Download Biology 122L – Invertebrate zoology lab Cnidarian diversity lab

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Biology 122L – Invertebrate zoology lab
Cnidarian diversity lab guide
Author: Allison J. Gong
Figure source: Brusca and Brusca, 2003. Invertebrates, second edition. Sinauer
Associates, Inc.
INTRODUCTION
Cnidarians are some of the most conspicuous and colorful tidepool animals, yet they
are often overlooked and unappreciated. To the untrained eye their radial symmetry
makes cnidarians seem more plantlike than animallike, and the colonial forms of
hydroids often have a "bushy" appearance that reinforces that mistaken first impression.
However, cnidarians are indeed animals, and a closer examination of their bodies and
behaviors will prove it to you.
The generalized body plan of a cnidarian consists of an oral disc surrounded by a
ring (or rings) of long tentacles, atop a column that contains the two-way gut, or
coelenteron. This body plan can occur in either of two forms: a polyp, in which the
column is attached to a hard surface with the oral disc and tentacles facing into the water;
and a medusa, in which the column is (usually) unattached and the entire organism is
surrounded by water. In the medusa phase the column is flattened and generally rounded
to form the bell of the pelagic medusa.
Cnidarian tentacles are armed with cnidocytes, cells containing the stinging
capsules, or cnidae, that give the phylum its name. Cnidae are produced only by
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cnidarians, although they can occasionally be found in the cerata of nudibranch molluscs
that feed on cnidarians: through a process that is not understood, some nudibranchs are
able to ingest cnidae from their prey and sequester them, unfired, in their cerata for
defense against their own predators. Cnidarian polyps and medusae are passive
predators that hang cnidocyte-laden tentacles in the water column and catch whatever
prey blunder into them. Most cnidarians feed on small prey – copepods, worms, larvae,
etc. – but some can immobilize large animals such as fishes. In fact, one of the deadliest
toxins in the animal kingdom is produced by the cubomedusa Chironex fleckeri; every
year on the Great Barrier Reef a few people are killed by the sting of these sea wasps.
There are currently three recognized classes of cnidarians. The Class Hydrozoa
contains the freshwater Hydra as well as marine hydroids and hydromedusae; the Class
Scyphozoa contains the large marine medusae, or jellies. The Class Anthozoa is the most
diverse class and contains the corals, sea anemones, soft corals, and gorgonians.
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CLASS HYDROZOA
Individual hydrozoan polyps are usually very small, on the order of a few
millimeters, but colonies can be quite large, consisting of hundreds of genetically
identical polyps. Similarly, a single polyp is morphologically very simple, comprising a
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mouth on a stalk (manubrium) surrounded by one or more rings of filiform or capitate
tentacles. If a colony is reproductive the polyps may carry medusa buds or gonophores,
which are degenerated medusae that are retained on the polyp. The distal end of a
hydroid polyp, which bears the mouth and feeding tentacles, is called a hydranth.
Hydranths are usually raised above the level of the basal stolons by a length of
hydrocaulus.
All hydroid colonies contain some amount of perisarc or periderm, a chitinous
exoskeleton that surrounds the living tissue, or coenosarc, of the stolons and stalks. The
perisarc may be transparent and difficult to distinguish from the coenosarc, or it may be a
golden-brown color and have a noticeably tougher texture than the coenosarc. In thecate
hydroids, the perisarc extends beyond the end of the stalk to form a cup, or theca, into
which the hydranth can withdraw; some hydrothecae are equipped with an operculum
that can be pulled down to shut the hydranth inside the theca. Obelia and Orthopyxis are
local examples of thecate hydroids. Thecae are usually completely transparent, and may
not be visible unless you get the light shining at exactly the right oblique angle. They
may be wineglass-shaped or ovoid. In athecate hydroids there is no theca and the
hydranth is continually exposed. Tubularia is an athecate hydroid with large pink
hydranths bearing two rings of filiform tentacles and long (up to 10 cm) stalks. Sarsia is
another athecate hydroid and is much smaller; its hydranths are stubby and bear capitate
tentacles.
Colonies, on the other hand, can be quite complex, growing horizontally over
surface nooks and crannies or vertically to produce bushy upright forms. All of the
polyps in a colony are connected by a shared gastrovascular cavity (GVC). Food
ingested by one or a few polyps is circulated and shared by all polyps in the colony. The
sharing of food resources has allowed some polyps to specialize in functions other than
feeding. This remarkable phenomenon is known as polymorphism. In polymorphic
colonies, non-feeding polyps are morphologically and functionally differentiated to carry
out duties such as defense (dactylozooids) and reproduction (gonozooids). You may be
able to observe polymorphism in some of the colonies in today's lab. If so, how many
types of polyps can you identify? You should note, however, that not all hydroid
colonies are polymorphic.
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Hydroid colonies are formed by a clonal process called budding. A primary polyp
buds to produce other polyps, which may arise from a stolon, a hydrocaulus, or the
junction between a stolon and a hydrocaulus. Colony growth forms can be broadly
described as either stoloniferous or upright. Stoloniferous colonies branch in only one
plane, the basal plane along which the stolons grow; these colonies tend to grow as
encrusting mats over surfaces. Upright colonies branch in the basal plane, but also
branch up and away from the basal surface; these species form the bushy colonies that
superficially resemble plants in their morphology and growth. A given species has either
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a stoloniferous or an upright morphology. Although some details of colony growth can
be modified experimentally by pruning, the overall growth pattern is consistent within a
species.
Polyps represent the asexual, or clonal, phase of the general cnidarian life cycle.
The sexual phase is represented by a pelagic medusa. In some species, the sexual phase
of the life cycle is retained by the polyp stage, either as the sessile gonophores of
Tubularia or the polymorphic gonozooids of Hydractinia. In species that have a
planktonic medusa stage, medusae are produced by polyps in structures called medusa
buds. In the Monterey Bay area, medusae of the thecate hydroid Obelia are found yearround in coastal plankton samples, and the hydroid itself can usually be found on pilings
and docks in local harbors.
CLASS SCYPHOZOA
Scyphozoans are the large pelagic medusae that often occur in swarms (called
"smacks") in coastal waters. The medusa may be the conspicuous stage for these
animals, but it plays a relatively minor role in the life cycle. Medusae are ephemeral,
appearing in the water column on a seasonal basis, but the benthic polyp phase of the
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scyphozoan life cycle is a persistent stage that produces medusae intermittently for many
years. Scyphozoan polyps, or scyphistomae, can sometimes be found on the undersides
of floating docks, where clones can cover large areas despite the polyps' small body size.
In lecture we discussed the scyphozoan life cycle and the relationship between polyp
and medusa. Keep this relationship in mind as you observe the different life history
stages of the moon jelly, Aurelia sp. The scyphistoma of Aurelia is a small funnelshaped polyp, usually 3-10 mm long, attached at its basal end by a few stolons. Unlike
hydroid polyps, scyphistomae do not have a protective coating of periderm and can
"walk" around a bit by selectively detaching and re-attaching individual stolons.
Scyphistomae have filiform tentacles that are well armed with batteries of cnidocytes,
which might be visible as tiny bumps on the fine tentacles. If you can look down onto
the oral side of a relaxed scyphistoma, you might be able to see into the GVC. The GVC
is a two-way gut whose single opening functions both as a mouth and an anus. The
interior cavity of the GVC is partially divided by septa into four compartments; these are
homologous to the gastric pouches seen in the medusae of Aurelia.
Scyphistomae are capable of two clonal activities: budding and strobilation.
Budding creates additional polyps and strobilation produces medusae. Scyphistomae bud
on a more or less regular basis which can vary with food availability; strobilation appears
to be a more seasonal phenomenon, although some clones of polyps strobilate at any time
of the year. With any luck I will be lucky enough to have strobilating polyps (strobilae)
and ephyrae for you to observe. A strobila is a polyp that has temporarily stopped
feeding and is undergoing the process of producing medusae. It divides transversely to
form a stack of saucer-shaped ephyrae, which break off individually from the distal end
of the polyp and swim off into the plankton. After all of the ephyrae are released, the
polyp regrows its tentacles and begins feeding again.
The ephyrae of Aurelia are 8-armed creatures with a mouth and four gastric pouches.
Contraction of the bell propels the animal through the water and simultaneously traps
prey under the subumbrellar surface where they can be caught and eaten. This method of
locomotion and prey capture continues as the ephyra grows into a sexually mature
medusa. The cnidocytes of Aurelia are not particularly powerful, and this animal can
subdue and eat only small prey such as brine shrimp. The bell of the medusa is ringed
with a fringe of fine tentacles that are heavily laden with cnidocytes. Prey are killed and
caught by the tentacles, then the oral lobes sweep across the tentacles to accumulate food
and convey it via ciliary tracts to the mouth, which is located in the center of the
subumbrella on a short manubrium.
Be sure to observe and draw all the available life history stages of Aurelia. The
medusa is on display in the Seymour Center exhibit hall.
Stauromedusae (Order Stauromedusae) are undoubtedly the strangest of the
scyphozoans. Whether these animals represent polyps or medusae has been debated over
the decades; according to your text (Ruppert et al, 2004), they can be considered a hybrid
of the two body forms, namely, a polypoid base with a medusoid top. They are stalked
and live attached to algae or seagrasses. They can move around a bit because the basal
disc is not permanently attached. The systematics of this group are extremely crude;
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there are 2 (or maybe 3) genera that can be found around here, but identifying them to
species is a very difficult proposition.
CLASS ANTHOZOA
The anthozoans are the most numerous and largest cnidarian polyps. There is no
separate medusa stage, so sexual function resides in the polyps. Anthozoans can be
solitary (many sea anemones), clonal (other sea anemones), or colonial (reef-building
corals, gorgonians, etc.).
We have many examples of anthozoans along the central California coast. It is
difficult not to notice the large green anemones (Anthopleura xanthogrammica) in the
tidepools at Natural Bridges, or groups of the small clonal A. elegantissima at almost any
rocky shore. SCUBA divers will have seen the tall anemones of the genus Metridium on
pier pilings, and clusters of the corallimorpharian Corynactis californica on subtidal
reefs. We even have two local species of scleractinian corals, which secrete a CaCO3
skeleton; in tropical waters these are the zooxanthellate reef-building corals.
Many anthozoans harbor within their cells unicellular photosynthetic algae called
zooxanthellae. These algae are the encysted form of a dinoflagellate named
Symbiodinium. This relationship between cnidarian and alga is an example of symbiosis,
in which both partners benefit from the association between them. The animal benefits
from the presence of zooxanthellae by borrowing or stealing photosynthate from the
algae. If corals in the oligotrophic tropics did not have zooxanthellae within their cells,
they would not be able to catch enough food to support the metabolic demand of
secreting CaCO3 and building reefs. The algae benefit by being maintained up in the
photic zone and safe from herbivores. The animal also provides nutrients, in the form of
CO2 and nitrogenous wastes, to the algae.
We have a few species of zooxanthellate anemones (Order Actiniaria) along our
coast. The most conspicuous is A. elegantissima. The usual color of these animals is a
golden-brown, due to the presence of zooxanthellae (dinoflagellates typically have a
similar color from their own photosynthetic and other pigments). Anemones that are in
shaded locations often have a bleached appearance and are much paler because they no
longer have zooxanthellae. Lacking zooxanthellae is not fatal for A. elegantissima, but
likely has negative effects on growth rate and reproductive success. I hope to have both
normal and bleached anemones for you to observe.
Our local scleractinians (Order Scleractinia, the stony corals) are the orange cup
coral, Balanophyllia elegans, and the brown cup coral, Paracyathus stearnsi. Unlike
their scleractinian brethren, these are solitary corals that do not reproduce asexually.
Balanophyllia can be found in the low intertidal along exposed coasts, while Paracyathus
is primarily subtidal. When you observe Balanophyllia in the lab, notice the hard
skeleton the animal secretes. The living tissue sits on top of the secreted base and can
scrunch down into the depression in the center, but cannot actually withdraw into the
skeleton.
Lastly, we have the Order Corallimorpharia, exemplified by Corynactis
californica. Corynactis has white capitate tentacles and very large cnidocytes. It is
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called the "strawberry anemone," but occurs in various shades of pink, orange, and
brown. It reproduces asexually via longitudinal fission and often form easily
recognizable clones of same-colored polyps in large clumps subtidally.