The phylum Mollusca is one of the largest of the invertebrate clades,
both in the size of certain species and the number of species which have
been described. Early mollusks were abundant in Cambrian seas and the
long history of the group is reflected today in the variation among Molluscan
types. This variation also attests to the success and plasticity of the basic
Molluscan body plan.
Background
The basic molluscan body plan is bilaterally symmetrical,
unsegmented, and coelomate and the body is divided into a ventral muscular
foot, a dorsal visceral mass, and a mantle of epithelium and other tissue
which encloses the dorsal surface of the body. The cavity between the
mantle and visceral mass is termed the mantle cavity. The visceral mass is
provided with a blood circulatory system generally containing the oxygen
carrying copper pigment haemocyanin, a variably specialized and cephalized
nervous system with ganglia and ventral nerve cords, a well-developed
excretory system, and a distinct reproductive system. The mantle cavity
generally houses an efficient respiratory system. As will be seen in today's
and next week’s laboratory, however, among the molluscan clades (classes)
almost every one of the organ systems mentioned above shows a wide
spectrum of variation.
Molluscs apparently arose as creeping types, probably living on hard
surfaces and scraping their food from the substrate by means of a unique
organ, the radula, which is found in all modern clades except the Bivalvia
(Pelecypoda). Bivalves have extensively modified their gills (ctenidia) for
filtering particulate food from the water column. The molluscs are closely
related to the annelids. This affinity is seen in the similar developmental
patterns within the two groups, the trochophore larva, and the possible
vestiges of segmentation seen in some of the primitive molluscs.
Today’s lab will survey two common mollusk groups, the gastropods
and the bivalves.
Observations on gastropods
Nudibranchs.
1. The Berghia nudibranch feeds only on Aiptasia. We have a small
colony. This nudibranch is highly sensitive to all changes and so
graciously donated cerata for you to view. Many nudibranchs sport
cerata that contain toxins or nematocysts stolen from Cnidarians. In
the latter species, the digestive system extends into the cerata and when
the nudibranch eats the cnidarian, the stinging cells pass unharmed into
the cerata.
Obtain a cerata, place it on a slide. Gentle pressure on the
coverslip should allow the nematocysts to be released. Treat these
slides as you did slides of Aiptasia tentacles. Try to obtain pictures
of these nematocysts. Can you go back to your photographs of
Aiptasia nematocysts and confirm that the nematocysts in these
cerata came from Aiptasia?
2. Observe an egg case of an Berghia nudibranch. With a small
pipette, add a few eggs to a depression slide and look for movement
in the embryos. With your instructor’s help you may also be able
to capture some larvae that have already hatch in the bowl
containing egg cases. If so watch the larvae move. Berghia larvae
are released at the veliger stage. Those obtaining larvae should
share their observations with the rest of the class. Please make a
video of any movement you observe in embryos and larvae.
The main organ of locomotion of a larvae is a ciliated velum which is also
used to obtain food particles. Nudibranch veligers (and all other veligers)
swim in a constant orientation, with velum uppermost and leading. Part of
this may owe to the location of the velum and to the body’s centre of
gravity, but the veliger larva does have well-developed paired statocysts
located in the base of the foot.
Compare these larvae in your journals eventually with the glochidia
larvae for which we have slide specimens. Ask your instructor if you
should do it now or wait for the end of the laboratory if time is running
short.
Snails:
We have several species of snails available in the laboratory for
observation on locomotion and feeding.
Nassarius adults and babies to use for locomotion, feeding and shell
shpae.
Ninja star adult snails to be used for locomotion and shell shape.
Various Astraea spps. to be used for direction of coiling and shell
shape.
This last activity involves working with preserved specimens----do after all living observations, including those on bivalves but
before dissections---rinse hands after working with preserved
material.
3. Locomotion: Use baby Nassarius
Locomotion in most gastropods is accomplished by muscular
contractions of the foot aided by mucus secretion. Exceptions to this general
pattern include swimming gastropods and gastropods that use cilia to
locomote. In gastropods that move by the muscle/mucus method, there are
two specific ways by which movement is achieved: 1) direct muscular
waves where the posterior edge of the foot is lifted, moved forward and then
this advancing wave is propagated forward and 2) retrograde muscular
waves where the anterior end of the foot is stretched and attached and the
advancing wave is propagated backwards.
You should try to obtain a short video of your baby snails either
moving or feeding.
a. Watch your snails crawl across a glass surface. Observe and describe
the motion of the foot. Time the snail as it moves along the surface.
Calculate average speed. Calculate feet per minute and miles per hour.
Some convenient conversions:
cm/min x 1.97 = feet/hour; ft/min x 0.0114 = mi/hr;
cm/min x 0.6 = meters/hr.
b. Feeding: action of the radula.
Allow the baby or snails to start feeding on some fish food and then
flip it over gently so you can observe the radula. Describe feeding by
each species and try to get a photograph or video of the radula if you
can. Examine the marks let in the dried food by the scraping radula.
Count the strokes of the radula per minute if possible.
You may also try to feed our star snails. They are algae feeders. They will
need a bit of an algae sheet wrapped around a rock. Place a star snail on top
of the algae sandwich. Observe how it moves as it attempts to feed on the
algae.
Please leave these species upright. As beautiful as these snails are, they
cannot turn upright and will starve to death if not aided by aquarium keepers
if they find themselves upside down. Obviously peaceful bottom dwellers
who hide among the debris and rocks in their native habitat, no much is
known about their biology.
If you do not attempt to feed the star snails, attempt to feed an adult
Nassarius snail. In aquaria, the Nassarius is considered nearly
indispensable for keeping sand beds clean and healthy. It usually spend
most of its time in the sand only its siphon, allowing oxygen exchange,
protruding. Adding food to the aquarium quickly brings it to the sand
surface to feed.
Variation among species. Describe the shell of two species. One complete
circle of the shell is a whorl and the edges of each whorl are connected to
the next by suture lines. These lines are often sculptured and can form
spines. Like all mollusk shells, growth lines are visible on the surface of the
shell and the oldest part of the shell is the apex.
4. Look for the dish that has a large number of snails in it. Every group
takes 20 and determines how many turn to the right or left. Be sure
you tally class frequencies before leaving. Discuss in your notebook,
the basis for this phenomenon. These snails have been preserved in
alcohol, please rinse your hands and/or gloves after handling these.
5. Observations on bivalves:
Bivalves do not initially appear to have much in common with snails or the
primitive molluscan forms except for their protective shell. Bivalves are
generally sedentary. The gills, visceral mass, and mantle cavity dominate
the body, and the head is suppressed. Bivalves have developed from the
primitive molluscan form by enlarging the mantle and dividing it into
symmetrical halves hanging down on both sides of the body, enlarging the
gills in the now huge mantle cavity, and extending the foot downward
between the mantle folds as a blade-like structure. Bivalves have lost the
radula and the majority are ciliary feeders with large, platelike
food-gathering gills (ctenidia). In oysters, the foot is greatly reduced and
usually not even apparent if specimens are small on dissection.
Obtain some of the dishes that contain bivalves for observation.
We have a scallop and two oysters, wing and blood (because they contain
hemoglobin for storage) for observation. Simply view the differences in
shell shape and if you are lucky and one opens during observation, feet and
any parts of the mantle that may protrude. In scallops, simple eyes,
sometimes hundreds of them are found on the protruding mantle.
(If you have not observed the slides containing glochidia, this would be a
great time to do so.)
6. After observing these bivalves, dissect either a mussel or an oyster.
One pair should dissect an mussel, the other an oyster and share photos
and observations.
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