cnidarians part 2 lecture 5 animal diversity

Introduction

  • By C. Isabelle Aubé (she/her) contact: isabelle.aube@dal.ca

  • Lecture content adapted from J. Frail-Gauthier, J. Bennett, and McGraw Hill LLC.

  • Images adapted from Hickman et al. (2021) or attributed within.

  • Course: BIOL 2003 - Animal Diversity Lecture 5: Cnidaria (Part 2)

  • Dalhousie University acknowledges the Indigenous territories of the Mi’kmaw, Wolastoqey, and Peskotomuhkati Peoples, and the contributions of African Nova Scotians.

Phylum Placozoa

  • Animalia - Placozoa: Derived from Greek origins where 'plakos' means flat and 'zoon' means animal.

  • Main species: Trichoplax adhaerens (where "trich" means hair, "plax" means plate, and "adhere" suggests its adhesive capabilities).

Body Forms of Cnidarians

Dimorphic Body Forms:

  1. Polyp (Hydroid)

    • Typically sessile.

    • Form resembles anemones, may be solitary or colonial.

  2. Medusa (Jelly)

    • Swimming form resembling jellyfish.

Structure Overview:

  • Epidermis: Outermost layer.

  • Mesoglea: Gelatinous layer between the epidermis and gastrodermis.

  • Gastrodermis: Inner layer involved in digestion.

Major Features of Cnidarians

  • Have two main body forms: polyp and medusa.

  • Unique stinging cells known as cnidocytes.

  • Presence of a jelly-like layer called mesoglea.

  • Have a blind-ended gut with tentacles surrounding the mouth.

  • Reproductive strategies include both asexual and sexual methods.

  • Approximately 10,000 species, primarily aquatic and mostly marine.

  • Divided into 5 main classes.

Cnidocyte Structure and Function

  • Cnidocyte: A specialized cell with an invagination of epithelium.

  • Contains cnidae, which are capsules within the cell; the most notable type is the nematocyst.

    • Nematocyst: A harpoon-like structure capable of paralyzing prey via venom with a sting.

    • Cnidocil: A hair-like trigger that initiates discharge of the nematocyst.

  • Discharge involves rapid hydrostatic pressure and is a one-time event.

Developmental Overview

  • Cnidarians are diploblastic organisms, developing from:

    1. Zygote

    2. Blastula

    3. Gastrula, which eventually forms the gut

  • Early developmental stages include blastopore formation and differentiation into endoderm and ectoderm.

Reproductive Strategies

  • Often exhibit alternation of generations:

    • Asexually reproducing polyp stage leads to the formation of a sexually reproducing medusa stage.

    • Polyps can asexually produce medusae, while medusae undergo meiosis to produce gametes.

    • Gametes fuse to form a planula larva, which later attaches to form a new polyp.

Body Structures & Important Cells

Key Structures

  • Gastrovascular cavity: Central cavity for digestion and circulation.

  • Tentacles: Surround the mouth and contain cnidocytes.

Important Cells

  • Nerve and Muscle Cells: Present but not organized into a centralized nervous system.

  • Cnidocytes: As previously explained, critical for defense and prey capture.

Taxonomic Classification of Cnidarians

  • Phylum Cnidaria

    • Major classes include:

    • Anthozoa

    • Staurozoa

    • Scyphozoa

    • Cubozoa

    • Hydrozoa

Class Anthozoa

  • Characterized by its polyp form only.

  • Over 6000 species including anemones and corals.

  • Structures include tubular body and large gastrovascular cavity.

Subclass Hexacorallia

  • Features 6 axes of symmetry with examples like stony corals and anemones.

Subclass Octocorallia

  • Displays 8-part symmetry; includes gorgonians, which have flexible bodies with calcareous spicules in mesoglea.

Ecological Importance of Anthozoa

  • Coral reefs act as biodiversity hotspots and provide habitat for approximately 25% of marine life.

  • Support deep-sea ecosystems.

Mutualistic Relationships

Zooxanthellae

  • Photosynthetic dinoflagellates that have a mutualistic, endosymbiotic relationship with corals.

  • Provide oxygen and organic materials (sugars) to the corals; corals reciprocate with protection and nutrients, including carbon dioxide and nitrates.

  • Coral bleaching occurs when zooxanthellae are expelled due to stress.

Mutualism with Anemones

  • Fish and crabs benefit from anemones (e.g., clownfish benefit from protective mucus, and certain crabs are associated with anemones).

Class Scyphozoa

  • Also known as true jellies, with approximately 200-400 species.

  • Dioecious organisms mean they have separate sexes.

  • Characterized by large oral lobes extending from the mouth.

Notable species in Scyphozoa

  • Cyanea capillata (Lion's Mane Jelly)

  • Aurelia aurita (Moon Jelly)

Class Cubozoa

  • Notable for their square-shaped bells and tentacles extending from the corners.

  • Rhopalia: Sensory structures, some capable of image formation.

  • Chironex fleckeri (sea wasp) is a known fatal species.

Class Hydrozoa

  • A diverse group including the only known freshwater cnidarians (Hydra).

  • Hydra can bud off new individuals and do not have a medusa stage.

  • Many hydrozoans are colonial with distinct polymorphic forms, as seen in hydroids and siphonophores (like the Portuguese Man-O-War).

Siphonophores and Bioluminescence

  • Many siphonophores exhibit bioluminescence.

Conclusion of Lecture

  • The phylum Cnidaria exhibits significant diversity in form, structure, and ecological role.

  • The next lecture will cover Ctenophora and an introduction to Bilaterians and Xenacoelomorpha.

Review Questions

  • Group Cnidarians into their five taxonomic classes, providing examples and key facts for each class.