Cnidarians Overview

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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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Worms!

Cnidarians Overview

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