The first step in most biological studies is fieldwork — to collect needed specimens or fossils from field localities. For bivalves, collecting takes many forms — from simple collecting by hand, to shovel-and-sieving in sand or mud, snorkeling or scuba diving on reefs, or bottom sampling from large ships. Once collected, the specimens are preserved in whatever fluid is best for how they will be analyzed in the laboratory — alcohol or a special salt solution for molecular work, formaldehyde-based fixatives for dissection and histology, or glutaraldehyde for electron microscopy.
Bivalve morphology (the more technical term for anatomy) is studied using a variety of methods including simple dissection (with our without a microscope), electron microscopyElectron Microscopy:
A form of microscopy in which the interactions of electrons (instead of light) with a specimen are used to provide detailed information about structure., and histologyHistology:
The study of cells and tissues at the microscopic level.. These techniques reveal the structures of shells, organs, and tissues.
Dissection guides to use in your classroom can be found here.
Electron microscopyElectron Microscopy:
A form of microscopy in which the interactions of electrons (instead of light) with a specimen are used to provide detailed information about structure. uses high-energy electron beams (instead of light) to examine and photograph bivalve shells and anatomy. Scanning electron microscopy (SEM) provides high-resolution images of outside surfaces, like sculpture, a hinge, or the appearance of the siphons.
Transmission electron microscopy (TEM) provides very fine details of cross sections of tissues, such as sperm still under development within a bivalve's reproductive tract.
Histology is a more traditional technique of embedding tissues in paraffin wax or plastic, then sectioning the block, mounting the thin slices on microscope slides, and staining them to help distinguish one type of tissue from another. Histology allows scientists to examine the inner structures of soft anatomy, or to reconstruct the anatomy of species that are too small to dissect by hand.
DNA or RNA isolation and sequencing are very valuable methods to determine evolutionary pathways and relationships among bivalve species. In some ways, they can be more useful than morphology, because DNA is less influenced by external forces such as food, temperature, and the shape of the rock to which a bivalve is attached. Below are the basic sequencing steps but a more detailed animation can be found here.
The basic steps of gene sequencing are:
Phylogenetic analysisPhylogenetic Analysis:
Analytical method used to find a hypothesis of relationships among species, by coding the various states of homologous characters; also called cladistics. (also known as cladistics) uses anatomical traits and/or molecular sequences to determine a phylogenyPhylogeny:
Sequence of ancestors of a particular lineage. or history of an organism's lineage through time. This phylogeny is a hypothesis based on the taxaTaxon:
An organism or group of organisms of the same rank, e.g., members of an order, family, genus, or species. (pl. taxa) (the organisms under study, usually species) and charactersCharacter:
A single attribute of an organism. (the traits and sequences for each organism) used in the analysis. See the Cladistics Activity for a more complete description of this method.