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Round 2 - Mollusca - Bivalvia

(Sources - 1, 2, 3, 4)

Bivalvia is a class of molluscs whose bodies are enclosed by a pair of half-shells called valves, though some bivalves, like the Naked Clam (Chlamydoconcha orcutti) (image 2) have secondarily lost their shells. Bivalves have no head and no radula. Their gills have evolved into ctenidia, specialised organs for feeding and breathing. Bivalvia includes the clades Heteroconchia, Palaeoheterodonta, Protobranchia, Pteriomorphia, and animals commonly known as clams, oysters, cockles, mussels, and scallops.

Bivalves live in marine and freshwater environments. Most are filter feeders that bury themselves in sediment, lie on their side on the seafloor, or attach themselves to rocks or other hard surfaces. Some bivalves, such as scallops and file shells, can swim (see gif below). Shipworms bore into wood, clay, or stone and live inside these substances. Bivalve shells are composed of two calcareous valves joined along one edge by a flexible ligament that, usually in conjunction with interlocking "teeth" on each of the valves, forms the hinge, allowing the animal to open and close its shell. The animal secrets its shell from lobes on its mantle. They have a foot located at the front of their shell and two siphons in the back, which inhale and expell water. The shipworms, of the family Teredinidae, have elongated bodies but tiny, reduced shell valves, which function as scraping organs that permit the animal to dig tunnels through wood. Bivalves have sensory organs located on the margins of their mantle, usually mechanoreceptors or chemoreceptors, sometimes on short tentacles. All bivalves have light-sensitive cells that can detect a shadow falling over the animal, some have simple eyes on the margin of the mantle, and scallops have complex eyes with a lens, a two-layered retina, and a concave mirror. Most bivalves are filter feeders, using their gills to capture particles of food such as phytoplankton from the water. Protobranchs feed in a different way, scraping detritus from the seabed with mucus-covered tentacles. A few bivalves, such as the Granular Poromya (Poromya granulata), are carnivorous, eating larger prey like small crustaceans, though they will also scavenge. It does this though its inhalant siphon which is modified into a cowl-shaped organ, sucking in prey, and then inverting to bring the prey within reach of the mouth.

Most bivalves have separate sexes, though some are hermaphroditic. Fertilization is external in most species. Spawning may take place continually or be triggered by environmental factors such as day length, water temperature, or the presence of sperm in the water. Eggs hatch into free-swimming, planktonic trochophore larvae. These later develop into veliger larvae which settle on the seabed and undergo metamorphosis into adults. In some species, such as those in the genus Lasaea, females draw sperm in through their inhalant siphons and fertilize their eggs inside their bodies. These species then brood the young inside their mantle cavity, eventually releasing them into the water column as veliger or glochidia larvae or as crawl-away juveniles. The juveniles of freshwater bivalves will attach themselves parasitically to the gills or fins of a fish host. After several weeks they drop off their host, undergo metamorphosis and develop into adults on the substrate.

Bivalves first appear in the fossil record in the Early Cambrian. Possible early bivalves include Pojetaia and Fordilla, though these are probably stem-bivalves. True Cambrian bivalves may include Camya, Arhouriella, and Buluniella. Bivalves began to diversify during the Early Ordovician. By the Early Silurian, gills were adapting for filter-feeding, and during the Devonian and Carboniferous periods, siphons first appeared along with the newly developed muscular foot. At this point Brachiopods were still the most dominant filter-feeders in the ocean, but the Permian–Triassic extinction event hit both brachiopods and bivalves hard, but resulted in bivalves becoming the more common filter-feeders by the Triassic Period.

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Voting ended onDec 17, 2024

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📍 Location: Nags Head, North Carolina 🗓 Date: May 24, 2025 🐾 Media: Image 🌿 Species: Atlantic Calico Scallop (Argopecten gibbus) 📝 Notes: Small marine bivalve found in sandy or shelly substrates in shallow coastal waters of the western Atlantic. Shell is fan-shaped with radiating ribs and variable coloration, often showing mottled patterns of orange, red, purple, and white.

Unlike many bivalves, scallops are capable of limited swimming by rapidly clapping their shells together to create jet propulsion, allowing short bursts of movement when disturbed.

🔗 iNaturalist: https://www.inaturalist.org/observations/283707546

He devoted many pages to the flatfish of the North Sea – less lucrative than the herring, but also good eating. Sections like this are devoted to all manner of sea life: octopuses, squid, eels, scallops, shrimp, sardines, cod, salmon, trout, turtles . . . if it swam, dived, or drifted, Coenen wanted to know.