1. Phylum Ctenophora Comb jellies ~100 sp., all marine
- Similar to cnidarians: radial symmetry, simple nerve net,
jelly-like mesenchyme (middle layer)
- Monomorphic (one form throughout life), never colonial
- Adhesive colloblasts (similar to cnidae)
- Unique cydippid larva
- ciliated tracts called ctenes (comb plates) used in
movement
- the first animals?? or the first predators??

2. Most ctenophores are pelagic predators
Ecologically, can be the dominant predators in the plankton

3. Colloblasts
Colloblast cells develop into cluster of granuoles packed with
strong adhesive, at the end of a spiral filament
Spiral filament coils around the cell nucleus
On discharge, spiral filament wraps around prey; adhesive released

4. Ctenophore body plan Apical sense organ Aboral canal
Ctene, or comb plate
- have 8 evenly spaced rows
of cilia; each one is a ctene
Tentacle sheath
Pharynx
Tentacle
Mouth

5. Ctenophore body plan ctenes, the locomotory apparatus
- linear arrays of cilia
- 8 rows along oral-aboral axis
Apical sense organ contains a gravity- sensing statolith,
supported by 4 tufts of cilia called balancers
- ciliated grooves connect sense organ to each ctene, direct
beating and hence movement of animal
Digestive system: GVC + tiny, partially functional anal pores
- GVC is highly differentiated into system of canals thru body
Controversial whether their embryos have mesoderm or not

6. Ecological disaster: Mnemiopsis
In 1982, the Atlantic ctenophore Mnemiopsis
was accidentally introduced into the Black Sea
(Europe), likely from U.S. ship’s ballast water
Underwent explosive population growth,
consuming most fish larvae and causing
collapse of fisheries in 8 countries
- cost: >$350 million
In 1997 it was nearly eliminated itself,
by accidental introduction of its major
predator-- ctenophore Beröe ovata
Mnemiopsis leidyi
Beröe

7. Benthic ctenophores
deformable amoeba-like bodies; common on tropical green algae
bottom surface is pharynx, swallows prey caught by 2 branched tentacles; body filled with branches of 8 digestive canals

8. ctenophores & animal phylogeny
Given their similarities, ctenophores and cnidarians were long
thought to be sister to each other, forming a clade that was
itself sister to the bilaterian animals
bilateral symmetry evolved
symmetry
+ tissues
evolved

9. ctenophores & animal phylogeny
However, analyses of partial or whole genomes suggest that
ctenophores are basal animals (more primitive than sponges..?!)
implies the ancestor of all animals
had symmetry, tissues, and a GVC
Ryan et al. 2013, Science
Moroz et al. 2014, Nature
Whelan et al PNAS

10. ctenophores & animal phylogeny
symmetry, tissues, GVC lost?
This would mean:
- sponges lost tissues, GVC, symmetry
- the similarity of choanoflaggellate protists and sponge
choanocyte cells is coincidence, not co-inherited
- the 1st animals were predators, not bacteria-filterers

11. General agreement: ctenophores are simpler than cnidarians,
Gene presence/absence data indicate ctenophores are more
similar genetically to sponges than to other animals, but does
not answer who is sister to the rest of animals
genes
a b c d e f
x x x x + +
General agreement: ctenophores are simpler than cnidarians,
which share many more genes with Bilateria

12. I said 2 years ago: I’ll be shocked if this doesn’t turn out to be
genes
a b c d e f
x x x x + +
symmetry
+ tissues
new genes evolved
I said 2 years ago: I’ll be shocked if this doesn’t turn out to be
the correct phylogeny of animals (see above!)
How have the last two years of research changed our view...?

13. Re-analysis of the same datasets using other evolutionary models
of sequence evolution support sponges as basal, argue that
artifacts of computer analysis are to blame
Pisani et al. 2015, PNAS

14. Say this is the true tree, but both fungi and ctenophores happen
One artifact is long-branch attraction: most dissimilar sequences
tend to find each other and clump together on a tree
fungi
long
branches
find
each
other
choanoflaggelates
sponges
ctenophores
Cnidaria
bilaterian animals
Say this is the true tree, but both fungi and ctenophores happen
to have had an unusually high number of amino acid changes
in their proteins over the past billion years  long branches

15.  when more distantly related outgroups are included in a tree,
One artifact is long-branch attraction: most dissimilar sequences
tend to find each other and clump together on a tree
fungi
ctenophores
choanoflaggelates
sponges
Cnidaria
bilaterian animals
 when more distantly related outgroups are included in a tree,
it tends to artifically pull ctenophores to the base of the tree

16. Analyses of 3 different datasets (different genes, different species)
suggest that both long-branch attraction and poor models of
amino acid change are driving the placement of ctenophores
probability ctenophores
are most basal
probability sponges
are most basal
fungi
Pisani et al. 2015, PNAS