Isolation and Characterization of Feeding-Deficient Strains in Inbred Lines of the Hydrozoan Jellyfish Cladonema pacificum

Feeding behavior in cnidarians has been studied as a model experimental system in physiology and neurobiology. Although the feeding response in cnidarians, such as Hydra, is triggered by chemical signals, the underlying molecular mechanisms that ensure their precise execution are not well understood. It could be largely due to the lack of genetic analysis in cnidarian experimental systems. Cladonema pacificum is a hydrozoan jellyfish that is easy to maintain and cross for genetic analysis in the laboratory. To establish C. pacificum as a model experimental animal in cnidarians, we have been inbreeding strains of jellyfish. Here, we document our progress in developing C. pacificum inbred lines and feeding-defective strains that we isolated in the course of inbreeding. In the inbred lines, an increasing number of feeding-defective strains appeared as descending generations and finally all the F5 progeny showed a feeding-deficient phenotype presumably owing to inbreeding depression. Feeding behaviors of these strains were analyzed by video microscopy and we found that the feeding-defective strains captured prey, but could not kill them. After trapping prey, wild-type medusae contracted their tentacles tightly and then bent the tentacles to bring the prey to the mouth; however, feeding-defective medusae rarely contracted their tentacles and did not bend. These feeding-defective phenotypes are caused by lack of stinging nematocytes in their tentacle batteries. These findings furnish a clue to the regulatory aspects of feeding behavior, but also reveal the mechanisms of stinging nematocyte transport in tentacles.