Invertebrate Animal Diversity – Sponges and Jellies
We will begin our study of the diversity of marine animals by looking at two of the
simplest forms of animal life: the sponges and the jellies. Although these animals do not
appear to be very complex in a physical sense, they have evolved sometimes complex life
histories and are significant members of the ecosystems they inhabit.
Part 1 – Sponges (Ph. Porifera) [por-if-er-a]
{see pp. 116 – 118 in Castro & Huber text}
Sponges are the only animals to exhibit a cellular level of organization. This means that
sponges do not have true tissues in the sense of non-changing groups of cells that perform
a specific function for the organism. Instead the cells are more flexible in their roles and
can switch from one type to another. Nonetheless the cells in sponges do become
specialized for particular functions. Below is a list of the primary cell types and a
description of their form and functions.
Porocytes [por-o-sites] – hollow, tube-like cells that form tiny openings or pores
called ostia. It is the presence of these cells that gives the phylum its name
(Porifera = “pore bearers”)
Choanocytes [ko-an-o-sites] – cells with a ring of finger-like folds of cell
membrane that form a collar. A flagellum extends from the center of the collar
and is used to move water through sponge’s body while the collar collects food
particles.
Pinacocytes [pin-a-ko-sites] – flat, shingle-like cells that form the epidermis, or
“skin”, of the sponge
amoebocytes [a-mee-bo-sites]– have no permanent shape. They wander
around body producing the skeletal framework for the sponge and assist in the
distribution of nutrients
The body of a sponge is built by amoebocyte cells that produce a gelatinous matrix
surrounding a framework of hard crystalline pieces called spicules or softer spongin
fibers. The spicules are either calcium or silicon based and vary in their shape. Spongin
is a strong protein fiber similar to collagen found in other animals. The body forms range
from simple sacs to large, multi-chambered constructions.
Because all sponges are sessile their only means of acquiring food is to have it come to
them. Sponges are filter feeders. The beating action of the choanocyte flagella creates a
current that draws water in through the ostia into either a large central space called the
spongocoel or through canals into a series of small feeding chambers. Food particles are
then trapped by the collars of the choanocytes that line these internal spaces. The filtered
water then passes out through one or more large openings called oscula.
Gas exchange and waste removal are handled by the individual cells due to their
closeness to seawater.
Reproduction in sponges can involve both sexual and asexual methods. Sponges are
hermaphrodites meaning they can produce both eggs and sperm. The sperm are
modified choanocytes whereas the eggs are modified choanocytes or amoebocytes.
Fertilization can be internal or external depending on the species. The zygote develops
into a free-swimming embryo that eventually settles and grows into a new sponge.
Asexual reproduction can occur from mechanical damage that releases pieces of sponge
which in turn settle and develop into a new individual, or a sponge can produce a cluster
of unspecialized cells, called a gemmule, that is released similar to an embryo.
Obtain a microscope slide of the sponge Granita. First examine the circular
cross section at 40X, then 100X magnification. Make a sketch of the sponge at
100X and label the central spongocoel. Next examine a section of the body wall
at 400X. Find an area where you can see pinacocytes, porocytes and
choanocytes. Sketch what you see and label these cells. If you can distinguish
any spicules label those as well.
There are three classes within the phylum. Class Calcarea includes small sponges with
spicules made from calcium carbonate and with few, if any, spongin fibers. The sponge
Granita you viewed earlier is member of this class. Class Hexactinalida, or “glass
sponges”, use six-rayed spicules made of silica that are fused into an intricate lattice work
skeleton. Check out the glass sponge sample at the front of the room. Class
Desmospongiae is the most diverse group and contains the largest, most common
species. Their bodies are supported by large amounts of spongin as well as calcareous
spicules. Large desmosponges were once harvested commercially for bath sponges until
synthetic ones were developed. Examine the dried sponges at the front of the room.
These are just the dried spongin skeletons of once living sponges.
Part 2 – Stinging Jellies and kin (Ph. Cnidaria) [ni-dare-ee-a]
{see pp. 118 – 122 in Castro & Huber}
The animals most commonly called jellyfish, as well as their cousins the hydrozoans, sea
anemones and corals, are examples of radially symmetrical animals. Their general body
plan is that of a circular body with tentacles radiating out from a central axis where the
mouth is located. Having a round body means that the conventional terms of left/right
and front/back do not apply to these animals. The presence of a mouth does allow for
orientation with respect to that feature so the animal’s body can be described as having an
oral side (with the mouth) and an aboral side (without a mouth).
In addition to radially symmetry cnidarians are distinguished by having a fairly simple
body plan. The body consists of two layers of cells. The outer layer is called the
epidermis and contains the neuromuscular tissue that allows for body movement. The
inner layer is called the gastrodermis and is the tissue that lines the simple, sac-like
stomach known as the gastrovascular cavity. In between the two cell layers is an
acellular matrix called mesoglea or mesenchyme. It is this gelatinous material that give
jellies their characteristic appearance. The body is supported by the mesoglea as well as
by water pressure in digestive cavity. Cnidarians do not have a centralized nervous
system. Instead they use a network of interconnected nerve cells that are spread all over
the body. All behaviors are in direct response to various stimuli.
Although there is some variation, there are two basic body forms found in this phylum.
Polyps are usually recognized as being attached on the aboral side with the oral side and
tentacles oriented away from the substrate. Sea anemones and corals exhibit the polyp
body form. Polyps are generally sessile, but some solitary polyps can “creep” along the
substrate. Some species of anemones have even been observed detaching from the
substrate and thrash around in an attempt to “swim” away from advancing predators.
Medusae are free swimming and can orient their oral side and tentacles in any direction.
The typical jellyfish is a medusa. Medusae swim with jet propulsion by drawing water
into the gastrovascular cavity and forcing it back out the mouth.
All cnidarians are essentially carnivores. They are able to capture prey with the help of
stinging cells called cnidoblasts that are unique to the phylum. Inside the cnidoblast cells
are the actual stinging organelles called nematocysts. Nematocysts consist of a coiled
thread inside a capsule covered by a lid called the operculum. Physical contact with a
hair-like trigger causes a change in fluid pressure outside the capsule to force the thread
into the body of the prey. A neurotoxin is usually delivered to paralyze the prey.
Paralyzed food is then brought to mouth by tentacles and digestion occurs in the
gastrovascular cavity which breaks down the food and distributes it to the other parts of
the body. Because the gastrovascular cavity only has one opening anything that cannot
be digested is expelled back out the mouth.
Cnidarians are similar to sponges in one regard. Gas exchange and waste removal is still
handled by the individual cells because the epidermal cells are in direct contact with
seawater and the gastrodermal cells can make exchanges with the fluids of the
gastrovascular cavity.
Examine the microscope slide labeled Hydra. Although Hydra is a freshwater
organism, it will allow you to see the basic construction of a cnidarian polyp. First
sketch the entire organism at either 40X or 100X magnification. Label the oral
and aboral sides, mouth, tentacles, and the gastrovascular cavity. Next,
look at tentacle at a magnification of 400X. Try to identify the cnidoblast cells
with their nematocysts inside. Sketch what you see and label the nematocysts.
Reproduction in cnidarians generally follows a pattern of alternation of generations that
involves the two body forms described above. In general, polyps produce medusae
through asexual budding. The medusae engage in sexual reproduction, producing eggs or
sperm that combine to form a flat, oval planula larva. This larva lives a planktonic
existence for a while then eventually settles and grows into a new polyp.
The phylum Cnidaria is divided into four classes. Class Hydrozoa [hi-dro-zo-a]
consists primarily of organisms that produce colonies of interconnected polyps, giving
the organism a somewhat bushy appearance. Some polyps have the typical structure of
tentacles surrounding a mouth and are used for feeding. Nutrients are distributed through
a shared gastrovascular cavity. Other polyps do not possess tentacles and are specialized
for producing medusa buds. The Hydra you examined earlier is an atypical member of
this class because it lives as a solitary polyp that does not produce medusae.
Examine the microscope slide labeled Obelia hydroids at a magnification of
40X. Sketch what you see and label the feeding polyps and reproductive
polyps.
Now examine the microscope slide labeled Obelia medusa at a magnification of
100X. Note the bell-shaped body with the fringe of tentacles around the outer
margin. A short, tube-like extension called the manubrium should stick out from
the center. The mouth is located here. You should also note the four radial
canals that are branches of the gastrovascular cavity. In between the radial
canals should be four circular masses of tissue that are the gonads. Sketch
what you see and label the structures written in bold face.
Class Scyphozoa [sky-fo-zo-a] is considered the class of the true jellyfish. The medusa
stage is the dominant form and the one most often encountered by humans. Scyphozoan
jellies do produce a polyp stage, but it is reduced in size and frequently inhabits small
crevices and rock overhangs where they are less conspicuous.
Using a dissecting microscope, examine a specimen of Aurelia, or moon jelly.
Turn the specimen so that you can observe the oral side. A short, cone shaped
manubrium is surrounded by four oral arms that help transfer food to the mouth
from the fringe of tentacle around the bell’s outer margin. Aurelia has a fine
network of canals that extend from the gastrovascular cavity for the distribution
of nutrients. You should also note the four horseshoe-shaped gonads located
near the center of the bell. Sketch what you see and label the structures written
in bold face.
Class Anthozoa includes the sea anemones and corals. In this class the polyp is only
form and is responsible for sexual reproduction as well as asexual methods of
propagation. Sea anemones live as solitary polyps, although some species form colonies
of clones that were produced through binary fission. Corals are colonies of
interconnected polyps, but these produce much larger forms than their hydrozoan
cousins. Examine the samples of sea anemones and coral skeletons on the side
counter.
Class Cubozoa includes the jellies known as “sea wasps” because they have the most
potent toxins of all jellies. Although they are essentially medusa shaped, these animals
have a more cube-like appearance that gives them their alternate name of box jellies.
They also have the most sophisticated visual organs of any cnidarian allowing them to
“see”, although without a central brain to process visual information, there’s no telling
what the organisms do with these structures. We do not have any specimens in this
class for you to examine.
Part 3 – Comb jellies (Ph. Ctenophora) [teen-o-for-a]
{see pp. 122 – 123 in Castro & Huber text}
The comb jellies or sea gooseberries are very similar to cnidarians in basic body
construction. They mostly exhibit radial symmetry, consist of two layers of tissue
(epidermis and gastrodermis), and lack a centralized nervous system. However there are
major differences that necessitate their being placed in their own phylum. First and
foremost ctenophores lack nematocysts for capturing prey. Instead they may have
adhesive colloblast cells on a single pair of tentacles that are used to trap small plankton,
or they may just swallow larger prey. This phylum is actually distinguished by the
presence of stiff cilia arranged in rows (usually eight) of comb-like structures that are
used for a gliding style of locomotion. Reproductively, ctenophores are also different in
that they do not use an alternation of generations pattern. Most ctenophores are
hermaphrodites. Their zygotes develop into free-swimming larvae that never settle
down. We also do not have any specimens of ctenophores for you to examine
.
Lab Summary Questions
1. Why are sponges considered to be the simplest members of the animal
kingdom? _____________________________________________________
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2. Describe the basic mechanism for feeding used by sponges. _____________
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3. Describe the general body plan for cnidarians. ________________________
_____________________________________________________________
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4. Describe how alternation of generations works in cnidarian reproduction. ___
_____________________________________________________________
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5. Compare and contrast ctenophores with cnidarians. ___________________
_____________________________________________________________
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6. List the features that are unique to each of the phyla you studied in today’s
lab.
Ph. Porifera ______________________
Ph. Cnidaria ______________________
Ph. Ctenophora ___________________