Re: How do comb jellies differ from true jellyfish?

Ctenophores (comb jellies) and medusae (pelagic cnidarians) share a superficial resemblance in many ways - they both have transparent gelatinous bodies, (more or less) radial symmetry, and tentacles used to catch their prey. However, these similarities are what evolutionary biologists call "convergences"; that is, they have been evolved separately by each group because they work in the pelagic realm these animals inhabit.

The ctenophores and cnidarians used to be lumped into a single phylum called the Coelenterata. However, we now know that they represent two very different groups of animals whose body plans have converged on a common design because they share independently derived adaptations to their pelagic existence. The key features that separate ctenophores and medusae into different phyla are the cells that each group uses to catch prey. Comb jellies (Phylum Ctenophora) use sticky cells called colloblasts to tangle and ensnare prey. For example, the sea gooseberry (Pleurobrachia) has two long, retractable tentacles that can be splayed out in the water column when the animal is "fishing." Here's a link to some photos and good general information on Pleurobrachia.

Medusae (Phylum Cnidaria) do not have colloblasts. The diagnostic cell for this phylum is called a cnidocyte. The cnidocyte contains an organelle called a cnida, which in turn contains a coiled thread called a nematocyst. It's the nematocyst that provides the sting that makes some jellies so notorious. Only cnidarians produce cnidocytes. There are some other animals that contain cnidocytes in their tissues, but they don't generate the cnidocytes themselves. Instead, they very cleverly steal the cnidocytes from their cnidarian prey and sequester them, unfired, in their own tissues to make themselves unpalatable to their own predators. Nudibranchs (sea slugs) are particularly noted for this activity. But I digress....

So the major taxonomic feature that distinguishes comb jellies and medusae is the cell that each uses to catch prey. There are also morphological differences between the groups. Ctenophores are called comb jellies because many of them have 8 rows of stiffened cilia that function as paddles to propel the animal through the water. In beasts such as Pleurobrachia , the cilia refract the light and are quite beautiful, as in the photographs in the link above. Cnidarians do not have comb rows; medusae swim by contracting and releasing the bell, in a sort of gentle jet propulsion. So while ctenophores glide smoothly through the water, medusae swim with a more jerky pulse-glide-pulse-glide motion.

Radial symmetry is considered an adaptation for living in the open waters of the seas because it allows the animal to encounter its environment in all directions. Although medusae and ctenophores appear to have the same kind of radial symmetry, a closer look at their respective anatomies reveals differences. Medusae have more or less truly radial symmetry; click here to see a stained medusa, and note the many planes in which you could slice it to obtain mirror images. On the other hand, ctenophores such as Pleurobrachia have bi-radial symmetry. The pharynx of the animal is an elongate slit, which creates one plane of bilateral symmetry, and the plane of the two tentacles is at right angles to that of the pharynx. The pharyngeal and tentacular planes of bilateral symmetry are both superimposed over the radial symmetry produced by the 8 comb rows, hence the term "bi- radial symmetry."

Unfortunately, I couldn't find a nice diagram of bi-radial symmetry on the Web, but you should be able to find one in any invertebrate zoology textbook. The two that I use when writing lectures are:

Pearse and Buchsbaum, 1987. Living Invertebrates, The Boxwood Press, Pacific Grove.
Ruppert and Barnes, 1994. Invertebrate Zoology, sixth edition. Saunders College Publishing.

Allison J. Gong
Mad Scientist