Arthropoda
Invertebrates
in
the
Plankton:
Arthropoda
The
phylum
Arthropoda
encompasses
a
tremendous
range
of
marine
species
and
body
plans,
as
well
as
an
amazing
diversity
of
terrestrial
insects
and
spiders.
Coastal
and
nearshore
marine
habitats,
including
the
plankton,
feature
a
number
of
these
forms,
such
as
copepods,
crabs,
and
barnacles.
While
the
mobile,
highly
jointed
bodies
of
crabs
and
copepods
may
seem
quite
different
from
the
sessile,
immobile
carapaces
of
barnacles,
these
groups
share
important
traits:
a
chitinous
exoskeleton
and
a
need
to
molt
that
exoskeleton
in
order
to
grow
larger.

In
our
observations
of
the
plankton,
we
found
both
larval
and
fully
planktonic
species
to
be
very
abundant:
during
our
night­
lighting
from
the
Lab
dock,
we
caught
many
hundreds
of
two
crab
larval
stages,
zoea
and
megalopae.
And
in
our
daytime
plankton
tow
in
the
Straits
area
close
to
Friday
Harbor,
we
caught
great
numbers
of
several
different
types
of
copepods.
Photos
of
these
and
our
other
discoveries
follow.

Brachyuran
Crabs
This
crab
megalopa
(
4
mm
in
length)
was
one
of
hundreds
captured
by
night­
lighting
during
the
last
week
of
June.
It
swam
strongly
at
the
water's
surface,
its
legs
occasionally
splaying
energetically.
This
megalopa
is
the
final
larval
stage
of
true
crabs,
preceding
the
benthic
juvenile
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depts.
washington.
edu/
fhl/
zoo432/
plankton/
plarthropoda/
plarthropoda.
html
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Arthropoda
stage
and
following
several
zoeal
stages.
One
of
the
most
recognizable
brachyurans
(
also
known
as
true
crabs)
along
the
Pacific
coast
is
the
Dungeness
crab
(
Cancer
magister),
a
species
with
considerable
commercial
and
cultural
value
in
the
region.

Anomuran
Crabs
This
zoea
of
a
porcelain
crab
(
Petrolisthes
eriomerus,
3
mm
in
length)
is
well
defended
from
potential
predators;
notice
its
long,
inflexible
rostral
spine.
The
porcelain
crab
belongs
to
the
group
known
as
anomuran
crabs,
which
include
hermit,
porcellanid,
lithodid,
and
galatheid
crabs.
The
zoea
uses
its
first
two
pairs
of
thoracic
appendages,
the
maxillipeds,
to
swim,
making
it
one
of
the
more
agile
planktonic
animals.
The
zoea
will
develop
into
a
megalopa
form,
sometimes
referred
as
a
glaucothoe,
and
then
into
a
functional
adult.

http://
depts.
washington.
edu/
fhl/
zoo432/
plankton/
plarthropoda/
plarthropoda.
html
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Arthropoda
Copepods
Copepods
are
the
most
abundant
multicellular
animals
in
the
world,
living
in
both
freshwater
and
marine
environments.
The
most
abundant
free­
living
marine
copepod
is
the
calanoid
(
seen
on
the
right);
it
is
recognizable
by
the
long
first
antennae,
which
it
flicks
in
swimming
locomotion.
This
type
of
copepod
uses
mouth
parts
called
maxillae
to
scoop
phytoplankton,
small
invertebrates
(
including
other
copepods),
or
fish
larvae
from
the
water.
Copepods
are
also
the
primary
consumers
of
phytoplankton
and
are
an
important
food
source
to
fish,
baleen
whales,
and
other
invertebrates.
Copepods
have
separate
sexes.
In
some
species
one
can
distinguish
males
from
females
by
the
differences
in
antennae
or
appendages.
In
calanoidsk,
attraction
to
the
opposite
sex
occurs
through
detection
of
chemical
pheromones
in
the
water
column.
When
courting,
males
will
swim
in
a
somersaulting
fashion
which
is
thought
to
increase
the
opportunity
for
contact
with
the
females.
Females
might
not
be
as
excited
to
reproduce
and
may
struggle
against
the
males,
which
use
their
modified
appendages
to
grasp
and
transfer
sperm
packaged
into
structures
called
spermatophores.
Normally
only
one
spermatophore
is
deposited
per
female
but,
at
times
when
there
are
alot
of
male
competitors
some
species
of
females
will
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washington.
edu/
fhl/
zoo432/
plankton/
plarthropoda/
plarthropoda.
html
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Arthropoda
mate
with
several
males
and
store
multiple
spermatophores.

In
all
copepods,
fertilization
occurs
when
the
egg
passes
out
the
female
genital
tract.
Many
copepods
carry
the
eggs
in
sacs
on
the
body;
calanoids
carry
one
egg
sac
as
seen
above.
Development
of
the
eggs
depends
strongly
on
sea
temperature
(
see
development
of
eggs
at
different
temperatures),
species,
and
egg
size.
The
reproductive
season
depends
on
the
species
and
its
location.

Barnacles
This
larval
stage,
called
a
nauplius,
is
found
in
barnacles,
copepods,
penaeid
shrimps,
and
ostracods.
It
swims
by
using
its
jointed
appendages
that
are
moved
by
muscle
contraction.
These
larvae
have
a
nauplier
eye,
made
up
of
photoreceptor
cells
that
are
thought
to
give
rise
to
the
photoreceptors
in
some
adult
forms.
They
frequently
molt
(
shed
their
hard
outer
skeleton)
to
http://
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washington.
edu/
fhl/
zoo432/
plankton/
plarthropoda/
plarthropoda.
html
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Arthropoda
enable
growth
during
this
phase.
Barnacles
are
hermaphrodites.
They
release
their
larvae
into
the
water
column
at
the
nauplius
stage.
After
several
moltings,
the
final
larval
stage
before
settling
is
called
a
cyprid.
The
cyprid
stage
is
non­
feeding.
For
more
information
on
adult
barnacles
see
the
Cattle
Point
page.

Cladocerans
An
example
from
a
group
of
crustaceans
called
cladocerans
is
seen
here.
These
adult
animals
swim
using
the
second
antennae
and
they
feed
by
either
filtering
microscopic
algae
or
preying
on
small
animals.
Most
cladoceran
females
reproduce
by
parthenogenesis,
which
produces
males
and
females
without
sexual
recombination.
However,
they
also
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fhl/
zoo432/
plankton/
plarthropoda/
plarthropoda.
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Arthropoda
have
sexual
reproduction
that
produces
females.
This
marine
representative,
Evadne.
uses
the
carapace
as
a
brood
chamber
(
a
place
to
store
eggs).
A
more
commonly
known
freshwater
species
is
Daphnia.

Plankton
Home
Annelida
Chaetognatha
Chordata
Cnidaria
&
Ctenophora
Echinodermata
Mollusca
Invertebrates
2000
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washington.
edu/
fhl/
zoo432/
plankton/
plarthropoda/
plarthropoda.
html
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