Gastropods and Bivalves PDF

Parts of a Gastropod & Bivalve Shells Shell identification guides become easier to use with some background on the morphological features present in different shell types. Check out this background information on parts of a Gastropod and Bivalve...

Parts of a Gastropod & Bivalve Shells Shell identification guides become easier to use with some background on the morphological features present in different shell types. Check out this background information on parts of a Gastropod and Bivalve shells to help you identify shells to that species level identification! Parts of a Gastropod Shell There are key areas of a gastropod shell that can be useful in identifying and differentiating species. Some of these characteristics include: Protoconch Shell whorl Spire Aperture Outer lip Siphonal Canal Shell Sculpture Chirality What is a shell whorl? A shell whorl is one 360 degree rotation of this shell spiral. These can often be seen as "layers" down the spire of the shell. Each shell has a single Body Whorl. This is the largest section of the shell where the gastropods body resided at the latest stage of growth. The smaller shell whorls make up the Spire of the shell. What is a shell aperture? The aperture is the primary opening in the shell that the mollusk body, foot, and head enter and exit in order for it to interact with the environment. Living gastropods have an operculum, a hard calcified "door" that they can use to plug up the aperture. The operculum allows them to seal of from the outside protecting them from predators or other dangerous environmental conditions. Shell Sculpture The texture on the outside of a shell is called the shell sculpture. Shell sculpture is typically oriented in one of two ways, parallel or perpendicular to the axis of coiling (shown in red). Sculpture arranged parallel to the axis of coiling is called axial sculpture while sculpture arranged perpendicular to this axis is spiral sculpture. What is chirality? Most gastropods have spiral shells. This spiral shape allows them to extend the shell from the aperture to make a continuously larger compartment for their bodies as they grow. Chirality refers to the direction in which the shell spirals. To determine a shells chirality hold it with the aperture facing you and the spiral of the shell pointing up. If the aperture is on the right side of the shell it is Dextral(right-handed) and if its on the left the shells is Sinistral (left-handed). Bivavle shells Bivalves have two shells that are often referred to as valves. These shells can be equal in size and shape or be different with some varying greatly in size. Valves that differ in size and shape are called inequivalve while valves that are equal in size and shape are called equivalve. Parts of a Bivalve Shell The inside of the valves contain much of the important information used in species identification. However the sculpture of the valve exterior can also be useful. Learning the parts of these valves can give insight into the animals these shells used to belong to. Important parts of a bivalve shell include: Valve Ligament Hinge Teeth Umbo Adductor Muscle Scars Pallial Line Pallial Sinus Shell Sculpture What is the hinge? The bivalve's two shells are hinged together with a horny ligament. The ligament is black or brown in color and has elastic-like properties. It is stretched when the valves open and compresses when they close. This ligament can be characterized by its location, form, and growth pattern. This hinge also often includes a series of interlocking teeth that create a stronger hinge joint. These teeth can be similar on both valves or different, with one valve having the teeth and the other sockets for the teeth. The arrangement of the hinge teeth is useful in bivalve identification. Some shells have large central teeth called Cardinal Hinge Teeth and smaller teeth on the sides called Lateral Hinge Teeth. What are the Adductor Muscles? On the interior of the shell, you can usually find the scars where muscles used to be attached. The major muscle scars you'll find are from the adductor muscles. The adductor muscles were used to pull the valves closed. There are typically two adductor muscle scars, however some groups have only one. When the adductor scars are nearly equal in size, they are called isomyarian (or sometimes homomyarian). When there is a significantly larger adductor muscle it is called heteromyarian. When there is only one adductor muscle it is called monomyarian. What are the Pallial Line and Pallial Sinus? The pallial line marks where the mantle retractor muscles attach to the shell. This marks the farthest point where the bivalve’s soft body is actually attached to its shells. Some shells also have a pallial sinus that connects to the pallial line and marks where the siphonal muscles attach to the shell. The depth of the pallial sinus is often correlated to the size of the siphon in the living animal. So, a very large pallial sinus means the living animal likely had a large siphon! What are Byssal Threads? Some bivalves secrete strong fibers from their shell, called byssus or byssal threads, that they use to anchor themselves to both soft and hard surfaces. These byssal threads are secreted as a liquid that solidifies when it contacts water. The byssal threads of some bivalves like Pinna nobilis have been used to make a fine clothe called sea silk! Learn more about Sea Silk in the following video! TAXONOMY AND ANATOMY Introduction Some knowledge of bivalve biology is necessary to understand operations of a bivalve hatchery and to assist in solving problems that arise. It is not the intention here to give a detailed description of bivalve biology but to provide a brief resume of information pertinent to operations of a hatchery. There are several excellent texts on molluscan biology readily available and there are extensive reviews of groups and individual species of oysters, scallops, mussels and clams. The reader is directed to these publications at the end of this section for additional information. Bivalves belong to the phylum Mollusca, a group that includes such diverse animals as chitons (chain shells), gastropods, tusk shells, cephalopods (squid and octopus) as well as clams, oysters, mussels and scallops. The phylum has six classes of which one is Lamellibranchia or Bivalvia. These animals are compressed laterally and the soft body parts are completely or partially enclosed by the shell, which is composed of two hinged valves. The gills or ctenidia of animals in this class are well developed organs, specialized for feeding as well as for respiration. External anatomy The most prominent feature of bivalves is the two valves of the shell that may or may not be equal and may or may not completely enclose the inner soft parts. They have a variety of shapes and colours depending on species. The valves are composed mostly of calcium carbonate and have three layers; the inner or nacreous layer, the middle or prismatic layer that forms most of the shell, and the outer layer or periostacum, a brown leathery layer which is often missing through abrasion or weathering in older animals. Internal anatomy Careful removal of one of the shell valves reveals the soft parts of the animals. The differences in general appearance of bivalve shells internal anatomy The soft tissue anatomy of the European flat oyster, Ostrea edulis, and the calico scallop, Argopecten gibbus, visible following removal of one of the shell valves. Key: AM - adductor muscle; G - gills; GO - gonad (differentiated as O - ovary and T - testis in the calico scallop); L - ligament; M - mantle and U - umbo. The inhalant and exhalant chambers of the mantle cavity are identified as IC and EC respectively. Crustaceans Taxonomy Crustaceans are one of the organisms living on the earth that is important to know forms (e.g., larvae, morphs, adults, sexes) and units. In the current era, crustaceans (crab, shrimp, and lobster) emerge as one of the most demanding seafood than other types of seafood, and therefore, natural implication of worldwide fisheries is more dominated by crustaceans. Because a large number of crustaceans taxa have been identified and described, it is necessary to definite groups. Crustaceans are one of the world’s oldest and most diversified arthropods, as well as one of the most successful invertebrate groups, with an assessed 40,000 living species and more than 150,000 identified. The names “shrimp” and “prawn” do not relate to any specific taxonomic groups. Although “shrimp” is often used to refer to smaller species and “prawn” is more commonly used to refer to larger ones, there is no clear distinction between the two terms, and their usages are sometimes misunderstood or even reversed in various regions. This chapter aims to focus on the taxonomy of the crustacean and their contribution and importance in fisheries. Crustaceans are a varied group of arthropods (phylum Arthropoda) that includes Brine shrimp, copepods, ostracods, Sand hoppers, crayfish, crabs, lobsters, barnacles, prawns, krill, and mantis shrimp. Crustaceans have branched (biramous) appendages, chitin and calcium exoskeleton, two pairs of antennae projecting in front of the mouth, and paired appendages that function as jaws with three pairs of biting mouthparts. Carcinologists have long argued the taxonomic classification of crustaceans, with five, six, or even 10 classes sometimes assigned to the group at the phylum, subphylum, or superclass level. The Crustacean has been classified as a class by others. Taxonomy, systematic, and classification The term taxonomy is derived from the Greek taxis (it means “order” or “arrangement”) and nomos (it means “law” or “science”) (A.P. de Cadolle, 1813). Taxonomy is the rehearsal and application of classification and can be used ass taxonomic units also familiar as taxa. It originally can be denoted only to the classification of organisms but up-to-date it is used in a fuller sense to the principle fundamental such as a classification. Taxonomy essentially copes with the naming and classification of taxa and forms only a part of biological systematics known as biodiversity. In taxonomy, the smallest unit of classification is called species, which consists of classification and nomenclature. Based on the relation of the organisms, it can be grouped or placed known as classification. The system of classification is developed and used by Carl Linne, 1735, the term systematic can be used in taxonomy. Crustaceans Basic Anatomy