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Download Lecture 13, Annelids + Molluscs - Cal State LA
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Pay attention to the progression from life originating in water to colonizing dry land in animals, just like in plants - what pre-adaptations were already present in aquatic ancestors that made the transition to land easier? - what limitations of the body plan had to be overcome by adaptive evolution? (in other words, how did evolution solve problems associated with life out of water) Especially think about these highly successful groups: - annelid worms - molluscs - arthropods - vertebrates Coelomates Coelom = fluid-filled space surrounded by muscle - acts as a hydrostatic skeleton for muscles to push off of (like having water balloons instead of bones) Protostomes Lophotrochozoa Ecdysozoa Protostomes are further divided into 2 groups on the basis of DNA sequence analysis Deuterostomes Platyhelminthes (flatworms) Protostomes Annelid worms Molluscs Nematodes Arthropods Deuterostomes Phylum Annelida ~17,000 species Coelom acts as hydrostatic skeleton allowing worms to crawl (marine polychaetes) or dig (earthworms) effectively Segmentation: body is divided into many repeating blocks - blocks of segments are specialized for different functions Complete digestive system, closed circulatory system Chetae = spines sticking out of epidermis; help in movement “polychaetes”, marine worms 25 Orders, 87 Families - numerous chetae (bristles) on each segment - parapodia (paddle-like appendages) can act as walking paddles, or gills for respiration - reproductive structures often simple and even temporary Cross-section of a polychaete segment Dorsal blood vessel Circular muscle Longitudinal muscle intestine Coelom chetae Parapodia - can act as gills, feet, or hold worm in its tube Ventral nerve cord ventral blood vessel worm movement Circular muscles squish or expand different parts of the body important for tunneling and other movement in annelids nematodes lack circular muscles + a coelom, so can’t do this show vids oligochaetes (earthworms) 3 Orders - chetae reduced, but important for burrowing; no parapodia - circular muscles + hydrostatic skeleton allow distinctive “oozing” method of movement - clitellum is the obvious band; after mating, it produces a mucus cocoon that slides along body, picking up sperm deposited by the mate and eggs released from the body clitellum Anatomy of an earthworm 5 “hearts” which are muscular vessels that surround esophagus & pump blood Septa divide body into isolated segments Digestive, circulatory systems pipe right through the septa Oligochaete ecology “Worms have played a more important part in the history of the world than most persons would at first suppose.” from The formation of vegetable mould, through the action of worms, with observations of their habits. Published in 1881 by C. Darwin Earthworms consume soil as they burrow beneath the ground - organic material is partly digested as it passes through worm Worms then back out of their burrows and deposit nutrient-rich “castings” on top of the ground - creates fertile topsoil, essential for healthy plant growth - over 16,000 pounds of soil processed per acre, in a year! Tunnels also give roots access to fresh air, for gas exchange Deep-sea vent worms + their bacteria Giant tubeworm Riftia lives next to superhot, deep-sea hydrothermal vents Adults have no digestive tract; instead have a special organ full of mutualistic chemo-autotrophic bacteria bacteria use energy from toxic sulfide (H2S) to fix CO2 make sugars that feed worm 8 ft tall worms create habitat for 100’s of species, all living in complete darkness Riftia pachyptila Phylum Mollusca ~100,000 species Soft body divided into 3 parts: a) foot, for locomotion b) visceral mass = internal organs c) mantle, a flap of tissue that secretes the shell - Open circulatory system spouts blood through a body cavity called hemocoel (replaces coelom, not needed due to shell) - Ctenidia = flaps used as gills, for respiration (getting oxygen) - Radula = toothed ribbon used to scrape food - Develop through a larval stage called a trochophore, a feature shared with annelid worms Shells Special proteins are secreted by the mantle tissue, cause the mineral calcium carbonate to crystallize out of the ocean water - same material coral uses Shell provides protection (physical defense) against predators Acts as an external skeleton, to which muscles can attach and work off of - termed an “exoskeleton” - molluscs no longer needed the coelom as as internal skeleton, so their coelom is really reduced (tiny) Shells vary from class to class Chitons: 8 separate plates Bivalves (clams): 2 shells, hinged Gastropods (snails): 1 twisted shell with an operculum (trapdoor) Cephalopods: nautilus 1 shell internal shell octopus no shell squid Molluscan Body Plan radula is extended out, then pulled back to scrape surfaces for food water pumped through mantle cavity, over gills Radula Unique to molluscs: Ribbon of teeth made of chiton - tips may be hardened by iron May grow 5 new rows of teeth per day Can be used for... herbivory: scrape algae off rocks predation: cut tissue from animals injecting poison (cone snails) Phylum Mollusca: 4 major classes Class Polyplacophora - Chitons Class Gastropoda - Snails + slugs Class Bivalvia - Clams, mussels, oysters Class Cephalopoda - Octopus, squid, nautilus Phylum Mollusca: 4 major classes Class Polyplacophora - Chitons - scrape algae off rocks using radula tipped with IRON - shell is 8 distinct plates, held by mantle check out a chiton radula in lab! Class Gastropoda: Snails + slugs 70,000 spp. Asymmetrical body packed into a spiral shell Torsion: during development, body rotates 180o over the foot - moves viscera, anus over head - organs on right side lost; space taken over by gonads One of few invertebrate groups to successfully colonize land 1) operculum (trapdoor) to close body into shell = protection against desiccation & predators 2) muscular foot for movement on land Torsion before torsion after torsion Cone Snails In these hunters, the radula adapted into a harpoon to capture prey Injects nerve toxins into fish or molluscs to paralyze them Only “drug from the sea” so far is a back pain drug - a cone snail toxin! SEA SLUGS!!! Sea Hares -- includes world’s largest gastropod - herbivores eat algae, store toxins and color which they release by inking if molested - Nobel Prize in medicine awarded in 2002 for work on learning & memory in Sea hares Ultimate Vegetarians Some sea slugs exhibit kleptoplasty: - chloroplasts from food algae are stored, instead of being digested - continue photosynthesizing for up to 9 months in some species! made possible by transfer of genes from the algae’s nucleus into the slug’s chromosome: now part animal, part plant! Nudibranchs: Poison-eaters Nudibranchs can store toxic chemicals from sponges they eat gills Chromodoris Phyllidia Halgerda Glossodoris Bright colors & patterns serve as a warning to fish, crabs: “I am poisonous, don’t eat me” – aposematic coloration Hypselodoris iacula Hypselodoris kanga Chromodoris annae Hypselodoris fucata Hypselodoris nigrostriata Aeolid nudibranchs - feathery cerata on backs store cnidae from cnidarian prey - cnidae detonate in mouth of any fish that tries to eat the slug Godiva quadricolor turn your prey’s defenses into your own armaments Hermissenda crassicornis Class Bivalvia – clams, mussels ~20,000 species Shell with 2 valves, held together by powerful adductor muscles (the meat of a scallop) Shell Adductor muscle holds shell closed foot Ctenidia = gills, used also in filter feeding Foot used for digging into bottom Bivalves filter-feed with their gills to mouth Cilia (tiny hairs) beat to generate water currents (like sponge choanocytes) Trap tiny particles, which are then carried to the mouth Use their ctenidia (gills) for both filter feeding & respiration - clams burrow into mud/sand; use siphon to draw in water, pass it over their gills for both oxygen and food Mussels: Dominant Spatial Competitors use protein threads to glue themselves to rock surface Mussels cover rocks in the intertidal zone out-compete other organisms for space, unless their #’s are limited by predators (more on this to come..) Freshwater mussels (Unionids) edge of mussel’s mantle female mussel actual prey fish Freshwater mussels mimic small fish, insects with parts of their body to lure larger, predatory fish spray vampire-larvae into big fish’s gills, where the larvae drink its blood! Class Cephalopoda ~900 species Nautilus, squid, octopus Most intelligent invertebrates, w/ complex eyes - Only molluscs w/ closed circulatory system high speed - Zoom backwards by jet propulsion, for hunting - Defense without a shell: inking, color/texture change - Foot divided into tentacles w/ flexible suckers nautilus (external shell) squid (thin, internal shell) octopus (no shell) Chambered Nautilus Nautilus lives at great depths (600 meters), floats up to surface at night by adjusting pumping gas into sealed shell chambers Squid Fast-moving mid-water predators shoot out 2 extra-long tentacles to snag fish, using their suckers internal shell giant squid = world’s biggest invertebrate Glow-in-the-dark squid Bioluminescence can result from light-producing bacterial symbionts Counter-shading used by many mid-water squid to hide from predators lurking below produce same amount of light from your belly as hits your back, so you don’t cast a shadow visible to predators under you! essentially, you become invisible Euprymna scolopes Octopus - 8 arms - no shell - best vision - smartest invertebrate ~200 spp.
