Question

1. What is torsion? Why is it significant? What is detorsion? What taxa do these processes involve?

1. What are the synapomorphies of mollusks? For each of the synapomorphic traits, give an example of one or more molluscan species that don’t have that trait. You can use different taxa for different traits as the examples. When we have examples like this where the actual features of an animal don’t agree with the defined synapomorphic features, why might we still consider them to be part of the group? What evidence is used? What is hypothesized about the lack of the synapomorphic features?
2. Consider the field habitats that you saw in person in class and the invertebrate animals that you met there. Name and describe a biomedical discovery made from invertebrate animals that we saw during our rocky intertidal field trips? Same question for the fouling community at Hyde Street Pier (and other local marinas)—but give a different example animal for this habitat. And, for the sandy beach (e.g. Ocean Beach), what animal you saw there is giving insight into bioengineering and fluid dynamics? And, what east coast Atlantic Ocean invertebrate animal is of great biomedical significance and can be seen on sandy beaches laying eggs? For each of these cases (rocky intertidal, Ocean Beach, Hyde Street Pier, Atlantic beaches), name the organism (including phylum and other name) and describe the discovery or special feature of human use that is involved.
3. Which taxa that we have studied show the greatest powers of regeneration? What are they able to regenerate and under what circumstances?

Answer 1

1. What is torsion? Why is it significant?

  Torsion is the action of twisting of one end of an object relative to the other.

Gastopoda belongs to a large taxonomic class of invertebrates within the phylum Mollusk. This gastropods shows torsion during larval development. Torsion is the ancestral derived character of gastropods. The gastropods possess an asymmetrical body (Any line pass through the center of the body divides it in to 2 unequal halves).

Torsion is the rotation of the visceral mass, mantle, and shell 180˚ with respect to the head and foot of the gastropod.This rotation brings the mantle cavity and the butthole to an anterior position above the head.

Gastropods are the main class which shows torsion in phylum mollusk. all most all classes of mollusk exhibit varying degrees of torsion. But exceptions are there. Monoplacophora, class of mollusks, shows a complete absence of torsion.

Major significance includes,

• It helps to increases member shear strength, by adding shear stress to existing stress due to vertical and lateral applied loads.
• It allows foot to be retracted after the head for better protection of head.
• It positions the gills, anus, and opening from the excretory and reproductive system just behind the head and nerve cords.
• It helps to twists the digestive tract into a U shape.

2. What is detorsion?

It is just opposite of torsion phases and it occurs in some groups of gastropods. During evolution if shell is lost or a type of shell evolves that has openings on the opposite sides and in such situations twisting of visceral mass is not necessary. In this case detorsion happens.

In some groups of gastropods, rotation towards the original position happens, this may be only partial detorsion or full detorsion.

3. What taxa do these processes involve?

Kingdom Animalia consist animals without a backbone or bony skeleton.There are over 30 phyla dedicated to invertebrates. Members of Phylum mollusks shows both torsion and detorsion. Phylum mollusk consists 7 classes. They are  Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda. Members of Bivalvia, Gastropoda, Scaphopoda, and Cephalopoda shows torsion.

Detorsion occurs in some groups of gastropods. Mainly in Opisthobranchia, group of gastropod.

4. What are the synapomorphies of mollusks?

Synapomorphies are characteristics present in an ancestral species and shared exclusively (in more or less modified form) by its evolutionary descendants. simply it is the characters inherited exclusively from their common ancestor.

Synapomorphic characters are used for reconstructing phylogenetic trees.

In phylogenetic, apomorphy is derived character and synapomorphies are shared derived characters. In mollusks different synapomorphies are there. It includes the mantle, the foot, the radula, pectinate gills and torsion.

Torsion is a gastropod synapomorphy.

5. For each of the synapomorphic traits, give an example of one or more molluscan species that don’t have that trait.

Radula is is an anatomical structure used by mollusks for feeding. Class bivalves does not contain radula. They use cilia for feeding. Example are clams, mussels, scallops and oyster.

Monoplacophora, class of mollusks, shows a complete absence of torsion.

Snails, member of mollusks do not have pectinate gills.