Ctenophora form a group of marine organisms of nearly 150 species, found throughout the oceans. The presence of invasive species within this group has led to an increase in the number of marine ecology studies devoted to them. Classified apart from the clade of Bilateria and having remarkable physiological particularities, Ctenophora forms a key group for understanding the evolution of some attributes of the animal organization planes such as body symmetry, the neurosensory system or the mesoderm. Moreover, their extraordinary regenerative capacities make them excellent models for the study of stem cell properties and the determination of cell lineages.
Among Ctenaphora, Pleurobrachia pileus (also known as “sea gooseberry”) is one of the most widespread. Pleurobrachia pileus is a pelagic species, moving through the synchronous beating of the cilia, mounted on transverse plates. Today, research on Pleurobrachia pileus is oriented towards studies of gene expression profiles and functional genetics. The phylogeny of Ctenophora has been a subject of controversy, but recent studies position Ctenophora in a sister group to the Cnidarians, bringing the former group of Coelenterata (assembly of Cnidarians and Ctenophora) into line.
As nearly all ctenophores, Pleurobrachia pileus is hermaphroditic, possessing both male and female reproductive organs. When mature, the sexual material is emitted into the sea through small holes in the ectoderm called “gonopores”. Fertilization is external and self-fertilization is assumed to be rare. After fertilization, the egg-cell begins to divide. Egg-cell segmentation is complete, uneven and early determinable. From the second egg-cell division (4-cell stage), radius symmetry is apparent. During gastrulation, a mesodermal micromere lineage, precursor of muscle cells, is established. L’embryon se développe en une larve nageuse, appelée cydippide. La larve subit ensuite une métamorphose pour passer à l’état adulte. The embryo grows into a swimming larva, called a cydippid. The larva then undergoes a metamorphosis to become an adult. Planktonic ctenophores, are monomorphic: they don’t have a fixed stage.
Hernandez-Nicaise ML. 1991. Ctenophora. In: Harrison W, editor. Microscopic Anatomy of the Invertebrates. Volume II: Placozoa, Porifera, Cnidaria, and Ctenophora. New York: Wiley-Liss Inc. p 359-418.
[FR] Hernandez-Nicaise ML, Franc JM. 1993. Embranchement des Cténaires. Morphologie, biologie, écologie. In Grassé PP, editor. Traité de Zoologie. Anatomie, Systématique, Biologie. Tome III, fascicule 2 (Cnidaires, Cténaires). Paris : Masson. p 943-1055.
Tamm SL. 1982. Ctenophora. In: Shelton GAB, editor. Electrical Conduction and Behaviour in ″Simple″ Invertebrates. Oxford: Clarendon Press. p 266-358.
″ Insights into the early evolution of SOX genes from expression analyses in a ctenophore.″ Jager, M., Quéinnec, E., Chiori, R., Le Guyader, H. & Manuel, M. 2008. Journal of Experimental Zoology part B Molecular and Developmental Evolution. 310(8):650-667.
″ Phylogenomics revives traditional views on deep animal relationships.″, Philippe, H., Derelle, R., Lopez, P., Pick, K., Borchiellini, C., Boury-Esnault, N., Vacelet, J., Deniel, E., Houliston, E., Quéinnec, E., Da Silva, C., Wincker, P., Le Guyader, H., Leys, S., Jackson, D.J., Schreiber, F., Erpenbeck, D., Morgenstern, B., Wörheide, G. & Manuel, M. 2009., Current Biology 19 : 706-712.