Post on 02-Mar-2018
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Phylum Cnidaria
Lecture 2May 16, 2016
Professor Hofmann
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Great Barrier Reef in Australia:only 7% has escaped bleaching!http://www.npr.org/sections/thetwo-way/2016/05/14/477963623/new-photos-show-the-rapid-pace-of-great-barrier-reef-bleaching
Coralwatch
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Road map - Lecture 2
Intro to Phylum Cnidaria Distinguishing characteristics
Body plan
Functional information Gastrovascular cavity
Nematocysts
Symbiosis in cnidarians
Primitive nerve nets Hydrostatic skeleton
Biodiversity
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Who are they?
Phylum Cnidaria contains over 10,000 species
Anthozoans: corals & sea anemones Scyphozoans: Jellyfishes
Hydrozoans
Cubazoans
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Evolutionary relationships
Cnidarians next
splitfrom the
animal lineage after
sponges
Most cnidarians are
marine
Most distinct change
Two embryonic cells
layers
Distinct organsystems
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The Cnidarian Lifestyle
Simple carnivores
Some swim, some dont
Predators
Very low metabolic rates Can survive in cold and
nutrient poor waters (polar)Antarctic jellyfishDesmonema glaciale
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All cnidarians have!
A basic radial symmetry
Only 2 layers of living tissue
Epidermis and gastrodermis
A middle gelatinous layer the
mesoglea in between the 2living layers of tissue
Tentacles surrounding themouth
Only a single opening to thedigestive system
Gastrovascular cavity blind gut
Stinging Cells (containingstructures such asnematocysts) that account forthe stingingactivity that is
associated with these animals
Lions Mane JellyCyanea capillata
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Gastrovasuclar Cavity
Multiple purposes:
Digestion
Gas exchange (nogills)
Circulation
A diffusion-based
system
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A Few Specifics About
Cnidarian Physiology Gastrovasuclar cavity is
region of gas exchangeand digestion
Lack a circulatory system
Gastrovascular cavity canbe highly branched in largecnidarians
Extracellular digestion Cells in gastroderm contain
digestive enzymes Epithelial cells with
muscle fibers that allowmovement
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The stinging cells, cnidocytes
Cells are at the end of
tentacles
Eject a nematocyst
Literally,
threadbags
Can inject a toxin into
prey
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Cnidarian body wall crosssection
A. gastrovascular cavity
B. gastrodermis
C. mesoglea
D. cnidocysts
E. epidermis
(integument)
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An advance in the nerves
department!
Unlike sponges,cnidarians havenerves and muscles
First animal(Metazoan) to haveone
Not a true nervous
system Mesh of overlapping,
decentralized nervesin a network
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Examples of nerve nets
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Cnidarian Diversity: 4 main groups
Scyphozoans (jellyfish)
Anthozoans (corals and anemones)
Hydrozoans
Cubazoans (box jellies)
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Members of Class Scyphozoa
The jellyfishor G:
cup animals
Thick mesoglea
Large, effective
swimmers
All marine
Aurelia labiata, moon jellies
Monterey Bay Aquarium
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Large mesoglea in jellyfish
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Jellyfish locomotion:
more complex than we thought?
http://news.discovery.com/animals/brainless-animal-turns-out-to-be-world-class-swimmer-150122.htm
Article in Current Biology explores jellyfish orientation.
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Scyphozoan Life cycle
Alternation between
polyp and medusa
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Class Hydrozoa
Contains the only
freshwater cnidarians
+ marine forms
Sessile forms andcolonial swimmers
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Members of the Hydrozoa
Freshwater hydrozoan Hydra
Copyright 2002 Olympus Optical Co.,Ltd.
Obelia polyps
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Four types of zooids
Pneumatophore the
sail
Dactylozooid
For defense Fishing
10 meters long and
covered in
nematocysts
Gastrozooid for
feeding
Gonozooid -
reproduction
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Velella velella
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By-the-wind Sailor
Velella velella
Mass strandings on beaches in California
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Members of Anthozoa
G: flower animals
Corals & sea
anemones
Corals: Reef building
organisms
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Microdocs, Steve Palumbi, Stanford Universityhttp://www.stanford.edu/group/microdocs/index.html
What is a coral?
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Variable morphologies
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Coral-algal symbiosis: a key species
relationship in coral reef ecosystems Most reef-building corals contain photosynthetic algae, called
ZOOXANTHELLAE, that live in their tissues
the corals and algae have a MUTUALISTIC RELATIONSHIP: the coral provides the algae with a protected environment and
compounds they need for photosynthesis.
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Coral-zooxanthellae Symbiosis
Most reef-building coralsnormally contain: 1-5 x 106zooxanthellae/cm 2of
live surface tissue
>1010algal symbionts/m2
Relatively small biomass inrelationship to their ecologicaleffect = keystone speciesoncoral reefs
At right:
A coral with zooxanthellaephotographed under blue light,
which made the zooxanthellae'schlorophyll fluoresce red
Isolated zooxanthellae fromcoral tissue
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Coral Bleaching: stress @ high temperatures
BLEACHEDHEALTHY
STAGHORN CORAL (Acropora cervicornis)
The loss of symbiotic zooxantheallae and/or a reduction in
photosynthetic pigment in zooxanthellae residing within corals intermed CORAL BLEACHING
Note that the zooxanthellae give coral their color, in their absence only
the pale skeleton can been seen through the polyps transparent
tissues
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Coralwatch
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Much like organisms in the upper intertidal, many corals are already living
near upper critical limits for temperature This makes these corals especially vulnerable to global warming:
even small increases in temperature could trigger bleaching
sea temperatures are warmer (+0.7C) lower than at any other timeduring the past 420,000 years
rates of change in global temperature over the past century are 2 to 3
orders of magnitude higher than most of the changes seen in the past
420,000 years
HEAT STRESS AND CORALS
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Global Sea Surface Temperature
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1. Measure the number of consecutive weeks corals spend at
temperatures above summer maxima reflects cumulative heat stress2. Calculated over 12 weeks
3.
4 C-weeks significant bleaching is likely
4.
8 C-weeks cause significant bleaching and wide-spread mortality
See the live report:http://coralreefwatch.noaa.gov/satellite/index.php
DEGREE HEATING WEEKS
Monitoring the likelihood of bleaching
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Degree Heating Weeks on May 14th
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Resulting Bleaching Alerts
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Marine biologists debate the
nature of coral bleaching
Two schools, generallyspeaking:
Stress response
Adaptive BleachingHypothesis (ABH)
Premise: bleaching is aregulated mechanism thatcorals use to switch out
symbionts in response tovariable environmentalconditions
High risk ecologicalopportunity
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Next lecture
Worms!