BASAL ANIMALS Essay

Name: _______________________

BASAL ANIMALS

OBJECTIVES: Critically observe and identify different species representing the major lineages of basal animals and the ways they live. Describe common characteristics (anatomy, life cycle) of different sponge and cnidarian lineages. Observe the morphological structures of these groups, noting trends in body size, individualization, structural support (skeletons), and cellular and tissue-grade organization. Evaluate the function(s) of different morphological traits as they relate to life in different habitats, and practice developing ways to test these hypotheses. Discuss behaviors typically associated with animals, including mobility, feeding, and reproduction.

 

TECHNOLOGY REQUIREMENT: This lab requires internet access on a computer. For videos and digital images referenced below, check the Digital Atlas available on D2L.

 

 

Introduction to the basal animals

To most people, ancient animals such as sponges and jellyfish appear primitive and unsophisticated.^[1] But recall that in cladistics, the term primitive refers not to simple, less important or poorly functioning organisms, but to traits derived early in the phylogenetic history of a clade’s evolution. Although the bodies of sponges and jellyfish are simple and lack complex structural and behavior capabilities, don’t let that simplicity fool you. What these animals lack in “complexity” they more than compensate for in their elegant design. After all, their body plans have persisted in all marine environments since the evolution of the first animals approximately 600 million years ago, successfully feeding, reproducing, and adapting as other animal lineages evolved. Although “simple,” they remain evolutionarily very successful. And their “primitive” innovations—ability of cells to work cooperatively as multicellular organisms, use of specialized cells to feed heterotrophically and carry out essential functions, and a body organized into specialized tissues and organs—persists as their legacy throughout the entire Kingdom Animalia.

Part I. Phylum Porifera (Sponges)

Sponges are the most primitive multicellular animals, lacking true tissues and organs. Food, waste products, gas for gas exchange, sperm, eggs, and larvae enter or exit sponges through extensive systems of water canals. With their enormous surface area-to-volume ratio, their physiology is largely diffusional. Their bodies are supported with isolated spicules that function as an intracellular skeleton that helps support large body sizes and mechanical (structural) stability. Major sponge clades are identified by the biomineralization of these spicules, which can be made using calcium carbonate, silica (glass), or flexible proteins. Individual sponges are hermaphroditic, capable of producing both sperm and eggs. While most sponges are sessile (attached) on hard substrates, one interesting class of sponge is able to bore into calcareous materials like mollusk shells or coral. These are known as the boring sponges (although they are far from boring!). Sponges are mostly marine, although a few are found in freshwater. Approximately 10,000 living species have been described to date. Sponges feed by drawing water in through pores called ostia on the surface of the colony. Water flow is generated by flagellated collar cells called choanocytes, which also capture food particles as they pass through the sponge. (These flagellated cells are thought to be homologous to the choanoflagellate protists, which all animals are closely related to.) Water is eventually expelled through a large opening called an osculum that acts as a “chimney.”

Sponges don’t obey many of the “rules” we normally associate with multicellular animals. They lack tissue layers, and possess only a few cell types. However, these cells are rather specialized, with some functioning for feeding, others functioning in producing spicules, lining the ostia (pores), the body surface, and other functions. When needed, these specialized cell types can “de-differentiate” into a generalized amoeba-like form, travel to a different area of the sponge, and then re-specialize into a completely different cell type, a process termed totipotency.

materials: videos of living specimens; images of dead and preserved sponges (including Euplectella and Spongia), images of prepared slides of Grantia, in cross section and isolated spicules

methods: direct observations; observation in dissecting and compound microscopes

 

 

 

Sponges come in three basic body plans. Because they result from the requirements of a functional surface area-to-volume ratio, they are little phylogenetic usefulness. Water flow passes from the outside, passing laterally through incurrent ostia into internal chambers before passing out at the top in the oscula. Choanocytes line inner surfaces of the internal chambers.

Asconoid: The body wall is not folded to form chambers; choanocytes line a central chamber that exits at the osculum.

Syconoid: Folding of the body wall allows for a greater number of choanocytes.

Leuconoid: There is extensive folding of the interior body wall. Multiple ostia and oscula are present. Choanocytes line the numerous internal cavities.

1. Draw arrows on the three figures above showing how water flows through each sponge during feeding and respiration.
2. The vast majority of the sponges you will see in lab (or in the Jurica-Suchy Nature Museum) are leuconoid. In fact, all large sponges are leuconoid in form. Why do you think that this form allows for a larger animal?

 

 

 

 

 

Living sponge

Watch the video of living sponges showing how they move fluids through their body.

Water flow in living sponge

• Draw a sponge. Where does the water flow into the sponge? Where does it exit? About how long does it take a dye particle to make its way through the sponge?

 

 

 

 

4. Is the you drew sponge asconoid, syconoid, or leuconoid?

 

 

 

 

 

 

Grantia (cross-section)

Calcareous sponge Grantia

5. Examine the prepared slide of Grantia in cross section. Draw the cross section, drawing arrows where the water flow occurs, and labeling where the collar cells are located.

 

 

 

 

 

 

 

 

Grantia spicules

• Examine the prepared slides of Grantia spicules under a compound microscope. (You will need to do this under dark field or you will not be able to see the spicules clearly.) Describe the morphology of the spicules. How many types of spicules are present on the slide? Draw one of each type.

 

 

 

 

 

 

 

Euplectella, the Venus flower basket

Euplectella glass sponge

Euplectella is one of the most famous sponge genera. It produces spicules from silica (glass) and is one of the largest animals to have a glass skeleton. It lives in very deep water in the western Pacific Ocean, where there is no sunlight, little food and oxygen and carbon dioxide (but lots of dissolved silica), and very calm water currents. The osculum in mature specimens is enclosed in a “mesh” cap. Shrimp that live in these waters often inhabit the inner chamber of these sponges, and often there will be a monogamous breeding pair that, when capped, will spend their life within this sponge. (Because of this symbolism, such Euplectella specimens are a traditional wedding gift in Japan.)

7. Draw the intact body of a Euplectella Why might this organism push the limits of having a glass skeleton? (In other words, why might it be possible for this animal to be so large when most glass sponges are much smaller, and no other animals use glass for their skeletons?)

 

 

 

 

 

 

 

 

8. Why might it be advantageous for a breeding pair of shrimp to inhabit the inner chamber of such sponges? (What benefits might the shrimp gain by being stuck their entire lives in this unusual habitat?)

 

 

 

 

 

Other sponges

Representative sponge bodies

• Examine the preserved and dried sponge specimens to get an idea of the vast diversity of sponges colony shape. Make a sketch of several entire colonies, making certain to label the base, a few ostia, the osculum, and note whether asconoid, syconoid, or leuconoid.

 

10. It takes energy to manufacture mineral spicules. If you were designing two sponges, one to live in a rough, wave-swept environment, and another to live in a calm environment, which form might be evolutionary fitter to have elastic protein spicules versus ridig calcareous or glass spicules? What spicule chemistry and overall sponge morphology would be preferable in each environment (shallow and high energy versus deep, quiet water)? Explain why you think this.

 

 

 

 

 

 

Part II. Phylum Cnidaria (Jellyfish, sea anemones, corals, hydroids)

Cnidarians are radially symmetric aquatic animals with two body types: a free-floating medusa and a sessile polyp attached to substrate. All cnidarians have a central gut cavity with a tentacle-lined mouth as the only entrance, which also serves as the anus. This gut is sometimes termed a “one-way gut” or an “incomplete gut” because of its single opening. Cnidarians are carnivorous. Tentacles possess specialized stinging cells called nematocysts that subdue their prey, mostly zooplankton but also some fish.

As the earliest metazoan animals, all four animal tissues (epithelial, muscular, nervous, and connective tissues) are present, although their organs are generally simple and limited to sensory, digestive, muscular, and reproductive organs. There is no circulatory system and the nervous system is a nerve net. Reproduction can be sexual and asexual, and most life cycles include a free-swimming larval stage. There are more than 13,000 living species, almost all of them marine.

Hydrozoa

Anthozoa

Scyphozoa

Figure 2. The body organization of cnidarian classes, showing the arrangement of epidermis and endodermis surrounded by mesoglea (or mesenchyme), and the gastrovascular cavity (gut) with single opening surrounded by a ring of nematocyst-lined tentacles.

Cnidarians are a diverse group of animals that are united by several common characters. First, all cnidarians are composed of an outer, ectodermal epithelial layer of cells (the epidermis, which is just one cell layer thick!) and an inner, endodermal epithelial layer of cells (the gastrodermis). The two layers sandwich a jelly-like acellular layer called the mesoglea in most gelatinous cnidarians (such as the jellyfish); in some cnidarians (such as sea anemones), the middle layer is very collagenous and more muscular and is termed a mesenchyme. The amount and composition of the mesoglea affects the mechanical properties of the cnidarian body and varies in amount from very little (sea anemones, and other polyps) to lots (most medusae). Second, cnidarians possess either radial (or biradial or tetraradial) symmetry. Third, the oral surface of cnidarians is surrounded by a ring of tentacles. The tentacles, and occasionally other parts of the body, are covered with stinging cells called nematocysts. Finally, in most cnidarians there is a metamorphic “alternation of generations” (not to be confused with the plant life cycle!) between a polyp phase and a medusa phase. Both phases of the life history cycle are diploid and when both phases are present, the medusoid phase reproduces sexually while the polyp phase reproduces asexually, and is often colonial.

1. Class Scyphozoa (true jellyfish)

Scyphozoans are solitary marine cnidarians whose life cycles are dominated by the pelagic medusae. Scyphozoan medusae are larger and more advanced than the medusae of other cnidarians, with partly septate gut cavities and some sense perception. They are generally sexually reproducing with separate male and female individuals; well-developed gonads are shaped as rings located above gut. They are a small clade, with only 200 described living species.

materials: videos of live specimens of Cassiopea xamachana, the upside-down mangrove jellyfish; images of preserved jellyfish specimens (Aurelia, the moon jelly).

methods: direct observations using dissecting microscope.

 

Aurelia, the moon jelly

Aurelia

• Aurelia is a common jellyfish along north Atlantic shorelines. Observe anatomy, drawing the tentacles, swimming bell, gonads (found in rings near the gut), mouth, and gut. How many gonads are there? Is the symmetry radial, biradial, or tetraradial?

 

 

 

 

 

 

 

Cassiopea xamachana, the upside-down mangrove jellyfish

Observe the video of a living Cassiopea xamachana. Don’t worry, this animal isn’t sick! Unlike most scyphozoans, Cassiopeia lives its life upside-down with the top of its swimming bell resting on the substrate. It inhabits calm-water lagoons and mangroves in the Caribbean and Gulf of Mexico. Like many anthozoan corals, this jellyfish has zooxanthellae (mutualistic photosynthetic dinoflagellates, typically in the genus Symbiodinium) that live in the tentacles.

12. Notice when one is induced to swim. Describe how this jellyfish (and others) swim. What part(s) of the body is used when swimming?

 

 

 

 

 

 

13. Observe the living jellyfish. Most jellyfish are translucent and color-less. What colors are these jellyfish, and where is the color found on the animal? (Identify the colorations in the figure to the right.) What does this indicate about where its zooxanthellae are located?

 

 

 

 

14. Given the living orientation of this unusual xoozanthellate jellyfish, why might it be adaptive for it to live upside down in shallow, calm waters?

 

 

 

 

 

 

1. Class Hydrozoa (hydras, hydroids)

Hydrozoans are phylogenetically the most primitive cnidarians, most possessing both medusoid and polypoid forms during individual life cycles. Most hydrozoan animals are polypoid and colonial as adults, but there is much variability among subgroups. Some surround colony with a chitinous exoskeleton. The technical distinctions between hydrozoans and the two other groups of cnidarians include the following: 1) hydrozoans shed sperm and eggs directly into the water from epithelial cells, while the other two groups shed them into the gut cavity, from which they escape out the mouth, and 2) hydrozoans have nematocysts only in the epidermis and not in the gut cavity. 3,500 described living species.

materials: videos of live Hydra; images of preserved Obelia; images of prepared slides of Obelia; images of preserved specimens of Physalia (“the Portuguese Man-of- War”) and Gonionemus

methods: direct observations, dissecting or light microscope

Hydra

Hydra, the freshwater hydroid

Hydra is an unusual hydroid in that it lives in freshwater, is solitary as an adult, and lacks a medusa stage.

15. Observe the video of a living specimen in a compound microscope. Draw this organism, labeling important anatomical details.

 

16. How does it react to physical stimulus when disturbed? How do you hypothesize that this organism feeds? Does it appear capable of locomotion?

 

 

 

 

17. Watch the polyps feed when a few microcrustaceans are added to the water. Do your observations support or refute the feeding hypothesis you made above?

 

 

 

 

 

 

Obelia, a colonial hydroid

Obelia is a common marine colonial hydroid. The polyps are connected by a common gastro-vascular cavity (GVC), which presumably shares nutrients throughout the colony. Obelia is a “polymorphic” colonial animal, meaning that the individual polyp members of the colony have differing morphologies and perform different tasks. This is called “division of labor. ” In this way, Obelia is much like an ant colony that has workers, soldiers, queens, and drones that are all specialized for different tasks.

Remarkably, the polymorphic polyps in an Obelia colony are genetically identical despite their differing morphologies. This is because the colony grows by clonal budding. Hormonal or environmental cues turn on certain genes in some new polyps and different genes in others, resulting in differing morphologies.

Obelia colony of poylps

Examine the preserved specimens and microscope slides of Obelia. You should be able to find polyps that are specialized for feeding (they have tentacles) and polyps that are specialized for reproduction (they resemble tall vases and may have visible eggs inside). Draw (and label) these specialized polyps.

18. Note how the GVC connects with the rest of the colony. Does there seem to me a sphincter or valve restricting the GVC, or is this an open connection? How might this be beneficial to the entire colony?

 

 

 

 

 

 

Obelia-like medusa

• This colonial form is the polyp stage of Obelia. As with most hydroids, Obelia also has a solitary medusa stage, which serves a dispersal function. Obelia medusae are very small, but we can see the basic morphology using the medusa of another hydroid, Gonionemus. Observe the Gonionemus Draw a medusa, labeling important anatomical features.

 

 

 

 

 

 

20. As is typical with cnidarians, Gonionemus exhibits radial symmetry. Is this three-, four-, five-, or six-part symmetry? What sort of consequences might radial symmetry have on the direction this organism can move?

 

 

 

 

 

Physalia, the Portuguese man-of-war

Physalia, the Portuguese man-of-war, is a colonial, free-swimming hydrozoan composed of both polypoid and medusoid individuals that inhabits open oceans throughout the world. It belongs to a group of hydrozoans known as the Order Siphonophora. Division of labor is pronounced, with different individuals specialized for prey capture, feeding, and reproduction, among other functions. Stings of these organisms are potentially fatal to human swimmers because of severe allergic reactions, which can result in drowning.

Observe the Physalia specimens. Each hanging tentacle represents a single polyp loaded with nematocysts and specialized for prey capture. There are also polyps specialized for digestion and reproduction. Medusoid specialists may be used for locomotion. Even the large float is a specialized member of the colony.

21. Compare Physalia to the jellyfish observed earlier. Does Physalia appear to exhibit the same radial symmetry that is found in the true jellyfish?

 

 

 

22. Should Physalia be considered a single individual or a colony of separate individuals?

 

 

 

 

1. Class Anthozoa (sea anemones, corals, sea pens, sea pansies, sea whips, sea fans)

Anthozoans are the largest and most morphologically diverse of the cnidarian groups, with more than 6,000 living species described to date, including charismatic members like the sea anemones and corals. Anthozoans are sessile cnidarians entirely lacking a medusa form. Their polyps are more advanced than hydrozoan polyps, with a gut cavity partitioned into compartments by fleshy septa and nematocysts inside the gut cavity.

Sea anemones and hard corals belong to a group called the Subclass Hexacorallia (or Zoantharia). As the name would indicate, these organisms tend to be organized as six-part radially symmetrical animals. Another group of anthozoans belong to the Subclass Octocorallia (or Alcyonaria), and they have bodies with eight-part symmetry.

materials: videos of live specimens of sea anemones (Aiptasia) and hard and soft corals (Heteroxenia fuscescens, the pulsing xenia); images of preserved specimens of sea pansies, hard and soft corals, and anemones.

methods: direct observations

 

Aiptasia

Aiptasia, a sea anemone useful as a model organism

Aiptasia is a small “weedy” hexacorals sea anemone that can inhabit nearly any hard substrate and can reproduce quickly. This makes is a pest for salt water aquariums, but useful as a model organism. Like Cassiopea and many other anthozoans (especially those that build coral reefs), Aiptasia can be zooxanthellate.

23. Observe Aiptasia and draw and label important features.
24. What other organism that you have seen in this lab does the sea anemone resemble? Do you think that these organisms live similar lifestyles?

 

 

 

 

 

25. Aiptasia lacks hard parts. How do you think it keeps its body upright and stiff?

 

 

 

 

Other living anthozoans

Various living anthozoans

• Observe several living anthozoans, drawing several and practicing identifying them as hexacorals and octocorals.

27. Watch the video of corals feeding. Describe how the polyps feed. Do the colonial corals feed similar to, or differently than, sea anemones?

 

 

 

Skeletonized anthozoans

Anthozoans with hard parts have developed two main ways of providing structure: through external calcium carbonate deposits in stony corals and internal proteinaceous tissue masses in the soft corals (that is sometimes confused with wood or leather).

Look at the demonstration specimens of dead stony corals. The skeleton is secreted from the lower half of the body column. The living coral polyp sits in this skeletal cup. Stony corals deposit calcium carbonate year-round. Some parts of the year are more favorable for deposition, however, resulting in growth rings similar to those of woody dicots, which can also be used to date the age of corals. The buildup of calcium carbonate underneath stony corals is responsible for the massive coral reefs around the world.

Most reef-building corals are hermatypic, meaning they contain zooxanthellae within their epidermis. In the disease called “coral bleaching” caused by increasing water temperatures and other environmental stresses, the zooxanthellae abandon the coral’s skin, leaving the skin transparent, and with wounds liable to become infected.

28. Why might the loss of zooxanthellae be detrimental to the coral animal?

 

 

 

 

Various skeletonized anthozoans

• Observe the diversity of stony coral skeletons and draw several examples.

 

Gorgonians: Sea whips, sea fans, and sea pansies

The sea whips and sea fans are octocorals that are collectively known as gorgonian corals. Unlike stony corals, gorgonians build flexible skeletons out of a horny (proteinaceous) organic material. However, individual polyps within a colony are still connected with one another by pores within the skeleton.

30. Examine the sea whips. The dried skeletons are flexible. What sort of marine environment do you think that this flexibility is an adaptation for?

 

 

 

31. Looking at these animals, especially the sea whips, can you figure out where this name came from?

 

 

 

Examine the sea pens and sea pansies. Like Obelia and Physalia, gorgonians are colonial and polymorphic. The “body” of the colony is a single, greatly enlarged polyp called a rachis. The rachis is specialized for structural support and may be reinforced with spicules. These spicules are visible on the sea pansy. Much smaller polyps specialized for feeding grow out of the rachis. Finally, still smaller polyps specialized for water flow control the water pressure inside the rachis.

 

 

 

Gorgonian

• Draw a gorgonian, labeling important features.

 

 

 

 

 

 

 

 

 

 

 

Part VI. Live pulsing corals

Our lab on campus has a saltwater aquarium with several species of living corals living in it. View the live camera feed at https://www.youtube.com/channel/UCF9egDioqGvmkffZBG0lv7g

 

One of the most interesting corals in the tank is the “pulsing Xenia” coral, Heteroxenia fuscescens, a hermatypic soft coral. A recent study demonstrated that the pulsing tentacles improved photosynthetic rates.

 

33. Watch a few minutes of the live video. Briefly describe the pulsing behavior in these corals. Is it regular or sporadic? Affecting the entire colony or isolated polyps?

 

 

 

 

 

 

34. Explain a mechanism why pulsing tentacles might increase photosynthetic rates in calm water. (Hint: think about what reactants are needed for photosynthesis to occur and how they get to the corals.)

 

 

[1] Modified from Vodopich, D.S. and R. Moore. 2014. Biology: Laboratory Manual. McGraw Hill, New York.