When Astatotilapia burtoni females lay eggs, they don't simply turn their backs and make a quick exit. The soon-to-be mothers vacuum the eggs up into their mouths, where the youngsters develop and grow before they are ready to depart 14 days later. But the sudden weight shift, amounting to as much as 30% of the female's weight, could seriously destabilise brooding mums and the situation could become even more perilous as the larvae and fry become heavier and more negatively buoyant during development. Yet, the mothers seem to take the inconvenience in their stride and never topple forward. Knowing that the fish's swim bladder is divided into two compartments, Julie Butler, Prosanta Chakrabarty and Karen Maruska from Louisiana State University, USA, wondered whether the fish may adjust the distribution of gas between the two parts of the swim bladder to compensate when they become front heavy.
Butler, Sarah Whitlow and Polly Gwan collected female A. burtoni that were ready to spawn large eggs, at various stages over the 14 day brooding period, and 1 and 3 weeks after the fry sallied forth, and then X-rayed the fish. Comparing the X-ray images, the team could see that the overall area of the swim bladder did not alter during the development of the offspring; however, the distribution of gas between the two compartments did shift. The front section expanded while the rear section deflated when the females slurped up their eggs and remained so until the youngsters left their mother's mouths 2 weeks later. Then, the front section of the swim bladder deflated and the rear section reinflated. The team also monitored the fish's swimming posture when the fry were liberated, and found that the unencumbered females’ heads bob upwards, so that the fish swam at an angle of 7 deg for 5 min until they resumed their pre-release almost-horizontal posture. And when the team attached a small bead weighing roughly the same as a mouthful of 7-day-old fry to the underside of the jaws of females that were not brooding, they saw the same shift in the swim bladder shape 4 h later.
Considering the cichlid's rapid reaction to their sudden weight redistribution, the team suspects that the fish redistribute gas between the two parts of the swim bladder by opening and closing a ring of muscle in the diaphragm that separates the two. As tissue in the front section of the swim bladder produces gas, while tissue in the rear section removes gas from the swim bladder, the team suggests that the sphincter in the diaphragm closes when the fish suck the eggs into their mouths, allowing gas to inflate the front section to counteract the weight of the eggs. When the mothers release the fry, they can then restabilise their posture by rapidly opening the sphincter to drain gas into the rear compartment of the swim bladder. And the team suggests that a compartmentalised swim bladder could be an essential adaptation for fish that prefer to give their young a head start in their mouths.