Hello marine biology students.
This week we're going to be talking about
marine vertebrates and we are going to start
by discussing the diversity of fish.
[Intro Music]
So, in the videos for this week, we're going
to talk about the diversity of marine fish,
marine fish physiology and behavior, marine
reptiles, marine birds and non cetacean marine
mammals, and then everyone's favorite the
cetaceans.
Marine fish are vertebrates, meaning they
have a skull and a backbone.
Some have vertebrae made of cartilage and
others of bone.
More than half of the known species of fish
are marine.
They are the oldest and most structurally
simple of all living vertebrates.
They are the largest group of vertebrates,
in terms of species, measuring about half
of all vertebrate species, and in abundance
meaning, there are more fish and more different
types of fish than there are other types of
vertebrates.
The three major classes of fish are the Agnathans
or jawless fish, the chondrichthyans or cartilaginous
fish 
and the osteichthyans or bony fish.
And we'll talk a bit more about each of these
three groups.
Looking at their organization, the fish collectively
fall into these areas and they aren't necessarily
a monophyletic group, but we do see that they
have shared characteristics.
So starting with the agnathans, these are
the lampreys and the hagfish, some of my least
favorite marine organisms.
Being jawless, it's not that they don't have
a mouth, it's that their mouth is not supported
by a moveable bone.
Instead, it is muscular, a circular mouth
with circular rows of teeth.
Both hagfish and lampreys have long cylindrical
bodies
and they lack the paired fins and scales seen
in other fishes.
So, starting with the hagfish.
They are exclusively marine.
They tend to feed on dead and dying fish and
other marine organisms and carcasses in the
water.
They live in burrows in soft sediments and
one of their most distinctive traits is that
they produce large quantities of mucus from
skin glands for protection while they are
feeding on other organisms.
There are over 70 different species of hagfish.
Here we see a diagram of a hagfish.
It has gill slits, it has a mouth, and that
white line are the mucous gland pores.
The next type of agnathan are the lampreys.
They live both in freshwater and seawater,
and they are anadromous, meaning that they
start their life in fresh water, but then
go out to sea, and then return to fresh water
to breed normally, after which they would
die.
Instead of the hagfish, which feed on dead
and dying organisms, the lampreys feed on
the living.
They are parasites and they rasp onto the
sides of fish with their sucker like mouths,
consuming blood, tissue, and body fluids.
Really, they're things out of nightmares.
They are not pleasant and can end up having
a significant impact on the local fish populations
in the areas where they frequent.
Our next group of fish are the chondrichthyes
or cartilaginous fish.
These include sharks, rays, skates, and ratfish.
They have a skeleton made entirely of cartilage.
They have a movable jaw with well-developed
teeth and their mouth is always ventral, meaning
on the underside of their body, below their
snout.
Chondrithyes have a special type of scale
known as a placoid scale, which has a similar
shape and composition as teeth.
They also have paired fins.
In the chondrichthyes, most males have specialized
structures coming off of their anal fins known
as claspers and these are used during copulation
or sexual intercourse.
Chondrichthyans have five to seven gill slits,
which open directly into the water in most
species.
Many Chondrichthyans also have spiracles,
which are openings above their mouths which
allow water to enter into their gill cavities.
So here we see a diagram of a chondrichthyan,
in this case is shark.
We can see the paired fins, the placoid scales,
the ventral mouth and some of the other features.
Another aspect that we'll see in sharks that
we'll talk about is a heterocercal tail.
What that means is the two lobes of the tail
are of different sizes, so the first group
of Chondrichthyans we'll discuss are the Sharks.
They are primarily marine, but some species
do travel up in rivers as I had mentioned,
they have a heterocercal tail, meaning the
top lobe is larger than the bottom lobe.
Most species have two dorsal fins, with one
of them usually being the more prominent and
obvious, and enlarged pectoral fins to provide
lift.
Many sharks are carnivores, with powerful
jaws and triangular teeth.
Sharks have a lateral line, which is a sensory
system that we'll see in in many types of
fish, yet they also have these specialized
organs in their snout known as the Ampullae
of Lorenzini.
This structure allows sharks to detect electrical
currents in the water that often may be emitted
by small prey.
So here we see a picture of a sand tiger shark
showing off rows of triangular teeth and openings
of the ampullae of Lorenzini that would detect
those electrical signals, but there is variety
in shark diets, with some sharks being filter
feeders.
This includes the largest of the sharks which
are the whale sharks and the basking sharks.
These filter feeders make use of the gill
rakers, which are extensions from their gill
arches, for filtering water for their prey.
Sharks have a variety of reproductive strategies,
including laying eggs or being oviparous.
Laying eggs that then hatch within the females
reproductive system, which is ovoviviparous,
or in some species not laying eggs at all
but the young developing in the uterus and
receiving nutrients from the mother by an
organ that is almost like the placenta in
mammals.
These would be viviparous.
The next type of Chondrichthyan or cartilaginous
fish we want to talk about are the rays.
In some ways they are similar to sharks in
that their skeleton is made entirely of cartilage,
however, their pectoral fins are expanded
into wings and their entire body is dorsoventrally
flattened.
Like sharks, they have gill slits and mouth
and these are ventral or on the bottom.
And spiracles which allow the intake of water
are located at the top of the body.
Now, most rays spend much of their time on
the bottom.
They are demersal fishes and they spend that
time partially covered in sediments
as a form of disguise and protection from
predators.
Rays have large flattened teeth for crushing
molluscs, crabs, and other bottom fish, so
they aren't preying on fish or using the sediments
as a way to ambush their prey.
So here we see a stingray and a manta ray.
The stingray will be demersal, however manta
rays actually swim within the water column.
They are filter feeders and so they need to
filter large amounts of seawater to get their
planktonic food.
Most rays have long whip-like tails and some
even have a spine at the base of the tail
with an associated poison gland.
That's the case in stingrays.
Electric rays 
have organs that produce electricity on either
side of their head.
Manta rays are plankton feeders in mid-water.
Like the filter feeding sharks, they use their
gill rakers for filtering water.
All rays give birth to live young.
They are either ovoviviparous or viviparous.
Our next group of chondrichthyans are the
skates.
In a lot of ways, skates look like a combination
between rays and sharks.
Like rays, skates are dorsoventrally flattened,
with large wing-like pectoral fins, however
unlike rays, skates have a fleshy tail and
no spine on their tail.
Also unlike rays, skates always lay egg cases,
so they are oviparous.
Skates are also demersal 
and feed primarily on molluscs.
The last group of chondrighthyans that we'll
discuss are the Chimera or ratfish.
They're mostly a deep water species and some
of the differences between them and the other
chondrichthyans is that they have a cover
over their gill slits.
They feed primarily on bottom crustaceans
and molluscs and they have a heterocercal
tail like in sharks.
The last group of fish that we'll be talking
about are the Osteichthyans.
They have a skeleton composed of bones.
Now, most fish have a homocercal tail, meaning
that the two lobes of the tail are equal in
size, and that this typically provides their
forward thrust.
They will also have flat bony scales that
are ctenoid or cycloid.
There will be a bony structure known as the
operculum that covers the gills
and they will usually have a swim bladder
for buoyancy control, although some demersal
species will not have this structure.
Fish use a lateral line for sensory information
about vibrations in the water.
There are over 31,000 species known, both
freshwater and marine, and they equal about
half of all known vertebrate species.
So here we see a diagram of an osteichthyan.
The mouth is located terminally instead of
ventrally like in the chondrichthyans.
We see the operculum covering over the gills.
They do have paired fins, but the scales and
the tail are different than what we saw in
chondrichthyans.
The major divisions of the osteichthyans are
into ray-finned fish versus lobe-finned fish,
although to be fair, the vast majority of
bony fish are ray-finned fish, with only very
few being lobe-finned.
Now, the reason why we care about lobe-finned
fish is because they are believed to be the
ancestors of the first vertebrates to go into
land.
What makes a ray-finned fish different is
that their fins are made up of a bony ray
covered by skin, whereas the lobe-finned fish
have fins that have bones and muscles within
them.
And this is why the lobe-finned fish are believed
to be the ancestors of the amphibians, because
they had fins that could be modified into
limbs or appendages.
So that completes our discussion on the different
types of fish.
Now a question I want you to think about before
our next video is “What would you need to
do differently if you were living submerged
in seawater?”
We'll talk about that in the next video.
See you then.
