Phylum Cnidaria
Origin of Diversity
How has so much diversity been
possible in the Phylum Cnidaria
1. Polyp and medusa forms
– Provide the basic diversity by offering two
different ways of life
•
•
Polyp – sessile
Medusa – free-swimming
– Some taxa have both forms in life history
How has so much diversity been
possible in the Phylum Cnidaria
2. Colony formation
– Most common results of budding ability of
many polyps
•
Allows increase in size without expanding
mesoglea (in medusae), or support structures
and digestive surface area (in solitary polyps)
– Larger area can be swept for food
– Share food through a common coelenteron
How has so much diversity been
possible in the Phylum Cnidaria
• Colony formation
– Most common results of budding ability of
many polyps
• Allows for division of labor
– Ex. Feeding and reproductive polyps -Velella
– Ex. Feeding and stinging polyps are
differentiated in Siphonophores – Physalia
– Ex. Polymorphism in in Hydractinia
» Planula settles on shell and develops into
primary polyp: forms mat of branching stolons
» Buds into four types of polyps
Physalia
Siphonophores: Highly
developed polymorphism
among polyp types. Division
of labor between individual
polyps resembles organ
systems of more highly
organized animals.
Velella
Defense
Hydractinia
tentaculozooids
Defense
Nerve net connects all polyps
through stolons
Metamorphosis is triggered
by unknown cues from
bacteria found on the hermit
crab shell.
How has so much diversity been
possible in the Phylum Cnidaria
3. Coral Formation
– Extends range and size possible for
colonies
•
Milliporine corals (Hydrozoa)
– Skeleton is laid down outside the ectoderm of each
polyp
•
Alcyonaria (Anthozoa)
– Skeleton is laid down in the mesoglea by ectoderm
cell
•
Madreporaria (anthozoa)
– Skeleton is laid down underneath the polyps
Alcyonaria
Milliporaria
Madreporaria
How has so much diversity been
possible in the Phylum Cnidaria
4. Unusual lack on constraint on cell
movement and differentiation
– Allows for extraordinary complexity of
Cubozoans
– Degree of coordination of Hydractinia
colonies
5. Amazing genetic complexity
– Unlike much more than more elaborate
inverts
– Almost near that of vertebrates