7.1 Mollusks: bivalves, gastropods, and cephalopods

Mollusks are diverse marine creatures with unique adaptations. From bivalves' protective shells to cephalopods' advanced intelligence, these animals have evolved fascinating features. Their varied anatomies and behaviors allow them to thrive in different ocean environments.

Mollusks play crucial roles in marine ecosystems. As filter feeders, prey, and predators, they impact water quality and food webs. Some species, like oysters, even create habitats for other organisms. Humans also rely on mollusks for food and pearls.

Molluscan Anatomy and Adaptations

Anatomical features of mollusk classes

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• Bivalves
  • Two-part hinged shell provides protection from predators (clams, oysters)
  • Lack a distinct head which limits sensory capabilities
  • Possess a muscular foot used for burrowing into sediment or attachment to surfaces
  • Filter feeders using gills to strain plankton and organic particles from water
• Gastropods
  • Single, usually coiled shell offers defense and support for internal organs (snails, slugs)
  • Distinct head with sensory organs enables detection of food and mates
    • Eyes provide vision
    • Tentacles allow touch and chemoreception
  • Muscular foot facilitates crawling on surfaces or swimming in water column
  • Radula, a toothed ribbon-like structure, used for feeding by scraping algae or grazing
• Cephalopods
  • Soft body with a reduced or absent shell enhances flexibility and maneuverability (octopuses, squids)
  • Distinct head with complex eyes and a beak-like mouth enables active predation
    • Eyes among most advanced in animal kingdom
    • Beak used to tear flesh of prey
  • Muscular arms or tentacles with suckers or hooks aid in capturing prey and locomotion
  • Funnel, a tubular structure, used for jet propulsion and rapid locomotion

Ecological roles of mollusks

• Bivalves and gastropods serve as important primary consumers and prey for higher trophic levels
  • Clams and snails eaten by fish, crabs, sea stars, and marine mammals
• Bivalves filter water, removing particulates and improving water quality and clarity
  • Oyster reefs can filter up to 50 gallons of water per day per oyster
• Some mollusks, like oysters and mussels, create habitat for other species through reef formation
  • Oyster reefs provide shelter and feeding grounds for fish, crabs, and other invertebrates
• Economic importance
  • Many mollusks are important food sources for humans
    • Oysters, clams, scallops, and squid commonly consumed worldwide
  • Pearl production from certain bivalves generates significant revenue (pearl oysters)
  • Shells used for decorative purposes, construction materials (mother-of-pearl), and in traditional medicines

Mechanisms in molluscan biology

• Locomotion
  1. Bivalves: muscular foot used for burrowing into sediment or attachment to surfaces, some can swim by clapping valves (scallops)
  2. Gastropods: muscular foot used for crawling on surfaces or swimming in water column, some have modified feet for floating (sea butterflies)
  3. Cephalopods: jet propulsion achieved by expelling water through the funnel, fins aid in stabilization and maneuvering
• Feeding
  1. Bivalves: filter feeding using gills to capture plankton and organic particles suspended in water
  2. Gastropods: grazing or scraping algae and detritus using the radula, some are active predators (cone snails)
  3. Cephalopods: predatory lifestyle, using arms or tentacles to capture prey and a beak to tear flesh
• Reproduction
  • Most mollusks have separate sexes and engage in external fertilization, releasing gametes into the water
  • Some gastropods (nudibranchs) and cephalopods have internal fertilization and direct development
  • Bivalves and many gastropods have a planktonic larval stage that disperses in currents before settling
    • Trochophore larva develops into veliger larva before metamorphosis

Adaptations of cephalopods

• Intelligence
  • Complex nervous system and large brain-to-body size ratio enables advanced cognitive abilities
  • Ability to learn, remember, and solve problems demonstrated in laboratory studies
  • Demonstrated tool use (coconut shells for shelter) and social behaviors (communication, cooperation) in some species
• Camouflage and color change
  • Chromatophores (pigment cells), iridophores (reflective cells), and leucophores (white cells) in the skin allow rapid color and pattern changes
  • Used for communication, mating displays, and predator avoidance by blending into surroundings
  • Some species, like the mimic octopus, can imitate other animals (lionfish, sea snakes)
• Other adaptations
  • Closed circulatory system with a highly efficient copper-based blood pigment $\text{(hemocyanin)}$ enhances oxygen transport
  • Ink sac produces a dark, viscous liquid used for creating a smokescreen to confuse predators during escape
  • Ability to regenerate lost limbs, a useful adaptation to escape predation and recover from injury