Mollusca: Gastropoda

Mollusca Overview

  • Mollusca is a diverse phylum of invertebrate animals.
  • Major mollusc classes (excluding extinct groups) include:
    • Aplacophora
    • Polyplacophora
    • Monoplacophora
    • Bivalvia
    • Scaphopoda
    • Gastropoda
    • Cephalopoda
  • Detailed phylogeny of Aplacophora remains unclear.

Gastropoda Specifics

  • The class Gastropoda is characterized by the term "stomach foot."
  • Major subdivisions of Gastropoda include:
    • Orthogastropoda
    • Apogastropoda
    • Eogastropoda (includes true limpets and relatives)
    • Vetigastropoda (e.g., abalone, top-snails, keyhole limpets, turban shells)
    • Cocculinoidea
    • Neritopsina
    • Caenogastropoda (includes winkles, whelks, balers, heteropods, cowries, cones, etc.)
    • Heterobranchia (includes seaslugs, bubble shells, pulmonates, etc.)
  • Pechenik commonly categorizes gastropods into three groups:
    • Prosobranchs
    • Opisthobranchs
    • Pulmonates

Distribution of Gastropoda

  • Around 30,000 marine species
  • Approximately 5,000 freshwater species
  • Roughly 25,000 terrestrial species (most being pulmonates with a lung instead of gills)
  • About 500 species have been documented as extinct since the year 1500.

Key Biological Features

Torsion

  • Torsion refers to the twisting of the head and foot by 180 degrees during the development of the gastropod.
  • Digestive Tract Changes:
    • Ancestral Condition:
    • Water flows untorted and currents move around the mouth and foot to the gill.
    • Derived Condition:
    • Torted arrangement resulting in altered water flow and digestive system layout.

Shell and Operculum

  • Gastropods possess a shell equipped with an operculum, an anatomical feature that provides protection against predators and dehydration.

Dietary Habits

  • Gastropods exhibit diverse feeding habits, which include:
    • Herbivores
    • Predators
    • Detritivores
    • Scavengers
    • Parasites
  • Many gastropods also serve as intermediate hosts for parasites that affect both livestock and humans.

Life-style Adaptations

  • Gastropods may be categorized based on their lifestyle into:
    • Benthic-Crawling
    • Infaunal-Burrowing
    • Pelagic-Swimming

Reproduction

  • Gastropod reproduction strategies include:
    • Either dioecious or hermaphroditic reproductive systems.
    • Fertilization typically occurs internally.
    • Offspring can develop into planktonic larvae or crawl-away juveniles.

Larval Development

  • The Veliger larvae stage is significant in the development of many gastropods.

Major Gastropod Orders and Examples

1) Eogastropoda

  • Includes limpets and their relatives.
    • Characterized by a cone-shaped shell.
    • Generally classified as herbivores.

2) Vetigastropoda

  • A notable group characterized by various shell shapes and sizes.

3) Cocculinoidea

  • Known for unique shell features and adaptations.

4) Neritopsina

  • Examples include:
    • Neritopsis radula (Linnaeus, 1758) found in Indonesia (Moluccas).

5) Caenogastropoda

  • Includes important families like:
    • Conchs
    • Whelks
    • Cowries
    • Cones

6) Heterobranchia

  • Comprises three informal groups: lower heterobranchs, opisthobranchs, and pulmonates.
  • Opisthobranchs examples include:
    • Sea hares (Aplysia)
    • Nudibranchs
    • Bubble snails
  • In heterobranchs, features such as the shell and operculum can be reduced or lost, torsion may also be lost or reduced.
  • Employ various alternative defenses using nematocysts and chemicals.
  • Aplysia is notably used as a model organism in neurobiology studies.

Pulmonates

  • Primarily terrestrial and freshwater, notable members include:
    • Slugs
    • Land snails
  • Characterized by a highly vascularized mantle cavity that functions as a lung.