Hi marine biology students.
In this video we're going to talk about the
need for and several methods of reproduction
of marine organisms.
[Intro Music]
Living organisms are finite.
What I mean by that is a living organism has
a set lifespan.
It will live for so long and then it will
die.
Reproduction is required for the persistence
of a population.
For a population to continue and to stay at
a somewhat constant size, the death rate and
the birth rate has to be equal.
If it's a growing population, the birth rate
exceeds the death rate.
If it's a shrinking population, the death
rate exceeds the birth rate.
As we'll see through this next part of lecture,
different organisms have different reproductive
strategies.
The first category of reproduction we'll talk
about is asexual reproduction.
Asexual reproduction does not involve the
mating of two individuals, instead offspring
are produced 
by a single parent individual.
There's no need to find a fertile mate.
The offspring therefore are genetically identical
or clones of the parent.
Now, if the parent was able to survive to
adulthood in the current environment, we also
know that the offspring of asexual reproduction
will be adapted to their local environment.
With asexual reproduction there will be a
lack of genetic variability and this lack
of genetic variability can leave a population
susceptible to disease or changing conditions.
So, one example of asexual reproduction is
one organism dividing into two organisms.
We call this fission.
Fission can happen at the single cell level
and in fact most cell division is fission,
but there are certain marine organisms like
this sea anemone, that as an adult organism
it can literally pinch itself into two and
separate and become two individuals, though
its individuals are genetically identical
to each other, and so this is a form of asexual
reproduction, you now have two individuals
where you had started with one.
Fission is just a little different than our
next category, which is known as budding.
Budding is a process where a larger organism
forms smaller buds off of their body which
end up separating developing and living on
their own as their own individuals.
So, a big difference between fission and budding
is that in budding you have your parent organism
and small little buds form whereas in fission
that one parent organism simply divides into
two equal smaller organisms.
Asexual reproduction is actually quite common
in plants.
They can form runners that form their own
roots and separate and they can even become
their own plants.
In fact, asexual reproduction is so common
in plants that it is sometimes referred to
as vegetative reproduction.
Another method of asexual reproduction is
fragmentation followed by regeneration.
Now, when you look at these images of these
sea stars you might say “Wow!
That's kind of weird?
Why did that one leg or appendage grow so
large?”
Well, it turns out that in this case, that
one leg was severed from a larger organism
and not only did that appendage end up regrowing
on that organism, but the severed appendage
actually regenerated the rest of a body as
well.
As humans, we don't have this level of regenerative
ability.
If someone's arm is amputated, that arm is
not going to start developing a fetus on the
other end of it to replace that individual
and likewise the individual who lost that
arm would not be able to regrow that arm back,
but fragmentation followed by regeneration
is a form of asexual reproduction.
Now, you may not be aware, but sea stars are
actually voracious predators in their environment.
They eat many different forms of clams and
oysters and different bivalves.
And so, this reminds me of a story.
Over on the East Coast of the United States
there were some oyster farmers.
They were trying to grow oysters out on their
enclosures and these farming structures and
sea stars are a predator.
And so, the fishermen, the oyster farmers,
they would go out there and every time they
would find a sea star, they would cut it in
half and throw both halves back in the water,
thinking that they were eliminating this predation
on their supply, but little did they realize,
they were actually doubling the number of
predators in the environment because those
two halves could end up regenerating and regrowing.
So that practice ended up stopping once they
were in conversation with a marine biologist
about the problem they were having with the
sea stars out in their oyster farms.
So we talked a bit about asexual reproduction,
we're going to talk about how sexual reproduction
is different.
In sexual reproduction, this normally involves
two individuals
or at the very least it requires two different
types of gametes.
There are some cases of hermaphroditic marine
organisms that are capable of self-fertilization,
where the one individual produces both the
sperm and the egg, but normally under ideal
situations, sexual reproduction is between
gametes from two different sources.
So, the parents produce the gametes or the
sex cells and they unite during fertilization,
and this produces a new genetically unique
individual different from either parent and
likely different from the other siblings as
well.
For sexual reproduction to work, there need
to be ovaries which are organs that produce
eggs and testes which are organs that produce
the sperm.
Now, many marine organisms release their eggs
and sperm directly into the seawater, which
is known as broadcast spawning.
Sometimes this is because the adults are sessile,
they cannot move and so they simply release
their gametes into the seawater because they
could not go to find a mate themselves.
It could also be that there aren't a lot of
mates in the immediate area and so broadcast
spawning simply releases the gametes into
the seawater in hopes that sperm and egg from
the same species will find each other and
fertilize.
For broadcast spawning to be effective, millions
of gametes must be released into the water
at roughly the same time to ensure fertilization
will occur.
Many broadcast spawning species will synchronize
the release of their eggs to tides or moon
phase, water temperature, some environmental
cue, that will ensure success or least ensure
that egg and sperm will be released into the
sea water simultaneously.
So in this first image we see a coral releasing
eggs at a synchronous time.
So this coral and many other of the neighboring
corals are all releasing eggs into the water
column, simultaneously these corals are also
releasing sperm into the water column so that
egg and sperm will meet in the water and subsequent
embryonic development will happen there.
In this other slide we see an ascidian known
as Ciona intestinalis.
So, these sea squirts, they spawn each morning
at dawn and if kept in an artificial environment
these Ciona will accumulate gametes, both
sperm and egg, as long as they remain under
constant illumination.
Then, if lights are turned off for at least
4 hours and then turned on again, when those
lights are turned back on the Ciona will release
their gametes.
This is an example of one of those organisms
that is hermaphroditic, so will release both
sperm and egg and if kept in isolation, in
an individual cup or chamber, it is capable
of self-fertilization.
There are other marine organisms that rely
on internal fertilization.
With internal fertilization there is a copulatory
organ or a penis that insert sperm directly
into the female’s reproductive tract.
Now, it turns out that there are some sessile
or non-motile organisms that are able to reproduce
in this way.
One example is a barnacle.
The male barnacle has a penis that is more
than three times the length of the rest of
its body and so with this it can end up reproducing
with neighboring female barnacles.
With internal fertilization, this does require
contact between the parents, but fewer gametes
are required for successful reproduction.
As I had mentioned previously, hermaphrodites
are organisms that have male and female reproductive
tissue or organs either simultaneously or
at different phases during their life.
So, here we see another sea squirt, this is
Ciona savignyi and you can clearly see in
this picture, the bright orange tube is filled
with eggs and the bright white tube is filled
with sperm, so this individual produces both
sperm and eggs simultaneously.
That takes us to the end of this video.
Now, before our next video I want you to consider,
“Do you think we're more closely related
to sea cucumbers or squids?”
We'll talk about that in the next video.
