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Sponges are suspension feeders using choanocytes inside the sponge’s body to pump water through the animal. As the water passes through, the choanocytes have a second job, filtering food particles from the water before it leaves the sponge.
Fig. 1.
Main features of a poriferan choanocyte cell. The beat of the flagellum propels water away from the cell and draws it in towards the base of the flagellum. Water passes through the mesh of the microvilli but food is trapped on the surface and engulfed by phagocytosis. © BIODIDAC
To get this job done a sponge has one of three body plans, or architectures, and each is defined by water’s path through the sponge. In general it flows from outside, into the sponge and past the choanocytes, and back out through the osculum . But it’s not always that simple. Depending on how elaborate the architectures is, water moves through a variety of additional channels and/ or chambers collectively called the aquiferous system . It’s these differences that distinguish one sponge’s architecture from the other.
Along with the different paths that water takes through the sponge are differing efficiencies for extracting food from the water. The simplest, the asconoid plan, has a central choanocyte lined spongocoel that gives these sponges a vase-like appearance. The most complex architecture is in leuconoid sponges where the central spongocoel has disappeared and the animals appear much more solid. Intermediate between the two is the syconoid form that retains its central spongocoel and vase-like appearance but with choanocytes embedded in the body wall instead of lining the spongocoel.
In this lab you should trace the flow of water, and identify the different parts of the aquiferous system in each of the different architectures. We will be using these architectures to group our specimens. Remember that the architecture of a sponge has nothing to do with its taxonomy - a story of spicules... More on that a little later.
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Leucosolenia (A word of warning don’t fall into the trap of thinking that Leucosolenia is a leuconoid sponge because of the similarity between the genus name and the architectural type!) is an example of the asconoid architecture, one of the simplest found in the phylum. This also means that they are also the least efficient at trapping food from the water passing through them.
It also limits their size, why?
Fig. 2.
Leucosolenia is an example of the simple, asconoid architecture (right). The sponge itself is colonial (left) and appears to be nothing more than a mat of tissue.
Float it in some water to find the tip of one of the sponges. © BIODIDAC
These small, white, delicate sponges live in colonies with new individuals budding from the base and branches of the colony. Each member of the colony is connected to the next by hollow branches, extensions of the spongocoel. Leucosolenia is commonly found just below low tide levels and is anchored to just about any substrate it can hold onto.
The branching nature of the colony results in dense mats making it difficult to distinguish one individual from the next and the preserved material looks nothing like the usual illustration of a leuconoid sponge. If you want to see the characteristic asconoid body plan you’ll have to use a fine pair of scissors to cut off a piece of the colony. Place it in a small petri dish with some tap water, and shake it gently in the dish to try and spread the tissue out. You can also try gently teasing it apart using a probe or needle. Do this while watching under the dissecting scope and you’ll be less likely to tear the sponge.
It will be hard to tell the difference between a torn sponge with its central spongocoel exposed and an osculum , but one difference between the two is the presence of tissue damage disrupting what would be the circular opening of the osculum. Whether you see the osculum in your specimen or not, the spongocoel should be visible along with perhaps some new budding individuals. Look closely at the body wall of the sponge and you’ll be able to see the outline of the spicules embedded there.
Leuconoid sponges differ from the other two types of sponges in how water enters the sponge.
A single porocyte surrounds each incurrent pore. In other sponge architectures s a series of cells forms the opening more appropriately called an incurrent pore , dermal pore or ostium .
If you weren’t able to find the osculum, or porocyte in the preserved specimens be sure to take a look at the microscope slides with whole mounts of Leucosolenia . The spicules will also be easier to see in these stained specimens.
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The cross-sections and longitudinal sections are also available and may, at first, be difficult to interpret. Remember that these are thin-walled animals with sharp calcareous spicules embedded in their body walls. As the microtome makes the thin tissue slices on your slide, the spicules shatter and tear the specimen. You’re best bet is to work your way around the perimeter of the section as you make your observations of the different cell types. You’ll have to be sure that you light source is properly aligned, and oil immersion will be advantageous as well. As you look at these slides identify the location of the choanocytes and their flagella. Depending on the quality of the slide, and how well you’ve set up your microscope, you may also be able to see the collar of these unique sponge cells. Can you identify the spongocoel or see any other types of cells?
Be ready to compare this cross-section with that of Grantia ( Scypha ) in the next part of the lab.
Scypha and Grantia are examples of the syconoid sponge architecture where choanocytes no longer line the spongocoel and instead, are found in the radial canals that open into the spongocoel. Scypha and Grantia are different genera but appear similar to each other. If your interested, their differences lie in the a circle of spicules that surround the osculum in Scypha and a circular body compared to a more flattened body in Grantia without the ring of exposed spicules around the osculum. Unlike Leucosolenia both species are generally solitary, although they may be found in clusters, and are a little large than their asconoid counterpart. Depending on the availability of specimens you may have one, the other, or maybe even both. I’ll use
Scypha in the descriptions that follow but feel free to substitute Grantia if that is the specimen you have.
Fig. 3.
Scypha is an example of the syconoid architecture (right). Unlike
Leucosolenia these sponges are solitary. They can reproduce asexually by buds at the resulting in clusters of individuals (left) © BIODIDAC
Like asconoid sponges, syconoid sponges are also vase-shaped with a central spongocoel that opens to the outside through an osculum. Once you’ve located the osculum on your specimen find the attachment site, or hold fast , located at the opposite end of the animal. The outer surface of the sponge is covered by small finger-like projections and inside these are radial canals lined with choanocytes . Schypa has what is best described as modified syconoid architecture . The outer surface is not smooth and there are no distinct ostia leading to incurrent canals .
The best way to explain this is by using the fingers on your hand. Squeeze together your thumb and first three fingers. There’s a small opening where their tips meet, it’s analogous to the ostia.
The minute space that runs down the length between your fingers and thumb would be
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equivalent to the incurrent canal. The radial canal would be inside each of your fingers. How does water get from the incurrent canal to the radial canal, from the radial canal to the spongocoel?
Fig. 4.
The body wall of Scypha is a modified syconoid architecture and doesn’t have the smooth outer surface. Instead it’s covered with finger-like projections of the radial canals. The space between the “fingers” is the incurrent canals.
Take a specimen of Scypha, insert the tips of a pair of fine scissors in the oscular opening, and make a cut down the length of one side of the body wall. Fold the body wall back to expose the inner wall of the spongocoel. Place the tissue in a small petri dish with some water, and use the dissecting microscope to examine the inner surface of the spongocoel and the outer surface of the body. On the inner surface you can see openings of the radial canals where they enter the spongocoel as the apopyles . Be sure you understand the relationship between incurrent canals, radial canals, apopyles and prosopyles . It’ll make understanding the cross sections that much easier.
On the outer surface you can see the tips of monoaxon spicules that stick out from the body.
Look closer and you should be able to see the triaxons forming the mesh that supports the sponge, and characteristic of the calcareous sponges.
Both cross sections and longitudinal sections of Scypha are available, and you should be able to identify the following: incurrent canal, radial canal, spongocoel, pinacoderm , choanoderm , mesohyl , spicules, prosopyles and apopyles on the slides. Not all of these will necessarily be visible on the same slide and you may have to look at a few before you find all nine structures.
First, you have to be able to tell the difference between a radial and incurrent canal. The easiest way is to find an opening of the body wall into the spongocoel, once you’ve found that you’ve located the radial canal. You’ll also notice how the body wall is thicker at the junction of the radial canal and spongocoel where the mesohyl is thicker. As was the case with the
Leucosolenia, preparation of the material has shattered the spicules and you can see only their remnants. Not only were the spicules shattered they also tore the specimen and you’ll have to crawl along the surface of the radial canal with a well aligned microscope to locate choanocytes with their thick cell bodies and delicate flagella , often seen as a fine mesh above the cell body.
Underneath the choanocytes is the mesohyl then the pinacoderm lining the incurrent canal. As you move along the surface you should be able to find the prosopyles connecting the incurrent and radial canals. Compare the organization of these elements with Leucosolenia , what is common and what is unique and how does that relate to the way that water flows through the two specimens?
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Collar and flagellar components can also be seen on choanocyte slides that have been specifically stained to show these structures. You will need to use oil immersion.
There may be some amoebocytes visible in the mesohyl but one of the more obvious features are the eggs or developing larvae that form from the amoebocytes. They appear as dark staining, spherical structure embedded in the body wall of the sponge. Be sure you see them.
Of the three sponge architectures, the leuconoid is the most common. It’s design is the most efficient at capturing all particulate material flowing through the animal. The aquiferous system is elaborate and divided into choanocyte chambers connected to each other by small canals.
There is no longer a single osculum, instead multiple oscula can be seen across the surface of the sponge and they collect water from the excurrent canals that should also be visible.
Depending on the specimen, you may be able to see the dermal pores through which the water enters.
Dried sponges are usually used to demonstrate this form of architecture and as a result the living material has disappeared. This makes it hard to see the various paths the water takes through sponges with this type of architecture. In each of the examples examine the general external structure then take a close look at a sample of the sponge to see the calcareous , silica spicules and the spongin characteristic of the Demospongiae which are all leuconoid sponges.
Most commercial sponges belong to one of two genera; Hipposponia sp. or Euspongia sp.
sponges and live in warm tropical seas around the world. At one time in the Florida Keys the industry was more lucrative than the manufacture of cigars from tobacco from neighboring
Cuba. The advent of synthetic sponges has resulted in a major decline in the industry.
In a living sponge a nonliving membrane with openings that lead to the dermal pores covering the external surface. When sponges are harvested they are dried and then soaked to remove the outer membrane and any of the living material. The result, in a dry specimen you’ll only be able to identify the larger oscula and the excurrent canals connected to it. Take a close look at the supporting mesh of spongin under the dissecting microscope. These sponges have no silica spicules and the few if any calcareous spicules are removed when the sponge is prepared for commercial use. What remains is a fine net of spongin that gives the sponge its superior water absorbing qualities.
Compare the bath sponge to any of the other dry sponges. Do you see why this is the preferred sponge species for bathing?
The glass sponge Euplectella , or Venus Flower Basket as it is commonly called, is an example of a Hexactinelid sponge with a leuconoid architecture. There is some debate about this and some feel that the architecture is somewhere between syconoid and leuconoid. In part the confusion is the location of the choanocyte chamber and whether it empties directly into a central spongocoel, or excurrent canals. Euplectella is found in the deep oceans, usually 100m or deeper, which explains the use of silica in its supporting skeleton of spicules. Why?
Like commercial sponges, the living tissue has dried and is no longer visible. When alive, the skeleton you see was covered in a syncytial layer of tissue with the spicules located intracellularly. The woven web of the skeleton is formed from unique hexaxons connected at their tips and these connections make it hard to see the typical hexaxon of the class. The oscula are located at the tip in a sieve like plate; openings to the incurrent canals cover the sides of the sponge. At the base spicules are modified to allow it to anchor into the shifting sediments at the ocean bottom.
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Sponge spicules are made of crystals of either silica or calcium carbonate forming a crystalline mesh that supports the sponge’s body. In addition to spicules spongin, a collagen-like protein also helps a sponge maintain its external shape. The three major classes of Porifera; Calcarea,
Hexactinellida, and Demospongiae, can be distinguished from each other by the shapes of their spicules.
Calcarean sponges, as their name implies, have spicules made of calcium carbonate. The use of this material isn’t restricted to only the Calcarea and calcareous spicules are also found in the
Demospongiae. In non-calcarean sponges the calcium carbonate spicules are hollow and never form the three and four pointed spicules characteristic of the Calcarea. Silica is also used in deep-sea varieties but the hexaxon is unique to the Hexactinellida.
Place some small pieces of the dry commercial sponge under the microscope and examine the spongin and calcareous spicules found in these organisms. Locate the diagnostic spicules of calcarean sponges in the Scypha preparation that is available. Spongin isn’t a spicule but shares the supporting skeletal function with its silica and calcareous counterparts. Spongin is also available on microscope slides.
Freshwater sponges, such as Spongilla , cannot survive through the winter and instead produce gemmules. These overwintering structures contain archeocytes surrounded by a protective outer covering of spicules. Examine the gemmule slides that are available. Do marine species produce gemmules?
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