The Reproductive Biology and Spawning of Zebrafish in Laboratory Settings - Zebrafish: Methods for Assessing Drug Safety and Toxicity - page 4

prompt male mating behavior that immediately precedes and elicits oviposition and
spawning (Hurk and Lambesrt, 1983). Pheromonal release in some cases also appears
to suppress reproduction, as holding water from “dominant” female zebrafish has
been shown to inhibit spawning of subordinate females (Gerlach, 2006).
Reproduction in zebrafish is also influenced by photoperiod. Ovulation most
typically occurs just prior to dawn (Selman et al., 2005) and spawning commences
within the first few hours of daylight (Spence et al., 2006; Engeszer et al., 2007).
However, spawning is not strictly limited to this time period. Zebrafish will breed in
the laboratory throughout the day, particularly during the evenings, although spawn-
ing is most reliable and intense in the early morning (personal observation). In the
wild, zebrafish have also been observed spawning during the afternoon following
the onset of heavy rain (Spence et al., 2008).
1.2.3 Reproductive Behavior
Zebrafish display ritualized courtship behaviors prior to and during spawning. During
courtship, males swim in tight circles or hover, with fins raised, above a spawning site
in clear view of nearby females. If females do not approach, males will chase them to
the site, snout to flank. When spawning, a male swims parallel to a female and wraps
his body around hers, triggering oviposition and releasing sperm simultaneously
(Spence et al., 2008). This ritualized mating behavior and the fact that males are
known to establish and defend territories indicate that females are selective (Darrow
and Harris, 2004; Spence and Smith, 2005). This is supported by the fact that females
will produce larger clutches and spawn more frequently when paired with certain
males (Spence and Smith, 2006).
Females may exert choice on the basis of several combined factors. The quality of
a spawning site is clearly important, as both male and female zebrafish show a strong
preference for oviposition site, selecting and preferentially spawning over gravel
versus silt in both laboratory and field-based experiments (Spence et al., 2007b). If
given the choice, fish will also spawn preferentially in vegetated versus nonvegetated
sites (Spence et al., 2007b) and in shallow versus deep water (Sessa et al., 2008;
Adatto et al., 2011).
Male defense of territories may be one cue that females use to select males. Spence
andSmith (2005, 2006) found that territorialmales had amarginally higher reproductive
success than nonterritorial males at low densities, though there was no difference at
higher fish densities, and that male dominance rank did not correlate with female egg
production. This fact, coupled with female preferences for substrate, depth, and
structure for spawning, suggests that male defense of desirable spawning locations
over which females are choosy may be the basis to the zebrafish mating system.
Females appear to select males based on their genotype. Many fish, including
zebrafish, use olfactory cues to differentiate between kin and nonkin, and this
mechanism may be utilized during breeding to avoid inbreeding. Zebrafish also
appear to use olfactory cues to make social and reproductive decisions. Using odor
plume tests, Gerlach and Lysiak (2006) showed that adult female zebrafish chose the