Clams: Characteristics, Behavior, Mating and Unusual, 500-Year-Old Ones

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A) blood ark clam (Anadara ovalis)
B) West Indian cardita (Carditamera gracilis)
C) leafy jewel box clam (Chama macerophylla)

Clams are bivalve mollusks with two shells held together by a hinged joint and ligament. They have a soft body and siphon, which they use to filter-feed from the water. Their shell can open and close. Clams can be divided into two types: hard-shell and soft-shell. Hard-shell clams are smaller and have thicker shells that are harder to open. Soft-shell clams are usually larger and have a thin, brittle outer shell. Clams play a vital role in ecosystems they inhabit and an essential link in the marine food chain. They help to provide a source of food for other animals. Clams are a biological filter of the sea, helping to keep our waters clean and healthy. [Source: American Oceans]

Clam is a common name for several kinds of bivalve molluscs that live most of their lives buried halfway in the sand of the seafloor or riverbeds. Sometimes the word is often applied only to those that are edible. Clams have two shells of equal size connected by two adductor muscles and have a powerful burrowing foot. They live in both freshwater and marine environments; in salt water they prefer to burrow down into the mud and the turbidity of the water required varies with species and location; the greatest diversity of these is in North America. [Source: Wikipedia]

Clams do not live attached to a substrate like oysters, scallops and mussels and do not move around on the sea bottom like scallops do. Some clams have life cycles of only one year, while others live for hundreds of years. All clams have two calcareous shells or valves joined near a hinge with a flexible ligament and all are filter feeders. Because they feed directly on the abundant material floating in seawater clams can form colonies of massive size and density, especially in inner bays where the sand and mud substrates they like are abundant. They also grow fast. [Source: Kevin Short, Daily Yomiuri]

There are more than 400 species of clam. Like oysters they can produce pearls by compressing sand for long periods. However, clams are more unlikely to produce pearls than oysters and generally their pearls are not as pretty.. Only about 1 in 5,000 clams can produce a pearl of any noticeable size.

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal

Clam Characteristics

Like oysters, mussels, and scallops clams are bivalve mollusks, and have a hinged shell. They have slow growth rates and can live 12 to 20 years on average and up to 40 years. The outer concentric growth rings on the shell can be counted to determine a clam’s age. Clam are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature) and bilateral symmetry (both sides of the animal are the same).

Sphenoceramus steenstrupi, an 83 million years old clam that was 1.83 meters long

Quahogs are very common type of clam found in the U.S. Their thick shells are ridged and grayish green to white in color. Adult hard clams are just less than 7.5 centimeters (3 inches) but can reach up to 12.5 centimeters (5 inches). The inside of the shell is white with violet markings.

According to Animal Diversity Web: Northern quahogs has a fairly large and thick shell with uneven, elevated hinges on the anterior . Both shell halves are approximately even in size and sub ovate, or triangular in shape . The shell is increased by a daily layering of aragonite secreted by the outermost fold of the organism, giving it numerous concentric lines that are closely space near the margins of the shell and widely spaced at the umboes. Its shell is a composition of proteins and calcium carbonate . The shell is joined at a hinge called the umbo and is held closed by two pairs of adductor muscles located on each side of the shell. The clam opens its shell by relaxing the adductor muscles and contracting a pair of ligaments located on each side of the umbo . [Source: Bradford Burdette, Animal Diversity Web (ADW) /=]

Northern quahogs contain three well-developed teeth located on the edge of the shell, which serve to enhance its tightness when closed . The external color is dirty white or gray, while the interior is usually white with distinct violet areas near the umbo. The clam contains a foot, which allows it to burrow into the sand . The clam also contains a set of long siphons, which stretch from the clam's mantle, the membranous sac that contains the internal organs and constitutes the body of the clam, to the surrounding medium outside of the shell . It uses these siphons for respiration and gathering food.

Clam Shells

Clam shells can be found on many beaches and coastlines. They are sometimes used as decorative pieces in homes and offices and as jewelry. Clamshells also have a long history. Indigenous cultures and ancient peoples who used them for tools, weapons, utensils, and jewelry. Clam shells come in a wide range of sizes, colors, and patterns. Common types and kinds with interesting shells include giant clams, bear paw clams, whole pearl clams, Cardium heart,s jumbo arks, and heavy cockles. [Source: Heather Hall, AZ Animals, December 27, 2022]

A clam's shell consists of two (usually equal) valves, which are connected by a hinge joint that can be internal or external. The outline of the shell is more oval or circular in shape than mussel shells. Among the most common and recognizable clam shells are from Quahog clams, Manila clams, Atlantic razor clams, Cockle clams, Pacific Razors and Geoduck clams. These are edible. [Source: Nicholas Argent, Citrus Reef]

Coquina clams (Donax variabilis) are valued by shell collectors. They are known for their highly variable color patterns, which include shades of blue, white, pink, yellow and or mauve. These shells can be found buried just under the surface of the sand in swash zone (wave-swept area) of beaches worldwide as well as underwater. A typical coquina, measures only about one to 2.5 centimeters (0.4- to one inch). The animals in these shells are very active, migrating up and down the beaches.

The clam shell can be a symbol of Mary in Christianity, and means the divine conception of Jesus Christ in her body. [Source: Father Johann Roten, S.M., Daton University]

Clam Feeding and Behavior

modified from Markus Ruchter

Clams burrow into the sediment, leaving only their siphons exposed to feed. Adults are sessile (fixed in one place) and inhabit both intertidal and subtidal areas. They prefer saline water and cannot survive if the salt content is too low.

According to Animal Diversity Web: Clams are suspension feeders which means that they feed on small plants and animals called plankton which are drawn in with water. When the clam buries itself under a layer of silt and mud it sticks its siphons straight up through the surrounding muck. The inhalant siphon draws in water, which is passed over the gills. Millions of tiny cilia, hair-like structures, move the water across the gills and any food particles are caught in a mucous sheet that coats the gills. This food-mucous mixture is passed along a groove above the foot to a pair of muscles called the palps, which force the material into the mouth . It then follows the digestive tract consisting of a stomach, intestine, and anus to be excreted through the exhalent or excurrent siphons, as pseudofeces. .[Source: Bradford Burdette, Animal Diversity Web (ADW) /=]

Due to the fact that many clams spend their lives in an immobile and isolated state, their behavior isn't that extensive. In case of danger they are capable of retracting their siphons and some clams have been known to migrate small distances. They are capable of burrowing deep under the sediment layers using their foot structure. Clams are preyed upon by such organisms as oyster drills, moon snails, whelks, drums, skates, pufferfish, rays, and certain water fowl

200-Year-Old Clams

Business Insider also reported: “A Florida man and his family found a gigantic clam at Alligator Point, Florida. They were planning to cook it but realized it was more than 200 years old. Blaine Parker told the Tallahassee Democrat that when he found the clam over Presidents' Day weekend, he thought it was just big enough for two servings of chowder. "We were just going to eat it, but we thought about it a while and figured it was probably pretty special. So, we didn't want to kill it," Parker told the Tallahassee Democrat. [Source: Cheryl The, Business Insider, March 3, 2023]

Parker, a member of the volunteer group AmeriCorps, ended up bringing the clam to the Gulf Specimen Marine Lab in Panacea, Florida. Parker is also a specimen collector for the lab, the Tallahassee Democrat reported. The lab realized Parker's find was a six-inch, 2.6-pound clam, estimated to be more than 214 years old. Realizing how old it was, Parker named the clam Aber-clam Lincoln. "Age can be calculated by the number of layers on the shell, with each layer representing a year; with this, Blaine counted 214 layers on Aber-clam Lincoln's shell, meaning this clam was born in 1809, the same year as Abraham Lincoln, hence its name!" the lab wrote in a February 21 post on Facebook.

The lab added that most Ocean quahog clams weigh around half a pound. This makes Aber-clam Lincoln five times the weight of an average clam. The Tallahassee Democrat separately reported on February 28 that Parker had sent Aber-clam Lincoln back to his home under the sea. Parker released the clam back into the Gulf of Mexico about a week after he found it at Alligator Point.

Ming the 500 Year-Old Clam

Ming the Clam

An clam named Ming lived to be over 500 years old. According to Business Insider: Ocean quahogs, a type of clam, typically live 100 to 200 years. One of these clams, nicknamed Ming, was 507 years old when it was found in 2006 off the coast of Iceland. To calculate Ming's age, researchers counted the bands in its shell. Ming happened to be part of a group of 200 clams that were dredged from the ocean and frozen as part of a research project about climate change. It was nicknamed "Ming" because it was estimated to have been born in 1499, during the Ming Dynasty in China. [Source: Zoë Miller,Azmi Haroun, Business Insider, December 25, 2022]

Steven Austad, a biology professor at the University of Alabama at Birmingham, told Live Science: "Mollusks like Ming spend most of their lives living in really cold water burrowed in the mud and covered by a thick shell," Austad told Live Science. "Living at the bottom of the ocean is very stable, and being in the mud probably adds a layer of safety as well as having a shell." [Source: Jennifer Nalewicki, Live Science, March 31, 2023]

Austad suspects that when animals aren't prone to predation or to the vagaries of a harsh or chaotic environment, evolution favors physiology that lasts a long time. Bivalves also draw heat from their surroundings rather than generating it themselves the way humans and other mammals do. This may lead to creatures like Ming that are better protected from oxidative stress, Austad hypothesized. (Oxidative stress, or damage to tissue by chemically reactive oxygen compounds, has long been linked to aging.)

To test his theory, Austad and his students brought a variety of mollusk species into their lab, including bay scallops (Argopecten irradians), which live an average of two years; table clams, which can live up to a century; and a handful of super-aging ocean quahogs like Ming, and introduced oxygen-radical generating chemicals to their tanks. The scallops succumbed within two days, while the table clams held on for 11. Two weeks into the experiment, the quahogs remained "happy as a clam" despite living in tainted water, Austad said. This suggested that the quahogs were recovering from or preventing oxidative stress. "As humans, we can't replicate their living conditions, but we can figure out how they do it," Austad said. "[There are] no doubt some genetic tricks, but it could also be something that we could replicate pharmacologically if we understood it well enough."

Clam Living Inside the Esophagus of a Sea Cucumber

Entovalva lessonothuriae is a clam species with an an extraordinary life style: it lives inside the esophagus of the sea cucumber, Holothuria pardalis. These sea cucumbers are normally found in sand under boulders on coral-reef flats within the intertidal zone. The clams have been found nowhere else, they seem to occur only in this host species. Entovalva lessonothuriae is only known to live around Japan. It was first discovered living in Kasari Bay on Amami Island and in Urazoko Bay on Ishigaki Island in the Ryukyu Archipelago near Okinawa. They range in length from around 0.7 to 3.7 millimeters. [Source: Jingchun Li, Animal Diversity Web (ADW) /=]

The relationship between the clam and the sea cucumber is described as endosymbiont rather than parasitic because the clam does not harm its host. All species in the genus Entovalva seem to live inside other organisms. Entovalva nhatrangensis is a species of small marine bivalve mollusc, first described in 2010, that was originally found in Nha Trang Bay in Vietnam living in a species of sea cucumber there

From Barnegat Bay Shellfish, Assateague Naturalist

According to Animal Diversity Web: Species in the genus Entovalva all have very distinct morphologies compared to other bivalve species, presumably because of adaptations to an endosymbiotic life style. Their shells are small, thin and completely enclosed in expanded mantle tissue. The gills are well developed and extend outside the shell. Entovalva lessonothuriae has a bilaterally flattened foot that bends to the left. This is thought to be an adaptation for attachment to the host holothurian's esophagus. The bivalve has also retained autonomous locomotion, respiration and feeding abilities, which is unusual compared to other endosymbiotic taxa.

How Clam Eats and Reproduces Inside the Sea Cucumber

Entovalva lessonothuriae’s well-developed ctenidia (“gills”) suggest that Entovalva lessonothuriae is still a filter feeder, even though it lives within the digestive tract of its host holothurian. Specific predators of this clam are unknown. Its endosymbiotic life style probably protects it from many predators, though also makes it vulnerable to predators that attack its host. In Kato 1998's study, 19 percent to 33 percent of the holothurian hosts examined harbored Entovalva lessonothuriae and did not contain any other symbionts. This suggests that Entovalva lessonothuriae might discourage competitors, but no other evidence of this has been discovered. It is not known whether Entovalva lessonothuriae has any effect on the host's nutrition. [Source: Jingchun Li, Animal Diversity Web (ADW) /=]

Entovalva lessonothuriae are ovoviviparous, meaning that eggs are hatched within the body of the parent, and engage in internal reproduction in which sperm from the male fertilizes the egg within the female. The species is a brooder — the female keeps fertilized eggs inside her mantle cavity (suprabranchial chamber) until they develop into veliger larvae. Her mantle cavity is greatly enlarged so it can be used to brood larvae. It's very likely that individuals communicate chemically, so that males and females can locate each other within a host.

This species is a sequential protandric hermaphrodite, meaning they tart out reproducing as males and later reproduce in both sexes. These clams are mostly found in pairs inside one host and consist of one big female and a small male. The male is typically found attached to the female clam, not the host sea cucumber, while the female is attached to the sea cucumber. When only one individual is present in the host, that individual is always a female.

The fertilized eggs are brooded in a suprabranchial chamber of the female until they develop into D-shell veliger stage larvae. The larvae are then released and are thought to pass through the host's intestine and out. To survive they must then be ingested by a new host during the host's deposit-feeding activities.

Calyptogena magnifica

Deep Sea Clams That Live at Hydrothermal Vents

Discovered in 1977, Calyptogena magnifica are clams that lives in deep-sea hydrothermal vent areas in the East Pacific Rise and the Galapagos Rift. They thrive in rich ecosystem surrounding the vents. Using their foot and byssal threads as anchors, they group themselves together in clumps — known as “clambakes” — in the crevices of basalt on the ocean floor. Within the cracks, they are exposed to warm, carbon-dioxide- and hydrogen-sulfide-rich vent water that is about 10-15̊C (50-60̊F).

The oceanic vents have an average depth of around 2000 meters (6562 feet). There is no sunlight here and the pressure is over a thousand atmospheres. The soft body mass of the animal experiences a temperature of about 2-4̊ C (36-39̊F), due to its location above the foot. The siphons of the clam are used to tap into the ambient oxygen and carbon dioxide above the valves. Population density increases with increasing concentrations of hydrogen sulfide. Calyptogena magnifica individuals have lifespans between 3.5 to approximately 25 years. The hydrothermal vents where they anchor themselves last only a few dozen years, thus strong colonization abilities are favored over longevity since adults stay in one place.

Elizabeth Kolbert wrote in The New Yorker: “Some of the seas’ most extraordinary animals live around hydrothermal vents — the oceanic equivalents of hot springs. Through cracks in the seafloor, water comes in contact with the earth’s magma; the process leaves it superheated and loaded with dissolved minerals. (At some vents, the water reaches a temperature of more than seven hundred degrees.) As the water rises and cools, the minerals precipitate out to form crenellated, castlelike structures.” Animals that live there are “fundamentally different from other creatures. At the bottom of the vents’ food chains are microbes that have come up with their own novel survival strategy. Instead of using photosynthesis, which harnesses the energy of photons, they rely on chemosynthesis, which uses the energy stored in chemical bonds. [Source: Elizabeth Kolbert, The New Yorker, June 14, 2021]

Deep Sea Vent Clam Characteristics

Calyptogena magnifica are heterodont bivalves (bivalves joined together by “hinge teeth,” strong and flexible ligaments situated on the hinge line the shells). They range in length from 10 to 26 centimeters (four to 10 inches). They are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature) and heterothermic (have a body temperature that fluctuates with the surrounding environment)

Calyptogena magnifica for the most part are sessile (fixed in one place) and sedentary (remain in the same area). They are marginally motile (can move around). The valves are white and fairly elliptical in shape, and individuals grow an average of four centimeters per year. Typically an individual’s foot is deep in the mud and the siphon extends out toward the overlying water.

Calyptogena magnifica development is characterized by metamorphosis and indeterminate growth (they continue growing throughout their lives). arvae are lecithotrophic (feeding on egg yolk or other materials put in the egg by the mother), and nonplanktonic. Larvae are free-swimming and rely solely on their stored yolk reserves for energy. Once contacting a substrate, the larvae metamorphoses into the adult form. These clams have indeterminate growth (they continue growing throughout their lives),

Deep Sea Vent Clam Mating and Reproduction

Calyptogena magnifica engage in year-round breeding and employ internal reproduction in which sperm from the male parent fertilizes an egg from the female parent. Females and males both start to reproduce at one to 4 years when they reach around six centimeters in lenth. There is some pre-fertilization provisioning. The yolk reserves of the larvae are the only known maternal support the offspring receive.

Calyptogena magnifica are polygynandrous (promiscuous), with both males and females having multiple partners. Once sexually mature, the clam’s gametes are released into the environment continuously and in small numbers by all individuals. Egg cells range from 105-195 micrometers, and the heads of sperm cells are about three micrometers in diameter. Fertilization results from any successful union of an egg cell and a sperm cell. Therefore, mating is random and results in high gene flow and genetic variability.

When the clams reach 9-10 centimeters in length their gonads start filling. Complete sexual maturity is achieved at 12-14 centimeters. Spawning is continuous at least in part because environmental stimuli are largely absent. Large clams remain almost always sexually active as only a small proportion of gametes are released at any one time. Although dispersal distance is limited, these clams no problem effectively dispersing gametes.

Calyptogena magnifica communicate with chemicals usually detected by smelling and sense using infrared and heat, touch and chemicals usually detected with smelling or smelling-like senses. According to Animal Diversity Web: Calyptogena magnifica have synchronized but not always consistent release of sperm and eggs. The males detect an increase in water temperature and release sperm through their exhalant siphons. In response, the females release eggs from their exhalant siphons when a threshold of sperm or associated chemicals is detected. However, the water current must be slow for the females to detect the high concentration of chemical cues. If either or both conditions are not met, the females will not release their eggs. The neurotransmitter serotonin is commonly responsible for the stimulation of the release and re-initiation of meiosis in the oocyctes and may be one of the chemical cues responsible for egg release in C. magnifica. Calyptogena magnifica has an inhalant siphon that is used to sense the chemical environment from the incoming flow of water. /=\

How Deep Sea Clams Live at Hydrothermal Vents

Calyptogena magnifica have a mutualistic relationship with the sulfur-metabolizing bacteria located on their gill tissue, foot, and mantle. They depends on these sulfur-metabolizing bacteria to receive its nutrients from the rich hydrogen sulfide environment of the hydrothermal vent. According to Animal Diversity Web: The bacteria located on the outer layer of the foot and mantle also provide peripheral defense by detoxifying the sulfide as soon as it enters the body. High molecular weight factors in the blood bind the sulfide tightly to extract the sulfide from the environment. The sulfide is then transported to the bacterial symbiont via circulation. As a result, low concentration of free sulfide is found in the blood, protecting the aerobic respiration of the organisms from poisoning by sulfide due to its sensitivity to cytochrome-c oxidase and the enzymes involved in carbon fixation in the symbiont. (Powell and Somero, 1986) /=\

Calyptogena magnifica suspension-feed on particles rich in nitrogen and lipid compounds present in the hydrothermal fluid. It also receives nutrients through a symbiotic relationship with sulfur-metabolizing bacteria that are located on the outer region of its gill tissue. An individual’s single foot is highly vascularized and extends; it functions in taking in sulfide and transporting it through the blood to the sulfur-metabolizing bacteria.

Unique features that help this clam survive in deep sea vents includes the communities of sulfur-oxidizing chemolithotrophic bacterial symbionts within the gills and other tissues. According to Animal Diversity Web: The bacteria produce organic carbon and nitrogen, which serve as nutrition for the clam. As a result, the digestive system and labial palps of the bivalve are extremely reduced, and the foot and gills are highly vascularized to better facilitate gas exchange and hydrogen sulfide uptake. The visceral mass is a conspicuous red color due to intracellular hemoglobin, and the circulatory system is about 44 percent of the clam’s weight. /=\

Direct predation of Calyptogena magnifica has been observed by mobile grazers such as small gastropods, amphipods, and crabs. These predators consume newly settled larvae and juveniles. At the same time, indirect predation has also been observed by the removal of bacterial film on rocks by these mobile grazers. Without the bacterial film on rocks, larvae lose the marker that indicates where to settle during development.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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