Biology 320 Invertebrate Zoology Fall 2005

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Biology 320Invertebrate ZoologyFall 2005

• Chapter 13 Phylum Annelida
• Part One

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Phylum Annelida

• Name means ringed
• Mostly marine, but also freshwater and
  terrestrial representatives
• Size varies greatly
• Microscopic 3 m (giant Australian earthworm)
• 12,000 spp.
• Segmented
• Diverse modes of feeding

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Body Regions

• Three main regions (anterior to posterior)
• Prostomium
• Head with sensory organs
• Peristomium with mouth
• Trunk
• Majority of body segments
• Pygidium
• Anus
• Some sensory appendages
• Have annulations (superficial segments), but also
  true segments
• Internal compartmentalization
• Growth zone is located just anterior to pygidium
• Segments added posteriorly
• Oldest segment is most anterior

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• Some organs / structures are found in every
  segment
• Appendages
• Coelomic cavities
• Nephridia
• Gonads
• Other organs are parts of integrated systems
• Digestive system
• Nervous system
• Hemal system
• Musculature
• Segments are separated transversely by septa
• Bilateral coelomic cavities are also partitioned
  longitudinally by mesenteries

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Generalized Body Wall

• Cuticle
• Protective layer that resists abrasion and helps
  prevent desiccation
• Annelids are still restricted to moist
  environments
• Epidermis
• Glandular for production of mucus
• Previously discussed functions of mucus apply
  here
• Produce chaetae
• Chitinous bristles that project from epidermis
• Used for traction
• Not to be confused with setae of arthropods,
  which are sensory hairs
• Connective tissue
• Musculature
• Circular and longitudinal

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Generalized Nervous System

• CNS
• Brain is located dorsally in prostomium
• Two ventral longitudinal nerve cords with giant
  axons
• Impulses travel up to 40X faster than neurons of
  average diameter
• Facilitate the escape response
• Segmental ganglia
• Commissures give the nervous system a ladder-like
  appearance
• Sensory structures
• Typical unicellular receptors distributed on
  head, body, and appendages
• Polychaetes posses eyes and nuchal organs (more
  later)

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Coelomic Cavities

• Two lateral coelomic cavities in each segment
• Transverse septa
• Two longitudinal mesenteries
• One ventral and one dorsal to gut
• Benefit is that musculature and hydrostat of
  individual segments can be precisely controlled
• Coelomic cavities may be lined with mesothelium
  containing chlorogogen cells
• Tissue appears yellow or brown in color
• Has important metabolic functions
• Synthesis and storage of glycogen and lipids
• Detoxification
• Hemoglobin synthesis
• Protein catabolism and formation of nitrogenous
  wastes

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Segmentation

• Advantages
• Burrowing
• Isolation of hydrostat and muscular contractions
• Studies have compared fluid pressures in
  segmented worms vs. non-segmented worms that are
  burrowing
• Pressure is transmitted throughout coelom in
  non-segmented
• In order to prevent aneurisms, all muscles must
  contract in order to antagonize pressure

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Generalized Hemal System

• Dorsal and ventral blood vessel, which are
  connected by capillary beds
• Blood is pumped by
• Vessels
• Muscular contractions
• Enlarged portions of anterior dorsal blood vessel
  which function as hearts
• Blood may contain some hemoglobin, but typically
  larger concentrations are contained in amoeboid
  coelomocytes
• WBC RBC hybrid
• Gas exchange occurs across body wall, appendages,
  and gills

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Generalized Excretory System

• Paired segmental nephridia
• Funnels or terminal cells project into coelomic
  fluid
• Highly coiled tubule
• Large surface area for secretion and absorption
• Nephridiopore opens to exterior of animal

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Generalized Reproduction

• Clonal
• Fragmentation
• Budding
• Paratomy
• Sexual
• Gonochoric
• External fertilization
• Gametes exit through nephridiopores
• Gonads are segmental, and housed in coelomic
  cavities
• Trochophore larvae, with growth zone just
  anterior to the telotroch

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Class Polychaeta

• Polychaetes or bristleworms
• Name means many chaetae
• 8000 spp.
• Marine, a few freshwater, and a few tropical
  terrestrial varieties
• Diverse feeding ecology, locomotion, and chaetae
  structure
• Also abundant (13,425 / m2 in Tampa Bay, FL)
• 2 mm to 3 m (Eunice)

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Body Form

• Posses appendages called parapodia
• Lateral outgrowths of body wall
• Supported by chitinous rod
• Each segment bears one pair
• Posses a chaetal sac which secretes a bundle of
  chaetae
• Chaetal structure is closely related to lifestyle
  and locomotion
• Errant (motile)
• Leg shaped crawling
• Paddle shaped swimming
• Shovel shaped digging
• Sedentary
• Reduced burrowing
• Hook shaped also for burrowing

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Tubes

• Only annelids that secrete and occupy tubes
• Constructed from
• Fibrous proteins
• Foreign materials
• Calcified by animal to form a shell
• Tubes may be attached to a surface or partially
  buried in sediment
• Several functions
• Protection
• Hide-out from which prey can be ambushed
• Brooding chamber
• Sometimes tubes wash up on beaches

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Nervous System

• Typical of annelids
• Segmental pedal ganglia associated with parapodia
• Sensory organs
• Nuchal organs
• Unique to polychaetes
• Pair of eversible ciliated chemosensory pits on
  prostomium
• Ocelli
• Best developed in errant varieties, but useful
  for detecting shadows in sedentary worms
• Statocysts
• Useful to burrowers
• Sensory appendages
• Antennae prostomium
• Palps prostomium
• Cirri peristomium, parapodia, and pygidium

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Digestive System

• Typical of annelids
• May have a protrusible pharynx
• With jaws for grasping prey
• May be used for burrowing
• Esophageal ceca secrete digestive enzymes
• Many strategies for waste removal, based on
  lifestyle
• Errant leave wastes behind while moving
• Sedentary interesting adaptations and behaviors
  to help avoid waste contamination
• Some live upside down in burrows
• Some invert themselves just while defecating
  (sensory appendages on pygidium are useful here)
• May create unidirectional water flow through the
  burrow tube

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Nutrition

• Once again, related to lifestyle
• Errant
• Carnivores often have protrusible pharynx with
  jaws
• Herbivores or scavengers
• Burrowers
• Deposit feeders
• Tube dwellers
• Deposit feeders
• Sedentary (attached)
• Suspension feed using feather crown with large SA
  
• Parasites
• Endoparasites of polychaetes and sea stars
• Ectoparasites that suck the blood of eels

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Gas Exchange

• Body wall
• Gills
• Exposed, delicate outgrowths of body surface
• Modified portions of parapodia
• Feeding crowns in sedentary polychaetes, such as
  feather-duster worms, double as gills

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Circulatory System

• Smaller varieties lack a hemal system, but
  possessed by larger animals
• Typical annelid system plus lateral parapodial
  vessels
• Variety of oxygen binding pigments can be found,
  especially in larger animals
• All function differently and have different
  oxygen affinities
• Intertidal polychaetes must cope with periods of
  emersion or stagnation of water
• Some undergo torpor suppress metabolism
• Rely on stored oxygen from neuroglobin or
  myoglobin

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Excretion

• Typically have segmental nephridia
• Occasionally localized in one or a few segments
• Feather duster worms have one pair of anterior
  nephridia that empty via one medial
  nephridiopore, located on the head
• Tubes are coiled to increase SA for secretion /
  absorption
• Osmoregulate in low salinity (brackish) water by
  increasing rate of filtration
• Removes excess water
• A few species have even colonized freshwater

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Reproduction

• Clonal
• All forms of clonal reproduction
• Excellent regenerators
• Chaetopterus can grow an entire worm from one
  segment
• Sexual
• Most only reproduce sexually
• Most gonochoric
• Gonads are segmental in abdomen
• Gametes released into coelom
• Ripe gametes are shed via nephridiopores or
  rupture body wall

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Epitoky

• Reproductive phenomenon characteristic of many
  polychaetes
• Worms exhibit two life phases
• Atoke benthic non-reproductive individual,
  which transforms into an
• Epitoke pelagic reproductive individual
• Epitokes arise from atokes via metamorphosis or
  budding
• Undergo several modifications and essentially
  become swimming sacs of gametes
• Enlargement of eyes, parapodia, and chaetae
• Become sexually mature, and gametes ripen

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• Epitokes reach sexual maturity at the same time
  and swarm
• Swim to the surface and release gametes
• Many different cues
• Female pheromone release
• Light cycles
• Lunar cycles
• Very predictable patters in some
• Some convert back into atokes after spawning
  some reproduce once and die
• All regulated by hormones

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Oviposition

• Many shed eggs into sea
• Some attach gelatinous masses of eggs to
  substrata
• Chimney of tube
• Some brood eggs
• Retain eggs in tube or burrow
• Sac attached to ventral surface of parent

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Development

• Trochophore larvae in most
• Growth zone just anterior to telotroch
• May pass through trochophore phase in egg before
  hatching
• Some direct development
• Life spans
• Some live one or two years, and reproduce once
  (called annuals)
• Some live and breed for more than one year
  (perennial)
• Short life spans that progress through several
  generations in just one year (multiannual)

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Diversity

• Lugworms
• Construct L-shaped burrows
• Deposit feeders
• Arenicola
• Bamboo worms
• Construct tubes of sand that resemble drinking
  straws
• Deposit feeders
• Live upside down in tubes and ingest substrate
  from below
• Clymenella

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• Paddle worms
• Paddle-like portions of parapodia are used as
  gills
• Errant carnivores that hunt prey by crawling
  around
• Eteone tracks prey by following mucus trails
• Ragworms
• Errant crawl, burrow, and swim
• Eversible, muscular pharynx with jaws
• Lots of cephalic sensory structures
• Some reach 1.8 m
• Nereis

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• Bloodworms
• Live in shallow sedimentary burrows
• Ambush predators
• Capture prey with extremely long eversible
  pharynx
• Have poison glands
• Glycera
• Scaleworms
• Two rows of overlapping scales (resemble fish
  scales) on dorsal surface
• Scales are actually cirri
• Aphrodite (sea mouse)
• Covered with extremely long chaetae that
  resembles felt or hair

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• Fireworms
• Brightly colored
• Resemble caterpillars
• Feed on corals and barnacles
• Calcified chaetae break off when touched
• Causes a painful, burning sensation
• Amphinome
• Shaggy tube worms
• Chimneys of tubes are ornamented with foreign
  materials collected by the worm
• Camouflage for tube
• Ambush predators
• Diopatra

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• Parchment-tube worm
• Chaetopterus
• Lives in U-shaped burrow
• Filter feeder
• Seines plankton with mucus net it creates
• Net is rolled into a ball and swallowed
• Mucus is bioluminescent to ward of potential
  predators
• Spaghetti worms
• Occupy burrows
• Parapodia are reduced to grab sides of burrow
• Cluster of extensible tentacles
• Extend over sediment like active spaghetti
• Can be 1 m or more in length
• Mucus on tentacles traps particles
• Amphitrite

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• Feather-duster worms
• Secrete calcareous tubes or construct from
  foreign materials
• Crown of tentacles (known as radioles)
• One radiole (most dorsal) is modified and acts as
  an operculum when crown is withdrawn
• Suspension feeders
• Beard worms
• Also called pogonophorans
• 80 spp.
• Live deep in ocean typically near hydrothermal
  vents
• Up to 1.5 m in length
• Prostomium bears beard of tentacles
• Also live in tubes
• No mouth or anus
• Symbiotic chemoautotrophic (sulfur oxidizing)
  bacteria live in modified gut (trophosome)