Hello marine biology students.
In this video we're going to talk about the
diversity of arthropods.
[Intro Music]
Our next group is phylum Arthropoda, the arthropods.
This is the animal phylum with the largest
number of species.
In fact, 75% of all animal species on earth
are arthropods.
These include insects, which are a very diverse
group on land, but as we'll see not very common
in the oceans, and also crustaceans, spiders,
and etc.
The prefix arthro means jointed and pod or
podía refers to appendages.
So they have jointed legs and other appendages.
They also have a chitin exoskeleton, which
is a hard outer shell.
And if these individuals are to grow or increase
in size, that shell has to be shed through
a process called molting.
The basic body plan of an arthropod is that
it's broken up into a head thorax and abdomen
and so it shows segmentation.
So, we'll talk about several groups of arthropods,
the most important and abundant being the
crustaceans.
This is the dominant marine arthropod and
this includes marine organisms you're familiar
with like shrimp, and lobsters, and crabs,
but also some that you may be less familiar
with like the copepods amphipods and isopods.
One defining characteristic of crustaceans
is that they have two pairs of antennae, which
are distinct from insects you have only one
pair of antennae.
You will also find the head and thorax is
usually fused as a cephalothorax.
They are typically covered by carapace.
Among these crustaceans, some are filter feeders,
others are scavengers, some are parasites
and some are carnivores.
The appendages of crustaceans are specialized
for different functions
including feeding, swimming, carrying eggs
and etc.
During reproduction the males usually deposit
sperm directly to the females to ensure reproductive
success.
Here we see some diagrams of some of the crustaceans
you may be less familiar with.
Copepods are members of the plankton community
and have two long antennae that are used for
swimming.
Amphipods are sometimes on shore and sometimes
in marine sediment environments, and isopods
have some terrestrial relatives in the forms
of roly-polies.
A crustacean you might not often put into
this group or think about are barnacles.
For a while barnacles were thought to be a
type of mollusc based on their external anatomy,
but it's through studying of their embryological
development that we see that they are in fact
crustaceans.
Barnacles are also one of the few examples
of a sessile adult which still has internal
fertilization.
Most barnacles are hermaphroditic, so the
penis of one barnacle can deposit sperm into
its neighbors.
Lobsters are another typical marine crustacean
and we can see it has specialized appendages
both in the form of claws and walking feet
and then also swimmerets.
Related to crustaceans, but distinct, are
the horseshoe crabs.
They're in the class Merostomata.
They have five pairs of legs, the first pair
of which is modified in males for reproduction.
Mating pairs come on to beaches each spring
to breed and lay their eggs in the wet sand.
Horseshoe crabs are among the oldest creatures
on earth, not the individuals, but just that
the species has been around for so long.
They've remained virtually unchanged for millions
of years.
They live in burrows in soft sediments 
usually near shore where they scavenge and
feed on other invertebrates.
Here we can see what these horseshoe crabs
look like and they just appear prehistoric.
These are not found on the Pacific coast of
the United States but are present on the Atlantic
coast.
A somewhat obscure group of marine arthropods
are the sea spiders or
the Pycnogonids.
They usually have four or more jointed pairs
of legs and like the ribbon worms we talked
about earlier they have a proboscis for feeding
on soft invertebrates like sea anemones and
Hydrazoans.
They are more common in colder deeper water
but they can be found worldwide.
Since insects are the most diverse arthropods
on land you may be wondering where the sea
insects are, but it turns out that there aren't
really there.
Even though there are many species of insects
on land and even in some freshwater environments,
very few insects live in the ocean.
They'd be more common on shore living on seaweeds
that have washed up at the high tide mark,
but they don't tolerate well to being submerged
in water.
In fact, the closest to it, there are some
marine Water Striders that have been found
out at sea on the surface of the water, but
not beneath it.
There are a few terrestrial crustaceans and
isopods though.
So this concludes our discussion on arthropods.
Now, before our next video I would like you
to think about “What if your mouth was shaped
different than any other organisms?”
Well, we'll talk about that.
