external surface of the chorion of several species of killifish is covered entirely by an alveolar-like design adorned with numerous filamentous structures in non-seasonal species such as Aphyosemion gardneri and Epiplatys fasciolatus from Africa and Anablepsoides rubrolineatus from South America while other seasonal species from both continents present spike-like structures. Several genera such as Notholebias minimus from South America present complex tower-like appendages flattened at their apex to form a platform. A comparison of the variety of designs between African and South American killifish, reveals that the latter present a larger variety especially amongst the annual species, (Fig. 2).
From the SEM observations, the external chorionic layer presents complex designs that are not always related to phylogenetic traits, geographic distribution, or ecological parameters. This is even more obvious when comparing the internal structure of the chorion where it is difficult to find a logical relation between the external design and its internal organization. Perhaps comparisons employing a larger sample of species could reveal or clarify some potential or consequential relationships, either from phylogenetic aspects or from an ecological perspective at different taxonomic levels.
Analysis of the gut contents of several populations of three species of non-seasonal killifish from West Africa (Epiplatys spilargyreius, Aplocheilichthys normanni, and Scriptaphyosemion geryi), revealed that they feed mainly on small crustaceans, formicidae and chironomid larvae. Thus it seems that these fish are sufficiently versatile to adapt to a variety of ecological situations and have a varied nutrition.The predatory capacity of these fishes was tested in laboratory experiments while observations were made in their local environment yielding conclusions that the motion and size of the larvae played a significant role with preference being for the younger stages of Culicidae.
The peak of feeding activity occurs during the two hours before dark. Tests recording feeding habits reveal that these fish prefer, in descending order, Aedes, Anopheles, and Culex larvae. Field studies were undertaken with Scriptaphosemion geryi in order to overcome drawbacks inherent in laboratory experiments. They lasted several months during the rainy season and part of the dry season, between July and December, and were performed in small bodies of water that mimic well the natural habitats of killifish and the mosquito in Senegal.
These bodies of water contained a large population of the mosquito larvae of different species with the first four months corresponding to an important proliferation of all mosquito larvae (Fig. 3A). In the bodies of water with fish, the dynamics of mosquito larvae was different from the controls (without fish), the younger stages of larvae were the most affected with an important predatory pressure of S. geryi on stage II, (Fig. 3B).
The interesting outcome of these observations is the notable reduction of older mosquito larvae, especially during the rainy season when they are most abundant. These observations, undertaken over several months, represent the initial studies of the predatory capacity of Notobranchiidae species both in the laboratory and in natural conditions.
Ecological adaptations to extreme variations in water supply through the specialized anatomical structures of their chorions render killifish a very interesting subject for biological investigation. In shallow African waters, their predatory capacity with respect to mosquito larvae could prove highly useful in countries suffering from mosquito-related diseases.
These findings are described in the article entitled Study of the chorion of seasonal and non-seasonal Africa and Neotropical oviparous Cyprinodontiforme fishes, recently published in the journal Environmental Biology of Fishes. This work was conducted by Nadia Messaddeq, Josiane Hergueux, and Jean-Luc Weickert from IGBMC and Raymond Romand from the ICUBE Laboratory.