Deuterostomes: Overview and Phylogeny

Introduction to Deuterostomes

Deuterostomes are a major lineage within the animal kingdom, characterized by their embryonic development and unique anatomical features. They include three main phyla: Echinodermata, Hemichordata, and Chordata. Understanding their evolutionary relationships and distinguishing features is essential for studying animal diversity.

• Bilaterians: All deuterostomes are bilaterally symmetrical at some stage and possess three tissue layers (triploblasty).

• Coelomate Ancestor: All bilaterians descend from a coelomate ancestor, though some groups have lost or reduced the coelom.

• Main Phyla: Echinodermata, Hemichordata, and Chordata.

Protostomes vs. Deuterostomes

The distinction between protostomes and deuterostomes is based on embryonic development, specifically the fate of the blastopore and cleavage patterns.

• Protostomes: The blastopore becomes the mouth; spiral cleavage during early development.

• Deuterostomes: The blastopore becomes the anus; radial cleavage during early development.

Triploblasty and Coelom Formation

Three Tissue Layers

Triploblasty refers to the presence of three primary germ layers in animal embryos: ectoderm, mesoderm, and endoderm. These layers give rise to various tissues and organs.

• Ectoderm: Forms the outer layer (skin, exoskeleton).

• Mesoderm: Forms muscles, blood cells, and organs.

• Endoderm: Forms the inner lining of the digestive tract and associated organs.

Coelom Types

The coelom is a fluid-filled body cavity derived from the mesoderm. Animals are classified based on the presence and type of coelom.

• Acoelomate: No coelom (e.g., flatworms).

• Coelomate: True coelom surrounded by mesoderm (e.g., annelids).

• Pseudocoelomate: Coelom not fully lined by mesoderm (e.g., nematodes).

Major Deuterostome Phyla

Echinodermata

Echinoderms are exclusively marine invertebrates with unique anatomical and physiological features.

• Endoskeleton: Composed of calcium carbonate ossicles.

• Water Vascular System: Used for locomotion, feeding, and nutrient circulation; includes tube feet.

• Radial Symmetry: Adults exhibit pentaradial symmetry, while larvae are bilaterally symmetrical.

• Reproduction: Both sexual (external fertilization) and asexual (fragmentation).

• Ecological Roles: Sea stars are carnivores; sea urchins are herbivores.

Key Synapomorphies of Echinodermata

• Radial symmetry (in adults)

• Endoskeleton of calcium carbonate

• Water vascular system and tube feet

Major Lineages of Echinodermata

• Asteroidea: Sea stars

• Echinoidea: Sand dollars, sea urchins

• Crinoidea: Feather stars

• Ophiuroidea: Brittle stars

• Holothuroidea: Sea cucumbers

Hemichordata

Hemichordates are marine organisms, including acorn worms and pterobranchs.

• Body Plan: Divided into proboscis, collar, and trunk.

• Pharyngeal Slits: Used for filter feeding.

• Larval Development: Tornaria larva similar to echinoderm larvae.

• Feeding: Heterotrophic; detritus feeders or filter feeders.

Chordata

Chordates are defined by four key features present at some stage in their life cycle.

• Dorsal Hollow Nerve Cord: Develops into the brain and spinal cord.

• Notochord: Flexible, supportive rod beneath the nerve cord.

• Pharyngeal Slits: Openings in the throat region; function as gills in aquatic species or are modified in terrestrial species.

• Post-anal Tail: Extension of the body past the anal opening.

Chordata: Subphyla and Characteristics

Cephalochordata (Lancelets)

Lancelets retain all chordate features throughout their life.

• Notochord: Functions as a stiff endoskeleton.

• Movement: Fish-like locomotion via muscle contractions.

• Feeding: Suspension feeders through pharyngeal slits.

• Habitat: Adults burrow in sand on the ocean floor.

• Reproduction: External fertilization.

Urochordata (Tunicates)

Tunicates are marine suspension feeders with distinct larval and adult forms.

• Feeding: Capture food particles using pharyngeal slits inside the siphon.

• Body Covering: External coat of polysaccharide (tunic).

• Reproduction: External fertilization.

• Mobility: Adults are sessile; larvae are mobile.

• Body Form: Sac-like adult body; tadpole larva.

Vertebrates

Vertebrates are chordates with a backbone and complex organ systems.

• Vertebral Column: Replaces the embryonic notochord; protects the spinal cord.

• Skull: Encloses and protects the brain; supports complex sense organs.

• Endoskeleton: Internal skeleton made of cartilage and/or bone; grows and repairs itself.

• Cephalization: High degree of development of the head region.

Summary Table: Deuterostome Phyla and Key Features

Additional info:

• Deuterostomes are essential for understanding animal evolution and diversity, as they include both invertebrate and vertebrate lineages.

• Chordates are the only deuterostome phylum to include vertebrates, which are the most complex animals.