A recent phylogenomics study carried out by Cassey Dunn and colleagues placed Ctenophores ( comb jellies) at the base of metazoan tree , meaning a place much lower than phylas occupied by more morphologically simple animals like Porifera ( sponges ) and Placazoa (Trichoplax adhaerens).Before this study by Dunn et al, it was always believed that group’s evolutionary history is somewhat related to animals morphological complexity , which gradually increases from Placozoa,through Porifera to Cnidaria and finally Ctenophora, being more complex among the non bilaterian group of animals.But the real shocker was disclosed after the results of phylogenomics were out, Ctenophores branched off first from rest of the animals ,followed by porifera (sponges),then Placozoa and finally Cnidarians,branching just before bilaterians in the tree of metazoans.

As we have seen earlier on this blog that Ctenophores are morphologically more complex ( Read in detail about comb jellies in one of my previous post ) than sponges and Trichoplax indicating that that both sponges and placozoans are secondarily simple ,meaning sponges of olden times could have been more complex than the ones we find in today in oceans.
However Ctenophores might have become morphologically more complex independently during the course of evolution. One thing to keep in mind is that , phylogenomics study conducted by Dunn et al, didnot consider genome of Trichoplax and another study performed recently by Srivastava et al. did not include Ctenophora. So its rather unfortunate that so far that none of the recent large-scale studies have included all of the lower metazoan phyla. This new phylogenetic positions are quite important in understanding animal evolution as they will help us understand–How first animals evolved and what morphological features they had? Was the ancestor of all animals resemble ctenophores of today in being a direct developing, holopelagic animal with mesodermally derived muscles and nervous system or ctenophores evolve these features independently in their long isolated evolutionary history?

So in order to gain more insights into common ancestor of all animals one needs to re run the dataset by dunn et al, with the addition of genome of Trichoplax. Another important aspect to consider is that till now the genome sequence of Ctenophores is not completed ,but so far no obvious molecular criteria contradict this view of ctenophore as most primitive animal group.If you consider the example ,as in the case of sponges (but unlike cnidarians), ctenophores appear to lack Hox-like and ‘ParaHox’ genes. Even though Trichoplax lack hox genes but it para hox genes are represented in the form of Trox-2 ,which is clearly related to cnox2 genes of cnidarians, Drosophila Ind and the vertebrate Gsx genes.

Ctenophores were also in the news for wrong reasons, by devastating ecological impact as invasive species. Black sea is not the same in last 50 years and thanks to the fishermen ,who took away loads of carnivorous fish at the top of the food chain, leaving their prey species, like comb jellies multiply with little resistance. Mnemiopsis leidyi is a comb jelly native to the Atlantic coastal region and brings in the imbalance in ecosystem by eating huge amount of zooplankton, eggs, and the larvae of fish and invertebrates that would otherwise support populations of more desirable species. Another species of ctenophores, Beroe ovata also belongs to same general region as Mnemiopsis. Its rather interesting that unlike other ctenophores, who dine on crustacean zooplankton, Beroe prefers dining on other ctenophores. This is the reason why ecologists purposely introducing Beroe to overco the menace of Mnemiopsis in the Caspian Sea. Due to drastic increase in population of comb jellies in Black and Caspian Seas leading to huge declines in local fisheries.

Hence understanding biology of ctenophores will be advantageous on two counts -One it will helps us prevent or overcome these invasions , definitely helps from economy point of view and second obvious reason for studying comb jellies is because of the important phylogenetic position it holds at the base of metazoan tree.

Further reading :
Invasion of the Jellies: Unwelcome Visitors to the Black Sea
Keith Bayha, University of Delaware

Ctenophores
Kevin Pang and Mark Q. Martindale
Curr Biol. 2008 Dec 23;18(24):R1119-20.

Image Credit: Bzibble / FlickR

Related Posts:

• Will the most primitive animal group please stand up
• Sponges regain their tag of most primitive animal on Earth
• New tree of animals suggests nervous system evolved only once in animal history
• New tree of life divides diploblastic and triploblastic animals
• Functional equivalence of proteins among animals

SHARE THIS ARTICLE