Biologists from the USA have described the fusion of two injured comb jellies into one - with one mouth and two aboral ends and anuses. Having noticed accidentally fused individuals, the scientists conducted experiments and forced nine pairs of comb jellies to fuse. The animals fused very quickly: after two hours, their common body contracted and relaxed synchronously, and the food they ate entered both digestive systems. The results were published in Current Biology.
Comb jellies Mnemiopsis leidyi are small animals that look like jellyfish. They live in seawater and feed on zooplankton. The nervous system of comb jellies is structured somewhat differently than that of most animals, and some scientists even talk about its independent origin. Their subepithelial nervous network is a syncytium - that is, neurons are connected to each other by cytoplasmic bridges, not synapses. In other parts of the nervous system of comb jellies there are both synapses and gap junctions, but they do not have most of the neurotransmitters typical of true multicellular organisms.
Recently, researchers at the Marine Biological Laboratory in Woods Hole, where a population of M. leidyi was kept, witnessed an unusual sight. They noticed an unusually large specimen in the aquarium, with two aboral ends and two apical organs. The scientists speculated that this specimen was the result of a fusion of two others that had been damaged during a recent collection and had been kept very close together in a small tank for some time.
Building on this observation, as well as an earlier study that showed that tissue transplanted from one comb jellies to another survives, scientists led by Kei Jokura conducted an experiment. They took ten pairs of comb jellies, cut off a section of each individual's lobe (the paired body parts located near the mouth), and placed them in pairs, cut-side-down, overnight.
Nine out of ten attempts were successful - the pairs of comb jellies grew together. All the fused individuals survived, living in the aquarium for at least three more weeks. After just one night, the border between the fused individuals became continuous, and the epithelium and mesoglea became smooth. When the scientists mechanically stimulated one lobe of the fused individual, the entire body contracted, including the opposite lobe. The scientists concluded that the nervous systems of the comb jellies had also grown together.
The researchers then decided to observe the process of fusion of the two individuals. During the first hour, the blades of the two comb jellies relaxed and contracted asynchronously, but after two hours the movements were synchronized. The scientists also noticed that the meridional canals, parts of the digestive system, were continuous in the place where the individuals had fused. The scientists then began feeding the fused individuals fluorescently labeled brine shrimp through one of their mouths. Food particles were found in both digestive systems, and waste products were excreted through both anuses, and at different times. Apparently, the digestive systems of the fused individuals became connected not only physically, but also functionally, and excretion control remained independent.
Since not all physiological responses were synchronized in the fused ctenophores, whose neural network is mainly represented by many syncytial neurons, the authors suggested that ctenophores must also have separate functional neural units. Future research, according to the scientists, should focus on this issue.
The authors also concluded that M. leidyi comb jellies lack an allorecognition mechanism, meaning they probably cannot differentiate between their own and other cells. The scientists will now test whether individuals of different species will fuse together.
Previously, scientists found that invasive M. leidyi actively reproduce in the north of its range, even when food becomes increasingly scarce, in order to then wait out the cold period, feeding on its larvae.