Lecture 27: Cnidarians, placozoans and ctenophores

● List and describe key features of cnidarians

● Compare the development and body plan of cnidarians to other animals

● Describe the functions of cnidocytes

● Contrast the role of the polyp and medusa stages in the life cycle of cnidarians

● Explain the use of muscles and nerves in cnidarians in comparison to other animals

● Explain the symbiosis between coral and dinoflagellates, and its role in coral reefs and their sensitivity to perturbations

● Describe main features of Ctenophores and Placozoans

● Cnidarians Form.

● Most are dimorphic. What does this mean?

● What are key features of these morphs?

● Are both morphs always present?

• Dimorphic: Alternate bw two stages

  • Asexual Polyp Stage: oral ends up w/ stalk attaching to surface, usually small; can form colonies. Produce medusa asexually

  • Sexual Medusa Stage: motile, free-swimming, “sea jelly”, oral end down, sexually reproductive stage

● Cnidarians Form.

● Do cnidarians have muscles?

form from a mix of endoderm and ectoderm

appear to be a seperate origin from those of other animals

● Do cnidarians have nerves? If yes, what type of system? If yes, does their system have a common origin to that in other animals?

• have nervous net: decentralized systems (no brain) used to sense and respond to stimuli (eg prey capture movement)

• evolved independently

● Do cnidarians have symmetry? If yes, what type?

radial symmetry

● Which cnidarians have eyes?

○ What are cnidocytes?

single-use cells triggered by touch or chemistry found in cnidarians

○ What are nematocysts?

stinging organelles inside cnidocytes that contain toxins

○ What do nematocysts and cnidocytes have to do with feeding?

• most cnidarians are carnivorous

• used to capture prey

○ How do some organisms protect themselves from cnidocytes?

clownfish have protecting coating that prevents nematocysts from firing (form of mutualism)

● Cnidarians Life cycle

○ Sketch out the general life cycle

○ What is coral?

• are sessile and colonial Anthozoans (no medusa stage)

• polyps secrete organic molecules on which they deposit calcium carbonate (CaCO3) which form a “skeleton”

• living polyps form a layer on top of skeletal remains which forms coral reefs and islands

○ What are coral reefs? Why are they important?

• major marine foundational habitat

• habtat 25% of world’s fish species

• major food source for humans

• structurally buffer shorelines

○ How can some coral supplement the energy they get from predation?

• many corals supplement diet via mutualistic symbiosis with dinoflagellates

• these dinoflagellates are photosynthetic, unicell eukaryotes that live endosymbiotically w/ in coral cells

• dinoflagellates provide sugar to their hosts

• most reef-forming corals are restricted to clear surface waters w/ enough light to support photosynthesis by dinoflagellates

● Where does ctenophores name come from?

named for their “ctenes” (or combs)

● How do ctenophores move?

their “combs” which are stacks of cilia used for locomotion

● How do ctenophores feed? Do ctenophores have stingers?

cells on feeding tentacles discharge adhesive material to capture prey

● Do ctenophores have nerves? If yes, what type of system? If yes, does their system have a common origin to that in other animals?

have nerve nets like cnidarians

● Do ctenophores have muscles?

have muscles made up of endo/ectoderm

● Do ctenophores have symmetry? If yes, what type?

radial symmetry

● How many cell layers do ctenophores have?

are diploblastic (2 layers)

● What is mesoglea?

is gelatinous and the 2 cell layers are separated by it

do ctenaphores have a complete gut

yes

● How many species are there of placozoans?

only 4

● Do placozoans have a nervous system? Muscles? gut? mouth?

do not have any

how do they move

with cilia

● What type of symmetry do Placozoans have?

asymmetrical

● Are placozoans primitive? Ancient?

yes

● How many cell layers do placozoans have?

only a few cell types; diploblastic; are small

● Is placozoan simplicity an ancestral or derived state?

likely due to loss of features