Bivalves PDF

GROUP 9 BIVALVES Phylum Mollusca CASABAR, DELA CRUZ, MADARANG, MASCARDO LEARNING OBJECTIVES Identify the distinguishing features of Bivalves Understand the basic anatomy and physiology of Bivalves Outline the life cycle of a bivalve including their feeding and reproduction  OVERVIEW B...

GROUP 9 BIVALVES Phylum Mollusca CASABAR, DELA CRUZ, MADARANG, MASCARDO LEARNING OBJECTIVES Identify the distinguishing features of Bivalves Understand the basic anatomy and physiology of Bivalves Outline the life cycle of a bivalve including their feeding and reproduction  OVERVIEW Bivalves (also known as “pelecypods” or, in older literature, “lamellibranchs”) are a very diverse and abundant group of mollusks. They live today throughout the world’s oceans and fresh waters, where they are of major ecological importance as a food source for other organisms and for their water-filtering capabilities. Bivalves are also of major economic importance to humans, as sources of food and other products, and as damaging invasive species. GENERAL CHARACTERISTICS 1. Shell: Bivalves possess a shell composed of calcium carbonate, a common feature among many mollusks. 2. Mantle: The mantle, a sheet of tissue covering the body, is the only major feature shared by all mollusks. In bivalves, it covers the viscera and gills and secretes the shell. 3. Head: Unlike other mollusks, bivalves lack a distinct head with associated organs such as tentacles and a radula. 4. Valves: Bivalves have two shells, or valves, that enclose their laterally compressed soft body. These valves are typically connected by a hinge and a ligament. 5. Gills: Bivalves possess lamellar gills for respiration, which are also used for feeding in many species. 6. Habitat: All bivalves are aquatic organisms, living in or near water. 1ST REPRESENTATIVE ORGANISM Northern quahog or cherry stone Mercenaria mercenaria Phylum: Mollusca Class: Veneroida Superfamily: Veneroidea Family: Veneridae Genus: Mercenaria ANATOMY Body is entirely enclosed within a mantle that secretes a calcium carbonate shell Divided into lateral lobes, covering the foot and visceral mass; connected to the shell by pallial muscles Valves, consisting of two shells, are connected by an elastic hinge - ligament that allow movement when adductor muscles relax made of calcium carbonate (CaCo3) in the form of minerals aragonite and/or calcite Meeting of two valves is called commissure or shell margin Umbo is located near the hinge and the earliest-formed part of the valve that is knob-like and sometimes-pointed; features concentric growth lines (“break”; umbones - plural) Two siphons (excurrent and incurrent) facilitate water intake for respiration and feeding; one brings water and the other expels filtered waste Separate sexes; has specialized reproductive organs to produce eggs and sperm Younger quahogs can produce byssus threads for attaching to surfaces this capability decreases with age PHYSIOLOGY Filter feeders; incoming water passes over the gills where food particles are trapped and transported to the mouth Gils are important for oxygen absorption and waste expulsion by water flow through the siphons Shell growth happens through the deposition of calcium carbonate crystals on an organic matrix during aerobic respiration Has a rudimentary system that consist of ganglia and nerve cords that coordinate body functions and movements REPRODUCTION Spawning Occurs annually in spring, summer, or fall. Stimulated by water temperatures above 79°F (26°C). Egg Production Each female can spawn between 1-5 million eggs during a single event (some producing up to 40 million eggs annually) Fertilization Externally in the water column after sperm is released by males Larval Development Lasts up to 8 to 15 days, with larvae achieving maximum sizes of 200 to 275 microns before settling. LIFE CYCLE Larval Stage Eggs hatch into veliger larvae after around 24 hours; swim freely and feed until they settle to the bottom when they develop their shells Settling During the 6th to 10th day; larvae develop into a stage where they settle and choose a substrate (usually a mix of sand and mud for burrowing) Juvenile Development They maintain their limited mobility and grow until they reach commercial harvest size (after settling) 2ND REPRESENTATIVE ORGANISM Blue mussel Mytilus edulis Phylum: Mollusca Class: Bivalvia Family: Mytilidae Genus: Mytilus  ANATOMY & PHYSIOLOGY Anatomy: 1. The most prominent feature is the two-part shell made of calcium carbonate. The upper valve is slightly convex, while the lower valve is flatter. 2. The mantle is a thin, muscular membrane that lines the inside of the shell and secretes it. It also houses the gills, heart, and other organs. 3. The foot is used for both locomotion and to secrete and lay down the byssus. 4. The gills are feathery structures that extend from the mantle cavity. They are used for both respiration and feeding. 5. The entire body is enclosed by a two-valved shell which is closed and opened as a result of the antagonistic action of the adductor muscles and elastic hinge. Physiology: 1. Mytilus edulis is a filter feeder. It pumps water through its gills, trapping suspended particles such as phytoplankton and detritus. These particles are then transported to the mouth and ingested. 2. The gills are also used for respiration. Oxygen is absorbed from the water and carbon dioxide is released. 3. Mytilus edulis is an osmoconformer, meaning its internal body fluids are in osmotic equilibrium with the surrounding seawater. This allows it to survive in a wide range of salinities. 4. Mytilus edulis is a dioecious species, meaning individuals are either male or female. Fertilization is external, and the larvae develop into free-swimming veligers before settling to the bottom and undergoing metamorphosis.  LIFE CYCLE Trochophore The ciliated embryo develops into a freeswimming trochophore (70–75 μM) in about 12 h (24 hpf). At this stage, the larva forms a mouth and a shell gland Early Veliger the early veliger (75–100 μM) forms a shell rudiment. By 60 hpf, a shell is formed giving the larva a ‘D’-shape Mid-Veliger the larva is 100–150 μM long. The body of the larva now possesses a distinct, disk-shaped velum, which is ciliated along its edge. Using its velum, the D-stage larva swims in a helical ascent followed by intermittent passive sinking. At this stage, the larva has rudimentary gills while the mouth, esophagus, stomach, intestine and digestive gland are well developed.  LIFE CYCLE Late Veliger larva grows substantially to reach lengths of about 150–200 μM, it is characterized by a pronounced umbo on the larval shell. The mantle is spanned over the length of the anterior edge of the animal. The velum stops growing and gradually begins to reduce in size and shift anteriorly. The three pairs of striated muscle velum retractors and shell anterior adductor are developed Pediveliger Prior to metamorphosis, free-swimming veligers form a pair of pigmented eye spots and an elongated foot with a byssal gland. This stage (after 20 dpf) is called a pediveliger (200–250 μM), the onset of the stage demonstrates that metamorphosis is about to start. This stage commences when the foot is fully developed and the animal begins to sink and crawl. After metamorphosis, the posterior smooth adductor and functional gills are already well-developed. After the pediveliger settles (the early juvenile stage), the velum gradually degenerates.  REFERENCES Blue Mussel (Mytilus edulis). (n.d.). INaturalist. https://www.inaturalist.org/taxa/117650-Mytilus- edulis Bivalvia | Digital Atlas of Ancient Life. (n.d.). https://www.digitalatlasofancientlife.org/learn/mollusca/bivalvia/ Eggermont, M., Cornillie, P., Dierick, M., Adriaens, D., Nevejan, N., Bossier, P., Van den Broeck, W., Sorgeloos, P., Defoirdt, T., & Declercq, A. M. (2020). The blue mussel inside: 3D visualization and description of the vascular-related anatomy of Mytilus edulis to unravel hemolymph extraction. Scientific Reports, 10(1), 6773. https://doi.org/10.1038/s41598-020-62933-9 Bonham, V., & Roberts, D. (2022). Mytilus edulis (common blue mussel). CABI Compendium, CABI Compendium. https://doi.org/10.1079/cabicompendium.73755 Mytilus edulis. (2019). Animal Diversity Web. https://animaldiversity.org/accounts/Mytilus_edulis/ MacKenzie, J.. C. L., Morrison, A., Taylor, D. L., Burrell, J. V. G., Arnold, W. S., & Wakida-Kusunoki, A. T. (2002). Quahogs in Eastern North America: Part I, Biology, Ecology, and Historical uses. http://hdl.handle.net/1834/26358 SerNat. (2023). Clam Labeled Diagram | Anatomy and Structure. Labeled Science Diagrams. https://sciencediagrams.com/clam/