The Reproductive Biology and Spawning of Zebrafish in Laboratory Settings - Zebrafish: Methods for Assessing Drug Safety and Toxicity - page 2

1.2 OVERVIEW OF ZEBRAFISH REPRODUCTIVE
BIOLOGY AND BEHAVIOR

1.2.1 Natural History

Zebrafish are native to South Asia, and are distributed primarily throughout the
lower reaches of many of the major river drainages of India, Bangladesh, and
Nepal (Spence et al., 2008). This geographic region is characterized by its
monsoonal climate, with pronounced rainy and dry seasons. Such seasonality in
rainfall profoundly affects both the physicochemical conditions in zebrafish
habitats and resource availability. These factors also shape reproductive biology
and behavior.
Data gathered from the relatively small number of field studies suggest that
zebrafish are primarily a floodplain species, most commonly found in shallow,
standing, or slow-moving bodies of water with submerged aquatic vegetation and
a silt-covered substratum (Spence et al., 2008). Environmental conditions in these
habitats are highly variable in both space and time. For example, pooled environ-
mental data from zebrafish collection sites in India in the summer rainy season
(Engeszer et al., 2007) and Bangladesh in the winter dry season (Spence et al., 2006)
show that pH ranges from5.9 to 8.5, conductivity from10 to 2000 S, and temperature

from 16 to 38 C. These differences, which reflect changes in seasonality and
geography, provide strong evidence that zebrafish are adapted to wide swings in
environmental conditions. Results of laboratory experiments demonstrating their
tolerance to both thermal (Cortemeglia and Beitinger, 2005) and ionic (Boisen
et al., 2003) fluctuations support this hypothesis.
Zebrafish feed mainly on a wide variety of zooplankton and insects (both aquatic
and terrestrial), and to a lesser extent, algae, detritus, and various other organic
materials (McClure et al., 2006; Spence et al., 2007a). Gut content analyses of wild
collected animals indicate that they feed primarily in the water column, but also take
items off the surface and the benthos (Spence et al., 2007a).
Zebrafish are a shoaling species, most often occurring in small schools of 5-20
individuals (Pritchard et al., 2001), although shoals of much larger numbers have
been observed (Engeszer et al., 2007). Reproduction takes place primarily during
the monsoons, a period of resource abundance (Talwar and Jhingran, 1991). Fish
spawn in small groups during the early morning, along the margins of flooded water
bodies, often in shallow, still, and heavily vegetated areas (Laale, 1977). There has
also been at least one report of fish spawning during periods of heavy rain later on in
the day (Spence et al., 2008). Females scatter clutches of eggs over the substratum,
and there is no parental care. The eggs, which are demersal and nonadhesive,
develop and hatch within 48-72 h at 28.5 C. After hatching, larvae adhere to
available submerged surfaces by means of specialized cells on the head (Eaton and
Farley, 1974). Within 24-48 h post hatch, they inflate their gas bladders and begin to
actively feed on small zooplankton. Larval fish remain in these nursery areas as they
develop, and move into deeper, open water as they mature and floodwaters recede
(Engeszer et al., 2007).

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