What is a Killifish?

Development in annual killifish differs from other teleosts in two major ways:

1. Dispersion-reaggregation

Cell movements during early development separate the formation of an embryonic axis from the process of epiboly. Epiboly in annual killifish is accompanied not by formation of an embryonic axis as in other teleost embryos, but by dispersion of the deep blastomeres around the yolk (Wourms 1964, 1972a,b). These blastomeres then subsequently divide and reaggregate over a span of 4–8 days to form the embryonic axis (Wourms 1972a,b).

2. Diapause

Embryonic diapause is a state of suspended metabolic, developmental, and cellular activity. There are three distinct stages of diapause, which may occur during development, termed diapause I, diapause II, and diapause III (Peters, 1963; Wourms 1964, 1967, 1972a, b, c).

Which species of annual killifish does our lab study?

Most of our work focuses on the species Austrofundulus limnaeus. These fish are found in the Maracaibo basin of Venezuela (Lilyestrom & Taphorn, 1982).

The image to the right is a typical rainy season pond from the coastal desert regions of the Maracaibo basin. At the initiation of the rainy season, embryos hatch, and the larvae quickly reach sexual maturity. Adults spawn daily while water is available, burying their eggs in the pond sediment. 

The image to the left depicts a typical dry season pond inhabited by A. limnaeus. The ponds dry completely, resulting in the death of adults and larvae. The high clay content of the soils causes deep cracks to form as the sediments dry.

Entry specifically into embryonic diapause, especially diapause II, is responsible for their ability to endure for extended periods of time encased in the dry sediments (Peters, 1963; Wourms, 19721, b, c; Matias, 1982). As the ponds fill with water again during the next rainy season, embryos hatch and the cycle continues.

(Images courtesy of A. C. Terceira, © 2012)

Why are we interested in diapause in A. limnaeus?

1. Diapausing embryos experience a profound arrest of cellular metabolism, growth, and proliferation. Understanding how these feats are accomplished could help to understand how to slow or arrest development of human cancers.
2. Embryos of A. limnaeus can survive for months in the complete absence of oxygen despite being composed largely of brain and heart tissue (Podrabsky and Culpepper, 2012). Understanding the molecular underpinnings of anoxia tolerance may aid in the prevention and treatment of stroke and heart attack in humans.
3. Despite the astonishing feats of survival these fish can perform, we still understand very little about the physiological and biochemical adaptations that are associated with diapause.
4. Two major reasons for this lack in progress are probably the unavailability of the preferred experimental species (e.g., Austrofundulus) and difficulty in obtaining enough embryonic tissue suitable for biochemical and physiological studies. A. limnaeus has been chosen as a model organism for studies of development and diapause in annual killifish because they are relatively easy to rear in captivity, have a relatively large adult size, large eggs (~2 mm diam), and high reproductive output.