One would expect fish that scatter their eggs and offer no parental care to be cavalier about where they lay them, but a study published in a recent issue of the Journal of Zoology shows that they do give some thought to this matter.
Daniel Welsh and Rebecca Fuller conducted experiments to test whether female Bluefin killifish (Lucania goodei) prefer to lay eggs in spawning substrates that already contain eggs from other females.
The authors placed pairs of Bluefin killifish in aquariums containing four spawning mops. They manipulated the position of the mops (top vs. bottom) and the presence of eggs (empty vs. full), such that two top mops (one full and one empty) floated near the surface of the water and two bottom mops (one full and one empty) lay at the bottom of the tank; the full mops were created by having killifish held in other tanks spawn over them, with at least eight eggs in each full mop.
The experimental pair were allowed to spawn over the mops and the number of eggs laid in each mop counted.
A total of 30 successful spawnings were carried out, and after the authors analysed the results statistically, they found that females preferred to lay eggs in mops that already contained the eggs of other females.
While such preferences have already been documented in other fishes, they have not been demonstrated in a species that lacks male parental care and does not build a nest. The authors also found the preference for females to lay eggs in areas already containing eggs to decrease as the total number of eggs increased.
The authors propose several hyoptheses to explain their findings. In the first hypothesis, they posit that the egg mortality in the field is high and that this leads to selection to disperse eggs widely over time and space. Consequently, eggs laid among other eggs may face a smaller risk of predation or cannibalism if some eggs should be eaten, because the risk is ‘spread out’ among the entire nest. This benefit is the greatest when a small number of eggs are added to a larger mass, and this may also explain the decreased female preference with increasing clutch size.
The second hypothesis is that the presence of older, developing eggs that have not been preyed upon may indicate that the present location is good at concealing eggs from predators, thus prompting females to lay their eggs near other eggs.
For more information, see the paper: Welsh, DP and RC Fuller (2011) Where to place your eggs: the effects of conspecific eggs and water depth on oviposition decisions in Bluefin killifish. Journal of Zoology 284, pp. 192–197.
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