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Phylum Mollusca Mollusks, Annelids, and Arthropods • All have bilateral symmetry (at least as juveniles) • All are coelomate (alternative term: eucoelomate) • All have a complete digestive tract (mouth and anus) Mollusks (Phylum Mollusca) • unsegmented • muscular foot (may be highly modified) • mantle – membrane present in all; in most, secretes and maintains one or two calcerous valves (shells) • most have a radula (rasping tongue with chitinous teeth) Mollusks (Phylum Mollusca) • Diverse group, over 100,000 living species (2nd ranking in number among animal phyla) • Of 7 classes, 4 classes covered: • Polyplacophora (chitons) • Gastropoda (gastropods) – snails, slugs, and nudibranchs (sea slugs) • Bivalvia (bivalves) – scallops, oysters, clams, etc. • Cephalopoda (cephalopods) – octopi, squid, cuttlefish, etc. Symmetry of the Mollusk • Most mollusks are bilaterally symmetrical A Closer look at Mollusca The foot - a broad, flat muscular organ that is adapted for locomotion and attachment The visceral mass - contains the internal organs The mantle - a fold of tissue that drapes over the visceral mass; space between the mantle and the visceral mass is called the mantle cavity Mollusks – Body plan • Usually divided into two parts 1. Head-foot 2. Visceral mass • Head-foot contains – Head – contains mouth – Sensory structures – Foot - large muscle used for movement • Visceral mass contains – – – – Heart Digestive organs Excretion organs Reproduction organs The Mantle of the Mollusk • Mantle – covering of the visceral mass that secretes the shell – Positives! • Protection – Negatives! • Reduce surface area for gas exchange • Solved by having gills for gas exchange The Shell • The mantle is responsible for secreting the shell. • The shell is comprised of three layers: • The outside of the shell is covered by an organic layer periostracum • The middle prismatic layer is characterized by densely packed prisms of calcium carbonate laid down in a protein matrix • The inner nacreous layer is composed of calcium carbonate sheets laid down over a thin layer of protein Nervous Tissue of the Mollusk • Ganglia – – Cluster of nerve cells used for… • Feeding • Locomotion • Sensory information – Some are incredibly intelligent, more on this later! Gills • The gills of the mollusca are often indicated as one or more pairs of bipectinate gills, - flattened filaments attached to a longitudinal axis on either side Digestive System of the Mollusk • Radula – tongue like tissue covered with sharp teeth for scraping food • Complete Digestive System – Some mollusks filter feed, while others are predators The Radula • The mouth cavity of mollusca possesses a specialized rasping organ called the radula; sits on a cartilaginous structure - odontophore • Particles of food brought into the mouth are bound in mucous secreted by the salivary glands Circulation of the Mollusk • Most mollusks, (except cephalopods), have an open circulatory system – Hemolymph is the fluid that is contracted into sinuses and “washes” over body organs for nutrient and gas exchange. – In other words, unlike a closed circulatory system, the blood is not kept in capillaries, veins, and arteries while it moves throughout the body Reproduction of the Mollusk • All Mollusks reproduce sexxually – Depending on the species of mollusk, some reproduce externally or internally – Dioecious- Have male and female – Some are hermaphroditic Molluscan Larval Stages • Most molluscs produce a freeswimming ciliated larvae called the trochophore larvae • In some molluscs the trochophore develops into the adult, but in other molluscs (e.g., gastropods) there is a second larval stage called the veliger Mollusk Larvae • Trocophore Larvae – Free-swimming larval stage (first form) • Veliger larvae – Free-swimming larval stage with foot, eyes, tentacles, and shell Class Polyplacophora • Chitons! – Many-plated mollusk – Lives in shallow marine water – Feed on algae attached to rock Mollusks (Phylum Mollusca) • Polyplacophora – chiton (display only): • eight overlapping valves • muscular foot • thick mantle • gills Class Polyplacophora (Chitons) • Common on the rocky surfaces of the intertidal zone • Head is poorly developed; ventral surface occupied by a broad, flattened foot • Has a dorsal shell composed of 8 overlapping plates, arranged linearly along the anterior-posterior axis • Within the grooves lie many bipectinate gills Class Gastropoda • Single external shell • Radula for scraping food • Moves in wave like contractions through slime • Can be hermaphrodites, though commonly not • Ex) Snails, slugs Class Gastropoda • Snails live on land or in fresh or salt water – They have eyes on tentacles on their head • Slugs live on land and Sea Slugs (a.k.a. nudibranches) live in the ocean – Slugs do not have shells – Have exchange of oxygen (diffusion) across their entire body Gastropoda • More active than mono and polyplacophorans – Highly cephalized: tentacles, eyes • Gonochoristic • Veliger larva (an advanced version of the trochophore larva) Class Gastropoda • Three evolutionary innovations occurred among the gastropods: changes in the shell, increased development of the head, the embryonic process of torsion 1. Changes in the Shell • The shell became higher and conical with a reduced aperture • The shell also became coiled • Shells initially were planospiral - bilaterally symmetrical shell with the whorls lying in the same plane • Modern day shells are asymmetrical - each successive coil is a little outside and offset a little above the one below Torsion is unique to gastropods Most gastropods are dextral Pretorsion Post torsion 3. The Embryonic Process known as Torsion During embryonic development, 1 side of the visceral mass grows at a much faster rate than the other. • Causes the visceral mass to rotate 180 degrees relative to the headfoot. • Advantages: head retracted first; gills receive water currents; the osphradium is now directed anteriorly • Disadvantage: may cause fouling Adaptations to Avoid Fouling • Improved separation of inhalent and exhalent water flow • In some of the more primitive gastropods (keyhole limpets), the shell contains a hole at the top through which the exhalent water stream exits • In the more advanced gastropods, water is brought into the mantle cavity on the left side, passes over a single gill, and exits the right side Shell • Most have a single, spiraled shell and can move the entire head and foot into this shell for protection. • Also, many gastropods have a hardened plate called the operculum on the back of the foot that plugs the shell aperture when the body is withdrawn Nutrition • Many gastropods are herbivores and use their radula scrap algae from surfaces of rocks • Some gastropods are active predators and in these the radula is often highly modified, e.g., as a drill (oyster drills) or harpoon (venomous gastropods) Cone snail Respiration • Aquatic gastropods possess gills for respiration • Terrestrial gastropods obtain oxygen via a well vascularized mantle Vascularized mantle gills Pulmonata • Highly vascularized mantle for gas exchange (lung) • 17,000 spp: slugs, pond snails Snail infestations! Sea Slugs! Sea Slugs! Mollusks (Phylum Mollusca) • Bivalvia – clam: • two valves, connected by dorsal hinge ligament • adductor muscles (used by living clam to close the shell) • gills in mantle cavity • wedgelike foot Bivalves • Sessile • Filter-feeders • Extend the muscular foot into the sand to anchor it • No specific head – but has ganglia • Class Bivalvia – Two Shells • Three layers make up the shell of a bivalve – Inner most protects the body of the animal – Middle layer strengthens the shell with calcium carbonate – Outer layer protects against acid in the water • Pallial muscles insert on the underside of the shell and are attached to the free edge of the mantle; pull the mantle under the shell • Muscles fused across the width (from left to right) at 1 anterior and posterior position and form adductor muscles; connect the 2 shell across their width; close the shell • When relaxed, shell swings open due to elastic ligaments of the hinge Class Bivalvia • Shells divided into 2 equal halves or valves • Mantle tissue is indented in the anteriorposterior margins, with 2 centers of calcification • Shells joined at the dorsal midline by a non calcified protein ligaments called the hinge Bivalves • No radula • sexual reproduction • Incurrent and excurrent siphons – Help filter water in and out of the clam Movement of the Ventilating Currents Adaptive Radiation of Bivalves con’t Unattached Surface Dwellers • Rest unattached on the substrate • Capable of limited locomotion by rapid clapping of their valves using a powerful adductor muscle; forces a jet of water out of the mantle cavity Hard Bottom Burrowers • Several species of bivalves are capable of burrowing into hard surfaces such as rock, coral, wood • Use the anterior margins of their shell to chip away at the rock; some secrete chemical to breakdown rock Adaptive Radiation of Bivalves Reproduction • Most are dioecious • Marine forms usually produce free swimming trochophore and veliger larvae • Many of the freshwater bivalves have a different life history pattern; produce larvae called glochidia • Glochidia are housed in the outer gills; they use there outer gill as a brood camber marsupium • When the glochidia are released they parasitize the fins and gills of fishes Mollusks (Phylum Mollusca) : • Cephalopoda – squid: • muscular foot with tentacles (with suction discs) • thick mantle but no external shell (true for most cephalopods) • excurrent siphon (for jet propulsion) • large eyes Class Cephalopoda • Means “head foot” • Includes octopuses, squids, cuttlefishes and chambered nautiluses • Free swimming and predatory • Tentacles with suction cups to grasp prey • Have jaws called beaks to destroy their prey • Has the largest invertebrate brain Cephalopods • Can learn to perform tasks • Has complex eyes • Closed circulatory system • Are sexual - internal • Have ink to confuse predators Cephalopoda • • • • • • • • Swift, agile carnivores Closed circulatory system, 2 hearts Separate sexes Foot modified to form arms, tentacles, siphon Brain, cranium, complex image-forming eye 700 extant spp, 10,000 extinct spp Arose from limpet-like monoplacophorans Ergo, ventral became function anterior, etc Cephalopods Cephalopoda • Endocochleate cephalopds – Reduce internal shell, or shell absent – Squids, cuttlefish, octopi Class Cephalopoda • Fast moving predators of the marine environment • Cephalopods evolved following major readjustments in the mollusca body plan: • Dorso-ventral axis became elongated and the anterior-posterior axis became compressed • Migration of the head to the ventral part of the body where it fused to the foot • The foot is modified as a series of prehensile tentacles or arms • A circle of 8 or 10 tentacles surround the head; studded with suckers and are used to capture prey. Class Cephalopoda • Fast moving predators of the marine environment • Cephalopods evolved following major readjustments in the mollusca body plan: • Dorso-ventral axis became elongated and the anterior-posterior axis became compressed • Migration of the head to the ventral part of the body where it fused to the foot • The foot is modified as a series of prehensile tentacles or arms • A circle of 8 or 10 tentacles surround the head; studded with suckers and are used to capture prey. Locomotion • Cephalopods are excellent swimmers: streamlined body; tentacles and fins as stabilizers • Swim by means of jet propulsion, using the highly modified muscular mantle and the siphon – By relaxing the mantle the mantle cavity is expanded and water can be drawn in – By contracting the mantle water can be forced out of the mantle cavity by means of the small siphonal opening Feeding • Cephalopods are carnivores • Have a powerful parrot like beak that is used to tear prey apart. • They also have a powerful radula • In some of the octopuses the salivary glands are modified poison glands Shell • Primitively the cephalopods possessed a shell; the fossil record indicates both coiled and non-coiled shells • Extant members with coiled shells include Nautilus • Some cephalopods (cuttlefishes) have an internal shell - cuddle bone •The octopods have lost the shell entirely Other General Features • For protection, they possess an ink sacs • Cephalopods have well-developed sense organs, including a camera type eye • Some have well-developed brains and show a remarkable capacity for learning. • Cephalopods are the only molluscan class with a closed circulatory system Cephalopoda Posterior surface Right Ventral Dorsal Left Tentacle Arm Funnel (siphon) Collar Eye Fin Shell (Pen) Systemic heart Branchial heart Ctenidium Funnel Hectocotylus (sperm-bearing arm in males) Reproduction: trochophore and veliger are bypassed and hatch into planktonic juveniles eye Optic lobe Buccal ganglia Cerebral ganglion statocyst esophagus Brachial nerves Brain is surrounded by a cranium Cephalopod eye Iris Retina Optic nerves Lens Cornea Squid and octopus: basic facts A standard squid has: Two fins A mantle A head 8 arms and two tentacles, each endowed with hooks and/or suckers and sucker rings A standard octopus has: A mantle A head 8 arms endowed with one or two rows of suckers (but never hooks or sucker rings); they have no tentacles Giant optopus Examples of Cephalopods • Close-up view of an unknown species of bathypelagic squid encountered by ROV Tiburon at 3,380 meters depth off the coast of Oahu. • This animal was estimated to be four to five meters in length. • Different from other squids in that their eight arms and two tentacles are roughly equal in length and thickness. • A giant squid (3.15-metre-long) has netted off the UK coast; first time in 15 years. • The squid, believed to be female and three years old, did not survive being brought to the surface. The Mimic Octopus Cephalopoda • Ectocochleate cephalopods – Have external shell with internally subdivisions used for buoyancy control – This ancestral group is almost completely extinct – E.g. Nautilus Nautilus is the only cephalopod with an external shell and lacking chromatophores Chromatophores (color cells) Iridocytes (reflective cells) - Millions of these allow rapid changes in color, polarized signals - Also have photophores for bioluminescence Cephalopods except Nautilus have ink sac Types of Cephalopods • Squids have 10 tentacles • Jet propulsion through excurrent siphon to move • The colossal squid is the world’s largest invertebrate • Octopuses have 8 tentacles • Jet propulsion to capture prey but crawl along ocean floor Blue-ringed Octopus -When threatened, faint blue rings become vivid to warn predators to back off -it’s one of the most deadly venomous animals -bites prey with its jaws -injects a venomous saliva into the wound - venom contains tetrodotoxin, one of the most potent neurotoxins known -Human death from heart & respiratory failure occurs within minutes. This non-aggressive octopus only bites people in self-defense. Reflection: • Mollusks are some of the most diverse phyla that we have seen so far. • Reflect on how the differences in adaptations across the different classes can help the individual species in each class to survive!