🐠Marine Biology Unit 6 – Marine Invertebrates: Sponges & Cnidarians

Key Concepts

• Sponges and cnidarians are two of the most primitive animal phyla in the marine environment
• Sponges lack true tissues and organs, while cnidarians have simple tissue-level organization
• Both phyla are predominantly sessile or have limited mobility as adults
• Sponges are filter feeders that rely on water currents to obtain food and remove waste
• Cnidarians possess specialized stinging cells called nematocysts for prey capture and defense
• Sponges have a unique skeletal structure composed of spicules or spongin fibers
• Cnidarians exhibit two distinct body forms: polyp and medusa

Evolutionary Context

• Sponges are considered the oldest surviving animal phylum, with fossil evidence dating back to the Precambrian era (over 600 million years ago)
• Cnidarians emerged during the Cambrian explosion, a period of rapid animal diversification (approximately 540 million years ago)
• The evolution of sponges and cnidarians marked the transition from single-celled organisms to multicellular animals
• Sponges lack true tissues and organs, representing a primitive level of animal organization
  • This suggests they diverged early in animal evolution before the development of complex body plans
• Cnidarians are the first animals to develop true tissues and a nervous system
  • The presence of a nerve net allows for coordinated behavior and responsiveness to stimuli
• Both phyla have undergone adaptive radiation, giving rise to diverse species adapted to various marine habitats (coral reefs, deep sea, polar regions)

Anatomy & Physiology

• Sponges have a simple body plan consisting of two cell layers: the outer pinacoderm and inner choanoderm
  • The space between these layers is filled with mesohyl, a gelatinous matrix containing amoeboid cells
• Water enters sponges through tiny pores called ostia, passes through a system of canals, and exits through the osculum
  • Choanocytes, collar cells lining the choanoderm, create water currents and filter food particles
• Cnidarians have a basic body plan with two cell layers: the outer epidermis and inner gastrodermis, separated by a jelly-like mesoglea
• Cnidarian polyps are cylindrical, with a mouth surrounded by tentacles on the upper end and a basal disc for attachment
  • The mouth leads to a gastrovascular cavity that serves as a stomach and circulatory system
• Medusae, the free-swimming form of some cnidarians, have a bell-shaped body with tentacles hanging from the margin
  • Contractions of the bell allow for locomotion through water
• Cnidarians possess a unique cell type called cnidocytes, which contain nematocysts
  • Nematocysts are harpoon-like structures used for prey capture, defense, and attachment

Diversity & Classification

• Sponges are classified into four classes based on the composition of their skeleton: Calcarea (calcareous sponges), Hexactinellida (glass sponges), Demospongiae (horny sponges), and Homoscleromorpha (encrusting sponges)
  • Demospongiae is the most diverse class, containing over 90% of all sponge species
• Cnidarians are divided into four main classes: Anthozoa (sea anemones and corals), Hydrozoa (hydras and siphonophores), Scyphozoa (true jellyfish), and Cubozoa (box jellyfish)
  • Anthozoa is the most speciose class, with over 6,000 described species
• Sponges exhibit a wide range of shapes, sizes, and colors, adapted to various marine habitats
  • Some notable examples include barrel sponges, tube sponges, and encrusting sponges
• Cnidarian diversity is showcased by the variety of polyp and medusa forms
  • Polyps can be solitary (sea anemones) or colonial (corals), while medusae range from tiny hydromedusae to large scyphozoan jellyfish

Ecological Roles

• Sponges play a crucial role in marine ecosystems as filter feeders, removing bacteria, phytoplankton, and organic debris from the water column
  • This filtration process helps maintain water clarity and nutrient cycling
• Many sponge species form symbiotic relationships with microorganisms (bacteria, archaea, and microalgae) that contribute to their nutrition and chemical defense
• Cnidarians, particularly coral reefs, provide habitat and shelter for a wide variety of marine organisms
  • Coral reefs are among the most diverse ecosystems on Earth, supporting an estimated 25% of all marine species
• Some cnidarians, such as jellyfish and siphonophores, are important predators in pelagic food webs
  • They consume zooplankton and small fish, transferring energy to higher trophic levels
• Cnidarians also engage in symbiotic relationships, most notably the mutualism between corals and zooxanthellae (dinoflagellate algae)
  • Zooxanthellae provide corals with nutrients through photosynthesis, while corals offer protection and access to sunlight

Adaptations & Survival Strategies

• Sponges have evolved a highly efficient water pumping and filtration system that allows them to thrive in nutrient-poor environments
  • Some species can filter up to 20,000 times their body volume of water per day
• The presence of spicules or spongin fibers in sponges provides structural support and deters predation
  • Some sponges also produce secondary metabolites with antimicrobial and antifouling properties
• Cnidarians have developed specialized stinging cells (nematocysts) that enable them to capture prey and defend against predators
  • The toxins in nematocysts can range from mild to highly potent, depending on the species
• Many cnidarians have evolved the ability to regenerate lost body parts, allowing them to recover from injury or fragmentation
  • Some species, like hydras, exhibit remarkable regenerative capabilities and can even regenerate from dissociated cells
• Cnidarians employ various reproductive strategies, including asexual budding and sexual reproduction, to ensure population growth and dispersal
  • Some species have a complex life cycle alternating between polyp and medusa stages, enhancing their adaptability to different environmental conditions

Human Impact & Conservation

• Sponges and cnidarians face numerous threats from human activities, including habitat destruction, pollution, and climate change
  • Coral reefs are particularly vulnerable to rising sea temperatures, ocean acidification, and physical damage from fishing and tourism
• Overharvesting of sponges for commercial purposes (bath sponges, biomedical research) has led to the decline of some species
  • Sustainable harvesting practices and aquaculture initiatives aim to mitigate this impact
• The global decline of coral reefs has far-reaching consequences for marine biodiversity and the livelihoods of coastal communities
  • Conservation efforts focus on establishing marine protected areas, reducing greenhouse gas emissions, and promoting sustainable reef management
• Invasive species, such as the crown-of-thorns starfish, pose a significant threat to coral reefs by consuming large amounts of coral tissue
  • Control measures, including manual removal and biological control, are being implemented to manage these outbreaks
• Citizen science initiatives and public awareness campaigns play a crucial role in monitoring sponge and cnidarian populations and promoting their conservation
  • Programs like Reef Check and CoralWatch engage volunteers in data collection and reef health assessment

Cool Facts & Research Highlights

• Some sponge species can live for over 2,000 years, making them among the longest-living animals on Earth
  • The glass sponge Monorhaphis chuni has been estimated to reach ages of up to 11,000 years
• Sponges have a remarkable ability to regenerate from small fragments, a process known as dissociation-reaggregation
  • This has led to research on sponge cell culture and tissue engineering applications
• The immortal jellyfish (Turritopsis dohrnii) can revert from its adult medusa stage back to its juvenile polyp stage, essentially achieving biological immortality
  • This unique ability has attracted scientific interest in understanding the mechanisms of cellular rejuvenation and aging
• Coral reefs are often referred to as the "rainforests of the sea" due to their high biodiversity and productivity
  • They cover less than 1% of the ocean floor but support an estimated 25% of all marine species
• Some cnidarians, like the Portuguese man-o-war (Physalia physalis), are not single organisms but colonies of specialized individuals called zooids
  • Each zooid has a specific function (feeding, reproduction, defense) and cannot survive independently
• Sponges and cnidarians are rich sources of bioactive compounds with potential pharmaceutical applications
  • Compounds derived from these animals have shown promise in treating cancer, viral infections, and inflammatory diseases
• Recent research has highlighted the importance of sponge and cnidarian microbiomes in their health, adaptability, and response to environmental stressors
  • Understanding these microbial communities may provide insights into coral reef resilience and conservation strategies