Non-Bilaterians

NON-BILATERIANS

INTRODUCTION TO PHYLOGENY

  • Phylogenetic Tree of Animals

    • Common Ancestor: Evolutionary tree representing relationships among various animal groups.

    • Unique traits of Non-Bilaterians include:

    • Unique cell junctions.

    • Presence of collagen and proteoglycans in extracellular matrix.

    • Formation of nerve nets.

  • Main Groups:

    • Ctenophores (Comb Jellies)

    • Porifera (Sponges)

    • Placozoans

    • Cnidarians

  • Distinction between:

    • Diploblastic Animals: Organisms with two germ layers (Ectoderm, Endoderm).

    • Triploblastic Animals: Organisms with three germ layers (Ectoderm, Mesoderm, Endoderm).

    • Symmetry:

    • Radial Symmetry: Present in Cnidarians and Ctenophores.

    • Bilateral Symmetry: Present in more evolved groups (e.g., Bilaterians).

CTENOPHORES (COMB JELLIES)

Phylogeny

  • Ctenophores do not resemble the ancestral animal, Choanoflagellate.

  • Evolved since a common ancestor approximately 700 million years ago.

  • Characteristics:

    • Fully marine, around 250 species.

    • Genome sequences suggest Ctenophores were the earliest lineage to diverge from other animals.

Body Plan

  • Structure:

    • Diploblastic body plan with two cellular layers separated by inert mesoglea.

    • Complete gut: Food enters through a mouth and waste is expelled through two anal pores.

Reproduction

  • Ctenophores are hermaphroditic.

  • Most species lack a distinct larval stage; entire life cycle occurs in plankton.

Special Features

  • Movement primarily through ctenes (comb-like rows of cilia).

  • Capture prey using tentacles equipped with colloblasts, which discharge an adhesive material upon contact with prey.

Nervous System

  • Organized into nerve nets (non-centralized).

  • Biochemical makeup differs from other animals, lacking serotonin and dopamine; possess different neurotransmitters.

  • Possibility of independent evolutionary origin of nervous systems from those in other animals.

PORIFERA (SPONGES)

Body Plan

  • Comprising 8,500 species, sponges are the simplest animals.

  • Lacking distinct tissue types and anatomical symmetry.

  • Possess specialized cells, but no embryonic cell layers or true organs.

  • Most body cells are totipotent, capable of changing form and function.

  • Choanocytes closely resemble choanoflagellates, retaining ancestral features.

Ecology

  • Most sponges act as filter feeders, trapping bacteria and microorganisms.

  • Primarily marine, with some freshwater species.

  • Body forms are adapted to varying water currents.

Reproduction

  • Reproduction may be sexual or asexual:

    1. Sexual Reproduction: Sponges are generally hermaphroditic, producing both eggs and sperm. Sperm is transported to eggs via archaeocytes, fertilization occurs in the mesenchyme, leading to ciliated larvae.

    2. Asexual Reproduction: Via budding or fragmentation.

Special Features

  • Sponges have skeletal elements called spicules:

    • Glass sponges and demosponges contain silicon dioxide spicules.

    • Calcareous sponges possess calcium carbonate spicules.

Unusual Sponges

  • Some sponges are predatory, using hook-shaped spicules to trap prey. These specimens are often found in deep-sea environments.

Antimicrobial Molecules

  • Sponges produce bioactive compounds that serve as antimicrobials, offering potential for drug discovery due to their evolutionary adaptations to challenging environments.

PLACOZOANS

General Features

  • Placozoans are asymmetrical, flat, disk-shaped organisms that adhere to surfaces, such as aquarium walls or ocean substrates.

  • Fully marine, with weakly differentiated tissue layers and few cell types.

  • Lack a mouth, gut, or nervous system but exhibit basic functions such as feeding, digestion, and reproduction (both sexual and asexual).

Diversity and Taxonomy

  • The phylum has historically been monotypic, originally comprising only Trichoplax adhaerens, but may include approximately 100 species across several genera.

CNIDARIA

Classification as Eumetazoans

  • Together with Bilaterians, Cnidarians are classified as Eumetazoans characterized by:

    • Body symmetry.

    • Presence of a gut.

    • Presence of a nervous system.

    • Organized tissues in organs.

Body Plan

  • Structure:

    • The gut is a gastrovascular cavity involved in digestion, circulation, and gas exchange, supported by a hydrostatic skeleton.

    • Features include:

    • Body stalk.

    • Mouth/anus.

    • Tentacles.

    • Gastrodermis.

    • Mesoglea.

    • Epidermis.

Nervous System

  • Cnidarians have nerve nets that are derived independently from those in Ctenophores and Bilaterians.

Reproduction

  • Cnidarian life cycle includes:

    1. Motile Medusa Stage: Free-swimming and produces gametes for sexual reproduction.

    2. Sessile Polyp Stage: Attaches to substrates and primarily engages in asexual reproduction through strobilation.

Special Features - Nematocysts

  • Specialized cell structures that:

    • Inject toxins into prey.

    • Assist in holding onto prey.

  • Nematocysts are capable of rapid deployment:

    • Operculum opens.

    • Water rushes in.

    • A harpoon-like structure shoots out within less than a millionth of a second.

    • Remains capable of firing even if the organism is no longer in contact or is deceased.

Major Classes of Cnidarians

  1. Anthozoa (e.g., sea anemones, corals)

  2. Scyphozoa (e.g., true jellyfish)

  3. Hydrozoa (e.g., hydroids, siphonophores)

  4. Cubozoa (e.g., box jellyfish)

Class Anthozoa

  • Comprised of solitary sea anemones and colonial corals. Corals:

    • Secrete organic matrices and calcium carbonate, forming skeletons.

    • Form reefs primarily in shallow tropical waters where photosynthesis is supported by symbiotic dinoflagellates (zooxanthellae).

    • Threatened by oceanic changes such as increased CO2 concentrations leading to bleaching.

Class Scyphozoa

  • Commonly known as “true” jellyfish.

  • Around 200 species discovered with sizes ranging from 2 mm to over 2 m in diameter.

  • Characterized by a dominant medusa stage and possessing specific structures:

    • Rhopalia for balance.

    • Statoliths for orientation.

    • Ocellus for light perception.

Class Hydrozoa

  • Includes colonial hydroids which may stretch up to 30 m in length, where:

    • Each individual performs specific roles (pneumatophores, nectophores, dactylozooids, gastrozooids, gonozoids).

  • Includes bioluminescent species which helps in predation detection.

Class Cubozoa

  • Box jellyfish, characterized by a square-shaped body and well-developed eyes.

  • Fast swimmers; reproduce through unique mating behavior.

  • Venom from nematocysts can be life-threatening to prey.

SUMMARY

  • Non-Bilaterians are a paraphyletic group of early-branching animals characterized by:

    • Lack of symmetry or presence of radial symmetry.

    • Diploblastic or minimal tissue organization.

    • Varied ecological roles, from sessile filter feeders to active predators.

  • Their potential for bioactive compound production is significant.

  • Understanding their evolutionary backgrounds aids in comprehending broader animal evolution and functional relationships.