BIOL 2003: Lesson 3 (Cnidarians)

Placozoa

• ‘flat animal’

• metazoan

  • only 1 known species

• not diploblasts

• no epithelial tissues

• no germ layers 

Placozoa tissue

• cell tissue

Placozoa symmetry 

• asymmetrical

Placozoa body cavity 

• no body cavities

Placozoa development

• stops at blastula stage

Placozoa segmentation

• no true segmentation

Tissue level of organization

• true tissue secretes from an extracellular matrix, which helps to keep the cells organized

• derived from embryonic germ layer

• highly coordinated unit 

Cnidarians 

• two main body forms (dimorphic)

  • polyp (sea anemone)

  • medusa (jellyfish)

• jelly layer called mesoglea 

• blind-ended gut with tentacles around the mouth 

  • therefore has a mouth opening, but no anus 

• asexual and sexual reproduction 

• 10,000 species 

Polyp (hydroid)

• normally sessile

  • though some are able to jump/flip themselves slowly

  • anemone form

  • solitary or colonial forms

Medusa (jelly)

• motile

• jellyfish form

Radial symmetry

• oral (mouth end) and aboral (side without a mouth)

• sense environment ‘from all sides’

  • no head

  • good for sessile or free floating animals 

Diploblastic development

• gets past the blastula phase

• zygote → blastula → gastrula 

  • the single-layered ball of cells (blastula phase) folds inward to form a multilayered structure with three germ layers: the ectoderm, mesoderm, and endoderm

• for cnidarians, they are diploblasts with 3 layers

  • endoderm (embryo) → gastrodermis (adult)

  • ectoderm (embryo) → epidermis (adult)

  • jelly (non-living) → mesoglea

  • all layering a gastrovascular ‘cavity’

    • not a true cavity

Nerve/muscle cells

• epitheliomuscular cells

  • skin/muscle cells 

  • contractile 

• nerve cells are one/two way synapses

  • unlike the strictly one way human synapses 

• no true brain, rather a nerve net 

Cnidocyte

• the ‘nettle animal’s’ cell 

• pocket of the epithelium which contains a cnidae capsule within the cell 

  • most notable cnida is the nematocyst

• cnidae is released by the hair-like trigger, cnidocil, in response to hydrostatic pressure increase

• one time ‘fire’ only mechanism

Nematocyst

• harpoon-like cnida which paralyses prey with venom stings

Reproduction

• alternation of generations 

  • asexually-reproducing polyp will make medusa

  • medusa sexually makes gametes (via meiosis)

  • gametes fuse to a planula larva (zygote)

  • planula settles to make a new polyp

  • polyp asexually makes medusa (cycle repeats)

Polyp polymorphism

• different polyps have differentiated roles, and therefore many forms 

• mostly in class hydrozoa 

• clone colony in a single animal 

  • gastrozooids

  • gonozooids

  • dactylzooids

Gastrozooids

• specialized, feeding polyps in colonial cnidarians like the Portuguese man-of-war and Obelia

Gonozooids

• reproductive zooids, or specialized individuals, in colonial marine animals like tunicates, bryozoans, and hydrozoans that produce gametes (sperm and eggs)

Dactylzooid

• tentacleless polyps heavily armed with nematocysts that seem primarily concerned with defense

Portugese man o war

• not a single animal, rather a colony of specialized polyps, each with a specialized role 

  • see; dactylzooid, gonozooid, gastrozooid

Class Anthozoa

• polyps only

  • anemones, corals

• tubular body and pharynx, large gastrovascular cavity

• subclasses:

  • hexacorallia (individual, 6 axis symmetry)

  • octocorallia (colonial, 8 axis symmetry)

Ecological importance of Anthozoa

• coral reefs are biodiveristy hot spots

• habitats to nearly 25% of marine life

• able to create a marine oasis in otherwise uninhabitable deep sea ecosystems 

Zooanxthellae/anthozoa symbiotic relationship

• zooanxthellae are photosynthetic protists

• engage in a mutualistic, endosymbiotic relationship with corals

  • provide ‘food’ for corals, and in return, corals provide protection, and ‘food’ in return 

• expulsion of zooanxthellae due to inadequate environment (often warming waters) leads to coral bleaching 

Class Staurozoa

• polyp phase (therefore, non-swimming)

• 8 extensions (arms) with adhesive pads ending in tentacle clusters 

• rather than having long tentacles, like jellyfish, staurozoa have white spots on their bodies with nematocysts 

Class Scyphozoa

• true jellies

• 200-400 species

• dioecious

  • separate sexes

• large oral lobes extending from mouth

Scyphozoa strobilation and reproduction

1. scyphistoma

   • upside down polyps

2. strobila

   • stack of future buds of medusa

3. ephyra

   • baby jellies

Class Cubozoa

• square shaped bells 

  • unlike the circular bell of scyphozoa 

• beginning of eye-like structures, called rhopalia

Class Hydrozoa

• many different forms

• most are colonial and exhibit polymorphism 

  • different forms for different functions 

• see term for Portuguese man o war

Cnidaria organization

• true tissues

Cnidaria symmetry 

• radial symmetry 

Cnidaria body cavity

• no true cavity 

Cnidaria development

• diploblasts

Cnidara segmentation

• no segmentation 

Cnidaria classes

• Anthozoa (only non Medusozoa)

• Staurozoa 

• Scyphozoa 

• Cubozoa 

• Hydrozoa