"As
we learnt last week, the land hermit crab starts its
life in the sea. After hatching from eggs laid in the
ocean by their mother, the young become part of the
plankton of the sea. A month or so later when they have
developed into a crab they crawl ashore in search for
a shell. We will now learn a little more about this
fascinating lifecycle and then learn the names of the
body parts of a land hermit crab, as well as their uses."
Make
sure to have a labelled image of a land hermit crab
out of its shell such as that in the anatomy section
of my website, found here.
Stages
of Development of the Land Hermit Crab
Eggs
and Larvae: "As the eggs develop,
they change in color from dark reddish brown to pale
blue or gray. After about three weeks, the eggs are
ready to hatch. At low tide, the female moves towards
the sea, but does not enter the water. using her fifth
legs, she picks up small
clusters of fully developed eggs from the egg mass attached
to her pleopods.
She takes the eggs further out of her shell with her
maxillipeds
and claws. Then she drops or flings the eggs onto the
wet rocks where they hatch when the ocean water washes
over them"
Zoeae:
"When the eggs hatch, the individuals do not look like
hermit crabs. Hermit crabs, like other crustaceans,
must first go through several larval
stages of development before
they become adults. The newly hatched hermit crab larvae
are called zoeae.
They measure about 1/8 of an inch (3 mm) and are visible
without microscope. The zoeae have large bulging eyes
and long shrimplike bodies. They spend most of their
time swimming in the ocean as part of the plankton.
Many of the zoea are eaten by larger sea animals such
as fish. While they are larvae, hermit crabs are carnivorous
and feed on other tiny animals. As larvae,
hermit crabs drift on ocean currents. Sometimes they
are dispersed to other areas from those in which they
where initially hatched. In other cases, the ocean's
eddies and currents keep them in the same island location.
Glaucothoe:
"The zoeae grow and develop by molting
through a number of stages. The number of stages varies
with the species of crab. For the purple
claw hermit crab, there are
usually between four and five, although sometimes there
are up to six. Each zoeal stage can last almost a week.
At each molt, the zoeae grow larger and sometimes add
more appendages.
The zoeal larval development takes about 26 days, after
which a transformation point is reached and the larvae
molt to a postlarval stage called the glaucothoe or
megalops. At this stage, the animal looks more like
an adult hermit crab and both swims and walks about.
After at least another month, the glaucothoe metamorphoses
to become the juvenile land hermit crab. While still
living in the ocean, the glaucothoe
begin to search for shells in which to live. Newly metamorphosed
juvenile crabs can find their first shells underwater
by detecting clues from dead snails. Tiny crabs that
crawl ashore without shells will usually die. When the
Ecuadorian
crabs first come ashore, they are transparent and measure
about 1/5 of an inch (5mm).
Juvenile
crabs: "The crabs then move to the
land, where they mainly lead a nocturnal
existence. During the day, they seek shelter in cracks
under ledges or logs, or bury themselves in the sand.
Sometimes they are active during the day, such as in
humid
conditions or in rain. After a few more molts,
the little crabs can move further and further away from
the ocean. Typically, purple
claw hermit crabs are ready
to breed by their second year. By this time they have
passed through a molt called the puberty
molt, where the pleopods
and other structures needed for reproduction
are fully developed."
Fox,
S. (2000, p. 155)
*
Questioning/Recall:
"Today
we have learned that hermit crabs go through many stages
before they become fully formed. How does this compare
with other animals we have studied, the (such as frogs,
butterflies etc)."
OR
"Who
can name some of the stages the hermit crab goes through
before they move to land?"
"In
which stage would the mother crab flick the eggs into
the sea?"
"In
which stage did the hermit crab move on land?"
"What
was the importance of finding a shell?" OR "What would
happen if a juvenile hermit crab found a shell before
making its way onto land?"
*
Recording:
Write
an overview of the above lesson on the blackboard/whiteboard
or prepare a simple worksheet for students to glue into
their books/put into folders. If you have time, try
to make up a lifecycle puzzle in which the students
put the lifecycle stages into order and then glue them
into their books.
*
Conclusion :
Overview
of what we have learned today and return workbooks,
glue and scissors
Extentions:
Have
students draw/paint/write about the lifecycle and compare
with the lifecycle of a frog or other animal. If possible,
find photos of plankton magnified for viewing and research
animals that eat plankton and how hermit crab zoea are
part of the food chain for many marine life.
Reference:
Fox,
S. (2000) Complete Pet Owner's
Gude : Hermit Crabs. Barron Books
Notes:
You
will most probably have to adapt the stages text to the
level of your class. Use the glossary to find the meanings
of the words and make sure to have the clearly labelled
diagram of a land hermit crab out of its shell so that
you can point to each body part that is mentioned to give
the students an understanding of the description. Have
a blown up version made into a poster on the wall and
visible at all times so that the words will become more
familiar, and if possible incorporate some of those words
(based on the level of your class) into the spelling words.
[Lesson
1] [Lesson
2] [Lesson
3]
[Lesson
4] [Glossary]
[Hermit
Crab Books]
http://www4.tpg.com.au/users/vanessap/hermit/lesson2.html
