ZOO 2040 Biology of Animals Topic 8. Molluscs

Transcription

1 Phylum Mollusca Second-largest phylum: 50, ,000 living species, over 35,000 fossil species. Extremely variable in:! Size (80% are <5 cm, but can range up to 18 m and >1000 kg)! Morphology (snails/slugs, octopods/squids, clams and other bivalves).! Habitat (evolved in the sea and are still mostly marine, but many brackish and freshwater species, some gastropods are terrestrial)! Ecology (filter feeders, herbivores, carnivores, and parasites). Bilaterally symmetrical. Have a true coelom (eucoelom), formed by the splitting of embryonic mesodermal masses (schizocoely), but their coelomic space is limited to a small chamber around their heart. Development is protostomous. Important economically as food and as the source of pearls. Some cause damage to man-made structures, and some are agricultural pests. Gigantic cuttle-fish hooked by the French steamer Alcton! off Teneriffe. From Cassell (ca. 1875) Story of the Sea Major body regions of molluscs: Head-foot: contains feeding, cephalic-sensory, and locomotor organs. Visceral mass: contains digestive, circulatory, respiratory, and reproductive organs. A protective mantle or pallium (formed by two folds of skin that are formed from the dorsal body wall). Head-Foot Most mollusks are strongly cephalized, with a well-developed head with the mouth, brain, and a variety of sensory organs: A fairly complicated nervous system is present, including several pairs of ganglia and a system of nerves, but nervous systems are generally simpler than annelids. In air-breathing species, nervous system produces growth hormones. Multiple simple eyes, or two complex eyes capable of imaging prey. Tentacles may be present. Posterior to the mouth is the foot: Usually ventral: can function for! attachment to substrate or for locomotion.! Secreted mucus can aid in attachment;! helps some to glide on cilia. The foot can be extended hydraulically by! engorgement with blood: burrowers extend! the foot into substrate, enlarge the tip as an anchor, and then pull the rest of the body forward. Modified forms include attachment discs of limpets, hatchet-foot of clams, siphon jet of squids, and wing- or fin-like swimming organs. The mantle cavity and organs of the visceral mass The mantle cavity is the space between the body wall and the mantle. It contains the leaf-like, ciliated gills (ctenidia) or a pulmonate organ ( lung ) in terrestrial gastropods: Gas exchange is via these gills, lungs, and/or the body surfaces (including the lining of mantle cavity). In aquatic species, a continuous flow of water brings in oxygen and food, and flushes out wastes. In most, two ctenidia on opposite sides form an incurrent and excurrent chamber. Cephalopods use the head and mantle cavity to create jet propulsion. Except for the cephalopods, all mollusks have an open circulatory system: Blood bathes all of the organs in the visceral mass directly and is not confined to vessels. There is a contractile heart, blood sinuses, blood vessels, capillaries, and respiratory pigments. Countercurrent blood flow in gills efficiently absorbs oxygen. Excretion by kidneys (nephridia) that empty into the mantle cavity. Sperm and eggs are generally discharged through a portion (or a modified portion) of the excretory ducts. The Shell In most species, the mantle secretes a protective! shell enclosing the visceral mass. Three layers: Periostracum: outer horny layer of conchiolin, a! stabilized ( tanned ) protein (a thick periostracum! protects freshwater species from organic acids! generated by decaying plants). Prismatic layer: middle layer of closely-packed! prisms of calcium carbonate. Nacreous layer: inner mother of pearl layer next! to mantle that is laid down by blood cells in thin! layers. Pearls are formed when irritating particles or parasites! are trapped between the mantle tissue and the shell: These irritants are encapsulated in nacre secreted by the mantle. Only about 1 in 1000 oysters naturally produces a valuable! pearl. The process is aided when humans insert pieces of! freshwater mussel shells or plastic beads into the oysters. After! 5-7 years, a pearl is formed. Reproduction Most are dioecious, but some are protandric (changing! from male to female), are simultaneous hermaphrodites,! (e.g., freshwater snails) or are parthenogenetic. In cephalopods, one tentacle of the male may be! modified for transferring sperm and allowing! internal fertilization of the female. Egg hatches into a free-swimming trochophore larva,! similar to larvae of marine turbellarians, nemertines,! phoronids, and annelids. Possibly represents! a link between these groups ( Trochozoa ) In chitons, this larvae directly metamorphoses into small juveniles In many gastropods and bivalves, a uniquely-derived intermediate veliger larva precedes metamorphosis into the adult. In advanced ( pulmonate ) gastropods, the larval! stage is often suppressed, and development takes! place entirely within the egg capsule, or within! the brood chamber of the female. Only cephalopods have no free-living ecologically! and morphologically distinct larva. They protect! and ventilate eggs until tiny young emerge. Page 1

2 The Radula Feeding organ unique to mollusks, present in all! groups except Solenogasters, bivalves, and! Scaphopods. Usually a protruding, toothed organ: Its structure may be very complex, consisting of a! ribbon-like membrane supported by cartilage! (odontophores), with up to 250,000 backward-! pointing teeth that are replaced as they wear away. Generally functions as a rasp-like scraper that files fine! particles from surfaces and acts as a conveyer belt to! move them to the digestive tract. Radula is adapted to a wide variety of feeding modes! in different species In cephalopods, it forms a parrot-like beak for crushing and! tearing prey In some gastropods, it is specialized for stabbing, or for! boring through the shells of prey Bites of some cephalopods (e.g., the Australian electric blue-! ringed octopus) and stabs of gastropods (e.g., some Conus! spp. ) may be venomous, and have killed humans Molluscan Evolution First mollusks probably evolved in the! Precambrian. Because of their spiral embryo cleavage,! formation of mesoderm from the 4d! blastomere, and trochophore larva,! mollusks are considered to be! protostomes related to annelids and! arthropods: Because segmentation appears to be secondary,! they likely evolved before annelids and! arthropods. Molecular evidence also supports! this: mollusks and annelids are more closely! related to each other than either are to arthropods Ancestral mollusk : Was likely worm-like, with a ventral gliding surface. Probably lacked a shell or crawling foot. Probably had a dorsal mantle, posterior mantle cavity, radula, two gills, a chitinous cuticle, calcareous scales, open circulatory system with a heart, and a ladder-like nervous system. Caudofoveates and solenogasters probably branched off before development of a shell. Polyplacophorans probably branched off before development of a veliger larva. Gastropods and cephalopods appear to be related to monoplacophora Molluscan Phylogeny Class(?) Aplacophora Currently divided into two subclasses or classes: Caudofoveata (= Chaetodermomorpha, 3 families, ca. 70 spp.): Worm-like, burrowing, marine, mm. Orient vertically, with mantle cavity and gills at entrance to burrow. Resemble the presumed ancestral mollusc. Filter-feed on microorganisms and detritus. No shell, body covered with calcareous scales. Have an oral pedal shield (with no spicules or! scales) for food selection and intake. Have a radula, but it is often reduced. Separate sexes. Chaetoderma canadense Solenogastres (= Neomeniomorpha, still! called Aplacophora by some, 21 families, ca. 250 spp.): Resemble Caudofoveates, but with a more reduced head and no radula, gills (may! have secondary respiratory structure), or! Helicoradomenia sp. nephridia. Foot has a midventral furrow (pedal groove). Do not burrow: live on bottom (benthic) and! feed on cnidarians. Hermaphroditic. Class Monoplacophora Formerly considered extinct: found in 19th! Century, but were thought to be gastropods! until 1952 (19 extant spp., one family). All marine. Live at depths 2000 m. Small (<1 mm to 37 mm shell diameter). Mantle tissue secretes a single, rounded, unhinged,! cap-like, calcareous shell (resembles shell of! gastropods known as limpets, but is flattened! and not spiral, although the larval shells are).! No spicules. Radula present. Mantle cavity forms two lateral grooves. Some organs are repeated, but this is considered to! be a secondarily-derived metamerism: 3-6 pairs of ctenidia (may not be the same as typical molluscan gills). 2 pairs of auricles ( hearts ). 6 or 7 pairs of nephridia. 2 pairs of gonads pairs of pedal nerves and pedal retractor muscles. Class Polyplacophora! ( many plate-bearing ) Chitons ( coat of mail ), not chitin! (polysaccharide): 13 families, 800 spp. Eaten by early Native Americans (taste! like fish, but are difficult to chew and! even harder to collect). Small (3-10 cm long). All marine:! live near shore or in intertidal.! Must have hard substratum: coral! or (preferably) rocks: Foot attaches them to rocks by strong suction, resists tides/wave action. Locomotion by waves of foot muscle contractions ( pedal waves ). Feed on crustose algae, radula is reinforced with iron mineral to scrape algae. Pharyngeal sugar glands release amylases into stomach to help digestion. Dorsal shell of (usually 8) overlapping and articulating plates, partially or! mostly embedded in mantle tissue Mantle extends around the body margin, mantle cavity restricted to space between mantle edge and foot. A linear series of about 80 bipectinate ( double-combed ) ctenidia are suspended from roof of mantle cavity on either side of the foot. Water currents are created by cilia on the gills (countercurrent blood flow within gills), grooves form a closed chamber so that water flows from anterior to posterior. Page 2

3 Class Polyplacophora (cont.) Blood pumped by a three-chambered! heart travels through the aorta,! sinuses, and ctenidia. A pair of nephridia carries wastes! from the pericardial cavity to the exterior. Ladder-like nervous system, with four! cords running anterior to posterior! and numerous transverse nerves Head reduced, many sense organs: Photosensitive aesthetes (similar to eyes)! penetrate the plates. Two sense organs (osphradia) sample water in mantle near anus. Tactile receptors on mantle margin. Statocysts in the foot. Chemoreceptor (subradular organ) extends from roof of mouth (buccal cavity) to detect food, additional chemoreceptors around mouth. Dioecious: external fertilization. Produce a swimming trochophore larvae, but no veliger before metamorphosis. Fossil record extends back about 500 Mya. Probably diverged from other mollusks early in molluscan evolution. Class Gastropoda ( stomach-foot ) Snails, slugs, limpets, whelks, periwinkles, conches, nudibranchs ( sea slugs ), sea hares, and sea butterflies: >330 families, at least 40,000 living and 15,000 fossil spp. (75-80% of mollusks). Few mm to 75 cm in shell length (some fossil shells exceeded 2 m). Range from primitive marine forms to more advanced air-breathing terrestrial species. Adaptations of radula allow variety of ecological roles: Many are herbivorous and graze, browse, or are phytoplankivorous Some are active carnivores that can tear flesh or stab! and envenomate prey. Some scavenge on decaying flesh. Some are detritivorous,! may collect debris in a mucus ball or cast a mucus net. Some are parasitic (others are intermediate hosts of a! variety of important parasites). Generally have a one-piece, univalve, coiled or! uncoiled shell: Oldest part of the shell is at the tip (apex). Whorls become larger approaching the aperture, spiraling! around a central axis (columella). Many have an operculum that covers the aperture. Visceral mass attached to the shell by columellar muscle, which! retracts the body into or protrudes it from the shell. Class Gastropoda (cont.) Respiration by ctenidia in mantle! cavity in aquatic species: Prosobranchs lost one gill and half! of remaining gill, which is attached! to the mantle cavity to improve! respiratory efficiency. Pulmonates have highly vascularized! area in the mantle that acts as a lung,! opening to the outside via a small! opening (pneumostome). Aquatic pulmonates surface to expel! a gas bubble and grab an air bubble by curling the body, forming a siphon. Well-developed circulatory and nervous systems, often with elaborate behavioral or chemical defenses. Many sense organs on head: Eyes range from simple cups with photoreceptors to complex structures with a lens and cornea. Osphradium at the base of the incurrent siphon has chemo- and/or mechanoreceptors Tactile organs and statocysts present. Both monoecious and dioecious species. Internal fertilization in some dioecious species. Eggs laid singly or in clusters (often in egg capsules). Young may emerge as veligers, or pass this stage in the egg or in the female s body (some are ovoviviparous). Gastropod Taxonomy Three Subclasses :! Prosobranchia: most marine and some freshwater and terrestrial species Mantle cavity anterior due to torsion, ctenidia anterior to heart. Water enters from left side and exits from right. Long siphons may separate incurrent and excurrent water flow. One pair of tentacles. Dioecious, usually with an operculum. Opisthobranchia: marine sea slugs, sea hares, sea butterflies & canoe shells: Shallow-water species, hide under rocks and seaweeds. Partial or complete detorsion; anus and gill(s)! displaced to right. Foot of pteropods modified into a fin for swimming. Two pairs of tentacles. Shell reduced/absent. All! monoecious. Pulmonata: all terrestrial slugs and most! freshwater snails and slugs: Ctenidia lost; vascularized mantle acts as lung,! filled with air by contractions of mantle floor. Anus and nephriopore open near pneumostome,! wastes forcibly expelled. Aquatic spp. have 1 pair of tentacles; terrestrial spp. Have 2 pair (posterior pair has eyes. All monoecious. Class Bivalvia Clams, oysters, mussels, scallops and shipworms : >90! families, 7,650 spp. 1 mm to 2 m in shell length. Two-piece (bivalve) shell! held together by hinge ligament: Valves drawn together by strong adductor muscles. The bulge! (umbo) is the oldest part (and front) of the shell, growth occurs! outward in rings. Have no head (or any other evidence of cephalization) or radula. Mostly marine, some in freshwater lentic and lotic habitats: U.S. has the greatest diversity of freshwater species (>300 once! present: 12 are extinct, 130 are endangered or threatened due to! water pollution, sediment runoff, and introduced species). Zebra mussels are responsible for $billions in damages yearly. Most are sedentary filter feeders, use ciliary currents to! bring in food: Suspended organic matter enters the incurrent siphon. Gland! cells on gills and labial palps secrete mucus to catch particles.! Food in mucus masses slides to food grooves at the lower edge of! the gills. Cilia and grooves on the labial palps direct the mucus mass into the mouth. Some have endosymbiotes that feed them: shipworms (which excavate particles of wood) have bacteria that produce cellulase; giant clams have photosynthetic zooxanthellae; hydothermal vent species have chemosynthetic bacteria in their gills. Except for hydrothermal species with! no digestive systems, the stomach is! folded into ciliary tracts to sort! particles: A style sac secretes a crystalline style,! which is kept whirling in the style sac. This rotating style helps to free digestive! enzymes and roll up the mucus food mass. Dislodged particles are directed to a! digestive gland and are engulfed by! amebocytes. The visceral mass is suspended! from the dorsal midline, a foot is! attached at the front and bottom: Posterior edges of the mantle form! the incurrent and excurrent siphons. In burrowing species, the siphons! are elongated to reach the water above. Page 3

4 Mantle and gills perform gas exchange: Gills are derived from primitive ctenidia by! lengthening the filaments on each side. Filaments fused to form plate-like structures! (lamellae) with vertical water tubes inside.! These hang down on each side, covered by! a fold of the mantle. Water enters via the incurrent siphon,! enters the water tubes through pores,! moves dorsally to the suprabranchial! chamber, and out through the excurrent siphon. Three-chambered heart with two auricles! and one ventricle. Blood oxygenated in the! mantle returns to the ventricle through the! auricles, the rest circulates through sinuses,! kidneys, gills, and back to the auricles. Nervous systems has three pairs of widely-separated ganglia connected together. Sense organs poorly developed: Tactile tentacles and chemoreceptors, Statocysts in foot. Osphradia in mantle cavity Most have at least some pigment cells in the mantle for light reception. Some mantle eyes have a cornea, lens, retina, and a pigmented layer. Locomotion is by the foot: Foot extended out between the! valves. Blood is pumped into the foot;! it swells and anchors the bivalve. Shortening the muscles in the! foot pulls the animal forward. Scallops can also rapidly clap! their valves together to create a! jet propulsion ; the mantle! valves direct the water jets. Usually dioecious: Generally external fertilization.! Gametes discharged in the suprabranchial chamber and carried out in excurrent flow. Embryos develop as trochophore, veliger, and a unique final larva (spat). Freshwater clams (Unionidae), however, have internal fertilization: Sperm enter incurrent siphon. Fertilize eggs in water tubes of gills. Develop into a unique larval stage > Mimicry in Family Unionidae In Unionid clams (over 300 species in the U.S.), veligers are greatly modified into microscopic, non-swimming, shelled glochidium larvae that develop for weeks as fish ectoparasites (primarily on the gills). Glochidia cannot swim after their! hosts. Various mechanisms are! therefore used to ensure that! glochidia come into contact! with fishes: Some mussel species package! numbers of glochidia into long! mucus and/or mantle tissue forms (conglutinates)! that mimic worms, fishes, crayfish, or insects. These are extended into the water: when! eaten by a predatory fish, they burst,! infesting the host s gills and skin. Recently, a number of Unionids were! discovered to have superconglutinates:! groups of individual conglutinates that! can form into a fish-like lure, which is! extended out into the water on a strand! of mucus up to 8 feet long. Class Scaphopoda ( spade-foot ) Tusk or tooth shells: 8 families, spp. All marine:! subtidal to 6000 m. Live in sand and mud. Slender body covered with mantle. Tubular, curved shell open! at both ends: Foot protrudes from larger end: used to burrow into substrate. Water enters at narrower end: inflow of water driven by cilia! on ridges of mantle tissue, water expelled periodically back! through this opening by foot movements. Head lacks eyes and osphradia: has , thin tentacles! (captacula): Ciliated, sticky bulbs on the ends capture food (e.g.,! foraminiferans) from surrounding sediment and water.! Captacula extended by action of cilia on tips. Food transported to mouth by cilia if it is small, or captacula! retracts by muscles if food is large. Radula carries food to a! crushing gizzard. No ctenidia, heart or circulatory system. Blood circulates through! large hemocoel sinuses as foot rhythmically moves. Gas exchange! occurs through the mantle. Dioecious, with trochophore larva More recent than other mollusks, extending back only to middle! Ordovician (ca. 450 Mya). Probably shares a a common! ancestor with bivalves. Class Cephalopoda Range from 2cm to over 20 m (up to 1000 kg: largest! invertebrates known (past and present). All are marine! predators. Over 600 species in 44 families: Subclass Ammonoidea (ammonites): Chambered shells similar to nautiloids, but chamber walls! were more elaborate and frilled. Extinct since end of Cretaceous. Subclass Nautiloidea (nautiluses): two pairs of gills: Nautilus is only surviving genus; 5 or 6 spp tentacles that extend from open end of shell. No suckers, but produce adhesive secretions. Subclass Coleoidea: one pair of gills: Order Sepioidea (cuttlefishes): round body; 8 arms, 2 tentacles. Order Teuthoidea (squids):! squid cylindrical body; 8 arms, 2 tentacles. Order Vampyromorpha (vampire! squids): deepwater! species; 8 arms,! Octopus 2 tentacles. Order Octopoda! (octopods): 8 arms,! no tentacles. Promioceras (ammonite) Sepea (cuttlefish) Nautilus Vampyroteuthis! infernalis Around their mouth,! cephalopods have a ring of! 8 to 90 arms or tentacles that! often have powerful suckers! (sometimes edged with teeth).! Used to search out and grasp! food. Some are retractile. As large and active predators,! they need a good O 2 supply to! sustain aerobic metabolism.! Have a mostly closed! circulatory system (blood cells confined to blood vessels): One systemic heart (blood goes through systemic circulation before going to gills). Most species(except nautiloids) have a single pair of gills, which are ventilated by muscular actions of the mantle wall, there are therefore usually two accessory or branchial hearts (one at base of each gill to increase blood pressure). Most have a gland and sac that produces an ink-like substance (sepia): Emptied into the rectum and expelled when the animal is alarmed, providing concealment as the cephalopod escapes. Ink also appears to temporarily deaden the sense of smell of fishes. Deep-sea species may produce an ink that glows. Page 4

5 As their name suggests, the foot of cephalopods lies close to their head: Modified to form a funnel,! most move by forcefully! expelling water from the! mantle cavity through this! funnel or siphon. Resulting jet propulsion! allows very movement in! some species. Speed can be regulated by changing the force of water expulsion. Funnel can be flexed to allow changes in direction. Lateral fins of squids and cuttlefishes act as stabilizers (nautiloids use funnel and buoyancy only). Octopods mainly crawl over the bottom, but can jet for short distances. Some use webbing between tentacles to produce a medusa-like movement Reproduction Cephalopods are dioecious. Sperm are packaged in the seminal vesicle of the male and stored. Male uses one of his arms (specially modified as an intromittent! organ, or hectocotylus, for reproduction in some! species) to remove a spermatophore from his mantle! cavity and insert it into the female s! mantle cavity. Large, yolk-filled fertilized eggs! leave the female oviduct. Eggs are attached to the substrate! and are often tended by! the female. Eggs undergo meroblastic cleavage! (small animal pole and large yolky! vegetable pole, similar to! vertebrates). Hatch into juveniles! with no free swimming larval stage. An external shell is present in the nautiluses. Shells were also present in! the extinct belemnites (which! had straight shells) and! ammonites (which had coiled! shells). Shell is divided into chambers,! with the animal occupying the newest! chamber. A filament of tissue (siphuncle)! extends through all of the chamber walls through openings (siphonophores) to the oldest chamber. Allows chambers! to be filled with gas to adjust buoyancy: in Nautilus,! pressure is at 1 atm, while surrounding water pressure! may be many times that. More commonly, there is an internal shell of calcium! carbonate ( cuttlebone ) or conchiolin protein (pen)! embedded in the mantle that provides rigidity to the body. Nervous system and sensory organs Cephalopods have the largest and most complex brains of any of the invertebrates: Giant nerve fibers (up to 1 mm in diameter) increase transmission speed. Capable of learning by positive and negative reinforcement; can solve simple problems. Sense organs very well-developed: Most species have excellent eyes that are similar to those of! vertebrates (have corneas, lens and retina). The eyes of a! typical octopus have over 2 million photoreceptors: giant! squids have up to 1 billion. No hearing, but well-developed tactile senses. Chemoreceptors in their arms. Use chemical and visual signals to communicate: Chromatophores are cells in the skin that contain pigment granules. Contractions of muscle fibers attached to the cell boundary cause the cell to to expand and change the color pattern. These changes are very rapid, and individuals can send multiple color signals in different directions. Deep-water species have luminescent organs. Page 5