Annelids
On being a worm

Coelomate Protostome relationships

• Monophyletic
• Synapomorphies
  • Blastopore becomes mouth
  • True coelom via schizocoely
  • Spiral cleavage
  • Determinate development
  • Trochophore larva
• Three major Phyla
• Annelids and Arthropods united by segmentation

Phylum Annelida

• General characteristics and synapomorphies
• The coelom as a hydrostatic skeleton
• Segmentation and metamerism
• 3 classes: Polychaeta, Oligochaeta, Hirudinea
• Locomotion
• Feeding
• Reproduction
• Why be a worm?

The Annelid Bauplan

• General characteristics
• Segmented, Metameric
• Closed circulation
• True coelom as a fluid- filled cavity
• Circular and longitudinal muscles
• Probably began to burrow very early as they developed segmentation
• Figure 10-1

The Annelid Bauplan

• General characteristics
• Synapomorphies
• Annelid head
  • Prostomium
  • Peristomium
• Setae

The Coelom as a hydrostatic skeleton

• A fact: Muscles can only contract, not extend
• Extension due to action of antagonist
• Jointed skeletons
• antagonists have opposite effects on hard parts
• Hydrostatic skeleton
• antagonist takes advantage of the incompressibility of fluid
• Circular/Longitudinal muscles

Segmentation/Metamerism

• Different regions can
• Act independently
• Specialize for different functions
• Very important for evolution

Phylum Annelida - three major Classes

• Polychaeta
• Clam worms, lug worms, featherdusters
• Oligochaeta
• Earthworms, ice worms
• Hirudinea
• Leeches
• Oligochaeta + Hirudinea united by:
  • Hermaphroditism
  • Clitellum
  • Direct Development

Class "Polychaeta"

• >10,000 species
• Very common, mostly marine
• Monophyletic?
• Complex head
• Parapodia

Class "Polychaeta"

• >10,000 species
• Monophyletic?
• Retain trochophore larvae

Class "Polychaeta"

• >10,000 species
• Monophyletic?
• Retain trochophore larvae
• Two general types
• Errant (free moving)
• Sedentary
• Distinction between errant and sedentary blurry (e.g., burrowers)

Errant vs. Sedentary Polychaetes

• Errant
  • Large head
  • Jaws
  • Large parapodia
• Sedentary
  • Reduced head, usually
  • Reduced parapodia
  • Protective tubes, sometimes

Class "Oligochaeta"

• Paraphyletic
• Mostly FW, terrestrial, a few known from glaciers (ice worms)
• May exceed 3m!
• Direct development (eggs shed in cocoon)
• No parapodia
• Setae usually present, but reduced

Class Hirudinea

• Monophyletic
• > 500 species
• 75% parasitic (ectoparasitic), were used in Medieval medicine
• Mostly freshwater, some marine, some terrestrial
• Direct development (eggs shed in cocoon)
• No setae
• Up to 30 cm!

Polychaete locomotion

• Complex, reflecting variety of lifestyles
• Sedentary forms
• Retract feeding structures (giant nerve axons)
• Peristaltic movement within tube (pump water, many have modified parapodia)
• Parapodia and setae anchor the worm in tube
• Armored tubes
  • Calcium Carbonate
  • Protein
  • Cemented particles

Polychaete locomotion

• Errant polychaetes
• Peristalsis common (on surface but more commonly in burrowing; see discussion for Oligochaetes)
• Crawling
  • Parapodia and setae engage the substrate
  • Parapodial muscles contract for power stroke

Polychaete locomotion

• Errant polychaetes
• Peristalsis (especially burrowing)
• Crawling
• Swimming
  • Greater use of body wall muscles
  • Posterior to anterior wave of contraction on one side
  • Parapodia important, parapodia often large with long setae
  • Not very efficient

Oligochaete locomotion

• Peristaltic contraction
• Probably primitive for annelids
• Hydrostatic skeleton
• Coelom divided by septa
• Alternating contractions of
  • Longitudinal muscles
  • Circular muscles
  • against incompressible fluid
• Setae provide traction

Hirudinean locomotion

• Loss of setae, septa
• Leeches don't burrow
• Galloping
• Circular muscles contract with posterior sucker attached
• Anterior sucker attaches, posterior sucker detaches
• Longitudinal muscles contract with anterior sucker attached
• Swimming

Hirudinean locomotion

• Loss of setae, septa
• Leeches don"t burrow
• Galloping
• Circular muscles contract with posterior sucker attached
• Anterior sucker attaches, posterior sucker detaches
• Longitudinal muscles contract with anterior sucker attached
• Swimming
• Undulations simlar to galloping but without attachment
• Not very efficient

Polychaete feeding

• Raptorial
• Eversible pharynx
• Jaws
• Some venomous
• E.g., Nereis (clam worm)

Polychaete Feeding

• Raptorial
• Deposit feeders
• Direct
  • E.g., Arenicola (lug worm)
• Indirect
  • E.g., Terebella (spaghetti worm)

Polychaete Feeding

• Raptorial
• Deposit feeders
• Filter feeders
• Specializations
  • Head
  • Sabella
  • Body
  • Chaetopterus

Oligochaete Feeding

• Fairly simple
• Direct deposit feeding
• Scavenging
• Gut straight, with
• Gizzard
• Typhlosole

Hirudinean Feeding

• 25% predators, 75% parasites
• Usually penetrate skin with three bladelike jaws
• Secrete
• An anesthetic
• Hirudin - an anticoagulant
• Pump blood with muscular pharynx

Polychaete Reproduction

• Asexual in a few (bud new individuals)
• Usually sexual
• Dioecious -Male or female
• Coelom stores gametes
• Epitoky
  • "Epitoke" = specialized swimming forms budded off posterior or sides from benthic individual

Polychaete Reproduction

• Epitoky
• Specialized swimming form budded off posterior or sides from benthic individual
• Large parapodia, setae, eyes, (often lack digestive system)
• Swarming (often swimming and then burst open just after sunset or before sunrise; bioluminescent!)
• Pacific pololo worm, "fire worm"

Oligochaete Reproduction/Copulation

• Asexual reproduction (budding, fragmentation) common
• Usually sexual
• Simultaneous Hermaphrodites
• Copulation
• Pairs line up facing opposite
• Sperm exchanged
• Stored in each spermatheca
• Each produces cocoon

Oligochaete Cocoon formation

• Clitellum secretes
• Chitinous cocoon
• Albumin
• As cocoon is shed
• Eggs and sperm deposited in cocoon
• Fertilization external

Hirudinean Reproduction

• Similar to Oligochaetes, but
• No asexual reproduction
• Protandrous hermaphrodites (male first, change sex to female as grow older)
• Fertilization internal

Why be a worm?

• Many groups have converged on this body plan. Why?
• Bilateral symmetry and cephalization
• Sensory, feeding structures at one end
• Reduced cross-sectional area
• Good for burrowing, getting into protective crevices, insinuating into tissue spaces of organisms (parasitism)
• Large surface area/volume
• Gas exchange, waste removal, nutrient uptake
• Elongate digestive tract
• Regionalization; feces released far from mouth
• Conducive to development of segmentation