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. 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.
Watch the video of living sponges showing how they move fluids through their body.
|Water flow in living sponge|
Calcareous sponge Grantia
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.)
|Representative sponge bodies|
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.
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.
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
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.
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, the freshwater hydroid
Hydra is an unusual hydroid in that it lives in freshwater, is solitary as an adult, and lacks a medusa stage.
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.
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.
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, 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.
Other living anthozoans
|Various living 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.
|Various skeletonized anthozoans|
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.
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.
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.
 Modified from Vodopich, D.S. and R. Moore. 2014. Biology: Laboratory Manual. McGraw Hill, New York.
Delivering a high-quality product at a reasonable price is not enough anymore.
That’s why we have developed 5 beneficial guarantees that will make your experience with our service enjoyable, easy, and safe.
You have to be 100% sure of the quality of your product to give a money-back guarantee. This describes us perfectly. Make sure that this guarantee is totally transparent.Read more
Each paper is composed from scratch, according to your instructions. It is then checked by our plagiarism-detection software. There is no gap where plagiarism could squeeze in.Read more
Thanks to our free revisions, there is no way for you to be unsatisfied. We will work on your paper until you are completely happy with the result.Read more
Your email is safe, as we store it according to international data protection rules. Your bank details are secure, as we use only reliable payment systems.Read more
By sending us your money, you buy the service we provide. Check out our terms and conditions if you prefer business talks to be laid out in official language.Read more