BIOL 108 - Introduction to Biological Diversity

Topic 22: Lophotrochozoa

Introduction to Clade Bilateria

• Bilateria is a major clade of animals with true tissues and is characterized by:
  • Bilateral symmetry
  • Triploblastic germ layers
• Two main groups:
  • Protostomes
  • Deuterostomes
• Characteristics of Bilateria:
  • Bilaterally symmetrical body plan
  • Triploblastic animals
  • Most have a coelom and a digestive tract
  • Muscles and internal organs derived from mesoderm

Bilateral Symmetry

• Bilaterally symmetrical bodies are differentiated along the anteroposterior axis.
• Implications of bilateral symmetry:
  • Directional mobility
  • Cephalisation (formation of a head)
  • Concentration of neural ganglia
  • Digestive and reproductive structures typically posteriorly placed

Basal Bilaterians

• Phylum Acoela, considered basal to other bilaterians
• Characteristics:
  • Small, flattened body
  • Minimal cephalisation
  • Absence of a body cavity or complex organs
  • Simple digestive system

Introduction to Lophotrochozoa

• Clade includes about half of animal phyla (17/35)
• Characteristics:
  • Presence of lophophore in some taxa
  • Trochophore larval stage
  • Diverse body plans

Lophotrochozoan - Platyhelminthes

• Flatworms with ~20,000 extant species
• Characteristics:
  • Triploblastic animals lacking a fluid-filled body cavity
  • Incomplete digestive tract
  • Absence of circulatory or gas exchange systems

Free-living Planarians

• Characteristics:
  • Anterior cephalisation
  • Light-sensitive eyespots
  • Gastrovascular cavity with one opening
  • Hermaphrodites, capable of sexual and asexual reproduction

Parasitic Rhabditophorans

• Characteristics:
  • Suckers or hooks for attachment
  • Complex life cycle with at least two hosts

Tapeworms

• Endoparasites in the digestive tract
• Characteristics:
  • Absence of digestive system
  • Presence of scolex with suckers
  • Complex life cycle

Lophotrochozoan - Lophophorata

• Known for the lophophore structure
• Includes Ectoprocta and Brachiopoda

Ectoprocta

• Sessile, colonial animals
• Characteristics:
  • Hard exoskeleton in marine taxa
  • Suspension feeding with retractable lophophore

Brachiopoda

• Sessile, suspension feeders in marine environments
• Characteristics:
  • Attached to seafloor by pedicle
  • Digestive tract with or without anus

Trochozoa

• Includes Phylum Annelida and Phylum Mollusca
• Trochophore larvae are a common characteristic

Mollusca

• Over 100,000 extant species
• Characteristics:
  • Unsegmented body plan
  • Presence of trochophore larval stage
  • Gills, radula, open circulatory system

Annelida

• Over 22,000 species
• Characteristics:
  • Segmented bodies
  • Complete digestive system
  • Closed circulatory system

Annelida - Errantia

• Mobile, often predatory marine organisms
• Characteristics:
  • Parapodia with chaetae and bristles
  • Cephalisation with head and eyes

Annelida - Sedentaria

• Includes leeches and earthworms
• Characteristics:
  • Tube-dwelling or burrowing
  • Some lack trochophore larval stage

Reflective Questions

• Characteristics of major groups within Lophotrochozoa
• Resolving polytomy based on lophotrochozoan characteristics
• Understanding of Acoela and Platyhelminthes phylogeny
• Interpretation of trochophore larvae presence