LaB 3 Metazoa I: Systematics - Pacific Crest · LaB 3 Metazoa I: Systematics ... Classification of...

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LaB 3 Metazoa I: Systematics

Lab OutlineInquiry QuestionLab PreparationTree of LifeClassification of AnimalsPhylogeneticsCladograms

Activity 1: Inferring Character EvolutionActivity 2: Constructing Phylogenetic Trees Using Live Marine Animals

Activity 2A: Construct a Simple Tree (Tree 1)Activity 2B: Construct an Animal Phylogenetic Tree (Tree 2)

Activity 3: Clam DissectionQuestions to Explore

Humans are most closely related to: a) grasshoppersb) earthwormsc) starfishd) shrimp

In the next three labs, you will explore the animal kingdom from very simple to complex animals. Use your powers of observation developed in previous labs to compare the morphology and function of these animals to help you organize, categorize, and understand them.

Many animal species alive today are unknown to us. Who are they, and why should we care about them? Why not just focus on humans? Animals play many profound roles in the world’s ecosystems as members of food webs, as agents that control pest species, and some that may act as threats to human health, such as parasites. Our own well being is connected to other plants and animals in our world and it is in our best interest to understand them.

A

B

CD

EF

GX

YZ

H I

J

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BIO 204—Fundamentals of Scientific Inquiry in the Biological Sciences I

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Recently, with the aid of molecular techniques, we are discovering how intricately our ancestry is linked to all other members of our kingdom. In the July 2007 issue of Science, it was reported by Putnam et al. that one of Earth’s oldest animal species, the sea anemone, is surprisingly similar to vertebrates. The authors analyzed the DNA from the starlet sea anemone and compared it with the genome of other animals. They found that two-thirds of the gene families in humans and sea anemones are derived from an ancient common ancestor referred to as the “ancestral eumetazoan.” Other groups of animals such as fruit flies and worms have lost many of the genes from the ancestral eumetazoan. What implications does this knowledge have on our health and quality of life?

Metazoa Overview

Whether you are using genetic information or morphology to study animals, their similarities and differences can be used to organize them into distinct taxonomic groups. This first animal lab focuses on the methods used to organize the diversity of life called phylogenetics. You will use phylogenetic methods to classify animals from Stony Brook Harbor as an introduction to the diversity of the animal kingdom.

In the second lab, you will broaden your knowledge from direct observation with indirect measurements to learn key structural and functional characteristics of the harbor animals, as well as to understand their role in the environment. In the last animal lab, we will analyze a complex biological system using reductionism (taking living things apart and investigating their pieces).

At the end these three laboratories, you should be able to relate how patterns of structure resulting from evolutionary and developmental processes can be used to organize and understand animals across multiple phyla.

The Tree of Life (see Campbell, Chapter 25, pp. 491 -501, 506-507, pp. 508-Concepts 25.1-3, 25.5)

The Tree of Life represents the ancestral connections linking very early single celled organisms to multi-cellular algae, fungi, plants, and animals. Though life forms vary tremendously in size, shape, physiology, and ecology, they also resemble each other in ways that make it possible to classify them into groups. The groupings can be arbitrary, such as forming groups of all green or red things, or all living things larger or smaller than an iPod. But biologists have discovered that living things form “natural groupings.” For example, all mammals resemble each other in having hair or fur, just as all flowering plants resemble each other in bearing fruit. Why is this classification

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Metazoa I: Systematics — Lab 3

important? How would you identify an organism that you have never seen before and would your identification allow you to make inferences about the new organism? Classification is important in biology because it partitions new organisms into defined groups which contain other animals that have previously been well characterized.

Systematic Characteristics are used to Fit Organisms into the Tree of Life

The approach commonly used for classification is called phylogenetics or “cladistics” where organisms are placed on the “Tree of Life” based on phylogenetic characters. Certain characteristics called synapomorphies are features of a unique branch or clade of a phylogenetic tree. Notice that each branch connects to the rest of the tree by nodes. Tree nodes represent common ancestors that were points of divergence (where a new group emerges from an ancestral group).

Evolutionary relationships between species can be inferred using shared characteristics called synapomorphies or “derived” characters.

Figure 3.1

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These shared derived similarities have allowed systematists to classify species into larger taxonomic categories (taxa), such as genus, family, order, phylum/division, kingdom, and domain. It was Charles Darwin’s great insight that these resemblances are due to common ancestry and that speciation is due to descent with modification (evolution) from a common ancestor.

For example, Character Set A (Figure 3.1) represents synapomorphies that unite all of the living organisms in the group “Eukaryotes.” The common eukaryote ancestor (A) possessed histone bound DNA enclosed in a nucleus. Ancestral eukaryotes did not have chloroplasts so the common ancestors leading to Branch B acquired chloroplasts which gave rise to that unique clade. Character C represents the clade containing all organisms that possess the modified green chloroplast, the defining characteristic of green plants.

We will explore the synapomorphies that define major groups of animals collected from marine environments in this lab. This will provide you with an opportunity to develop your understanding of the evidence for the relationships that link branches in the Tree of Life.

Hierarchical Classification of Animals

These groups from largest to smallest are: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species.

Genus and species are used as “scientific names” and are called “specific epithets” or “systematic binomials.”

Which taxonomic groups are the same in humans

and cats?What characters might

define the “class” of humans and cats?

What characters might be used to differentiate the

“order” of humans and cats?

Domain (Super Kingdom) Eukaryota EukaryotaKingdom Animalia Animalia

Phylum Chordata Chordata

Class Mammalia Mammalia

Order Hominoidia Carnivora

Family Hominidae Felidae

Genus Homo Felis

Species sapiens domesticus

Common Name Human House cat

Species: The unit used to describe steps in evolutionary progression is called a species. Speciation is the process that leads to new “reproductively independent” groups of related organisms.

“A species is an actually or potentially interbreedingpopulation that does not interbreed with other such

populations when there is opportunity to do so.”—Ernst Mayr

Species Names: Each species has its own name in the form of a systematic binomial which includes a genus and a species name. For example, the systematic binomial for humans is Homo sapiens while the systematic binomial for dogs is Canis familiaris.Genus names are always capitalized, species names are lower case. In print, systematic binomials are usually italicized.

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Anagenesis QuestionFollow species evolution in the figure above A′ to A″. Think of an event that might have occurred that resulted in tick mark 1 (anagenesis).

This is an example of a cladogram, which is a branching diagram that represents a phylogenetic tree. A cladogram represents the best supported hypothesis of relationships based on data at hand.

In a cladogram, the tips represent extant, or living, species and nodes (branch points) represent extinct or ancestral species. Tips connected by a node are sister taxa (dog and cat). Sister taxa are more closely related to each other than either is to any other group on the tree. A branch connects two nodes, or a node and tip. The root of the tree represents its connection to the rest of the tree of life. A group that includes a node (ancestor) and all its descendents is monophyletic (e.g., closed triangle; including node 1, dog, and cat). A group that includes a node and some, but not all, of its descendents is non-monophyletic (e.g., a group including node 2, toad, and node 1, dog).

dog cattoad

root

node 1

tip

branch

node 2

Figure 3.3

Some time in the past, Species A consisted of several interbreeding orange individuals (o). A mutation to blue (b) in the population (A′) arose and displaced all “o” individuals in later generations (A″). This change from orange to blue is shown as a “tick” mark, a horizontal line marked 1. This is called a “Descent with Modification.” This evolutionary change occurred within one species (anagenesis).

Later in time subgroups of individuals in population A″ stopped interbreeding, and the population split into two groups B

and C. This is an example of speciation or cladogenesis. Species B and C evolved independently. Species C went from blue to red (r) (“tick” mark 2), and later underwent speciation (D and E). Today (top of arrow), we have three species B′´, D, and E. D and E are both red because they inherited that color from their most recent common ancestor, C. This is a shared derived trait. Another synapomorphy, other than color, resulted in speciation of D and E (character not shown). Species B′´ retains the ancestral trait, blue.

A

A'

A''

1

ooooo ooo

b b b b

B

B'

C

D

E

A

A'

A''1

2

ooooo ooo

oooo oob o

ooo oob

bbb bbbbbbb bbb

b b b b

b

b b b b

r r r

r r

r r

Tim

e

r r

Figure 3.2

Role of Phylogenetics in Studying the Tree of Life — How do we begin to develop a tree? How do we recognize branches and nodes? In what order do we place them? How long should each branch be?

Consider the development of the following tree.

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Cladogram rules:

1. The phylogenetic tree, rooted at the base, should be read tip to root or root to tip, but NOT across tips or branches.

2. The dimension between root and tips in a phylogenetic tree represents time—the root of the tree is the past and the tips are the present, therefore you may infer ancestor-descendent relationships from the tree.

3. The distance between nodes may or may not be drawn to scale. The trees in this laboratory manual are NOT drawn to scale (this means that short branches on our lab manual trees do not represent short time periods).

4. The dimension between tips of branches represents nothing. The branches may be drawn flipped without affecting the relationship between taxa.

Note: Cladograms are constructed so that the shortest tree possible that includes all of the synapomorphies is probably the correct one. This principle of parsimony is based on Occam’s razor. Einstein described Occam’s razor as “The supreme goal of all theories is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience.” Or said another way, theories should be as simple as possible, but no simpler.

To test your understanding of the cladogram rules, write your answers to the following questions in your lab notebook.

dog cat toad fish

3

2

dog cat toad fish

3

1

2

1

dog cat toad fish

3

1

2

dog cat toad fish

3

1

2

A B

C D

Figure 3.4

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Examine the cladograms in Figure 3.4 and answer the follwing questions:

1. Do any of the trees represent the same evolutionary history? Which ones? (Hint: start at the tips and identify sister groups to determine whether any two trees are the same.)

2. Encircle Ancestors 2, 3, toad, and dog in tree C. Is this group monophyletic?

3. Encircle Ancestor 3, cat, and dog in tree C. Is this group monophyletic? Be creative: what would you name this group?

4. Indicate a non-monophyletic grouping on one tree above.

The Tree Represents History: Inferring Character Evolution

Scientists discover phylogenetic relationships by detecting shared derived characters. A character is a set of alternative states (e.g., color of the eye) that can evolve from one state to the other. Characters used in systematics may be observations based on the study of morphology (external form), anatomy and histology (internal structure), cytology (cellular structure including chromosomes), biochemistry (e.g., variation in protein form and function), and molecular biology (e.g., variation in nucleotide and amino acid sequences).

See pp. 492–495 in Campbell. Why are characters from molecular biology, protein, and the fossil record used in addition to morphological characters?

Homologous vs. Analogous Characters

Homology is any feature or set of features that have the same evolutionary origin, which implies common ancestry where:

Anatomical features have the same evolutionary origin even though they may look different and/or have different functions.

Some DNA sequences in genes may be identical and have similar functions in organisms that may morphologically appear unrelated.

Analogy implies similarity of structure or function without common ancestry:

Convergent evolution occurs when unrelated species develop similar structures or genes, often under similar selective regimes. An often quoted example of convergent evolution is the origin of wings butterflies and birds. Recent molecular evidence from HOX genes has cast a different light on this interpretation. Airplane and bird wings are strictly analogous structures.

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•

•

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Tracing the evolution of characters on a tree

dog cattoad

furmammary glands

dog catt oad

furmammary glands

fish

limbs

The dog and cat both have fur and mammary glands, but the toad does not.The simplest (most parsimonious) explanation for this distribution of characters is that the dog and cat inherited their shared characters from a common ancestor (arrow). This gives a tree length of 2 (= 2 changes).

The dog, cat, and toad have four limbs, but the fish does not.The most parsimonious explanation is that the dog, cat, and toad share a common ancestor that had four limbs (arrow). This gives a tree-length of 3 (= 3 changes). It would be less parsimonious to infer that four limbs evolved independently in the toad, dog, and cat. Why?

Fur in the dog and cat are similar due to homology, since similarity of the biological feature is due to common ancestry, having evolved just once. The wing in insects and birds evolved independently and thus the similarity of form and function is due to analogy (including parallelism and convergence).

The Homologous Homeobox (Hox genes)

The nuclear genetic code of DNA is virtually universal in eukaryotes.

Do you think that humans share the same genes as plants? In other words, do plants and humans have similar sequences of DNA?

Evolutionary hypothesis: the genetic code originated in an ancestor common to all living things on earth.

What are the implications of the hypothesis above?

Do we have any evidence to support this hypothesis?

Read Campbell pp. 431-433; based on this reading, explain Figure 21.23.

Phylogenetic Questions

1. Give two reasons why classification is important in biology.

2. Define species. How does “speciation” occur?

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•

•

•

•

•

Figure 3.5

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3. What are homeotic genes? Why are HOX mutations relevant to the origin of vertebrates? (see Figure 24.19 in Campbell)

4. What is the “hierarchical” classification of animals? Give an example of a hierarchical classification of one animal species.

5. Contrast the terms homology and analogy.

6. Define the term “convergent evolution.”

7. Make your own interpretation of the meaning of the graph below (Figure 3.6).

% Average Nucleotide Sequence Differencesbetween Human and Chimpanzee Chromosomes

Ebersberger, I. et al., Am. J. Hum. Genet. 70:1490-1497, 2002.

Figure 3.6

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anagenesisanalogyancestral character state, ancestral traitbranch clade cladogenesiscladogramderived character state, derived traithomology lineage monophyletic groupnode

non-monophyletic group parsimonyphylogenyrecent common ancestorrootsister groupsspeciessystematicstaxon (singular), taxa (plural)tiptree

Below is a summary of information on the characters of four species of the genus Potatoheada

(P. longnosea, P. barefootii, P. muscularis, and P. longearea). The information is in the form of a matrix (table) that contains information on the 4 taxa (rows) and their characters (columns). In this activity, your lab group will interpret a data matrix and construct a phylogenetic tree.

Procedure

1. Build the 4 species using the matrix (Table 3.1).

2. Decide which species has the most ancestral characteristics. Which did you choose? Place this species at the end of the most basal branch and place the remaining species on the tree in order so the most derived species is positioned at the highest branch of the tree. Ignore variation in color for most of the characters.

LCA

B

A

Figure 3.7

Table 3.1

Mouth Ear lobe Hand Foot Nose Eye Mustache Hat

P. longnosea Tongue Attached Skinny Shod Long Round No No

P. barefootii Tongue Attached Skinny Bare Short Round No No

P. muscularis Teeth Attached Muscular Shod Short Round No No

P. longearea Teeth Free Skinny Shod Short Round No No

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3. Based on the 8 characters you used to construct your tree, how many were variable?

4. Assume that species P. longnosea shows all ancestral features inherited from the last common ancestor (LCA). Place P. longnosea on the lab bench to visualize the ancestor of the four extant (living) species. (Note: P. longnosea is not the LCA but it represents a species with character states compatible with the hypothetical LCA.) Write the character states of LCA in Table 3.2.

5. The position of B is the ancestor common to P. barefootii, P. muscularis, and P. longearea. State which, if any character(s) changed from LCA to B. Make the appropriate changes in LCA so that it “evolves” into B. Enter the character states of B in the matrix below. Repeat for A.

Table 3.2


LCA

B

A

TotalChanges

6. Use the 2 character matrices to determine which characters changed, and on which branches. Mark the changes as “tick” marks on the tree in Figure 3.7. The number of changes on each branch over the branch length is a measure of evolutionary change on that branch. The sum of these changes gives you the tree length, which is a measure of evolutionary change on the tree as a whole.

7. Calculate the length of the tree.

Discovery of a New Species: You discover a new species of Potatoheada that you describe and name (give it a name). Its character states are listed below. Build the new species and place it next to its closest relative on the bench. Assume that LCA is ancestral to this species as well.

Table 3.3


P. ______ Tongue Attached Skinny Shod Long Oval, lidded Yes Yes

8. Add your new species to the tree. To which species is your new species most closely related?

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9. Mark all changes that occurred in the branch leading to it. Is there a difference in the amount of evolutionary change in the 2 sister species? What does this suggest about the relative rate of evolution in the 2 species?

Interpreting character evolution: On further study of the genus (you are the leading expert on this taxon), you discover that P. longnosea, P. barefootii and the new species are insectivores, while P. muscularis and P. longearea are carnivores.

10. Where, on the tree, did carnivory evolve? Show this using a tick mark. Remember to include your new species.

11. Do you think any of the character/s that you studied may have enabled the evolution of carnivory? Which character/s? Show where this character/s evolved.

12. Briefly describe how this evolutionary change occurred (describe an evolutionary scenario), explaining the observations and inferences on which you base this hypothetical scenario.

Thought question: How might you test this evolutionary hypothesis?

Most of the animals in today’s lab live in Stony Brook Harbor, barely a mile from campus. They live near the shore between low and high tide (intertidal) or just below the low tide line (sub-tidal). Some of the most fascinating animals are so tiny you will need a dissecting scope or a compound microscope to see them.

Working with Live Animals: Observe live animals carefully. Give shy or disturbed animals some time to adjust to their observation conditions. If you are patient, you will be rewarded by seeing their behaviors; make keen observations and take notes about how they move, feed, and interact with their environment and each other.

In Activity 2a, you will construct a simple tree based solely on the morphology of 6 marine organisms. In Activity 2b, you will construct a phylogenetic tree based on morphology and functional synapomorphies from ten marine organisms.

Remember to cover your scope stages with plastic wrapto protect them from the corrosive effects of sea water.

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As a lab group, identify any 6 marine organisms by common name. For this activity, we are primarily interested in learning important concepts in making phylogenetic trees.

1. Select 6 marine organisms from the salt water tank.

2. Use the Table 3.4 (on the following page) to organize your specimens—use only 6 of the specimens listed.

3. Group the 6 species into pairs by morphological (anatomical) similarity—you will then have three pairs. Represent these three groups by a name or symbol of your choice. The diagram below includes group names indicated by shapes.

4. Then further group 2 of the most similar pairs—you will have one group of 4 species and a second group of 2 species. Give each of these 2 groups a name.

5. Finally, unite these 2 major groupings into 1 common group and name it. You have just constructed a 6 species sample of the “Tree of Life.” What features do all these animals have in common? Label each species in order by number and common name.

6. Identify and note those features that you used to unite each group in the figure to the right.

7. Submit “Tree 1,” with illustrations and their group defining characteristics, to your TA.

Figure 3.8 is a graphical representation of the ordered pairings used to unite 6 organisms into groups. For the organisms that you select, you can replace the symbols (shapes) and letter names with names for each group.

Figure 3.81

2

3

4

5

6

A

C

B

Use Figure 3.9 as a guide for listing the characters important in forming your groups.

A

B

C

Unique Species Characters

Characters Grouping Species Pairs

Characters ofGroups B and C

Characters Commonto All Species

Figure 3.9

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Species Organization: Use this table to sort your species into pairs and successively higher groupings.

Common Name First Grouping Second Grouping Third Grouping

Unsorted listChoose six

species only

Pair the most similar species

Group the most similar pairs from the first grouping

Pair the most similar groups from the

second grouping, and connect them to the

remaining group.

Hard shell clam

Mussel

Hermit crab

Sea anemone

Sea lettuce

Fucus

Red beard sponge

Sulfur sponge

Sandworm

Starfish

Hydra

Mud snail

Group Defining Characters

Table 3.4

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Remember to cover your scope stages with plastic wrapto protect them from the corrosive effects of sea water.

Purpose

Using the data that you collect, construct a phylogenetic tree of relationships among the species you observe. By making careful comparisons in today’s lab, you will discover important characters that will enable you to identify, group, and better understand marine animals.

Focus particularly on functional structures that animals depend on for feeding, reproduction, movement, sensory perception, and body support.

Procedure for Organizing Animal Phylogeny (Tree 2)

1. Set up a data collection process: Before you select any organisms, create a table to collect data to organize your specimens into phylogenetic groups.

2. Choose 10 organisms from the sea water tank.

3. Identify and record the common names of your specimens.

4. Identify key features that you will use to group the specimens.

5. Note physical characteristics and associate them with particular functions for each species. Descriptions and diagrams are provided for each phylum in the appendix.

6. Interpret structure and function, then question your interpretation. Attempt to turn your initial guesses about the ways these animals stay alive into a question you could potentially test in lab. For example, if you call a structure a “gut,” turn the label into a question, “how do I know it’s a gut?” “If this is a gut it might contain undigested and digested food; the fluids in the gut probably contain enzymes; if it is a gut, there must be a mouth.” Further observations lead to further questions; “what does this animal eat?”; “how does it digest its food?” and so on. Let each step in the cycle of observation and questioning lead to further facts about the organism.

7. Identify the major taxonomic group to which your specimen belongs. Use the descriptions and illustrations in the appendix to sort your animals into phylum and if possible into taxonomic class.

8. Do the activities associated with a few living members of the animal phyla to develop your understanding of their morphology and life style.

9. Construct a Phylogenetic Tree (Tree 2): Use the notes from your observations to construct a phylogeny for your 10 specimens. Place each specimen at the end of a branch, connect them at nodes and label each branch with observed characters that define each exclusive branch.

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Observations of Invertebrate Phyla

Phylum Porifera Red beard sponge, sulfur sponge, and bread crumb sponge

Phylum Cnidaria Observing live hydrozoansFeeding Hydra

Firing nematocystsPhylum Platyhelminthes

FlatwormsPhylum Nematoda

RoundwormsPhylum AnnelidaPhylum Mollusca

Clam dissectionSnailMusselsWhelks, chitons, squidSoft shell clams

Phylum ArthropodaHorseshoe crabs

Phylum EchinodermataStarfish

Phylum ChordataClass Urochordata: Colonial and solitary tunicatesClass Vertebrata: killifish

Note: When you are finished looking at the live specimens, return them to the sea water tank. Be careful not to expose the live specimens to the preserving fluids of the dissection material.

LIVE MATERIALS (AS AVAILABLE)Chiton (Katerina)Tunicates (Urochordates)Blue mussel (Mytilus)Ribbed mussel (Geukensia)Soft-shelled clam (Mya)Clam (Mercenaria)Scallop (Argopecten)Oyster (Crassostrea)Periwinkle (Littorina)Mud snail (Ilyanassa)Slipper shell (Crepidula)

Whelk (Busycon)Nudibranch (Doris)Feather duster worm (Eudistylia)Sea urchin (Arbacia, Strongylocentrotus)Sand dollar (Echinarachnius)Starfish (Asterias)Brittle star (Ophioderma)Hermit crabs (Pagurus)Shore crabs (Hemigrapsus)Bryozoans Tubularians (Tubularia)

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Grantia Dissection

Grantia is a simple calcareous sponge.

1. Observe a whole Grantia and look at the holdfast and the osculum. Grantia appears shaggy because the external surface of this sponge is highly folded into numerous peaks and valleys; the wall contains many tiny pores that enable water to flow from the outside of the sponge to the spongeocoel.

2. Learn how water circulates in Grantia –water enters through pores in the walls of the sponge and exits through the osculum at the top of the sponge. Water flow is kept in motion by specialized flagellated collar cells (choanocytes) which line the walls or canals of the sponge wall. Calcareous and glass sponges are supported by spicules (Figure 3.10).

3. Dissect Grantia by cutting it longitudinally. Observe the major features of this simple sponge. Know how water flows through the animal.

4. Compare the structure of preserved Grantia, a bath sponge, and the live red beard sponges. What evidence would you need to demonstrate a relationship among these specimens?

osculum

amoeboid cell

flagellum

Ostia andincurrent canals

epidermis

pore

spicule

Collar Cell

Figure 3.10

Porifera Activity

Examine the representatives of the 3 major classes of sponges—classified largely by skeleton composition and cell characteristics.

1. Identify living specimens of the common species of sponge from Stony Brook Harbor—red beard sponge, yellow sulfur sponge, and bread crumb sponge.

2. Examine the external structure of live sponges under a dissecting microscope. Can you detect water currents moving in or out of the sponge? A drop of food coloring next to the sponge may help.

3. Look at a “real bath” sponge skeleton—most “sponges” you buy in the supermarket are synthetic.

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Watching Hydra Feeding and Firing Nematocysts

Initial Observations: Obtain a small dish of freshwater and pipette up several Hydra (Figure 3.11) from the class culture jar. Allow time for the Hydra to recover from the transfer and keep the light as low as possible.

Hydra has a body column surrounded by a mouth cone, which is surrounded by a ring of tentacles. The “knobby” roughening seen on the tentacles with high magnification are clusters of nematocysts.

Look at whole mount Hydra with gonad-bearing individuals. The enlarged “bumps” on the body wall are gonads. There are male and female gonads distributed in separate regions of the Hydra. Male Hydra gonads located above female gonads.

1. Hydra (hydrozoa): Draw a picture of this animal. Label any distinguishing features. Label the features observed.

2. What may be the adaptive significance of the relative positioning of male and female reproductive organs?

3. Feeding: Use a dropper to add some brine shrimp that have been rinsed in fresh water to a Hydra on a slide. Try to observe food capture and ingestion under the dissecting microscope. Did the Hydra react to food at a distance? What happened when the Hydra first contacts its prey?

4. Nematocyst Firing: Place a Hydra in a drop of water on a clean microscope slide, cut off and remove the trunk (leaving only the tentacles), and add a cover slip. Examine the slide under a compound microscope. Notice the cnidocytes, which are visible as swellings along the tentacles (the cnidocytes may be easier to see if you stain them by adding a drop of 0.01% methylene blue stain to the edge of the cover slip). Look for nematocysts that have and have not discharged (Figure 3.12). Tap on the cover slip with a pencil and see whether you can observe any discharging nematocysts. Nematocyst discharge may also be stimulated by adding a drop of 1% acetic acid to the edge of the cover slip.

bud

gonad

gastrovascular cavity

Figure 3.11

Hydra

Undischarged Discharged

Figure 3.12

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Figure 3.13

Obtain a clam and place it on your dissecting tray. The clam’s shell is composed of two valves hinged together on their dorsal side. Study Figure 3.13 as well as the handout before proceeding with your dissection.

1. Two large muscles, the anterior and posterior adductor muscles, hold the shell valves together. Locate the positions of the adductor muscles, then carefully insert a scalpel blade between the valves and slice through the muscles along the inside of the left valve, trying to avoid damaging both the clam’s internal organs and yourself.

2. Open the clam and remove the left valve by twisting the upper valve to break the hinge ligament and free the mantle from the inside of the left valve. Completely cover the specimen with water before continuing with dissection and observations.

3. At the posterior end of the clam, locate the incurrent siphon and, just dorsal to it, the excurrent siphon. The siphons are used to take in and expel water.

4. Carefully cut away the left mantle, exposing the mantle cavity. Identify the foot, which extends anteriorly from the visceral mass (Figure 3.13). Locate the large, flaplike gills, or ctenidia. Remove the ctenidia to expose the visceral mass, which encloses most of the internal organs. Find the labial palps, which surround the mouth.

5. Place a drop of carmine and algae particles on the gills and observe the gill surface under your dissecting scope. Do the particles sort by size or type? Do the algae move anteriorly towards the labial palps and the mouth? Which way do the carmine particles move? User higher magnification and try to see the ciliary movements on the ctenidia surfaces that are sorting these particles on the gill surfaces. If you look closely, you will notice the incurrent and excurrent siphons at the posterior of the animal. Try and detect the direction of water currents at the end of the siphons. Which siphon is the incurrent and which is the excurrent?

6. Cut the visceral mass dorsal to the foot to expose the internal gonadal tissue and digestive tract.

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1. If you were given any five species to classify, what basic steps would you take to place them in order and organize them? How would you decide which specimens to place at the bottom and at the tips of the tree?

2. How would you construct a phylogeny for members of the invertebrate phyla to define characteristics so that new organisms can be classified.

3. What are the relative advantages of radial vs. bilateral symmetry?

4. Sponges do not have nerves, but cnidarians do. Based on your knowledge of hydra, why would a nervous system be necessary?

5. Compare feeding in sponges and sea anemones.

6. What possible advantage could be gained by having an extensively branched intestine like that of the larger turbellarian flatworms (e.g., planarians)? (Hint: Keep in mind the fact that flatworms lack a circulatory system.)

7. Is there an advantage to having a complete gut with a mouth and anus, as opposed to having a gastrovascular cavity with only one opening?

8. Why are skeletons or other supporting tissues important in multicellular organisms? What kinds of supporting structures are found in the organisms in this lab?

9. Do all the animals you have studied in today’s lab have specialized organs for respiration or excretion? If so what are they? If not, why not?

10. What advantages or disadvantages might colonial organisms have relative to solitary ones? Would you expect a colonial or solitary animal to grow faster? Or occupy surfaces more rapidly? Explain.

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