The zebrafish is a useful model organism for studies that include early development, behavior, ecology, cancer, and stem cells. In the laboratory, a setting that severely disrupts the natural environment in which the fish normally reside, the success of mating is a critical determinant of these types of research. Because the zebrafish appear to naturally prefer shallow mating areas, our study addressed whether we could increase embryo production in the captive setting by providing a depth gradient for mating pairs.
Fish that were set up in the typical fashion experience a uniform depth of water of 4
cm and express a particular set of mating behaviors. They include use of the entire tank space and water column with no strong preference for one particular end although edges and corners seem to be preferred over the middle area of the tank. However, when the insert is tilted to create a depth gradient of 4–0
cm (theoretical shore), the fish demonstrate a broader range of behaviors than in the untilted condition. For example, actions that are unique to the tilted condition such as waiting and circling may be intrinsic to the species, but not expressed previously in a lab environment due to the unnatural characteristics of the mating tank. Carrying out similar studies with wild-caught fish with more natural substrate and conditions would get us closer to the reasons behind the results observed.
Tilting increased absolute embryo production in both juvenile and adult matings, suggesting that this is a modifiable factor in laboratory settings. When using a cutoff of at least 10 embryos per mating, tilting did not increase the likelihood that a particular pair of fish would mate, only that the clutch size would be increased. Whether the chance of mating success in a given pair is influenced by the robustness of that particular strain is unknown, but future studies in obstinate maters will help define the general utility of this method.
The affinity for mating locations may be due to facilitating the male in his induction of mating. By pinning the female against the tank wall, the male can ease his work of pushing against the female either with his nose or during quiver. While in the shallow area, the friction caused by the female's belly on the bottom may also be of aid to the process and hence lead to the tendency to mate there.
An alternative explanation may be that the feeling of the insert bottom on belly of the female is similar to that of a preferred gravel substrate (Spence et al
). This particular type of surface combined with a shallower depth is thought to increase water exchange and oxygen availability, offers protection from cannibalism, and exposes them to warmer temperatures (Martin and Swiderski, 2001
; Spence et al
Some of the environmental factors known to affect fecundity in fish that are typically controlled in the lab are pH, food availability, and temperature (Winn, 1960
; Reisman and Cade, 1967
; Lee and Gerkin, 1979
; Wootton, 1999
). However, the aspect of topography and environmental stimulation is ignored in the lab mating setup. By overlooking factors such as substrate and presence of natural plant matter, both of which fish show a preference for (Stacey et al
; Spence et al
), we are limiting their natural behavior expression, and in doing so, we may be negatively affecting their lay rates.
Natural habitat items such as plants, substrate, and topography have all been shown to be preferred strata for fish mating. By simply altering the mating environment in lab zebrafish to include a more natural environment, we uncovered behaviors not previously described. This approach to exposing natural behavior opens doors to studying other inherent animal behaviors, why they've evolved, and how they are inherited. The results also have immediate implications for captive fish, which not only may facilitate research, but may also improve husbandry standards. By taking steps toward maximizing the sexual stimulation of the fish, we have not only shortened generation time and saved valuable tank space, but also increased embryo production, which is vital for the success of genetic screens, microinjections, and chemical screening. When combined with other variables such as food availability and vegetation, our aim is to further augment the integration of natural life history into the lab setting.