In lab today, you will be interacting with two species of marine bivalves; the Atlantic Bay scallop (Argopecten irradians) and the Blue mussel (Mytilus edulis). (General information about both species is listed below.) We will be comparing the functional anatomy of both species. Although both are bivalves, Argopecten is an active swimmer, while Mytilus is not and lives in coastal regions secured to the ocean floor. Both scallops and mussels are very important to the local economy via fishing and aquaculture, respectively. Interestingly enough, when you eat a mussel, you are eating the entire body of the mussel, but when you eat a scallop, you are eating only the adductor muscle. Today, we will be examining aspects of feeding and locomotion in two bivalves. All animals used in this laboratory were obtained from local suppliers, who sell the animals for human consumption.
Upon entering the laboratory you will find live scallops in small glass aquaria, and freshly euthanized scallops and mussels. The scallop shells (also known as valves) should be gaped open. Do not disturb the live scallops before you locate the following structures:
Centrally located adductor muscle.
Questions: How do the shells remain open? Try and identify where is the structure that keeps the shells open. Why do the animals need to keep their shells open most of the time? Remove a live scallop from the aquarium. It may attempt to jet when you lift it out – trying to escape the predator attacking it, so look out for the jets of water! Locate the following structures:
Jet apertures (may be obvious by now)
Surface riblets (Dr. DeMont and his students showed that these structures reduce the drag on the shell. If you are interested, a copy of the paper can be found on the bulletin board in the front of the room.)
Squeeze together the shells of both the mussel and the scallop and compare the shape of the shells in the context of their different locomotor abilities. Complete the Assignment.
Obtain a scallop that has been euthanized using procedures approved by the CCAC. In this case, scallops were placed overnight in 7% magnesium chloride at 4˚C.
Mussel dissection (Feeding structures)
Locate the adductor muscles using the Dissection Guide. Insert a scalpel between the two shells and slice the adductor muscles. Now gently pull the two shells apart. Locate the structures used for feeding, as listed in the Dissection Guide.
Examine the mussels in the large touch tank aquarium at the back of the lab. Observe the mussels closely; note the incurrent and excurrent siphon.
Scallop dissection (Locomotor structures)
Using the euthanized scallop, you will now measure the force that the adductor muscle has to generate to compress the hinge. The elastic energy stored in the compressed hinge is used to antagonize the muscle. Take your intact scallop to one of three stations with the force transducers. Insert the animal into the apparatus and turn the handle so that the apparatus is compressing the shells. (Where should the tip of the force transducer be located on the shell? Why?) Record the measured force on the Assignment sheet.
(DO NOT continue to compress the shells once they are closed – the force transducer could be damaged.) Adductor muscle and the elastic hinge
Locate the adductor muscles using the Dissection Guide. Insert a scalpel between the two shells and slice the adductor muscles. Now gently pull the shells apart. Notice the black pad of rubbery-like abductin, which provides the elastic antagonism to the muscle. You just measured the force required to compress it. Find both the smooth muscle component and the striped muscle portion of the adductor muscle. What is the function of each portion? Measure the cross-sectional area of the striped muscle using the vernier calipers. Most muscles can generate a maximum isometric stress of approximately 3.0 x 105 N/m2. Use this information to calculate the maximum force that the scallop adductor can generate. Show all your work on the Assignment Sheet. Now identify the other anatomical structures labeled in the Dissection Guide, especially those used in feeding.
Is there a difference between the force required to work against the abduction, and the force the adductor muscle can exert? Why do you suppose this is? Blue Mussel (Mytilus edulis)
Blue mussels are common throughout Nova Scotia, and shells are often seen washed up on beaches. Mytilus edulis is found worldwide in polar and temperate regions. Their habitat ranges from the preferred brackish water to full-strength seawater. Mussels are usually found in colonies attached to a solid substrate by byssal threads. The mussels are capable of pulling up the byssal threads and moving short distances to a new location.
There is little commercial fishery of mussels in Nova Scotia. However, mussels are actively cultured all over the province and other Atlantic provinces. Mytilus edulis larvae or “spat” from the wild settle onto lines or mesh. Once they have grown to a certain size the mussels are removed from the line and put into long mesh tubes called “socks”. The socks are taken to the farm and tied to long lines (~200m) that are anchored to the bottom. A mussel takes 18-24 months to grow to commercial size (about 50 mm) and at this size can filter 4 litres of water an hour.
Blue mussels are sedentary animals and relatively long lived (up to 20 years). For these reasons, as well as their wide range, they make excellent environmental indicators. While feeding, mussels will also take in contaminants. The toxins will bioaccumulate in the mussel’s body. In many locations mussels are harvested for contaminants testing. The following is a link to NOAA’s (US government, National Oceanic & Atmospheric Administration) mussel watch website. http://ccmaserver.nos.noaa.gov/stressors/pollution/nsandt/mw_details.html
Atlantic Bay Scallop (Argopecten irradians)
Argopecten irradiansis native from Maine to Texas, although there are a few naturalized populations in Nova Scotia. It lives in relatively shallow bays with beds of eelgrass present. Scallops go through the lifecycle above. The veliger is a planktonic stage that uses a ciliated membrane to move. One larval stage of Argopecten is called a “pediveliger” and is capable of independent movement. Once the pediveliger has found a good place to settle (usually rough surfaces) it will attach itself to this surface. Like mussels, these larval scallops also use byssal threads to attach themselves. They remain attached for several months before they start swimming. Bay scallops are hermaphroditic and are sexually mature after one year. However, they rarely self-fertilize as the sperm and eggs mature at different times. The maximum size of Bay scallops is usually around 7 cm shell height (shell height is measured from the bottom of the shell “hinge” to the furthest point perpendicular from the hinge). They don’t usually live for more than 2 or 3 years.
There is a large market for scallops and Bay Scallops are said to be tastier than sea scallops. The scallops we are using came from an aquaculture facility in Pictou County, Nova Scotia.