Biology 11 - Starfish Dissection Guide
References:
Animals w/o Backbones (p. 299-310)
Nelson Biology (p. 340)
Classification:
Kingdom Animalia
Phylum Echinodermata
Class Asteroidea - starfish
Purpose:
To observe the external and internal structures of a starfish.
Procedure:
Observe the demonstration of the dissection technique and follow the lab
procedure.
A.
General Information:
Sea stars and other members of the phylum Echinodermata (echino = spiny, derma = skin)
are really the archetypical marine animals. They are only found in the sea, and very often in
the littoral or intertidal zone. Biologically, echinoderms seem to be terribly improbable
animals — yet they have been successful in the Earth’s oceans since near the beginning of
the fossil record. Adults are often of moderate size, up to a few decimeters in diameter.
None is microscopic and none is parasitic. Most of them are benthic and while there are only
about 6000 species, they are often abundant and ecologically dominant.
Phylum Echinodermata: Diversity
Class Echinoidea - sea urchins, sand dollars
Class Ophiuroidea - brittle stars
Class Crinoidea - sea lilies
Class Asteroidea – starfish
Class Holothuridea - sea cucumber
B.
External structures and body plan:
The name of the phylum Echinodermata is derived from the Greek and means “spiny skin”.
There are six classes, but probably the most familiar to most of us is the common starfish,
Asteroidea. This genus is found along both coasts of North America where it crawls along
the shallow sea bottom, feeding on mollusks.
Many echinoderms exhibit radial symmetry, including the starfish. The body consists of a
central disc, from which five arms extend (an unusual number of arms is possible, but most
individuals have five). On the oral surface of the arms, an ambulacral groove is found,
containing small, fingerlike tube feet, which are the organs of locomotion. At the base of
the rows of tube feet may be seen a white nerve cord; at the tip of each arm is a light
sensitive eye spot, that is extended upwards during locomotion.
This image shows the aboral surface of a preserved starfish. Note the spiny skin, central
disc and madreporite. The madreporite is part of the water vascular system.
The image below shows the oral surface of a preserved starfish. Note the central mouth
and ambulacral groove that runs down the center of the oral surface of each arm. Extending
from each ambulacral groove are numerous tube feet that are used for locomotion,
attaching to the substrate and feeding. The spines on the aboral surface are extensions of
the skeletal plates. Surrounding each spine are skin gills, which function in respiration, and
tiny structures called pedicellariae. The latter are composed of two jaws (moved by
muscles) that open and shut, primarily to keep the body surface clean of debris.
C.
Internal Structures:
Place your specimen in the dissecting pan so that the aboral surface is facing up. Read this
whole section before proceeding with the dissection.
Use scissors to make a hole just through the body wall at point A. Insert scissors into this
hole and cut around the central disc (make cut as large as possible). Don’t cut too deeply.
Use forceps to gently pull up and remove the top of the central disc that you have cut.
Make sure you look for the very thin pyloric stomach which appears brownish and skin like
and is attached to the underside of the central disc. Gently cut it loose.
Choose a ray that is not bent or twisted. Start at points B and C. Cut along side to points D
and E and than continue cutting down the arm and the remove skin.
Identify all of the following structures.
D.
Skeleton:
The starfish endoskeleton consists of a set of small calcium carbonate plates that lie buried
below the skin of the starfish. The skeleton functions to both support and protect, which is
obvious when examining specimens. With the exception of those comprising the spines of
pencil urchins, all skeletal plates in echinoderms are covered with tissue. Unlike
vertebrates and most other higher invertebrates, locomotion is not achieved by muscles
attached to the skeleton elements. This skeleton is in some ways similar to that found in
chordates, such as fishes, as is the general physiology of the animals, indicating a relatively
close evolutionary relationship between these phyla. However, as a final bizarre note, no
echinoderm has an old age or senescence. In effect, unless they starve to death, get
diseased, get eaten or suffer from some environmental disaster, they have the potential of
immortality.
E.
Locomotion:
Starfish move around using a unique water vascular system. The internal canals of this
system include a circular ring canal and its extensions into each arm, called radial
canals. The stone canal links the ring canal to the outside through a hole on the aboral
surface, called the madreporite. From both sides of the radial canals, short lateral canals
arise; each contains a valve and terminates in a bulb called an ampulla and a tube foot - the
foot ends in a small sucker. The water vascular system is filled with a fluid composed of
seawater with added protein and potassium, and amoeboid cells. Its hydraulic actions
provide the mechanism for locomotion. When the ampulla contracts, the valve closes and
fluid is forced into the tube foot to elongate it. When the stretching foot makes contact
with the surface below the animal, the center of the sucker surface retracts to produce a
vacuum and cause the foot to adhere to the surface. After the foot sticks to the surface,
muscular fibers shorten the foot again and force fluid back into the ampulla. Each tube
foot is very small and moves the starfish only a small distance; however, the net movement
from the many tube feet is able to provide a forward motion for the animal.
F.
Digestion
The central disc contains the complete digestive tract, extending from a mouth on the oral
surface (ventral) to the anus on the aboral surface (dorsal). In between, the digestive tract
includes a short esophagus, a multilobed cardiac stomach, and an upper pyloric stomach.
There is no “intestine”, but small rectal caecae are located near the anus to provide
temporary storage of waste products. Food is digested in the stomach with the aid of
enzymes produced in digestive glands, tissues occupying much of the space in each of the
arms and draining their secretions into the stomach through pyloric ducts.
G.
Reproduction
Sea stars are capable of both sexual and asexual reproduction. Individual sea stars are male
or female. During the breeding season, the arms contain gonads that produce eggs or sperm,
which are discharged from the body through gonopores in each arm. Fertilization is then
external, and the larvae grow independently of the parents. The resulting fertilized
embryos form part of the zooplankton.
To increase their chances of fertilization, sea stars probably gather in groups when they
are ready to spawn, use environmental signals to coordinate timing (day length to indicate
the correct time of the year, dawn or dusk to indicate the correct time of day), and may
use chemical signals to indicate their readiness to each other.
Male and female sea stars look the same externally and internally (male and female gonads
look alike). To determine the sex of the starfish, the gonads must be examined
microscopically. To do this, remove a small piece of the reproductive organ and place it in a
drop of water on a slide. Add a cover slip and examine under a microscope at high power.
The testes of the male have flagellated sperm. The ovaries of the female produce spherical
egg cells that are considerably larger than the sperm. Use a microscope to determine if
you have a male or female starfish?
H.
Circulatory system
The echinoderms may be considered to be constructed of cavities within cavities
surrounded by cavities, all of which are lined with layers of fragile tissues. The cavity fluid
is not seawater and is physiologically maintained by intracellular “molecular” pumps that
regulate the animal’s ionic balance. This pumping system is delicate and depends to a great
extent on the relative ionic balances between the animal’s internal environment and the
surrounding ocean water. Under conditions of lowered salinity, most echinoderms lose the
capability to regulate their necessary internal conditions and die. For the aquarist this
means that full-strength salinity (between 35 and 37 ppt) is necessary for successful
maintenance of most echinoderms.
I.
Nervous system
The presence of radial symmetry implies no front end, and indeed, no head is found
anywhere in the phylum. Probably as a consequence of the lack of a head, they are truly
brainless, a characteristic shared with some other lower life forms. The remainder of the
nervous system is poorly known as the nerves are exceptionally small and diffuse.
Starfish Dissection Write up
1. On your chart, complete the example, phylum and general characteristics sections.
2. On your chart under external features, make an accurate drawing showing the oral
and aboral surfaces.
3. On your chart under external features, describe the function of tube feet,
madreporite and pedicellariae
4. On your chart under skeleton, describe the skeleton and the method of movement
of the starfish.
5. On your chart under reproduction, describe external fertilization and why it is an
adaptation for these organisms.
6. On your chart under digestion, describe how a starfish is adapted to eat a clam.
7. On your chart under circulatory system, describe how starfish control their internal
environments and how starfish may die.
8. On your chart under respiration, describe the location of the skin gills. (See
external features in dissection guide).
9. On your chart under nervous system, describe the nervous system of the starfish.