Lesson 14 Learning Objectives 1. Compare and contrast the ...
Lesson 14 Learning Objectives 1. 2. 3. 4.
Compare and contrast the morphology of the brachiopods and bivalves; ✔ Compare and contrast brachiopod and bivalve shell mechanics and feeding efficiency; ✔ Distinguish inarticulate from articulate brachiopods; ✔ List and describe the 4 main modes of life seen in the brachiopods and the 6 main modes of life seen in the bivalves; ✔ 5. Interpret the contrasting biological histories of the bivalves and brachiopods. ✔ • •
The modes of life for the majority of bivalves and brachiopods are independent of reef systems. Bivalves and brachiopods look similar on the outside, but they are very different internally and are not closely related.
Brachiopods • Marine filter feeders, majority of the 30,000 species are now extinct, only about 300 species remain. • Habitat: Ranges from intertidal zone to depths of 6,000 meters, and range from the tropics to the Polar Regions. • Symmetry: Contains two shells/valves, brachiopods are equilateral but inequivalved. This means each valve is symmetric, but the two valves are not mirror images of one another. • Morphology: o Respiration and feeding occurs using an organ called lophophore, it takes up 2/3 of the body cavity. o The rest of the body cavity contains organs responsible for digestion, reproduction, excretion, nervous/sensory, and circulation. o The pedicle is a fleshy stalk that protrudes from the animal used to attach the animal to surfaces. Also have adjustor muscles to reorient itself with respect to the pedicle. o The size of the animal is limited by the size of the lophophore, complex structures called brachidia help support larger lophophores, they are attached to the shell and are shaped like a loop or spiral. • Classification: There are two classes of brachiopods, the Articulata and Inarticulata. The difference between the two is in the system used for opening and closing the valves. o Articulata: Uses a hinge and muscle system, along with teeth and sockets to keep the valves closed. The adductor muscle is responsible for closing the valves, and the diductor muscle is responsible for opening the valves (up to 10°). The cardinal process is a projection at the posterior edge of the dorsal valve where the diductor muscle is attached.
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o Inarticulata: No hinge with a tooth and socket system exists, the valves are held together purely by muscles. To open the valves, muscles squeeze the body cavity, making it bulge out, therefore pushing the valves open (up to 10°). Modes of life: There are four modes of life for brachiopods. Three are epifaunal, including attached, cementing, and free-‐lying; one is infaunal, the burrowing brachiopods. o Attached: Most common mode of life for brachiopods. A pedicle protrudes from a hole on the ventral valve called pedicle foramen, and attaches to the substrate. An important characteristic of epifaunal attached brachiopods is loop-‐shaped brachidia. o Cemented: One valve is cylindrical in shape and cements to the substrate. The second valve forms a cap on top of the cylindrical valve. This group formed an important part of the Permian reef system. End-‐Permian mass extinction eliminated this species. o Free-‐lying: Juveniles are attached to the substrate via pedicles, however, adults are large enough to be able to sit under their own weight. Pedicle is lost and the foramen is sealed off. Their large size (15-‐30cm) requires spiral-‐shaped brachidia to support a larger lophophore. Ordovician to Jurassic. o Burrowing: Only one species in this group, the lingulids. They live in sandy/muddy substrates, burrowing head-‐first, then making a U-‐turn and coming back out the substrate. The animal attaches to the substrate by mucus on its pedicle. Lingulids do not have brachidia, the lophophore has two types of cilia, one for inhaling water and one for exhaling. Cambrian to present. History: o Large, complex, and abundant in Early Cambrian. Inarticulata were dominant until Middle Cambrian. o By Ordovician, Articulata were dominant. Cementing brachiopods appeared in the Devonian. End-‐Devonian mass extinction strongly affected brachiopods. Decline continued into Mississippian and Pennsylvanian. o Permian was a time of moderate diversity, brachiopods became framework organisms for reefs. End-‐Permian extinction exterminated entire families.
Bivalves • Became more prominent after the end-‐Permian extinction. 30,000 species exist today. • Habitat: Mostly marine, one species lives in fresh water. Examples include clams, mussels, and oysters. • Feeding: Majority are filter feeders, some are deposit feeders and some are carnivorous on larger prey. One genus is parasitic, living in the gut of sea cucumbers. • Symmetry: Equivalved and inequilateral. Meaning the two valves are mirror images of one another, but each valve is not symmetrical when a line is drawn down the middle. • Morphology: Bivalves have digestive, reproductive, excretory, nervous, and circulatory systems, in addition, they have a large gill, a foot, and adductor muscles. • Muscle scars: The muscle scars on the shell can tell us about a bivalve’s lifestyle. o Monomyarian: A single adductor muscle scar, generally found in epifaunal groups. o Dimyarian: Two adductor muscle scars, there are two types: Isomyarian and Anisomyarian, generally found in infaunal bivalves.
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Muscle system: Bivalve valves are connected at the top by an elastic ligament made of protein. This ligament naturally keeps bivalve valves open; opening the valves requires no muscles. Teeth and socket occur on both valves to stabilize the shell, and adductor muscles are used to close the valves. Pallial line: A membrane called a mantle surrounds the soft parts, it secretes the ligament, valves, and hinge teeth. Where the membrane attaches to the shell, near the exterior edges of the inside, there is a mark called a pallial line. Bivalve foot: Large, muscular foot that can be used to anchor the animal to the substrate, or move away from threats. Byssal threads can be secreted from the foot to permanently anchor to the substrate. Bivalve gills: Has many folded surfaces, leading to a huge surface area to volume ratio. This makes bivalves very efficient at respiration. Gills are covered with cilia, these generate current drawing water in for gas exchange and also for filter feeding. Modes of life: There are six modes of life for bivalves; four are epifaunal, including bysally-‐ attached, cementing, free-‐lying, and swimming. Two are infaunal. including burrowing and boring bivalves. 1. Bysally-‐attached: Animals are attached to hard substrates using byssal threads. The threads allow the animals to sway in currents, reducing the stress on the shell. They tend to be found in high-‐energy environments. Example includes modern day mussels. 2. Cementing: Bivalves that are cemented to the substrate. Example includes rudist bivalves, one valve was large and conical and cemented to the substrate or another bivalve, the other valve is smaller and forms a cap. Rudist bivalves first appeared in the Jurassic, and were an important framework component in Cretaceous reef systems. Eliminated by the end-‐ Cretaceous mass extinction. Modern day example includes oysters. 3. Free-‐lying: This group thickens the bottom of its shell by secreting calcium carbonate. They have zooxanthellae living in their tissue in a symbiotic relationship. Tridacna Maxima is the largest species, they can weigh up to 225kg and attain a length of 1.2m. They are generally found in shallow waters in a reef setting. Gryphaea is a Jurassic species, also known as “Devil’s toe nails”. 4. Swimming: Thin-‐shelled bivalves that live on the substrate, they can pressurize water internally, and jet-‐propel themselves when the valves are opened. This lifestyle has been around since the Silurian. Example includes scallops. Infaunal bivalves have two siphons that protrude out of the animal and through the sediment, exposed on the ocean floor. These are used to take in food and exhale waste material. When the animal moves, the siphons are retracted and are stored in the pallial sinus. Which is an inward bend in the posterior portion of the pallial line. 5. Burrowing: Tend to have compact, narrow, and elongated shells, which allows them to efficiently burrow into sediment. They burrow by contracting the two adductor muscles, which creates a rocking motion. Burrowing bivalves that have siphons have a permanent opening in the shell, which allows siphons to protrude out even when the valves are closed, this is called a gape. Shallow species tend to lack a siphon. 6. Boring: Tend to have thick shells, they bore into hard substrates using weak acids, or mechanical means by opening/closing the valves, or moving the valves back and forth creating a grinding action. The shell is generally not articulated, a hinge is unnecessary as they live permanently in hard substrates. They tend to live in near-‐ shore, tidal areas.
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History: o First evolved in the Cambrian, but was a minor player during this time. Diversified in the Ordovician, significant presence in the benthic fauna by late Ordovician. o By Permian, bivalves with siphons evolved, this allowed them to burrow into substrates more efficiently. o Triassic was a turning point for bivalves, end-‐Permian extinction provided many niche space for new species. o During Mesozoic, epifaunal and infaunal groups increased in diversity, following radiation of shell crushers and borers like snails and crabs. o Foray into reef framework during Cretaceous, strongly affected by end-‐Cretaceous extinction. Survivors provided major expansion for post-‐Cretaceous and remain highly successful today. Differences between brachiopods and bivalves:
Before the end-‐Permian extinction, brachiopods were more dominant than bivalves. However, 95% of brachiopods went extinct and only 60% of bivalves went extinct. Following the end-‐Permian extinction, and even more so after the end-‐Cretaceous extinction, bivalves became much more successful than brachiopods.
Compare and contrast the morphology of the brachiopods and bivalves; ✔ Compare and contrast brachiopod and bivalve shell mechanics and feeding efficiency; ✔ Distinguish inarticulate from articulate brachiopods; ✔ List and describe the 4 main modes of life seen in the brachiopods and the 6 main modes of life seen in the bivalves; ✔ 5. Interpret the contrasting biological histories of the bivalves and brachiopods. ✔ • •
The modes of life for the majority of bivalves and brachiopods are independent of reef systems. Bivalves and brachiopods look similar on the outside, but they are very different internally and are not closely related.
Brachiopods • Marine filter feeders, majority of the 30,000 species are now extinct, only about 300 species remain. • Habitat: Ranges from intertidal zone to depths of 6,000 meters, and range from the tropics to the Polar Regions. • Symmetry: Contains two shells/valves, brachiopods are equilateral but inequivalved. This means each valve is symmetric, but the two valves are not mirror images of one another. • Morphology: o Respiration and feeding occurs using an organ called lophophore, it takes up 2/3 of the body cavity. o The rest of the body cavity contains organs responsible for digestion, reproduction, excretion, nervous/sensory, and circulation. o The pedicle is a fleshy stalk that protrudes from the animal used to attach the animal to surfaces. Also have adjustor muscles to reorient itself with respect to the pedicle. o The size of the animal is limited by the size of the lophophore, complex structures called brachidia help support larger lophophores, they are attached to the shell and are shaped like a loop or spiral. • Classification: There are two classes of brachiopods, the Articulata and Inarticulata. The difference between the two is in the system used for opening and closing the valves. o Articulata: Uses a hinge and muscle system, along with teeth and sockets to keep the valves closed. The adductor muscle is responsible for closing the valves, and the diductor muscle is responsible for opening the valves (up to 10°). The cardinal process is a projection at the posterior edge of the dorsal valve where the diductor muscle is attached.
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o Inarticulata: No hinge with a tooth and socket system exists, the valves are held together purely by muscles. To open the valves, muscles squeeze the body cavity, making it bulge out, therefore pushing the valves open (up to 10°). Modes of life: There are four modes of life for brachiopods. Three are epifaunal, including attached, cementing, and free-‐lying; one is infaunal, the burrowing brachiopods. o Attached: Most common mode of life for brachiopods. A pedicle protrudes from a hole on the ventral valve called pedicle foramen, and attaches to the substrate. An important characteristic of epifaunal attached brachiopods is loop-‐shaped brachidia. o Cemented: One valve is cylindrical in shape and cements to the substrate. The second valve forms a cap on top of the cylindrical valve. This group formed an important part of the Permian reef system. End-‐Permian mass extinction eliminated this species. o Free-‐lying: Juveniles are attached to the substrate via pedicles, however, adults are large enough to be able to sit under their own weight. Pedicle is lost and the foramen is sealed off. Their large size (15-‐30cm) requires spiral-‐shaped brachidia to support a larger lophophore. Ordovician to Jurassic. o Burrowing: Only one species in this group, the lingulids. They live in sandy/muddy substrates, burrowing head-‐first, then making a U-‐turn and coming back out the substrate. The animal attaches to the substrate by mucus on its pedicle. Lingulids do not have brachidia, the lophophore has two types of cilia, one for inhaling water and one for exhaling. Cambrian to present. History: o Large, complex, and abundant in Early Cambrian. Inarticulata were dominant until Middle Cambrian. o By Ordovician, Articulata were dominant. Cementing brachiopods appeared in the Devonian. End-‐Devonian mass extinction strongly affected brachiopods. Decline continued into Mississippian and Pennsylvanian. o Permian was a time of moderate diversity, brachiopods became framework organisms for reefs. End-‐Permian extinction exterminated entire families.
Bivalves • Became more prominent after the end-‐Permian extinction. 30,000 species exist today. • Habitat: Mostly marine, one species lives in fresh water. Examples include clams, mussels, and oysters. • Feeding: Majority are filter feeders, some are deposit feeders and some are carnivorous on larger prey. One genus is parasitic, living in the gut of sea cucumbers. • Symmetry: Equivalved and inequilateral. Meaning the two valves are mirror images of one another, but each valve is not symmetrical when a line is drawn down the middle. • Morphology: Bivalves have digestive, reproductive, excretory, nervous, and circulatory systems, in addition, they have a large gill, a foot, and adductor muscles. • Muscle scars: The muscle scars on the shell can tell us about a bivalve’s lifestyle. o Monomyarian: A single adductor muscle scar, generally found in epifaunal groups. o Dimyarian: Two adductor muscle scars, there are two types: Isomyarian and Anisomyarian, generally found in infaunal bivalves.
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Muscle system: Bivalve valves are connected at the top by an elastic ligament made of protein. This ligament naturally keeps bivalve valves open; opening the valves requires no muscles. Teeth and socket occur on both valves to stabilize the shell, and adductor muscles are used to close the valves. Pallial line: A membrane called a mantle surrounds the soft parts, it secretes the ligament, valves, and hinge teeth. Where the membrane attaches to the shell, near the exterior edges of the inside, there is a mark called a pallial line. Bivalve foot: Large, muscular foot that can be used to anchor the animal to the substrate, or move away from threats. Byssal threads can be secreted from the foot to permanently anchor to the substrate. Bivalve gills: Has many folded surfaces, leading to a huge surface area to volume ratio. This makes bivalves very efficient at respiration. Gills are covered with cilia, these generate current drawing water in for gas exchange and also for filter feeding. Modes of life: There are six modes of life for bivalves; four are epifaunal, including bysally-‐ attached, cementing, free-‐lying, and swimming. Two are infaunal. including burrowing and boring bivalves. 1. Bysally-‐attached: Animals are attached to hard substrates using byssal threads. The threads allow the animals to sway in currents, reducing the stress on the shell. They tend to be found in high-‐energy environments. Example includes modern day mussels. 2. Cementing: Bivalves that are cemented to the substrate. Example includes rudist bivalves, one valve was large and conical and cemented to the substrate or another bivalve, the other valve is smaller and forms a cap. Rudist bivalves first appeared in the Jurassic, and were an important framework component in Cretaceous reef systems. Eliminated by the end-‐ Cretaceous mass extinction. Modern day example includes oysters. 3. Free-‐lying: This group thickens the bottom of its shell by secreting calcium carbonate. They have zooxanthellae living in their tissue in a symbiotic relationship. Tridacna Maxima is the largest species, they can weigh up to 225kg and attain a length of 1.2m. They are generally found in shallow waters in a reef setting. Gryphaea is a Jurassic species, also known as “Devil’s toe nails”. 4. Swimming: Thin-‐shelled bivalves that live on the substrate, they can pressurize water internally, and jet-‐propel themselves when the valves are opened. This lifestyle has been around since the Silurian. Example includes scallops. Infaunal bivalves have two siphons that protrude out of the animal and through the sediment, exposed on the ocean floor. These are used to take in food and exhale waste material. When the animal moves, the siphons are retracted and are stored in the pallial sinus. Which is an inward bend in the posterior portion of the pallial line. 5. Burrowing: Tend to have compact, narrow, and elongated shells, which allows them to efficiently burrow into sediment. They burrow by contracting the two adductor muscles, which creates a rocking motion. Burrowing bivalves that have siphons have a permanent opening in the shell, which allows siphons to protrude out even when the valves are closed, this is called a gape. Shallow species tend to lack a siphon. 6. Boring: Tend to have thick shells, they bore into hard substrates using weak acids, or mechanical means by opening/closing the valves, or moving the valves back and forth creating a grinding action. The shell is generally not articulated, a hinge is unnecessary as they live permanently in hard substrates. They tend to live in near-‐ shore, tidal areas.
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History: o First evolved in the Cambrian, but was a minor player during this time. Diversified in the Ordovician, significant presence in the benthic fauna by late Ordovician. o By Permian, bivalves with siphons evolved, this allowed them to burrow into substrates more efficiently. o Triassic was a turning point for bivalves, end-‐Permian extinction provided many niche space for new species. o During Mesozoic, epifaunal and infaunal groups increased in diversity, following radiation of shell crushers and borers like snails and crabs. o Foray into reef framework during Cretaceous, strongly affected by end-‐Cretaceous extinction. Survivors provided major expansion for post-‐Cretaceous and remain highly successful today. Differences between brachiopods and bivalves:
Before the end-‐Permian extinction, brachiopods were more dominant than bivalves. However, 95% of brachiopods went extinct and only 60% of bivalves went extinct. Following the end-‐Permian extinction, and even more so after the end-‐Cretaceous extinction, bivalves became much more successful than brachiopods.
