Sea Urchin Embryology
Objectives:
 To observe gamete production in organisms that reproduce sexually.
 To witness fertilization and early development.
 To observe and consider structures and adaptations of an organism with a body plan much different
from ours.
 To develop a greater appreciation for the diversity and similarity of life forms.
MICROSCOPE USE & CLEANUP:
 Always start with and return to the low power objective.
 ONLY use the FINE FOCUS knob with the 40x and 100x objectives. (focus with 10x first!)
 Wipe up any material spilled on your microscope with a Kimwipe.
 Cover microscope with dust cover and carefully return it to the shelf.
 Clean and dry slides and cover slips. If stored wet, they will stick together permanently.
 Dispose specimens in teacher assigned location.
Prelab Questions: Answer in COMPLETE SENTENCES on a piece of binder paper.
1. What phylum are Sea Urchins in?
2. Describe the characteristics unique to this phyla, including:
a. Symmetry
b. Body plan
c. Organ systems present
3. What would happen if the sperm could not get into the egg?
4. How do organisms develop from the size of a sperm and egg to an adult?
5. Why is there such a difference in the size of the egg and the sperm?
6. Water currents in the ocean are much stronger than any sperm. How do sperm and egg find each
other?
7. In humans there are no water currents and the volume of the female reproductive tract is relatively
limited. Then, why does the male have so many sperm?
8. Why do sea urchins have external fertilization? What are the adaptive advantages of external
fertilization? What are the adaptive advantages of internal fertilization?
9. What would happen if one cell was removed or damaged at the 2 cell, 4 cell, 8 cell stage?
10. What would happen if the two cells were separated after the first cleavage and allowed to develop on
their own?
11. We eat chicken eggs (a chicken gamete). In some cultures the gonads from sea urchins are regularly
eaten. Have you ever tried to eat a sea urchin gonad? What does it taste like? How does it make you
feel knowing you are eating gametes?
12. Who expends the most energy on Reproduction, male or females? Why?
13. In what way is the human body organized to facilitate reproduction and survival of our species? In sea
urchins, the gonads can be up to 80% of the weight of the urchin during breeding season. Imagine
what your own life would be like if that was true in humans as well. How would society be different?
14. The process of development is VERY complex with lots of possibilities for things to go
wrong. What would happen if:
a. the sperm could not get into the egg?
b. the male and female pronuclei did not fuse?
c. DNA synthesis did not occur?
d. the chromosome did not align properly at metaphase?
e. the actual division was not competed or happened at the wrong location?
f. gastrulation went out instead of in?
15. Copy the objectives above on to your paper.
Materials:
Potassium Chloride Solution, cups, Petri dishes, pipets, syringe with needles, seawater, live sea
urchins, hand lens, dissecting scopes, microscope, slides, paper towels
Procedure:
1. Load the syringe with 1.5 mL of 0.5 solution of Potassium Chloride. Carefully insert the needle
through the soft tissue surrounding the urchin’s mouth and inject the solution into a central cavity.
Swirl the urchin gently to mix the solution inside. Within 10 minutes, gametes will release from the
genital pores in the aboral side.
2. You will be able to differentiate male and female urchins when you see the gametes. Eggs are
yellow, and sperm are white. Eggs need to be collected in seawater and sperm need to be collected
“dry.” One approach is to invert the urchin over a dry cup and when you hear something begin to
plop in the cup, quickly take a pipet and examine the contents under a microscope. If it is female,
there will be many eggs, and you should immediately add about 50 mL of seawater to the cup. If the
urchin is male, there will be lots of active sperm and you will add no water to the cup. If the solution
is clear, wait; it was just a drop of water and the gametes have note yet been released.
3. Label each cup with its contents (sperm of eggs) and the student’s names. Label each pipet to match
each cup.
4. After the gametes have been shed and the sea urchins sex is known, there will be enough sperm and
eggs so that every student can share and observe gametes and the fertilization process.
5. To witness fertilization, place a drop of eggs on a concave slide and add a drop of diluted sperm to it.
(To dilute the sperm add a drop of sperm to 75-100mL of seawater, but do not do this until just before
use of the sperm. Using full-strength sperm suspension would likely cause polyspermy followed by
abnormal development of the zygote). Within a couple of minutes you should see the fertilization
coat lift from the egg. Continue to Observe.
6. The eggs in the cup will settle to the bottom. The seawater can be decanted or the eggs can be
pipetted into fresh seawater. This process is called washing the eggs.
7. Fill each Petri dish half full of seawater. With a pipet, add 1 squirt of washed eggs. The eggs should
not cover more than 1/8 of the bottom of the dish. Add 1 drop of diluted sperm suspension to the
eggs. Keep the dish covered and at room temperature and observe the contents with a microscope
every 15-30 minutes for evidence of fertilization and development. The first zygotic division should
begin about 80 minutes after adding the sperm suspension. Development will continue through a
series of divisions. Though the rate of development may vary due to environmental conditions,
generally, the blastula can be observed in 18-24 hours, early gastrula ion 30-36 hours, and early
pluteus in 72-80 hours. Every 24 hours, freshen the water in the dish by carefully drawing off at
least half of the water and replacing it with fresh, seawater.
8. Carefully examine the urchin’s external anatomy. It is easy to see the large spines and observe their
movement. With a hand lens or dissecting microscope, you can examine the soft tube feet that extend
in various lengths. Less easily seen are the extensible pedicellariae, which help keep the urchin’s
surface clean. The mouth, surrounded by soft tissue, is on the bottom, the ventral side. You may see
the mouth open and see the tips of “teeth” – part of an external structure known as Aristotle’s lantern.
This structure exhibits the five-sided radial symmetry typical of sea urchins and other echinoderms.
Five fused plates of bone comprise the skeleton of the urchin. This hard dome is called a test. On the
aboral surface, at the center of the test is the anus. In each corner of the five bony plates that converge
at the top is the gonopore, or gamete pore, in which gametes are released.
Internal anatomy can be observed after gametes have been released, With scissors, cut around the soft
tissue in the center of the oral surface and extract Aristotle’s Lantern.
Dismantling this structure gives some idea of how the sea urchin eats. The sea urchin is a very simple
organism with a mouth on one surface and an anus on the opposite one.
9. Dispose your sea urchin as instructed by your teacher. Make sure you CLEAN UP properly. No sea
urchin parts in the sink!!!!