Processing Shellfish Samples
This page outlines the steps for preparing shellfish composite homogenate samples for analysis. Wear powderless, disposable gloves when handling fish for tissue processing, changing gloves between each sample.
1. Inspect individual shellfish.
- Unwrap and inspect shellfish to ensure that they have been properly preserved during shipment and storage.
- Discard any specimen deemed unsuitable for further processing and analysis and note on the sample processing record.
2. Determine the sex of each shellfish (optional).
- Shellfish species sex determination is impractical if a large number of individual specimens are required for each composite sample.
- Bivalves - Sex determination is time-consuming. After a bivalve is shucked but before the edible tissue is removed, extract a small amount of gonadal material using a Pasteur pipette. Examine the gonadal tissue under a microscope to identify egg or sperm cells.
- Crustacean - Sex should be determined before removal of the edible tissues. For many species, sex determination can be accomplished by visual inspection. For example, the blue crab, Callinectes sapidus: the female has a broad abdomen suited for retaining the maturing egg mass or sponge, while the abdomen of the male is greatly reduced in width.
- Shrimp, lobsters, and crayfish: sexual variations in the structure of one or more pair of pleopods (abdominal appendages that function primarily for carrying the eggs in females) are common.
3. Note morphological abnormalities.
4. Rinse well with distilled water (or another form of contaminant-free water) to remove any loose external debris.
5. Remove the edible parts from each shellfish in the sample. The diagrams in Appendix L: General Procedures For Removing Edible Tissues From Shellfish (pdf) demonstrate how to extract the edible tissue.
- Minimize shellfish sample thawing during tissue removal to avoid loss of liquids.
- Only the tissues that the population of concern might eat should be removed as the edible portion. The edible tissue should be clearly defined in site-specific sample processing protocols.
- All fluids accumulated during removal of edible tissue should be retained as part of the sample.
- Bivalve mollusks - Remove the byssal threads from the mussel with a knife before shucking. Sever the adductor muscle, pry open the shell, and remove the soft tissue. The soft tissue includes viscera, meat, and body fluids.
- Crabs – Do not combine hard- and soft-shelled crabs in the same composite. For soft-shelled crabs, use the entire crab. For hard-shelled crabs, remove the edible tissues, typically all leg and claw meat, back shell meat, and body cavity meat. Remove internal organs from the sample except if the population of concern consumes the hepatopancreas.
- Shrimp and crayfish - Remove the cephalothorax and the tail meat from the shell. Only the tail meat with the section of intestine passing through the tail muscle is retained for analysis (Smith, 1985).
- Lobster - Edible tissues typically include the tail and claw meat. If the hepatopancreas and gonads or ovaries are consumed by local populations of concern, these parts should also be removed and analyzed separately.
6. Combine the edible parts in a glass or stainless steel container.
7. Weigh the sample.
- Edible tissue from all shellfish in a sample should be placed in an appropriate pre-weighed and labeled glass or stainless steel container. The mass of the empty container (tare weight) should be recorded to the nearest gram on the sample processing record.
- Re-weigh the container and record its mass to the nearest gram on the sample processing record after edible shellfish tissue added.
- If the sample is not going to be immediately homogenized, freeze and store at ≤-20 °C.
8. Homogenize the sample.
- Grind and homogenize samples to ensure even distribution of contaminants throughout tissue samples and to facilitate extraction and digestion of samples.
- Use a stainless-steel grinder, high-speed blender, or homogenizer to grind and homogenize samples. Parts of the blender or homogenizer used to grind the tissue (i.e., blades, probes) should be made of tantalum or titanium rather than stainless-steel.
- For best results, grind and homogenize when the biological tissue is still partially frozen. Additionally, chilling the grinder/homogenizer briefly with a few chips of dry ice will reduce the tendency of the tissue to stick to the grinder. These techniques will make grinding easier (Stober, 1991).
- Grind the sample into a stainless-steel or glass bowl until the tissues appear to be homogeneous. Repeat as necessary until the tissue is a uniform color and finely ground texture. Do not let the homogenate heat up in the process.
- Note the preparation specifics for each homogenate on the sample processing record.
- Label containers with aliquot identification number, sample weight (to the nearest 0.1g), and date aliquots were prepared.
- Freeze the remainder of each homogenate (i.e., tissue in excess of that needed for analysis) at ≤-20 °C to archive it. Record the designation "Archive" and the preparation date on the sample label. Record the location of the archived samples on the sample processing record under "Notes." Archived samples provide a valuable backup if samples are lost in transit or there are analytical problems.