Download Marine Biology Lecture 6

Marine Biology Lecture 6
April 14, 2006
I.
Phylum Mollusca (cont.)
A.
Class Gastropoda (stomach-footed); snails, slugs, nudibranchs.
1.
Major characteristic: undergo torsion
2.
Mantle cavity and visceral mass twists 180 deg.
3.
Occurs during larval development
a.
Trocophore differentiates into veliger, then undergoes torsion.
4.
Generally accompanied by loss of symmetry in organs.
5.
Advantages of torsion
a.
Can duck headfirst into mantle cavity
b.
Closes off shell with foot (operculum can serve as trap door)
c.
Sensory system for ctenidium (called the osphradium) now at front
6.
DISADVANTAGE OF TORSION
a.
possibility of pooping onto heads
b.
Groups of molluscs evolved various ways of dealing with this
"sanitation" problem.

Ex: keyhole limpets
B.
Clas Gastropoda: diversity
1.
Diodora aspera: rough keyhole limpet
a.
Appearance

Distinctive keyhole at top

Rough texture and ribbing distinguishes it from other keyhole
limpets
b.
Location

2.
middle to low intertidal, usually in protected crevices and
underhangs
c.
Other intersting details of life-history

Omnivorous, feeding primarily on bryozoans

Extends mantle over foot and shell, and above keyhole, to prevent
tube foot grip of predatory sea stars.
Lottia gigantea
a.
Appearance

Large limpet of middle to low intertidal

Relatively flat; weathered appearance
b.
Location

c.
middle intertidal; can be out on exposed rock
Other intersting details of life-history

territoriality

3.
4.
5.

mantle folds for respiration during low tide
Collisella digitalis
a.
Appearance

Recognizable by apex very far forward
b.
Location

High intertidal to upper middle intertidal
c.
Other interesting aspect of life-history

Feeds on microscopic films of algae

Restricted to vertical surfaces in areas of Oystercatcher predation
(why?)

Some individuals home, some don't

Recognizable by apex very far forward
Collisella pelta
a.
Appearance

Relatively high profile, but dependent upon habitat

Apex about 1/3 back
b.
Location

Associated with the kelp Egregia when small, and appear black.
Become more ribbed and mottled after moving to rock surfaces

Common in mussel beds
c.
Other interesting aspects of life-history

Feed on common, erect species of algae

Tidal rhythm of feeding activity
Tegula funebralis
a.
Appearance

Black shell; flattened bottom

Foot black on outside, white on inside, with long epipodial
tentacles
b.
Location

middle intertidal, often in pools are protected in crevices or
beneath algae
c.
Other interesting aspects of life history



6.
Lottia "farms" bare rock area, removing other organisms,
"using its shell like a bulldozer"
Littorina sp.
Move to tops of rocks during high tide at night only, and orient to
current (perhaps to catch drifiting algae?) Will retreat with strong
currents
Often aggregate in groups on rocks and in pools
Negative correlation with algal cover
a.
b.
c.
Appearance

Very small; ones here are mostly tiny and black
Location

High intertidal

Littorina planaxis occur higher than any other molluscan
species (can stay out of water for 2-3 mo.)

can survive submersion in fresh water

specialized "glue" to stick it to the rock so it can seal itself
off, close operculum
Other interesting aspects of life-history


7.
8.
feed on diatom films
erosion of tidepools (1 cm/16 yr)
Nucella sp.
a.
Appearance

Gray, spiral shell with yellow-orange operculum
b.
Location

Upper middle intertidal, primarily within mussel beds but may be
elsewhere in this zone
c.
Other interesting aspects of life-history

Predator on mussels and barnacles

Accessory boring organ on foot: dissolves shells of mussels and
barnacles with acidic secretions, then makes hole with radula and
consumes prey.
Nudibranchs
a.
Undergo detorsion
b.
reduction of shell, or no shell (nudibranchs have no shell)
c.
feeding: often on toxic sponges, cnidarians, tunicates. Co-opt poisons
for their own use.
d.
dorid nudibranchs (Order Doridacea): rear gill plume, forward
rhinophores (sensory)


Archidoris montereyensis

feeds on yellow sponges and stores their toxins
Rostangia pulchra

Feeds on red sponges, camouflaged against them, and
protected with sponge toxins

e.
Discodoris (Dialula) sandiegensis

circles on white body

Feeds on sponges
aoelid nudibranchs (Order Aeolidacea): extensions of mantle/gut
appear as cerata all over body. Location of gas exchange.

C.
Phidiana (Hermissenda) crassicornis

Feeds on cnidarians, stores cnidocytes for later protection
Class Bivalvia
1.
laterally flattened
2.
hinged shell
3.
enlarged mantle cavity
D.
4.
enlarged ctenidia: now serves both for respiration and filter feeding
5.
Reduced head
6.
Foot used for digging, more muscular, rather than wide, flat and slimy
7.
Mantle modified as siphons in some bivalves
Class Bivalvia: diversity
1.
Mytilus californianus: California mussel
a.
Appearance




b.
Raised ridges extend along length of shell
Posterior is somewhat angled
Anterior (pointy end) usually weathered
Byssal threads: secreted from foot, used to hold onto rock
Location


2.
Middle interitdal (especially lower middle)
Found in waveswept areas
c.
Other interesting aspects of life-history

Houses a community of organisms

filter 2-3 l of water/day. Their ingestion of algal spores/invert
larvae may help them in their competition for space

patchiness: large, old ones rip out, opening up space.
Mytilus edulis: bay mussel
a.
Appearance

Smooth surface, without ridges and less weathered

Posterior end is almost flat, with slightly rounded edges

Byssal threads: secreted from foot, used to hold onto rock

Has fewer and thinne byssal threads
b.
Location

Found in less turbulent water than M. californianus. This species
is collected off docks and in bays.
Study questions
1.
Know the basic features of the ancestral mollusc, and the functions of these major parts. Also,
be able to compare these features between the “hypothetical ancestral mollusc” (HAM) and each
of the major classes of the phylum, noting both similarities and differences. Also, be able name
each of the major molluscan classes, and list typical members of each class.
2.
What is torsion? Which class of phylum Mollusca has members which undergo torsion?
Describe the process, and discuss the major advantages and disadvantages of torsion.
3.
Know the difference between dorid and aoelid nudibranchs.
4.
Know aspects of the life histories of common intertidal organisms as discussed in class (and
included on this and previous, outlines.) For example, be able to describe what Nucella eats and
how it penetrates the shells of its prey. You might also have to name a particular animal, given
its description. You will have the list of scientific names (in alphabetical, not phylogenetic,
order) to help you. Common names not allowed.