Bivalve Pros & Cons: Should You Eat Mollusk?

March 23, 2018

From banquets to oyster bars to cocktail parties and beyond, bivalve varieties like clams, mussels and scallops are popping up just about everywhere. It’s easy to understand why; not only do they have a distinct and delicious flavor, but they are also versatile enough to add to pastas, soups and sauces.

While there’s no doubt that they’re tasty, many question the potential effects that this food may have when it comes to health. Uncooked oysters, in particular, are notorious for the spread of infectious bacteria, but other types of bivalves have also been known to pass on dangerous toxins that can have detrimental effects on health.

On the other hand, bivalves are an incredibly nutrient-dense food, providing a concentrated megadose of important micronutrients like vitamin B12 and selenium. They also tend to be high in protein and antioxidants, plus may be associated with some health benefits as well.

So should you eat bivalves, or should you start skipping the shellfish — whether you have a shellfish allergy or not? Keep on reading to find out what you need to know to help you decide for yourself.

What Are Bivalves?

Bivalves are a type of sea mollusk and are closely related to other types of mollusks, such as squid, octopuses, snails and slugs. However, the official bivalve definition encompasses any aquatic mollusk belonging to the class Bivalvia, such as oysters, scallops, clams and mussels. These animals are found around the world in both freshwater and saltwater and are one of the most commonly consumed types of seafood.

The most important aspect of bivalve anatomy is the two shells that protect the soft inner body. Bivalves can open and close the shells to help them move and eat but close them immediately in the presence of danger.

So how do bivalves move? They have a special muscle called the foot that works like a hinge to control the shells and is also responsible for most of their movement. The foot is especially effective when it comes to moving side to side and digging to help the bivalve burrow itself in the sand away from danger. For other types of movement, the bivalve generally depends on water currents to help carry them further distances.

Instead of a mouth, bivalves have tubes called siphons that allow them to breathe and eat. Food passes into the stomach where it can be digested, and oxygen from the water is passed to the gills, which aid in respiration. Waste products and carbon dioxide are also eliminated through a different siphon, known as the out-current siphon.

Edible Bivalve Food + Types of Bivalves

Bivalves are the second largest class of mollusks, right behind univalves, which are made up of slugs and snails. It’s estimated that there are about 10,000 living species of bivalves, with only about 20 percent found in freshwater sources. (1)

That being said, not all bivalves are edible. Some of the most common edible bivalve examples include oysters, clams, cockles, scallops and mussels.

Bivalves are often consumed grilled, steamed or cooked and served over pastas, soups or seafood skillets. The meat is typically removed with a fork, and the empty shell is discarded. Some types of bivalves are also often consumed raw, such as oysters. To eat an oyster, the meat is separated from the shell using a fork, the shell is picked up and the oyster is slurped down from the wide end.

Some types of bivalves are also used to make dips, sauces, broths and juices that can be easily incorporated into other recipes.

Bivalve Nutrition Facts

The nutritional value of bivalves can vary widely based on the type, but they are generally low in calories and high in important nutrients like zinc, vitamin B12, manganese and selenium. In particular, oysters are a food high in zinc, vitamin B12 and copper; mussels are a food high in vitamin B12, manganese and selenium; clams are a food high in iron, vitamin B12 and protein.

For example, a three-ounce serving of oysters (or about six medium oysters) contains approximately: (2)

57.1 calories
3.3 grams carbohydrates
5.9 grams protein
2.1 grams fat
76.3 milligrams zinc (509 percent DV)
16.3 micrograms vitamin B12 (272 percent DV)
3.7 milligrams copper (187 percent DV)
53.5 micrograms selenium (76 percent DV)
269 IU vitamin D (67 percent DV)
5.6 milligrams iron (31 percent DV)
0.3 milligram manganese (15 percent DV)
113 milligrams phosphorus (11 percent DV)
39.5 milligrams magnesium (10 percent DV)

Mussels, on the other hand, are higher in protein and contain a slightly different array of nutrients. A three-ounce serving of cooked mussels contains approximately: (3)

146 calories
6.3 grams carbohydrates
20.2 grams protein
3.8 grams fat
20.4 micrograms vitamin B12 (340 percent DV)
5.8 milligrams manganese (289 percent DV)
76.2 micrograms selenium (109 percent DV)
5.7 milligrams iron (32 percent DV)
242 milligrams phosphorus (24 percent DV)
0.4 milligram riboflavin (21 percent DV)
11.6 milligrams vitamin C (19 percent DV)
0.3 milligram thiamine (17 percent DV)
64.6 micrograms folate (16 percent DV)
2.3 milligrams zinc (15 percent DV)
2.6 milligrams niacin (13 percent DV)

Clams are also packed with protein and one of the best sources of vitamin B12 available. A three-ounce serving of cooked clams contains approximately: (4)

126 calories
4.4 grams carbohydrates
21.7 grams protein
1.7 grams fat
84.1 micrograms vitamin B12 (1,401 percent DV)
23.8 milligrams iron (132 percent DV)
54.4 micrograms selenium (78 percent DV)
0.9 milligram manganese (43 percent DV)
18.8 milligrams vitamin C (31 percent DV)
0.6 milligram copper (29 percent DV)
287 milligrams phosphorus (29 percent DV)
0.4 milligram riboflavin (21 percent DV)
2.3 milligrams zinc (15 percent DV)
534 milligrams potassium (15 percent DV)
2.9 milligrams niacin (14 percent DV)
485 IU vitamin A (10 percent DV)

Finally, scallops contain a good chunk of protein but are lower in many micronutrients than other types of bivalves. A three-ounce serving of cooked scallops contains approximately: (5)

94.2 calories
19.5 grams protein
23.4 micrograms selenium (33 percent DV)
283.8 milligrams phosphorus (27 percent DV)
1.2 micrograms vitamin B12 (18 percent DV)
2.4 milligrams zinc (18 percent DV)
2.4 milligrams iron (15 percent DV)

Potential Bivalve Benefits

1. May Protect Against Anemia

Anemia is a condition caused by a lack of healthy red blood cells in the body, resulting in anemia symptoms like fatigue, pale skin and dizziness. Anemia can stem from chronic disease, blood loss, certain gastrointestinal disorders, and most commonly, a deficiency in important vitamins and minerals involved in red blood cell production.

Most types of bivalves are jam-packed with both vitamin B12 and iron, two nutrients that can help prevent anemia. In fact, even just one serving of clams can meet and exceed your daily requirements for both iron and vitamin B12. Getting enough of these two micronutrients can protect against conditions like iron-deficiency anemia and pernicious anemia to keep your red blood cells healthy and prevent negative symptoms.

2. Lower Risk of Heavy Metal Contamination

One of the common concerns when it comes to eating seafood is an increased risk of exposure to heavy metals like mercury. This is especially common in certain kinds of fish, such as king mackerel, swordfish and tilefish, because they are higher up on the food chain and absorb a higher amount of mercury from their prey.

Mercury poisoning can cause symptoms like numbness, pain, rashes, tremors, memory problems and even death in extreme cases. Consuming a high amount of mercury during pregnancy can also increase the risk of birth defects, such as cognitive deficits, motor difficulties and sensory problems. (6)

Bivalves are considered filter feeders, meaning they’re toward the bottom of the food chain. While many people think this means that they eat the junk that builds up on the bottom of the ocean, such as parasites, feces and decomposing fish, this is actually not the case. So what do bivalves eat? While some types of bivalves are carnivorous, the types that are commonly consumed use their gills to draw in phytoplankton and algae while filtering out larger particles. Because of their unique diet, bivalves are less likely to accumulate heavy metals like mercury, making them a safer option than some other seafood varieties. (7)

3. Good Source of Protein

Certain types of bivalves are considered high-protein foods. Mussels, clams and scallops, in particular, are loaded with protein and can supply up to a whopping 22 grams in a single serving.

Protein plays a central role in nearly every aspect of health. Not only do proteins form the foundation of the skin, hair and nails, but they also function as antibodies to protect against infection, as enzymes to help catalyze chemical reactions in the body and as messenger proteins to transmit signals between cells. (8) Getting enough protein each day is also vital to everything from muscle growth to the regulation of blood pressure. (9, 10)

4. High in Antioxidants

Antioxidants are compounds that work to neutralize harmful free radicals and prevent damage to cells. Research shows that antioxidants can help reduce oxidative stress, minimize inflammation, and lower the risk of chronic conditions like heart disease, cancer and diabetes. (11)

Bivalves are high in several nutrients that act as antioxidants in the body. Selenium, for example, protects against oxidative stress and has been shown to have beneficial effects when it comes to immunity, heart health and cancer prevention. (12) Zinc can also decrease markers of inflammation and reduce the incidence of infection thanks to its antioxidant properties. (13) Additionally, vitamin C has also been shown to act as an antioxidant and can have a powerful effect on disease prevention. (14)

5. Aids in Weight Loss

Bivalves are low in calories but high in protein and essential nutrients, making them an ideal dietary addition if you’re looking to shed a few extra pounds. Mussels, clams and scallops, in particular, are especially rich in protein and can help keep you feeling full to ward off cravings and reduce appetite.

According to a study published in the American Journal of Clinical Nutrition, eating a high-protein meal can reduce levels of ghrelin, the hunger hormone, more effectively than eating a meal high in carbs. (15) Another study out of Seattle showed that increasing protein intake by just 15 percent increased satiety and decreased daily caloric intake by an impressive 441 calories, resulting in significant losses in body weight and fat mass. (16)

Bivalve Dangers and Side Effects

Although adding bivalves to your diet may come with some benefits to health, there are some side effects that need to be considered as well.

First and foremost, those with a shellfish allergy should definitely not be eating bivalves. Shellfish allergies are one of the leading causes of food allergies in the world, affecting an estimated 2.2 percent of adults in the United States. (17)

Common shellfish food allergy symptoms can include itching, hives, swelling, difficulty breathing, abdominal pain and dizziness. If you experience these or any other symptoms after eating bivalves or another type of shellfish, discontinue use immediately and talk to your doctor.

Eating certain types of raw seafood, such as oysters, can also come with some food safety concerns. Certain strains of Vibrio bacteria inhabit coastal regions where oysters are found. Eating raw oysters that have been infected with Vibrio bacteria can cause symptoms like diarrhea, vomiting and skin lesions. Cooking your oysters and bivalves thoroughly is the best way to prevent infection. (18)

Another health concern associated with bivalve consumption is the risk of dangerous toxins that can cause several types of shellfish poisoning. Because bivalves use filter feeding, harmful bacteria and toxins from the algae that they consume can build up in the tissues and be passed onto humans.

The three main categories of shellfish poisoning include paralytic shellfish poisoning (PSP), amnesic shellfish poisoning (ASP) and diarrhetic shellfish poisoning (DSP). These conditions are caused by toxins that can accumulate in bivalves, causing neurological, gastrointestinal and respiratory symptoms that can even be fatal in extreme cases.

Unfortunately, these toxins are not destroyed with cooking, making it all the more important to follow a few basic rules of thumb when eating bivalves. Here are a few guidelines to follow to minimize the potential dangers: (19)

Be sure to only eat bivalves from “open areas,” which are safe harvest areas that are regularly monitored and tested to lower the risk of contamination.
Avoid eating bivalves when traveling to developing countries where food safety standards may not be as high.
Keep bivalves frozen or refrigerated until ready to use.
Only purchase and consume shellfish from trustworthy, reputable suppliers.
Seek medical attention immediately if you feel ill after consuming bivalves.

How to minimize bivalve dangers - Dr. Axe

Bivalves vs. Other Shellfish

Shellfish can be broken up into two main categories: mollusks and crustaceans. Shrimp, crab and lobster fall into the crustacean group of shellfish while bivalves are a type of mollusk.

So what is a mollusk? The mollusk definition incorporates nearly 85,000 species of invertebrate animals belonging to the Mollusca phylum. Some mollusks characteristics include a soft body, an internal or external shell, and a muscular foot that aids in movement. In addition to bivalves, other mollusk examples include slugs, snails, squids, cuttlefish and octopuses. (20)

Crustaceans, on the other hand, have a segmented body and hard exoskeleton as well as three mouthparts, two eyes and two pairs of antennae.

Most people with an allergy to shellfish are allergic to both mollusks and crustaceans. In some cases, you may be able to tolerate some types of shellfish, but doctors generally recommend avoiding all shellfish if you have an allergy.

Bivalves and shellfish share some similarities in terms of nutrition. Like bivalves, most shellfish varieties tend to be low in calories but high in protein as well as micronutrients like selenium, niacin and vitamin B12.

Shellfish are a common dietary staple for many around the world. However, like bivalves, certain types of shellfish can come with significant health concerns.

Shrimp, for example, make the list of health foods you should never eat thanks to their content of additives and extra ingredients as well as the pesticides and chemicals used in industrialized shrimp production. (21) Not only that, but shrimp are considered bottom feeders, meaning they scavenge the ocean floor and feed on the waste that collects there.

Much like raw oysters, other types of uncooked shellfish can also be affected by strains of bacteria, such as Vibrio parahaemolyticus and Vibrio vulnificus. It’s important to cook shellfish thoroughly to minimize your risk of infection.

Bivalve Recipes + Healthier Alternatives

If you’re looking to enjoy your favorite bivalve recipes while minimizing the risk of consuming dangerous toxins and bacteria, have no fear. There are plenty of healthy and safe alternatives available that are equally rich in both nutrients and flavor. In fact, foods like king oyster mushrooms can effectively mimic the taste of scallops and clams while bringing their own dose of hearty health benefits.

Here are a few other ideas of some classic recipes to get you started:

History

Bivalves are believed to have been around for millions of years, with the first appearances of the bivalve fossil dating back to the Cambrian era, around 510 millions years ago. They are thought to have evolved from rostroconchs, a class of extinct mollusks with a single shell and a muscular foot similar to that found in the modern bivalve.

Bivalves have been harvested for consumption for many years. Oysters, for example, have been cultivated in Japan since 2000 B.C. and can be traced back nearly 10,000 years in Australia. Mussels, clams and scallops have also been enjoyed for thousands of years around the world.

Bivalves also hold historical significance in many forms of art, culture and religion. Scallops signify fertility and can be spotted in many prominent paintings, such as Botticelli’s “The Birth of Venus.” Clams, on the other hand, have been worshipped by the Moche civilization in ancient Peru and were even used as a form of currency by the Algonquin Indians.

Precautions

If you have a shellfish allergy, you should avoid consuming bivalves as well as other types of shellfish like shrimp, lobster and crab. A shellfish allergy can trigger symptoms like itching, swelling, abdominal pain and even anaphylaxis.

Additionally, be sure to cook bivalves thoroughly to reduce the risk of certain strains of bacteria, such as Vibrio. If you experience any negative symptoms after eating bivalves, seek medical attention immediately. In addition to bacteria, contaminated bivalves may also contain toxins that can pose serious health problems.

To reduce the risk of adverse side effects, be sure to only consume bivalves harvested from open areas that are regularly inspected, buy from a trusted and reputable supplier, avoid eating bivalves when traveling to developing countries, and keep them refrigerated or frozen until you’re ready to use them.

Final Thoughts

Bivalves belong to the mollusk phylum of animals. Some of the common bivalves characteristics include two shells that protect a soft inner body, tube-like siphons for eating and breathing and a muscular foot that opens and closes the shell.
Although there are over 10,000 species of bivalves, the most common edible types include mussels, clams, oysters and scallops.
Bivalves are low in calories but high in protein, vitamin B12, selenium, zinc and magnesium.
Potential bivalve benefits include protection from anemia, improved weight loss and a lower risk of heavy metal contamination.
However, bivalves can pass on dangerous bacteria and toxins to humans, many of which are not killed off with cooking.
Practicing a few basic rules of thumb is key to consuming bivalves safely and minimizing your risk of infection and negative side effects.