Transcript Lecture 13, Annelids + Molluscs - Cal State LA

Pay attention to the progression from life originating in water to
colonizing dry land in animals, just like in plants
- what pre-adaptations were already present in aquatic
ancestors that made the transition to land easier?
- what limitations of the body plan had to be overcome
by adaptive evolution? (in other words, how did evolution
solve problems associated with life out of water)
Especially think about these highly successful groups:
- annelid worms
- molluscs
- arthropods
- vertebrates
Coelomates
Coelom = fluid-filled space surrounded by muscle
- acts as a hydrostatic skeleton for muscles to push off of
(like having water balloons instead of bones)
Protostomes
Lophotrochozoa
Ecdysozoa
Protostomes are further divided into 2 groups
on the basis of DNA sequence analysis

Deuterostomes
Platyhelminthes
(flatworms)
Protostomes
Annelid worms
Molluscs
Nematodes
Arthropods
Deuterostomes
Phylum Annelida
~17,000 species
Coelom acts as hydrostatic skeleton allowing worms to
crawl (marine polychaetes) or dig (earthworms) effectively
Segmentation: body is divided into many repeating blocks
- blocks of segments are specialized for different functions
Complete digestive system, closed circulatory system
Chetae = spines sticking out of epidermis; help in movement
“polychaetes”, marine worms
25 Orders, 87 Families
- numerous chetae (bristles) on each segment
- parapodia (paddle-like appendages) can act as
walking paddles, or gills for respiration
- reproductive structures
often simple and even
temporary
Cross-section of a polychaete segment
Dorsal blood vessel
Circular muscle
Longitudinal
muscle
intestine
Coelom
chetae
Parapodia - can act as gills,
feet, or hold worm in its tube
Ventral
nerve cord
ventral
blood
vessel
worm movement
Circular muscles squish or expand different parts of the body
 important for tunneling and other movement in annelids
 nematodes lack circular muscles + a coelom, so can’t do this
show
vids
oligochaetes (earthworms)
3 Orders
- chetae reduced, but important for burrowing; no parapodia
- circular muscles + hydrostatic skeleton allow distinctive
“oozing” method of movement
- clitellum is the obvious band; after mating, it produces a
mucus cocoon that slides along body, picking up sperm
deposited by the mate and eggs released from the body
clitellum
Anatomy of an
earthworm
5 “hearts” which are
muscular vessels that
surround esophagus &
pump blood
Septa divide body into
isolated segments
Digestive, circulatory
systems pipe right
through the septa
Oligochaete ecology
“Worms have played a more important part in the history of the
world than most persons would at first suppose.”
from The formation of vegetable mould, through the action of worms,
with observations of their habits. Published in 1881 by C. Darwin
Earthworms consume soil as they burrow beneath the ground
- organic material is partly digested as it passes through worm
Worms then back out of their burrows and deposit nutrient-rich
“castings” on top of the ground
- creates fertile topsoil, essential for healthy plant growth
- over 16,000 pounds of soil processed per acre, in a year!
Tunnels also give roots access to fresh air, for gas exchange
Deep-sea vent worms + their bacteria
Giant tubeworm Riftia lives next to superhot,
deep-sea hydrothermal vents
Adults have no digestive tract; instead
have a special organ full of mutualistic
chemo-autotrophic bacteria
bacteria use energy from toxic sulfide (H2S)
to fix CO2  make sugars that feed worm
8 ft tall worms create habitat for 100’s of
species, all living in complete darkness

Riftia pachyptila
Phylum Mollusca
~100,000
species
Soft body divided into 3 parts:
a) foot, for locomotion
b) visceral mass = internal organs
c) mantle, a flap of tissue that secretes the shell
- Open circulatory system spouts blood through a body cavity
called hemocoel (replaces coelom, not needed due to shell)
- Ctenidia = flaps used as gills, for respiration (getting oxygen)
- Radula = toothed ribbon used to scrape food
- Develop through a larval stage called a trochophore,
a feature shared with annelid worms
Shells
Special proteins are secreted by the mantle tissue,
cause the mineral calcium carbonate
to crystallize out of the ocean water
- same material coral uses
Shell provides protection (physical defense)
against predators
Acts as an external skeleton, to which
muscles can attach and work off of
- termed an “exoskeleton”
- molluscs no longer needed the coelom as as
internal skeleton, so their coelom is really reduced (tiny)
Shells vary from class to class
Chitons: 8 separate plates
Bivalves (clams): 2 shells, hinged
Gastropods (snails): 1 twisted shell
with an operculum (trapdoor)
Cephalopods:
nautilus 1 shell

internal shell
octopus
no shell

squid
Molluscan Body Plan
radula is extended out,
then pulled back to
scrape surfaces for food
water pumped through mantle cavity, over gills
Radula
Unique to molluscs: Ribbon of teeth made of chiton
- tips may be hardened by iron
May grow 5 new rows of teeth per day
Can be used for...
herbivory: scrape algae off rocks
predation: cut tissue from animals
injecting poison (cone snails)
Phylum Mollusca: 4 major classes
Class Polyplacophora - Chitons
Class Gastropoda - Snails + slugs
Class Bivalvia - Clams, mussels, oysters
Class Cephalopoda - Octopus, squid, nautilus
Phylum Mollusca: 4 major classes
Class Polyplacophora - Chitons
- scrape algae off rocks using radula tipped with IRON
- shell is 8 distinct plates, held by mantle
check out a chiton radula in lab!
Class Gastropoda: Snails + slugs 70,000 spp.
Asymmetrical body packed into a spiral shell
Torsion: during development, body rotates 180o over the foot
- moves viscera, anus over head
- organs on right side lost; space taken over by gonads
One of few invertebrate groups to successfully colonize land
1) operculum (trapdoor) to close body into shell =
protection against desiccation & predators
2) muscular foot for movement on land
Torsion
before torsion

after torsion
Cone Snails
In these hunters, the radula adapted
into a harpoon to capture prey
Injects nerve toxins into fish or
molluscs to paralyze them
Only “drug from the sea” so far is a
back pain drug - a cone snail toxin!
SEA SLUGS!!!
Sea Hares -- includes world’s largest gastropod
- herbivores eat algae, store toxins and color which they release
by inking if molested
- Nobel Prize in medicine
awarded in 2002 for
work on learning &
memory in Sea hares
Ultimate Vegetarians
Some sea slugs exhibit kleptoplasty:
- chloroplasts from food algae are stored, instead of being digested
- continue photosynthesizing for up to 9 months in some species!
 made possible by transfer of genes from the algae’s nucleus into
the slug’s chromosome: now part animal, part plant!
Nudibranchs: Poison-eaters
Nudibranchs can store toxic chemicals from sponges they eat
gills
Chromodoris
Phyllidia
Halgerda
Glossodoris
Bright colors & patterns serve as a warning to fish, crabs:
“I am poisonous, don’t eat me” – aposematic coloration
Hypselodoris iacula
Hypselodoris
kanga
Chromodoris annae
Hypselodoris fucata
Hypselodoris nigrostriata
Aeolid nudibranchs
- feathery cerata on backs store cnidae from cnidarian prey
- cnidae detonate in mouth of any fish that tries to eat the slug
Godiva
quadricolor
turn your prey’s
defenses into your
own armaments
Hermissenda
crassicornis
Class Bivalvia – clams, mussels
~20,000 species
Shell with 2 valves, held together by
powerful adductor muscles (the meat of a scallop)
Shell
Adductor
muscle
holds shell
closed
foot
Ctenidia = gills,
used also in
filter feeding
Foot used for digging
into bottom
Bivalves filter-feed with their gills
to mouth
Cilia (tiny hairs) beat to
generate water currents
(like sponge choanocytes)
Trap tiny particles, which are
then carried to the mouth
Use their ctenidia (gills) for both filter feeding & respiration
- clams burrow into mud/sand; use siphon to draw in water,
pass it over their gills for both oxygen and food
Mussels: Dominant Spatial Competitors
use protein threads
to glue themselves
to rock surface
Mussels cover rocks in the intertidal zone
out-compete other organisms for space, unless their #’s
are limited by predators (more on this to come..)
Freshwater mussels (Unionids)
edge of mussel’s mantle
female
mussel
actual prey fish
Freshwater mussels mimic small fish, insects with parts of their
body to lure larger, predatory fish  spray vampire-larvae
into big fish’s gills, where the larvae drink its blood!
Class Cephalopoda
~900 species
Nautilus, squid, octopus
Most intelligent invertebrates, w/ complex eyes
- Only molluscs w/ closed circulatory system
high speed
- Zoom backwards by jet propulsion, for hunting
- Defense without a shell: inking, color/texture change
- Foot divided into tentacles w/ flexible suckers
nautilus
(external shell)
squid
(thin, internal shell)
octopus
(no shell)
Chambered Nautilus
Nautilus lives at great depths (600 meters), floats up to surface
at night by adjusting pumping gas into sealed shell chambers
Squid
Fast-moving mid-water predators
shoot out 2 extra-long tentacles to snag
fish, using their suckers
internal
shell
giant squid = world’s
biggest invertebrate
Glow-in-the-dark squid
Bioluminescence can result from
light-producing bacterial symbionts
Counter-shading used by many
mid-water squid to hide from
predators lurking below
 produce same amount of light
from your belly as hits your back,
so you don’t cast a shadow visible
to predators under you!
 essentially, you become invisible
Euprymna scolopes
Octopus
- 8 arms
- no shell
- best vision
- smartest invertebrate
~200 spp.