Lab Activity 2.4
Sexual Reproduction

  Directions

Study the instructional material below. Be sure to click on each of the photographs for an enlarged view in a separate window. The larger version is necessary to complete the assignment. It opens in a separate window which can be resized by grabbing the bottom right corner and dragging it. It can also be moved by grabbing the top heading bar and dragging it. Be sure to close the extra window by using the X in IBM, or the close box in MAC when you are finished using them.

  Introduction


Marine organisms have evolved a variety of ways to produce offspring. Some of these are sexual (include sex). Sexual reproduction is the production of new individuals with the admixture of genes from two parents. Each parent donates half the genes to the offspring. This mixing of genetic material is called genetic recombination and results in the formation of a variety of offspring each with a new combination of genes. In fact, except for identical twins, no two individuals are exactly alike. This variety among young assures survival in an unstable or changing environment since some of the young can survive even in environments where the parents cannot.

Top  Instruction

Study the information, photographs, and diagrams of the various aspects of sexual reproduction depicted below. Study the examples and formulate answers to the study questions you're asked.

Be sure to write about what you are learning in the lab section of your notebook. You will be expected to answer questions about the lab activity during the lab self test and lab quiz. It helps to have your text and coloring books open beside you for support.

 
Supporting Information
Refer to the Assigned Readings Below:
Marine Biology Textbook Chapter 4, pages 83 to 84
Marine Biology Coloring Book Plates 75, 84 and 88
 

Gametes

Gametes are the sex cells that combine to form a new individual during the process of sexual reproduction. They are produced by the parents and carry the genetic contribution of each parent. These cells are often specialized as large, sessile eggs and small motile sperm. During the conception of an offspring a sperm cell will swim to an egg and fuse with it to form a zygote. The process of fusion is called fertilization.

Starfish gametes sperm (left) and eggs (right):
  1. We obtained a prepared slide of starfish sperm and photographed the sperm under a compound microscope.
  2. We obtained a prepared slide of unfertilized starfish eggs and photographed the eggs under a compound microscope.




Study Questions:

  1. Describe the sperm cells.
  2. Describe the egg cells.
  3. In what ways are the sperm and egg cells different?

Development

Development is the process by which the zygote changes from a single cell into a multicellular embryo and ultimately into a mature adult form. This developmental process can take place externally in seawater or internally inside the female. In echinoderms, such as the starfish, development occurs externally. The embryo grows in size and develops in a sequence of stages starting with the zygote (fertilized egg) through cleavage, blastulation, and gastrulation to become a larva. The larva continues development first as a bipinnaria and then as a brachiolaria. The larva then settles to the bottom and develops into a tiny juvenile starfish.
Starfish Development:

We obtained prepared slides of starfish developmental stages and photographed them under a compound microscope. Observe the photographs of each succesive developmental stage. Compare it to the previous stage and to the next stage.
Unfertilized Egg:
Study Question:
  1. How does the unfertilized egg differ from the zygote?
Zygote:
Study Question:
  1. How does the zygote differ from the early cleavage stage?
Early Cleavage Stage:
Study Question:
  1. How does the early cleavage stage differ from the late cleavage stage?
Late cleavage stage:
Study Question:
  1. How does the late cleavage stage differ from the blastula?
Blastula:
Study Question:
  1. How does the blastula differ from the early gastrula?
Early Gastrula:
Study Question:
  1. How does the early gastrula differ from the gastrula?
Gastrula:
Study Question:
  1. How does the gastrula differ from the late gastrula?
Late Gastrula:
Study Question:
  1. How does the late gastrula differ from the bipinnaria?
Bipinnaria (early larva):
Study Question:
  1. How does the bipinnaria differ from the brachiolaria?
Brachiolaria (late larva):
Study Question:
  1. How does the brachiolaria differ from the juvenile starfish?
Juvenile Starfish:
Study Question:
  1. How does the juvenile starfish differ from the adult starfish?

Larvae

If the embryo develops externally it will develop into a larva. The larvae of nekton and benthos often live and develop as members of the plankton. The environment in which they must survive is very different than that of the adult. Therefore, the larval forms are often quite different from the adults. The end of larval development is often marked by a radical transition to the adult form and lifestyle. This transition is called metamorphosis.
Starfish Metamorphosis:
  1. We obtained a prepared slide of starfish brachiolaria larvae (late larval stage) and photographed a larva under a compound microscope.
  2. We obtained a prepared slide of starfish brachiolaria larvae (late larval stage) and photographed a larva under a compound microscope.

Study Question:
  1. How has the starfish changed as it metamorphosed from a brachiolaria larva into a juvenile starfish?


Blenny Fish Metamorphosis:
  1. We obtained a prepared slide of a Blenny larva. Blennies are small demersal fish whose eggs and larva are planktonic.
  2. We photographed the larva under a compound microscope.


Study Question:
  1. In what ways does the blenny larva differ from the adult?
  2. How has the blenny changed as it metamorphosed from a larva into an adult?