Lopsided Fish Show That Symmetry Is Only Skin Deep

Putting function before form, members of the Perissodinus genus of fish have developed a hugely lopsided jaw that provides a distinct feeding advantage. Research published in the open access journal BMC Biology describes how these scale-eating fish, called cichlids, develop mouths directed either to the left or the right -- enabling them to feed on the opposite side of their prey.

A research team from Syracuse University, led by Thomas Stewart and R. Craig Albertson, studied the facial evolution of cichlids from Lake Tanganyika in eastern Africa, where they feed on the scales of larger lake-dwellers. According to Stewart, "While most animals have bilateral symmetry and some show a radial symmetry, asymmetries are actually relatively common in nature. For example, humans can be either left handed or right handed, and one side of the brain is structured differently to the other. Often, both the trait and whether it is placed on the left or right are genetically determined, and the population as a whole tends to show the same characteristic. For example, most people have their heart positioned slightly to the left."
The research team found that there is a genetic locus that determines whether a cichlid will be a 'lefty' or a 'righty'. However, they add that these developmental divergences are more complicated than originally thought.
"Some scale-eaters start off with balanced mouths, and others do not. So, what happens as these symmetrical fish get older: do they develop asymmetries due to lateralized foraging behaviors? Do they develop asymmetries due to lateralized growth? Do they die because they are not effective hunters?" debates researcher Craig Albertson. He continues, "All three are possible, but we do not have data to support any one of these scenarios. It would seem, however, that in this group an innate, genetically-determined left/right difference has been accentuated to enable a bizarre predatory life style."

Helpful Fish: Cooperative Cichlids Boost Their Own Reproductive Success

Subordinate individuals living within a group of vertebrates sometimes assist a more dominant pair by helping to raise the dominant pair's offspring and this has been shown to occur among subordinate female cichlids. Dik Heg and colleagues at the University of Bern, Switzerland, and the Ohio State University, suggest that rather than engaging in an act of reciprocal altruism, these subordinate females actually benefit from the care-giving they offer as the more helpful subordinates are more likely to reproduce.

A number of hypotheses have been proposed to explain why subordinate female cichlids (Neolamprologus pulcher) often help to raise the offspring of unrelated dominant females (a form of alloparental care). Studying small groups of unrelated individuals, Heg and colleagues found no evidence that the helping behaviour of the subordinates was an example of reciprocal altruism, even though opportunities for reciprocation among cichlids are high, as the females lay eggs about every second week—there was no correlation between the amount of care that dominants provided for their subordinates' broods and subordinates' alloparental care for dominants' broods.
The results of the study suggest that while reciprocal altruism doesn't occur, the subordinate cichlids did receive reciprocal benefits in exchange for the care they provided for the offspring of dominant pairs—a behaviour that enhanced their own chances of successfully reproducing. More specifically, a subordinate's helping behaviour ensures she gains access to the breeding substrate, on which she lays her eggs.
Heg and colleagues show that subordinates may be prepared to increase their care "payments" in return for enhancements in their own current reproductive potential (which they call the "pay-to-reproduce" hypothesis). Finally, subordinates may adjust their level of help according to parentage, at least in females
The authors suggest that this finding may also apply to other species, such as the female yellow-bellied marmot, which also adjusts its social behaviour largely to get access to direct reproduction.
Journal reference:
Heg et al. Helpful Female Subordinate Cichlids Are More Likely to Reproduce. PLoS ONE, 2009; 4 (5): e5458 DOI: 10.1371/journal.pone.0005458
Adapted from materials provided by Public Library of Science, via EurekAlert!, a service of AAAS.

Fish Researcher Demonstrates First 'Non-visual Feeding' By African Cichlids

Most fish rely primarily on their vision to find prey to feed upon, but a University of Rhode Island biologist and her colleagues have demonstrated that a group of African cichlids feeds by using its lateral line sensory system to detect minute vibrations made by prey hidden in the sediments.The lateral line system is composed of a canal embedded in the scales along the side of the body of a fish, around its eyes and on its lower jaw, which contain small groups of sensory hair cells that respond to water flow. The lateral line system aids some fish in swimming upstream, navigation around obstacles, and the detection of predators and prey.According to Jacqueline Webb, a URI professor of biology, cichlids in the genus Aulonocara, which only live in Lake Malawi, have widened lateral line canals that are highly sensitive to vibrations and water flows. They feed by gliding through the water with their chin close to the sand like a metal detector, seeking out twitching arthropods and other unseen prey items.There are about 16 species of Aulonocara cichlids in Lake Malawi, all of which feed in the sand."These cichlids join a short list of fish that have been demonstrated to use their lateral line system to feed," said Webb. "Since most of the fish with widened lateral line canals are found in the deep sea, it's difficult to study them. These cichlids can now be used as a model system for studying widened canals, and we can apply what we learn from them to the fish in the deep sea."Webb analyzed video of the swimming behavior of the fish in response to live and dead brine shrimp located on the surface of the sandy substrate in a tank. She compared the fishes' ability to detect prey under light and dark conditions, and looked at their ability to detect prey when the lateral line system was chemically "deactivated."She found that the fish were able to find live prey easily, even in darkness, but not without a healthy lateral line system.Her discovery opens the door to the study of the convergent evolution of wide canals and raises the question of whether fish that feed non-visually have an ecological advantage over visual-only feeders. Webb was recently awarded a $334,000 grant from the National Science Foundation to study the development and behavioral role of wide lateral lines."We also hope that this work will allow us to determine whether the sensory biology of a species can be used to predict its ecological success," she said, "especially in environments where the water quality is poor or where there is increased turbidity. Do these fish have an advantage in water where it is difficult to see well?"To examine these questions, Webb will use microCT imaging to create a three-dimensional reconstruction of the skulls of cichlids, while also developing what she calls a "chin tickler" – an artificial stimulation delivery system – to standardize the stimulation provided from beneath the sand to the cichlid test subjects.Source: University of Rhode Island,