Phylum Mollusca

Phylum Mollusca Overview

  • Second largest phylum in the animal kingdom, after arthropods.
  • Approx. 100,000 living species and 70,000 fossil species, some dating back to the pre-Cambrian period (around 450 million years ago).
  • The name "Mollusca" comes from Latin, meaning "soft."
  • Tremendous morphological and ecological diversity:
    • Classes include Gastropoda, Bivalvia, Cephalopoda, Polyplacophora, etc.

Key Characteristics of Mollusca

  1. Body Plan:

    • Consists of four main regions:
      • Foot: Muscular structure for locomotion or clinging.
      • Mantle: Dorsal epithelium that secretes the shell.
      • Visceral mass: Contains internal organs.
      • Mantle cavity: Houses gills and facilitates respiration.

  2. Digestive System:

    • Complete digestive tract (mouth and anus).
    • Includes a radula (a feeding structure) except in bivalves.

  3. Reproductive System:

    • Mostly gonochoristic (separate sexes), some are hermaphroditic (e.g., certain gastropods).
    • Modes of reproduction include internal and external fertilization.
    • Development stages include trochophore and veliger stages.

Classes of Mollusca

Class Gastropoda
  • Examples: Snails, slugs, nudibranchs.
  • Includes terrestrial and marine species.
  • Most diverse mollusk class with 65,000 - 80,000 species.
Class Bivalvia
  • Examples: Clams, oysters, mussels, scallops.
  • Generally have two shells and lack a radula.
  • Important ecological role as filter feeders.
Class Cephalopoda
  • Examples: Squid, octopus, cuttlefish, nautilus.
  • Known for advanced nervous systems and complex behaviors.
  • Many are capable of camouflage and have sophisticated locomotion.
Class Polyplacophora
  • Example: Chitons.
  • Recognizable for having eight calcareous plates on their dorsal surface.

Importance of Mollusca

  • Economic Importance:

    • Source of food for humans (e.g., oysters, clams).
    • Cultured for mariculture and commercially harvested.
    • Historical use of shells for currency (wampum).

  • Ecological Importance:

    • Oysters and other bivalves provide habitat and improve water quality through filtration.
    • Considered foundation species, significantly affecting biodiversity and ecosystem structure.

Respiratory System

  • Ctenidia: Specialized gills found in aquatic mollusks.
  • Gills generally located in the mantle cavity, allowing for efficient gas exchange.
  • Counter-current flow: Maximizes oxygen diffusion efficiency into the blood by having blood flow in the opposite direction of water.

Shell Structure

  • Composed of three layers:
    1. Periostracum: Outer proteinaceous layer.
    2. Prismatic layer: Middle layer made of columnar crystals of CaCO3.
    3. Nacreous layer: Inner shiny layer (mother of pearl) made of sheets of CaCO3.
  • Pearl Formation: Occurs when irritants become encrusted by nacre layers, forming a pearl to protect the mollusk.

Laboratory and Exam Notes

  • Upcoming Lab Practical and Exam 1:
    • Ensure lab notebooks are submitted by the given deadlines.
    • Review the study guide posted on Moodle for comprehensive exam preparation.

Fun Fact

  • The variety of mollusks also demonstrates an exceptional range of adaptations, making them a model group for studying evolution and biological diversity.