Lean este artículo en español.
From time to time, when in the field or even via the internet, someone will exclaim “Look at this cool jellyfish” and be totally incorrect. Not that there is anything wrong with that. After all, the magnitude and breadth of marine biodiversity are so immense that we still don't know how many species inhabit our oceans, and being an expert in everything just isn't possible.
Jellyfish are adult swimming stages of animals in Cnidaria, the phylum that also includes stony corals, soft corals, anemones, hydroids, siphonophores, etc. Jellyfish, like most marine animals, have different stages during their lives. Most have a larval sausage-shaped stage known as a planula that creeps around until it settles and grows into a bottom-dwelling polyp. Polyps can range from solitary and huge like the anemones that provide homes for anemone fish to tiny and colonial, as in hydroids. Some cnidarian polyps produce medusae, or jellyfish, that swim away to reproduce, and all of these are classified within a group called Medusozoa. We all encounter them if we get into the sea and it seems that people have a reasonable gestalt understanding of what a jellyfish is. The animal above fits that picture. But if it isn't a jellyfish, then what is it?
It turns out that one small group on the non-medusozoan branch of the cnidarian tree of life, which is known as Anthozoa, has a swimming juvenile stage that looks remarkably like a jellyfish. This group is called Ceriantharia and it comprises the tube anemones, which as adults are long skinny polyps that burrow into soft sediments and secrete a rubbery tube in which they live. Just recently, this group was elevated to its own subclass within Anthozoa, equal in status to the octocorals (soft corals, sea pens) and hexacorals (stony corals, black corals, true anemones, zoanthid anemones, etc.).
For most benthic sessile species, the primary dispersal phase is associated with the earliest life history stages, like free-swimming embryos and planktonic larvae. However the recognition of most larval forms is not simple. In the case of ceriantharians, this is a particular problem because they have very distinct looking planktonic larval forms. Since the description of these planktonic stages by Sars (1846), a number of different authors have described and named many "species", not recognizing that most of these names simply refer to different life stages rather than to distinct lineages. Because these descriptions are based on inconsistent and variable characters (e.g. number and form of tentacles, body size, pigmentation), the result has been taxonomic confusion that is still being cleared up [partly through the use of genetics which can easily link up distinct looking life stages and which we will be featuring on No Bones at a later date].
The difficulties of ceriantharian taxonomy have even extended to other groups of Cnidaria. For example, the late, great Haeckel established an entire group of planktonic forms that he hypothesized were swimming relatives of the non-swimming, stalked medusae known as Stauromedusae (or Staurozoa). For nearly 100 years, the group was never seen again and presumed to not exist. However, in 1979 a new species, Tessera gemmaria, was described and classified within this group of pelagic Stauromedusae. The definition of this species and its classification was controversial because the described morphology was so different from all other forms within Staurozoa. And so it was not so surprising, when recently, this supposed jellyfish was identified as a ceriantharian larva of the genus Isarachnanthus, based on specimens collected in the same region.
Fast-Evolving Mitochondrial DNA in Ceriantharia: A Reflection of Hexacorallia Paraphyly? PLoS ONE 9(1): e86612.
Jellyfish and Comb Jellies at the Smithsonian's Ocean Portal.