Earliest molluscs date from the late Edicarn or Vendian rocks. Kimberelia is a bilateral symmetrical animall with a shell that probably was a mollusk. http://www.ucmp.berkeley.edu/vendian/kimberella.html
There are four small groups that allow us to speculate on how early mollusks may have looked.
Aplacophora, which live in the deep sea, are characterized by a worm like appearance and lack of shells. Their mantle secretes tiny calcareous spicules. Although lacking a shell and a well developed foot, this group does posses radulae. Most biologists feel that the earliest mollucs did not have a shell, but were forms that did not leave any fossils. Perhaps some species resembled modern day Aplacophorans.
Species can be hermaphroditic, but many also have two sexes. The females also release their eggs into the water, or hold them within their mantle cavity. In the latter case, inhalent water drawn into the female's mantle cavity contains sperm, which fertilizes the eggs held there.
Monoplacophora is a group that for a long while was only known from fossils, but that was rediscoved in 1952. In this group, the mouth is typically surround by a V-shaped lip and post-oral tentacles.
Below the head lies the semi-circular foot. In between the lateral sides of the foot and the ventral mantle edge, are found five or six pairs of gills. The serial alighment of the gills and other organs such as nephridia are considered primitive traits indicating the molluscs may have descended from a group that more resembled living Annelids. There are two sexes although nothing is known about development in this group.
Polyplacophora or chitons are more familiar and have eight valves on their shells. The valves are bordered by a thick girdle, often covered with spines or hairs, that forms from the mantle. There is a groove between mantle and foot containing numerous mollusc-type gills or ctenidia. Within the dorsal plates are "aesthetes". These can detect water movements and also contain simple eyes which can detect changes in shadow. They posses the typical open cirulatory system, a pair of kidneys and a simple nervous stytem.
Perhaps the early shells of some molluscs as of chitins consisted of overlaping plates. Chitins are all marine and for the most part feed on material such as algae, that are scraped up from the surface with their radula.
Chitons generally have two sexes and development in which a trochophore larvae delevops into a minature adult.
Scaphopoda or tusk shells are very small, unique mollusks with conical shells that curve, making them look like tusks. The "head" region lacks eys and has tentacles. The mantle is fused into a tube that surrounds the animal, but open at both ends. They are truely the odd man of the molluscan small clades.
Water is circulated around the mantle by various cilia. Most scaphopoda are predatory, and use their radula to crush or tear small prey such as forams. They have unique set of tentacles know a the captacula with sticky ends to capture prey. They use their foot to burrow. The animal pushes its foot into the ground, expands it and then pulls the shell in behind the foot.
Usually two sexes are present and upon release the larvae will go through a free-swimming trochophore and then veliger stage.
That leaves the three largest clades among the molluscs, the gastropods, bivalves, and cephalopods. We could as with the arthropods, spend all semester on these three groups. Because of time constraints we will only briefly examining those aspects of their biology that are of particular significance for understanding their lifestyle or usefulness to mankind.
The Class Gastropoda includes the snails and slugs.
Gastropods are by far the largest group of molluscs. Their 40,000 species comprise over 80% of living molluscs. Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior. Some graze, some browse, some feed on plankton, some are scavengers or detritivores, some are active carnivores.
Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups. Many snails have an operculum, a horny plate that seals the opening when the snail's body is drawn into the shell.
A few conch use their operculum defensively.
Gastropods often produce mucus that is very important to moving, especially on land.
View these two videos https://www.youtube.com/watch?v=JhRwcPRy6l4
Genetics
One of the first early mutations discovered was a maternal effect mutation that determined whether shells in the snail Limnaea coiled to the left rather than the right.
The direction of coiling is due to orientation of cells after early divisions in embryo. The mother transfer some signal to her embryos that determines the direction of coiling and so the direction of coiling is dependent on the mother's genotype.
Be able to explain the following genetics of coiling. Note the coiling direction does not depend on the embryos own genotype (middle column) but depend on the mother's genotype.
Scientists have begun to take a serious look at effects of parental environment on offspring phenotypes. Some correlations can be seen that cross generations even in humans. There is a claim that the quality of a grandfather's nutrition can influence lifespan in his grandchildren in humans. Interestingly, you live longer if your granddad was privy to poor nutrition. We have also know for quite some time that during embryonic life certain paternal genes at specific ( or maternal genes at different times) may have more influence on phenotype of the resulting child, even when alleles are inherited from both parents. This phenomena is known as paternal imprinting if you want to research it further.
Gastropods are characterized by "torsion," a process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted. Torsion takes place during the veliger stage, usually very rapidly. During torsion, most of the body behind the head, including the mantle, mantle cavity and visceral mass are twisted counter-clockwise (when viewed from the dorsal surface as in the figure below) through 180 degrees. Veligers are at first bilaterally symmetric, but torsion destroys this pattern and results in an asymmetric adult. Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms.
Nudibranchs are among the most beautiful of the gastropods and a group that has undergone detorsion. You used nudibranchs to examine phylogeny early in the course. Nudibranchs have soft bodies and most of them lack an external protective shell. Visit this website that is devoted entirely to nudibranchs. http://www.seaslugforum.net/find/detort
Some nudibranchs have numerous body projections (called "cerata") increasing the overall body surface and enhancing breathing. Some species extract nematocysts from the coelenterates on which they feed and store them in the special sacs at the tips of their ceratas called cnidosacs.
The lion maned nudibranch have has a large expandable oral hood which is fringed with sensory tentacles.
Many species of gastropods are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species involves an individual sending a dart into the tissues of the other.
"The love dart transmits a secretion produced by the mucous glands in the genital apparatus. The secretions contain a mix of hormones. Those hormones influence the spermoviduct of the receiving snail, the peristaltic movement of which now supports the sperm cells' way up the canal, which have to pass the spermoviduct to reach the sperm pouch at it's end, to be stored until fertilization. This way, a larger part of the sperm cells survive, and thus the chance to inherit the spender snail's own genes is improved. By application of a love dart, the fertilization chances of a snail can be doubled (Chase, R.; Blanchard, K. C.; 2006)."
"The love dart is a tool of male manipulation" (National Geographic, 2002). The Canadian scientist Dr. Ronald Chase has found out that snails do avoid the partner's love dart. He describes the mating play of brown garden snails (Cornu aspersum) sometimes to look like a medieval knights' tournament ("jousting"), both snails circling each other to avoid being hit.
Many marine species have veliger larvae or larvae with wings. Trochophore larvae can develop into veliger larvae.
Continue your examination of bivalves and cephalopods using this link.
