Basal Animal Lineages Study Guide

TOPIC 11: BASAL ANIMAL LINEAGES

General Overview

  • Discusses the fundamental animal lineages including key phyla: Porifera, Cnidaria, Ctenophora, and Bilateria.

  • Provides insights into cellular and anatomical features that define these groups.

Animal Lineages

  • **Lineages noted: **

    • Porifera (sponges)

    • Cnidaria (jellyfish, corals, and anemones)

    • Ctenophora (comb jellies)

    • Bilateria (animals with bilateral symmetry)

Choanoflagellates

  • Defined as the outgroup to all animals.

    • Characteristics:

    • Sessile organisms that can exist either solitary or colonial.

    • Have cells with a ‘collar’ structure that aid in filter feeding.

Transition from Choanoflagellates to Porifera

  • Unicellular to Multicellular Transition:

    • Unicellular Choanoflagellates are sessile protists, sometimes existing in colonies.

    • Multicellular sponges (Phylum Porifera) are sessile animals showcasing a complex structure.

    • Feeding Mechanism:

    • Choanocytes (feeding cells in sponges) create water currents to filter feed.

Phylum Porifera (Sponges)

  • Fossil Record: Dates back approximately 700 million years.

  • Lack of true tissues, organs, or symmetry.

    • Synapomorphies of Porifera:

    • Bosses** - Characteristic cells with flagella (choanocytes).

    • Spicules:

      • Provide structural support and defense for sponges.

      • Can be composed of calcium or silica.

      • Their structure and shape serve as significant taxonomic features.

Choanocytes

  • Defined as specialized sponge cells responsible for water pumping to facilitate filter feeding.

    • Key function includes:

    • Structural support

    • Pumping water to aid in filter feeding (Correct answer to function).

Cnidaria & Ctenophora

  • Both phyla are identified as diploblastic organisms:

    • Tissue layers:

    • Composed of only two layers: ectoderm and endoderm, lacking mesoderm.

Symmetry

  • Both Cnidaria and Ctenophora exhibit radial symmetry.

Gastrulation Process

  • Gastrulation results in the development of initial cell layers:

    • Forms two layers leading to:

    • Incomplete gastric systems (Cnidaria)

    • Complete gastric systems with a one-way gut for Ctenophora.

Phylum Ctenophora (Comb Jellies)

  • Unique features include:

    • Possess complete guts and mesoderm-derived muscles.

    • Muscle origins differ from other animal types.

    • Locomotion achieved through cilia and prey capture through specialized sticky cells known as coloblasts.

    • About 100 species are all marine and found in various marine environments.

Comparative Neurology

  • Ctenophores exhibit an integrated nervous system with a primitive central nervous system (CNS).

    • Cnidarians only feature a nerve net.

  • Ctenophores lack HOX genes and specific microRNA sequences which regulate gene expression.

    • Notably, sponges do possess microRNA.

Common Hypotheses in Phylogenetics

  • Discusses phylogenetic relationships among Choanoflagellates, Porifera, Cnidaria, and Ctenophora.

  • Ctenophores are positioned as sister taxa to all other animals, contrasting with various hypotheses.

  • Study reveals:

    • Ancient chromosomal syntenies conserved between animals and unicellular relatives, indicating evolutionary lineage.

    • Ctenophores share ancestral traits with unicellular eukaryotes, while other groups like sponges share derived traits, supporting monophyletic clades.

Notable Article Discussion

  • Article published in Nature discussing ancient gene linkages to support ctenophores as the sister lineage to other animals, highlighting complex systems' evolution:

    • Authors: Darrin T. Schultz, Steven H. D. Haddock, et al.

    • Mention of advancements in chromosome scale gene linkage analysis.

Phylum Cnidaria (Jellyfish, Corals, Anemones)

  • Dimorphic Lifecycle: Exhibits both polyp and medusa forms.

  • Complex structures including:

    • Stinging cells (Cnide)[c.nid.o.cyst] are utilized for prey capture and defense.

  • Feeding strategy is carnivorous, often forming symbiotic relationships.

    • Prey captured using cnidae are moved to the gastrovascular cavity; incomplete digestion occurs extracellularly followed by intracellular digestion.

Structure and Biological Features

  • Two-way digestive system characterized by:

    • Presence of stinging cells, articulated structures such as barbs, and characteristics defining the phylum such as nematocysts.

Differences Between Ctenophores and Cnidarians

  • Ctenophores are not grouped with Cnidarians due to:

    1. Differences in symmetry (Ctenophores have bilateral tendencies, not radial).

    2. Absence of stinging cells, utilizing cilia instead for movement.

    3. Habitat differences (Ctenophores are common in open waters).

Conclusion

  • The study of basal animal lineages unveils crucial evolutionary, anatomical, and functional differences within fundamental animal phyla, grounding our understanding in both historical and biological contexts. The phylogenetic relationships elucidate the deep evolutionary patterns that continue to inform modern biology and evolutionary studies.