🐠Marine Biology Unit 7 – Marine Invertebrates: Mollusks & Arthropods

Key Concepts

• Mollusks and arthropods represent two major phyla of marine invertebrates
• Mollusks include diverse groups such as bivalves (clams, oysters), gastropods (snails, slugs), and cephalopods (octopuses, squids)
• Arthropods encompass crustaceans (crabs, lobsters, shrimp), chelicerates (horseshoe crabs, sea spiders), and others
• Both phyla exhibit a wide range of morphological and behavioral adaptations to marine environments
• Mollusks and arthropods play crucial roles in marine food webs and ecosystems
• Many species of mollusks and arthropods hold significant economic and cultural value for humans
• Conservation efforts aim to protect these invertebrates from threats such as overharvesting, habitat destruction, and pollution

Classification and Diversity

• Mollusks are classified into seven classes: Gastropoda, Bivalvia, Cephalopoda, Polyplacophora (chitons), Scaphopoda (tusk shells), Monoplacophora, and Aplacophora
  • Gastropoda is the most diverse class, including snails, slugs, and limpets
  • Bivalvia includes clams, oysters, mussels, and scallops
  • Cephalopoda encompasses octopuses, squids, cuttlefish, and nautiluses
• Arthropods are divided into four subphyla: Chelicerata, Crustacea, Hexapoda, and Myriapoda
  • Crustacea is the most diverse subphylum in marine environments, including crabs, lobsters, shrimp, and barnacles
  • Chelicerata includes horseshoe crabs and sea spiders
• Mollusks and arthropods exhibit an incredible range of sizes, from microscopic to giant squid and Japanese spider crabs
• Both phyla have successfully colonized various marine habitats, from shallow coastal waters to deep-sea environments
• The diversity of mollusks and arthropods reflects their adaptability and evolutionary success in marine ecosystems

Anatomy and Physiology

• Mollusks have a soft body usually protected by a hard shell secreted by the mantle
  • The mantle is a layer of tissue that covers the body and produces the shell
  • Some mollusks, like slugs and octopuses, have reduced or absent shells
• Mollusks have a muscular foot used for locomotion, burrowing, or attachment to substrates
• Many mollusks possess a radula, a ribbon-like structure with rows of teeth used for feeding
• Arthropods have a segmented body covered by a hard exoskeleton made of chitin
  • The exoskeleton provides protection and support but must be molted periodically to allow for growth
• Arthropods have jointed appendages used for locomotion, feeding, and sensory functions
  • Crustaceans have two pairs of antennae and biramous (branched) appendages
• Both mollusks and arthropods have well-developed nervous systems and sensory organs
  • Cephalopods have complex eyes similar to those of vertebrates
  • Arthropods have compound eyes and various sensory setae (hairs) for detecting stimuli

Habitat and Ecology

• Mollusks and arthropods inhabit a wide range of marine habitats, from intertidal zones to the deep sea
• Many mollusks are benthic (bottom-dwelling), living on or in the sediment
  • Clams and oysters are infaunal, burrowing into the sediment
  • Snails and chitons are epifaunal, living on the surface of rocks or other substrates
• Some mollusks, like octopuses and squids, are pelagic, swimming in the water column
• Arthropods occupy various ecological niches in marine environments
  • Crustaceans can be benthic (crabs, lobsters) or pelagic (krill, copepods)
  • Barnacles are sessile, attaching themselves to rocks, shells, or other surfaces
• Mollusks and arthropods are important components of marine food webs
  • Many species are herbivores, grazing on algae or filtering phytoplankton
  • Others are predators, feeding on smaller invertebrates or even fish
  • Some, like clams and oysters, are filter feeders, straining particles from the water

Adaptations and Survival Strategies

• Mollusks have evolved various adaptations for protection, locomotion, and feeding
  • Hard shells provide defense against predators and environmental stressors
  • Some gastropods have an operculum, a hard plate that seals the shell opening when the animal retracts
  • Cephalopods have a highly developed nervous system and complex behaviors for hunting and camouflage
    • Octopuses can change color and texture to blend in with their surroundings
    • Squids use jet propulsion for rapid swimming and escape from predators
• Arthropods have adapted to diverse marine habitats through specialized appendages and behaviors
  • Crustaceans have gills for respiration and osmoregulation in seawater
  • Some crabs have modified claws for crushing shells or filtering food particles
  • Mantis shrimp have powerful, fast-striking appendages for capturing prey or defending themselves
• Both mollusks and arthropods have evolved chemical defenses to deter predators
  • Some sea slugs incorporate toxic compounds from their prey into their own tissues
  • Many crustaceans have cryptic coloration or camouflage to avoid detection

Evolutionary History

• Mollusks and arthropods have a long evolutionary history, with origins dating back to the Cambrian Period (~541-485 million years ago)
• The earliest mollusks were likely small, shell-less animals that later evolved hard shells for protection
  • The diversification of mollusks occurred throughout the Paleozoic and Mesozoic Eras
  • The extinction of ammonites, a group of cephalopods, at the end of the Cretaceous Period (~66 million years ago) is a well-known event
• Arthropods also originated in the Cambrian Period, with the appearance of trilobites and other early forms
  • The evolution of crustaceans is closely tied to the colonization of marine habitats
  • The appearance of land-dwelling arthropods (insects, spiders) occurred later in the Paleozoic Era
• The fossil record provides insights into the evolutionary history and past diversity of mollusks and arthropods
  • Exceptional preservation in Lagerstätten (sedimentary deposits) like the Burgess Shale has revealed intricate details of ancient species

Ecological Importance

• Mollusks and arthropods play vital roles in marine ecosystems as primary consumers, predators, and prey
• Bivalves, such as clams and oysters, are important filter feeders that help maintain water quality
  • Oyster reefs provide habitat and shelter for numerous other species
  • The filtration activities of bivalves can mitigate the effects of eutrophication and improve clarity in coastal waters
• Gastropods, like snails and limpets, are grazers that control algal growth on rocky shores
• Cephalopods, particularly squids, are key predators in pelagic food webs
  • Squids are also important prey for many marine mammals, seabirds, and fish
• Crustaceans, such as krill and copepods, form the basis of many marine food chains
  • These small crustaceans are the primary food source for baleen whales and other filter feeders
• Crabs and lobsters are significant predators in benthic communities, regulating populations of other invertebrates
• The ecological roles of mollusks and arthropods contribute to the overall functioning and stability of marine ecosystems

Human Interactions and Impact

• Mollusks and arthropods have long been important resources for human populations
  • Many species, such as oysters, clams, scallops, crabs, and lobsters, are commercially harvested for food
  • The shells of some mollusks, like pearl oysters and abalone, are used for decorative purposes or in jewelry
• Overharvesting and destructive fishing practices can lead to the depletion of mollusk and arthropod populations
  • Bottom trawling can damage seafloor habitats and harm benthic communities
  • Aquaculture of mollusks and crustaceans can help alleviate pressure on wild populations but may have its own environmental impacts
• Pollution, particularly from plastic debris and chemical contaminants, poses threats to marine invertebrates
  • Microplastics can be ingested by filter-feeding bivalves and other organisms, potentially affecting their health and entering food webs
• Climate change and ocean acidification are growing concerns for mollusk and arthropod populations
  • Increasing water temperatures can alter species distributions and disrupt ecological interactions
  • Ocean acidification can impair the ability of mollusks to build and maintain their shells
• Conservation efforts, such as establishing marine protected areas and implementing sustainable fishing practices, are crucial for the long-term preservation of mollusk and arthropod diversity and the health of marine ecosystems