Download LAB # 2. PORIFERA AND CNIDARIA Phylum

LAB # 2. PORIFERA AND CNIDARIA
Phylum Porifera
1. Overview
Poriferans (= pore bearers) are sessile, suspension-feeding metazoans. Most of the 10,000
or so species are marine. Despite their simplicity (they lack all known tissue and organ
systems), they often comprise a substantial component of many marine habitats (some
can be up to 1 m in diameter). It is thought that the sponges are an evolutionary ‘deadend’ and represent an early experiment in multicellularity that gave rise to nothing else.
There are two main features of Poriferan biology that we will consider in this lab: their
unique water vascular system and the totipotency of their cells.
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2. Essential components of the lab
describe the gross functional morphology of a local species of sponge
observe the functional morphology of the characteristic cell types in a typical sponge
examine representative specimens of preserved sponges which vary in the complexity
of their water circulation system (e.g. Scypha to Spongilla)
3. Classification
Class Calcaria
Scypha (prepared slides)
Leucosolenia (prepared slides)
Class Demospongia
Ephydatia
4. Sponge ultrastructure and functional ecology
Refer to first few figures in Chaper 4 of Pechenik to help you understand the basic
Poriferan body plan. Examine the preserved specimen of Scypha under the dissecting
scope. These are small, marine, colonial sponges with many tube-like projections. Note
the simple flask shape and the exhalent osculum. Note also the protruding spicules. See
the ‘strew’ of spicules on the slide preparation. Identify characteristic features such the
mesohyl, porocytes and sponge skeleton. Note the surface is interrupted by numerous
small projections, containing the choanocyte chambers. Under the compound
microscope examine the slide material (x.s. and l.s.) for the inhalant ostia (numerous
small openings), spongocoel, incurrent canals and choanocytes (often difficult to see,
but they are usually found as darkly stained cells lining the internal cavities), spicules and
perhaps amoebocytes. Pinacocytes line the incurrent canals that run perpendicular to the
long axis of the sponge. In the mesohyle region, look for amoebocytes and large granular
cells, possibly developing. You might have to check several slides and use the
illumination carefully.
Reproduction in the sponges verges on the bizarre. Scypha, for example, produces freeswimming larvae that are flagellated. In this case, sperm arise from choanocytes and
eggs arise from archaeocysts. Sperm are released in characteristic clouds from the
osculum and then travel into the water circulation system of a neighbouring sponge.
Sperm is then ‘captured’ by choanocytes and engulfed by phagocytosis. A zygote is then
formed and the embryo develops into a flagellated larvae. Ephydatia, at the northern edge
of its range, probably only reproduces asexually (via budding or the formation of
gemmules). Very little is known about the biology of this species in Alberta.
5. Biology of Ephydatia sp.
Unfortunately, we cannot collect our local species in winter. Normally, Ephydatia is very
common at local sites, especially around Park Lake. Examine sections of the preserved
sponge under the dissecting microscope for general form and function; be able to
interpret what you see with particular emphasis on the direction of water flow. Then,
prepare wet mounts (under the compound scope) of squashed segments of the body wall
and observe the spicules.
Make wet mounts of the gemmules (these are the spherical structures that are asexually
produced at this time of year as overwintering bodies). You will notice a hardened outer
layer, the inner cell mass and one or more exit tubes. Apply gentle pressure to pop one or
more gemmules. You should notice spicules, amphidiscs and amaebocytes. Contrast
these complex gemmules with those of Spongilla in your slide box.
6. Questions for discussion
1. What is the functional significance of Ephydatia within the Park Lake ecosystem?
How would you design experiments to address this question?
2. Like plants, sponges are sessile and have few options available to avoid predation.
What major strategies can you suggest that sponges may use to limit the risk of
predation? Likewise, why do you rarely find organisms such as barnacles and shellfish
settling on marine sponges?
3. Sponges show great inter- and intraspecific variation in colour (blues, pinks, yellows
and brilliant red). What mechanism might be responsible for producing variation in
colour? Can you think of the adaptive significance of such variation?
4. How do sponges grow? What factors might limit this growth? What is a sponge
‘individuals’? If you stepped on a sponge, would the resulting animal die, attain a
similar shape to the original, or reaggregate at random? Would the individual cells retain
their original functions?
Phylum Cnidaria
1. Overview
This phylum includes the hydras, jellyfish, anemones and corals. Most of the 9,000 or so
species are marine (Hydra is one familiar exception) and as such, they represent one of
the major invertebrate phyla in the oceans. Despite the functional and structural diversity
of this mixture of animals, they all share several features; all are carnivorous and
multicellular and all have an underlying radial symmetry. All possess unique cells called
cnidocytes; a characteristic which defines the group. They possess two layers of tissue
(the epidermis and gastrodermis), between which is the gelatinous layer called the
mesoglea. This group therefore represents the first group we have come across that is at
the tissue level of organization. The phylum contains some of the most beautiful marine
organisms, yet also some of the most deadly. Unlike the sponges, Cnidarians (= nettle
animals) feature a mouth and a gastrovascular cavity. There is no anus, and wastes exit
through the mouth.
Recall from Bio 1020 that there are two basic body forms - the polyp (hydroid) and the
medusa (jellyfish). Despite obvious differences in form, both are actually just sacs with
a gastrovascular cavity. The sessile polyp form is tubular with a mouth at the distal end,
surrounded by tentacles. The medusa is typically free-swimming and umbrella-shaped
with tentacles extending from the outer margin of the bell. The mouth is located at the
end of the manubrium. The mesoglea of the medusa form is much thicker than in the
polyp form. Polyps usually reproduce asexually by budding off additional polyps; they
may also produce mudusae by budding. Medusae are typically dioecous and reproduce
sexually. The fertilized egg gives rise to a planula larva that in turn settles down and
matures into a polyp. This typical life-cycle thus exhibits alternation of asexual polyp
and sexual medusa generations.
To supplement the lecture material, this lab requires you to focus on three aspects of
Cnidarian biology: 1) biology and function of cnidocytes, 2) life-cycles of representative
groups and 3) functional morphology of the specialized groups of cells (zooids) which
perform specific functions.
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2. Essential components of the lab
observe the feeding and gross morphology of live freshwater Hydra and the sea
anenome, Anthopleura
observe the symbiotic photosynthetic cells associated with feeding in Anthopleura
observe the functional morphology (especially feeding and reproductive) of
representative Hydrozoans, Scyphozoans and Anthozoans
3. Classification of the Cnidaria
Class Scyphozoa
Aurelia (slide material, preserved specimens)
Gonionemus
Class Hydrozoa
Hydra (slide and live material)
Obelia (slide material, live material if available)
Physalia (slide material, preserved specimens)
Class Anthozoa
Anthopleura (live)
Gorgonia (preserved skeleton)
Pennatula (preserved specimens)
preserved corals (skeletons)
4. Cnidarian ultrastructure and functional ecology
Class Scyphozoa
Aurelia is atypical of other Cnidaria in that it is not carnivorous. Rather, it is ciliarymucus feeder that collects small particles from the water column. Refer to figures in your
text to help you interpret the functional morphology and life cycle of Aurelia (slides and
preserved specimen). There is a lot of new terminology here, some of which is described
in more detail than required. Your focus should be on understanding the methods these
simple animals use for feeding, locomotion and reproduction. In other words, given their
notoriously simply body plan, how do these animals accomplish these tasks? Rather than
focusing on memorizing the different stages of the life-cycle figures, ask yourselves why
the Figure is so complex. How did it evolve? What are the various stages designed to
accomplish ? Your slide box contains examples of planula larvae, scyphistoma, strobila
and ephyra. On the preserved specimens, observe the rhopalia and within the
transparent body, the gonads and four gastric pouches. The gastrovascular cavity is
complex, involving a canal system which links the bell margin with the gastric pouches
and mouth.
Aurelia medusa are dioecous. Sperm are brought into the mouth and then into the
gastrovascular cavity where they fertilize the eggs. The eggs are brooded in the oral arms
until the planula are fully-formed, which are then released into the water. Shift to the
prepared slides to follow the rest of this amazing life-cycle.
Class Hydrozoa
All hyrdozoans have a simple gastrovascular cavity with both polyp and medusa stages
present in their life-cycles. Usually, the polyp morph dominates. Compared to the
Scyphozoans, Hydrozoan medusae are usually much smaller, and they possess a
distinctive velum.
Examine the slides listed above (Hydra, Obelia colony, Obelia medusa). You are
responsible for understanding the structure and life history of solitary (e.g. Hydra) and
colonial hyrozoans (e.g. Obelia, Physalia).
Hydra spp. are highly-specialized, atypical Hydrozoans. Not only are most species found
in freshwater, but their life-cycles do not include a medusa stage, and each planula forms
into a distinct male or female individual (rather than a typical colony such as Physalia).
Refer to figures in chapter 6 of your text to help interpret the functional morphology of
Hydra and Obelia. Examine the living Hydra by placing a healthy specimen in a small
dish. Allow the animal time to settle on the bottom of the dish. Then observe the unique
attack and feeding behaviour typical of many sessile Cnidarians. Add a few small water
fleas (Daphnia) to the dish. Under your dissecting microscope observe what happens
when a flea comes into contact with this predator. Observe ingestion and consumption.
Spend a few minutes on the histology of the body wall of Hydra. Use Fig. 6.4 and 6.13 in
your text to help interpret the c.s. and l.s. sections of Hydra from your slide box. Notice
the two layers, an outer epidermis and an inner gastrodermis. The mesoglea lies
between these two layers It is very thin in Hydra and may be seen as a line between the
two tissue layers. The large cavity in the middle of the cross section is the
gastrovascular cavity. The various cell types present are difficult to make out in these
preparations. The most abundant, large cells are epithelionutritive cells in the
gastrodermis and epidermal cells in the epidermis.
Obelia is a good example of a colonial hydroid. It is a common carnivore on our West
Coast and as far south as the Gulf of Mexico. Hopefully, we will have live specimens in
the Marine tank. It’s ‘stick-like’ appearance masks what, under the microscope, is truly
one of the more amazing animals we will see this term. See Fig. 6.14 in your text to help
you interpret the live material, and the material from your slide box.
Polyp stage – The erect bushy colony consists of a much-branched cylindrical
structure terminated at its distal end by the polyps. The tubes are composed of two parts,
an outer, non-living part and an inner living sac that is continuous throughout the colony.
The base of the colony is attached to the substrate via root-like structures (usually absent
in living material due to collection). There are two types of polyps (or zooids). The most
numerous are the feeding ones (gastrozoids), recognizable by the ring of tentacles
surrounding the raised hypostome which contains the mouth. Less common are the
reproductive types (gonozooids), lacking tentacles. At the end of these club-shaped
zooids are the meduscae in various stages of development. Obelia produces freeswimming medusae.
Medusa stage – Living medusae of Obelia are hard to obtain so we will be
restricted to the material in the slide box. The medusa is very small in comparison with
the colony, only about 5mm across. Each one consists of a flattened circular body with
short tentacles distributed equally around the bell. You will also see the short
manubrium and four radial canals with each, in turn, bearing a gonad. A shelflike velum
extends inward from the margin of the bell. This is not visible in these Obelia but may be
observed in some of the other Hydroid medusa. I have also included a slide preparation
of Gonionemus. This is one of the larger Hydroid medusas. The slide shows the gonads,
the characteristic -‘doughnut-shaped’nematocyst batteries, and at the base of the
tentacles, statocysts can be seen. This interesting medusa is benthic, usually using
adhesive pads on the tentacles to attach to plants and other material.
The Portugese-Man-of-War, Physalia, is one of the more familiar floating, colonial
hydrozoans. They are highly advanced and show the greatest polymorphism of any
Cnidarian. It should be noted that even within its Order, Physalia is unusual in its
morphology.
The colony consists of both polyp and medusa morphs. This species has a very large
pneumatophore that floats on the surface of the water. Its cormidia (groups of
individuals, each with repeated sets of zooids) are arranged horizontally on the underside
of the float. The complete anatomy of Physalia is difficult to sort out in preserved
specimens. At one end of the float, there is a pore which permits the enclosed gas to be
released from the float. Within the float, towards the end opposite to the pore, is the gas
gland, which secretes gas into the float. Each cormidium has a single prominent
dactylozoid with a separate ‘fishing’ tentacle. From its base grow a number of
gonozooids which lack tentacles. These can be extremely numerous (>500 in some).
Your slide box contains sections of tentacles of Physalia, on which you can observe the
extremely large nematocyst capsules.
Class Anthozoa
These are the sea anenomes and corals. All 6,000 or so species are marine. The medusa
form is completely lacking in this group. Gametes are produced by the polyp, forming a
planula, which then metamorphoses into another polyp. Refer to figs. 5.17 and 5.19 to
understand the basic anthozoan body plan (take note of the siphonoglyphs, mesenteries
and muscles). This highly diverse and ecologically significant class is divided into two
subclasses. It is convenient to begin lab work on this group by concentrating on the
anenomes, and progress from that base to understand the others.
Subclass Hexacorallia
This group includes the sea anemones and the ‘true’ corals. The two have very similar
body plans, only differing in that one is solitary (the anemones) and the other is colonial
(the corals). The name refers to the tendency for tentacles to be arranged around the
mouth in multiples of six.
The internal anatomy of the sea anemones is most conveniently learned from preserved
material and slides, while the external anatomy is most easily visible in living specimens.
Observe the demonstration dissection of Metridium. Compare with the appended figure
for basic features of its functional morphology. Be sure to be able to recognize the
cylindrical column, oral disk, hollow tentacles, siphonoglyphs, pedal disk,
mesenteries, retractor muscles, gametes, and acontia.
The true corals (ca. 2,500 spp.) represent the largest group in this subclass. Polyp
anatomy is very similar to the anenomes, but they lack syphonoglyphs and the animal
produces a calcareous exoskeleton. Corals may be solitary or colonial and either
hermatypic (reef-building) or ahermatypic (non-reef building). All of the former have
zooxanthellae in their tissues, most of the latter do not. Examine the dried examples of
some common tropical corals, observing the individual cups. Each of these houses, in
effect, a small sea anemone.
Subclass Octacorallia
This group, known collectively as the ‘soft’ corals, includes the sea pens, sea whips and
sea fans. There are always eight tentacles arranged around the mouth. All species are
colonial with some polyps functioning in different roles than others (some for feeding,
others for circulation). In species such as the sea fan (see demo), the skeletal matrix is
secreted by cells in the mesoglea rather than by symbiotic zooxanthellae. Sea fans are
usually attached to hard substrate by a basal plate. Observe the skeleton remains of
Gorgonia. The individual cups, each housing an 8-tentacled polyp, are easily seen.
Sea pen (e.g. Pennatula) colonies are much more complex (see below). All species live
on soft substrates with the ‘club’ anchored. The colony consists of a flattened, kidneyshaped structure (rachis) and a thin stem, the peduncle. The rachis lies at the surface of
the substrate; the peduncle anchors the colony to the substrate. The rachis buds off
secondary polyps on its upper surface. One type has the eight tentacles around the mouth
and is used for typical carnivorous feeding. The second, found near the base, have a
‘mouth’ but no tentacles. These function to pump water into the main polyp, via cilia.
5. The cnidarian cnidocyte
The single defining feature of the Cnidaria is the cell known as the cnidocyte. It contains
some of the most sophisticated cellular gadgetry and weaponry found in the animal
kingdom. Nematocysts function in killing the prey and tentacles then push the prey into
the mouth and digestion occurs in the gastrovascular cavity. Using the live Hydra and
Anthopleura, locate the cnidocytes and nematocysts by making a wet mount of a tentacle.
You should be able to identify at least two types of nematocyst in Hydra and up to three
in Anthopleura.
Although it will appear to you as though predation is by sheer brute force (probably
involving random contact with prey), it is actually much more specific. The actual
mechanism for the release of some nematocysts is partly chemically-mediated. When the
cuticle of their prey is pierced and the animal begins dying, it releases specific chemicals.
We can mimic this by adding synthetically-produced glutathione to the water.
Sometimes, one can observe Hydra literally eating itself in response to this chemical.
Add a few drops to the water containing Hydra and watch the show.
Supplement your observations from the wet mounts with material from your slide box,
the slow-motion video footage, the web site (SEM shots) and the appropriate diagrams in
your text. Make appropriate drawings, with the aim of trying to explain to your
classmates the functional significance of the different types. The figures below show a
sample of the different types of nematocysts which have been described from Hydra and
Anthopleura.
6. The biology of the Dinoflagellate/Sea anenome symbiosis
There are a great variety of protozoans that form spectacular symbiotic relationships with
marine animals. One of the most common of these is the dinoflagellate called
Symbiodinium microadriaticum. One of the most important symbiosis in marine biology
is the association of this species with stony corals. These corals are a major component
of coral reefs in warm, shallow seas. The very existence of the reef habitat is dependent
on the metaboloc activities of the coral symbionts, including photosynthesis, mineral
recycling, and the synthesis of the coral’s calcareous skeleton.
Dinoflagellates have been reported in all three classes of cnidarian and are especially
common in tropical marine habitats. Although these types of symbiotic association are
relatively rare in temperate areas, one of the best examples (and one of the easiest to
study) involves S. microadriaticum in the sea anenome, Anthopleura.
Recall that the process of photosynthesis ‘fixes’ carbon (i.e. it utilizes CO2 and to
produce sugar). To demonstrate a truly symbiotic interaction, researchers would need to
demonstrate that photosynthetically-fixed carbon was utilized by the host. The first
experiments to conclusively show this were with Anthopleura. By using 14C-tracer
methods, researchers showed that up to 35% of the carbon fixed by photosynthesis was
later recovered in the tissues of the anenome. What a brilliant strategy to obtain an
additional source of carbon for use in growth and reproduction ! Even more intriguing is
the discovery that the tissue of the anenomes actually releases a factor that stimulates the
release of the photosynthate from the dinoflagellate. Lastly, the dinoflagellates have also
been shown to take up host nitrogen (from wastes) and recycle it into a form that is again
useable by the host.
Remove a tentacle from one of the live sea anenomes and examine it under the dissecting
and compound scopes. Locate the dense brownish cells in the inner tissues. Try to isolate
a couple, and look at them under oil immersion. Make drawings where appropriate and
refer to the diagram below. Think about some of the issues described above when you
observe this fascinating symbiotic interaction. During the dissection, you will also see
many discharged nematocysts.
Observe the SEM images of this interaction on the side bench taken by a student in our
microscopy course.
7. Feeding and defense in Anthopleura and Hydra
Drop bits of minnow on the oral disc of Anthopleura and notice its response. How is
food moved to the mouth? How are the tentacles used? Try dropping bits of wood or
sand grains onto the oral disc. How are these materials moved across the disc? In the
dissected specimen, lay a piece of minnow beside the cut section. Notice the action of
the filaments associated with the mesenteries. How do you explain this action ?
In another dish, I will have set-up two individuals of Anthopluera such that when they are
fully expanded, they will come into direct contact. Notice the action of the acrorhagi
(small white bumps hidden between the column and the oral disc). When non-clone (i.e.
genetically different) individuals touch, you should be able to see these structures expand
and inflict damage on their neighbour.
8. Questions for discussion
1. Consider the body shape and composition of typical ‘jellyfish’. What would be the
adaptive explanation to describe why the nematocysts of some medusoid jellyfish contain
some of the most powerful toxins known ?
2. Virtually all green sea anemones in a given population contain zooxanthellae. Given
that the protozoans are non-flagellated and thus non-motile, how do you imagine that
they get into new, or young hosts?
3. How might you explain the fact that even coral reefs which contain few plants (such as
kelp and other algae) are surrounded by oxygen-rich water and are thus able to support
ecosystems which are notoriously high in biodiversity ?
4. Cnidocytes are the unique property of the Cnidaria. Yet other features of this group
are primitive to say the least. How do you envision that this extremely complex cell
evolved? Think about the relationship between Anthopleura and its dinoflagellates as
one way to answer this unanswerable question.
5. Explain the familiar clown fish/sea anenome symbiosis (see poster). How can it be
that clown fish don’t get stung by nematocysts while other fish do?
6. Suppose an oil spill resulted in reduced light penetration on a coral reef. Speculate on
the consequences of this to the survival of a species of coral which was restricted in its
depth requirements to 1m below the surface, where there were very few zooplankton.