"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)
"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?"
