There is another similar case, but one in which an oxygen-supply explanation seems
more likely. Some gouramis take in air, move below their bubble nest, and with
strong opercular action they squirt bubbles up through the nest. 31 This behaviour
probably brings oxygen to the eggs and must have evolved as an adaptation to the
warm, stagnant and therefore oxygen-poor waters in which these species eke out a
living.
The ease of ventilating eggs may be influenced by nest shape. In the common goby,
males build a nest for their future eggs by dumping sand over down-turned mussel
shells. In the lab they may do it over a clay flower pot. The accumulated sand partially
occludes the object’s opening. Normally the males try to make the opening relatively
small because this impedes the entry of egg predators such as crabs. But small
openings also mean less potential for circulating water. It has been observed that the
owners of nests with smaller openings fan more than others. Also, if the oxygen
content of the water is experimentally reduced, then the males build nests with larger
openings. Obviously the fish adapt their nest building and nest tending behaviour to
oxygen levels.
In many fish species, females prefer to lay eggs in nests that already contain other
eggs, perhaps a form of copying the choice that previous females have made for
particular high-quality males. But considerations relating to oxygen must be taken
into account. In sticklebacks for example, females prefer to spawn in nests that
already contain two or three clutches, but they disregard nests that are packed with
four or five clutches. This may be related to the fact that eggs in an overcrowded
nest have trouble getting enough oxygen for optimal development. In the same vein,
female common gobies normally prefer nests that already have eggs in residence, but
if the fish are kept in oxygen-poor water the females reverse their preference and now
favour nests that are empty. This new choice is probably adaptive because
There is another similar case, but one in which an oxygen-supply explanation seemsmore likely. Some gouramis take in air, move below their bubble nest, and withstrong opercular action they squirt bubbles up through the nest. 31 This behaviourprobably brings oxygen to the eggs and must have evolved as an adaptation to thewarm, stagnant and therefore oxygen-poor waters in which these species eke out aliving.The ease of ventilating eggs may be influenced by nest shape. In the common goby,males build a nest for their future eggs by dumping sand over down-turned musselshells. In the lab they may do it over a clay flower pot. The accumulated sand partiallyoccludes the object’s opening. Normally the males try to make the opening relativelysmall because this impedes the entry of egg predators such as crabs. But smallopenings also mean less potential for circulating water. It has been observed that theowners of nests with smaller openings fan more than others. Also, if the oxygencontent of the water is experimentally reduced, then the males build nests with largeropenings. Obviously the fish adapt their nest building and nest tending behaviour tooxygen levels.In many fish species, females prefer to lay eggs in nests that already contain othereggs, perhaps a form of copying the choice that previous females have made forparticular high-quality males. But considerations relating to oxygen must be takeninto account. In sticklebacks for example, females prefer to spawn in nests thatalready contain two or three clutches, but they disregard nests that are packed withfour or five clutches. This may be related to the fact that eggs in an overcrowdednest have trouble getting enough oxygen for optimal development. In the same vein,female common gobies normally prefer nests that already have eggs in residence, butif the fish are kept in oxygen-poor water the females reverse their preference and nowfavour nests that are empty. This new choice is probably adaptive because
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