9.1 Porifera and Cnidaria

Porifera and Cnidaria are ancient animal groups crucial to marine ecosystems. Sponges, the oldest known metazoans, have simple body plans and filter-feed. Cnidarians, including corals and jellyfish, have more complex structures and unique stinging cells.

Both groups play vital roles in ocean habitats. Sponges filter water and provide homes for other organisms. Corals build massive reef structures, supporting incredible biodiversity. Their fossils offer insights into Earth's past environments and evolutionary history.

Porifera overview

• Porifera, commonly known as sponges, are multicellular organisms that belong to the kingdom Animalia
• Sponges are among the oldest known metazoan animals, with a fossil record dating back to the Precambrian
• Poriferans play important ecological roles in marine ecosystems, acting as filter feeders and providing habitat for various organisms

Sponge body structure

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• Sponges have a simple body plan consisting of two cell layers: the outer pinacoderm and the inner choanoderm
• The body is permeated by a complex system of canals and chambers through which water flows for feeding and gas exchange
• Specialized cells called choanocytes line the choanoderm and create water currents using their flagella
• Mesohyl, a gelatinous matrix, fills the space between the cell layers and contains various cell types (amoebocytes, archaeocytes)

Sponge skeletal elements

• Most sponges possess a skeleton composed of either organic fibers (spongin) or mineral elements (spicules)
• Spicules are made of silica (in demosponges and hexactinellids) or calcium carbonate (in calcareous sponges)
• Spicule morphology varies greatly among sponge taxa and is used for classification and identification
• Spongin fibers provide flexibility and support to the sponge body

Sponge reproduction strategies

• Sponges can reproduce both sexually and asexually
• Sexual reproduction involves the production of gametes (eggs and sperm) and subsequent fertilization
• Asexual reproduction occurs through budding, fragmentation, or the formation of gemmules (resistant structures containing totipotent cells)
• Gemmules allow sponges to survive unfavorable conditions and disperse to new habitats

Sponge fossil preservation

• Sponge fossils are commonly preserved as spicules or entire body fossils
• Siliceous spicules have a higher preservation potential than calcareous spicules due to their resistance to dissolution
• Sponge body fossils can be preserved through permineralization, carbonization, or as molds and casts
• Exceptional preservation of sponge soft tissues has been reported from Lagerstätten deposits (Burgess Shale, Chengjiang)

Porifera classification

• Porifera is divided into four main classes: Demospongiae, Hexactinellida, Calcarea, and Homoscleromorpha
• Classification is based on skeletal composition, spicule morphology, and other morphological characters
• Molecular phylogenetic studies have led to revisions in sponge classification and revealed new relationships among taxa

Demosponges vs hexactinellids

• Demospongiae is the most diverse class of sponges, characterized by the presence of siliceous spicules and/or spongin fibers
• Hexactinellida, also known as glass sponges, possess a skeleton composed of six-rayed siliceous spicules (hexactines)
• Demosponges are found in various marine habitats, while hexactinellids are primarily deep-sea dwellers
• Hexactinellids often have a more symmetrical body plan compared to demosponges

Calcarea sponges

• Calcarea, or calcareous sponges, have a skeleton made of calcium carbonate spicules
• Calcareous spicules are typically smaller and less diverse in morphology compared to siliceous spicules
• Calcarea sponges are generally smaller in size than demosponges and hexactinellids
• Most calcareous sponges inhabit shallow marine environments

Homoscleromorpha sponges

• Homoscleromorpha is a recently recognized class of sponges, previously classified as a subclass of Demospongiae
• Homoscleromorphs possess a unique combination of morphological and molecular characters, setting them apart from other sponge classes
• They have a simple body plan with a thin pinacoderm and a well-developed mesohyl
• Homoscleromorphs have siliceous spicules with a distinct morphology (tetraxons, calthrops)

Porifera evolutionary history

• Sponges are considered to be among the earliest diverging metazoan lineages
• Molecular clock estimates suggest that sponges originated in the Precambrian, possibly as early as 600-800 million years ago
• The sponge fossil record provides insights into the evolutionary history and diversification of the phylum

Earliest known sponge fossils

• The oldest unequivocal sponge fossils date back to the Ediacaran Period (635-541 million years ago)
• Ediacaran sponge fossils include body fossils and isolated spicules (Coronacollina, Paleophragmodictya)
• Putative sponge biomarkers (24-isopropylcholestane) have been reported from even older Cryogenian strata (~650 million years ago)

Sponge diversification patterns

• Sponges underwent significant diversification during the Cambrian Explosion (~541 million years ago)
• The Cambrian sponge fossil record is diverse, with representatives of all main sponge classes present
• Sponge diversity continued to increase throughout the Paleozoic Era, with notable radiations in the Ordovician and Silurian periods
• Post-Paleozoic sponge evolution is characterized by the rise of modern lineages and adaptations to new ecological niches

Mass extinctions and sponges

• Sponges have been affected by major mass extinction events throughout Earth's history
• The End-Permian mass extinction (~252 million years ago) had a significant impact on sponge diversity, with many lineages going extinct
• Sponges showed resilience and recovery after mass extinctions, with some lineages diversifying in the aftermath
• The Cretaceous-Paleogene mass extinction (~66 million years ago) had a less severe impact on sponges compared to other marine invertebrates

Cnidaria overview

• Cnidaria is a diverse phylum of aquatic animals, including corals, sea anemones, jellyfish, and hydroids
• Cnidarians are characterized by the presence of specialized stinging cells called cnidocytes, which contain nematocysts
• Cnidarians play important ecological roles in marine ecosystems, particularly in coral reef habitats

Cnidarian body plans

• Cnidarians exhibit two main body forms: polyp and medusa
• Polyps are sessile and typically cylindrical in shape, with a mouth surrounded by tentacles (corals, sea anemones)
• Medusae are free-swimming and have a bell-shaped body with tentacles hanging from the margin (jellyfish)
• Some cnidarians alternate between polyp and medusa stages in their life cycle (hydrozoans), while others exhibit only one form

Cnidarian life cycles

• Cnidarian life cycles vary among different classes and orders
• Many cnidarians have a complex life cycle involving alternation of generations between polyp and medusa stages
• In the polyp stage, cnidarians reproduce asexually through budding or fission, forming colonies
• Medusae are the sexually reproductive stage, producing gametes that fertilize to form planula larvae
• Planula larvae settle on a substrate and metamorphose into polyps, completing the life cycle

Cnidarian nematocysts

• Nematocysts are specialized organelles found in cnidocytes, the stinging cells of cnidarians
• Nematocysts contain a coiled, thread-like tubule that can be discharged for prey capture, defense, or locomotion
• Different types of nematocysts exist, each with a specific function (penetrants, volvents, glutinants)
• The morphology and arrangement of nematocysts are used as taxonomic characters in cnidarian classification

Cnidaria classification

• Cnidaria is divided into two main subphyla: Anthozoa and Medusozoa
• Classification is based on morphological characters, life cycle patterns, and molecular phylogenetic data
• Recent molecular studies have led to revisions in cnidarian taxonomy, particularly at the class and order levels

Anthozoans vs medusozoans

• Anthozoa includes corals and sea anemones, which are characterized by the presence of only the polyp stage in their life cycle
• Medusozoa encompasses cnidarians that have both polyp and medusa stages (hydrozoans) or only the medusa stage (scyphozoans, cubozoans)
• Anthozoans have bilateral symmetry and mesenteries, while medusozoans exhibit radial symmetry
• Medusozoans possess a velum or velarium, a shelf of tissue that partially closes the bell cavity, which is absent in anthozoans

Hydrozoans and siphonophores

• Hydrozoa is a diverse class of cnidarians that includes hydroids, fire corals, and siphonophores
• Hydrozoans typically have a complex life cycle with alternating polyp and medusa stages
• Siphonophores are colonial hydrozoans that consist of specialized polyps and medusae (Portuguese man-o-war)
• Some hydrozoans have reduced or absent medusa stages and reproduce primarily through asexual budding

Scyphozoans and cubozoans

• Scyphozoa, commonly known as true jellyfish, are characterized by a dominant medusa stage and a reduced or absent polyp stage
• Scyphozoans have a complex life cycle involving a planula larva, a sessile polyp (scyphistoma), and a free-swimming medusa (ephyra)
• Cubozoa, or box jellyfish, are a small class of cnidarians known for their complex eyes and potent venoms
• Cubozoans have a life cycle similar to scyphozoans, with a cubopolyp stage giving rise to medusae

Cnidarian skeletons

• Many cnidarians, particularly anthozoans, possess skeletons that provide support and protection
• Cnidarian skeletons can be composed of organic compounds (proteinaceous, chitinous) or minerals (calcium carbonate)
• Skeletal composition and microstructure vary among different cnidarian taxa and are used as diagnostic features

Coral skeleton formation

• Scleractinian corals secrete calcium carbonate skeletons in the form of aragonite
• Coral polyps deposit aragonite crystals extracellularly, forming a basal plate and vertical walls (septa, thecae)
• The process of skeleton formation is controlled by specialized cells (calicoblasts) and organic matrix proteins
• Environmental factors, such as temperature and pH, can influence coral calcification rates

Coral skeleton types

• Coral skeletons exhibit a wide range of morphologies, reflecting adaptations to different environmental conditions
• Massive corals have solid, boulder-like skeletons that are resistant to wave action (Porites, Favia)
• Branching corals have tree-like or bushlike growth forms that are adapted to calm, well-lit waters (Acropora, Pocillopora)
• Encrusting corals have flat, plate-like skeletons that grow horizontally over the substrate (Montipora, Agaricia)
• Solitary corals, such as mushroom corals (Fungiidae), have a single, free-living polyp with a large, disc-shaped skeleton

Cnidarian biomineralization processes

• Biomineralization in cnidarians involves the controlled deposition of calcium carbonate by specialized cells
• The process is mediated by organic matrix proteins, which provide a template for crystal nucleation and growth
• Biomineralization is influenced by genetic factors, as well as environmental conditions (temperature, pH, salinity)
• Cnidarian biomineralization has evolved independently in different lineages, resulting in diverse skeletal structures and compositions

Cnidaria evolutionary history

• Cnidarians have a long evolutionary history, with fossils dating back to the Precambrian
• The cnidarian fossil record provides insights into the diversification and extinction patterns of the phylum
• Molecular phylogenetic studies have shed light on the relationships among cnidarian classes and the timing of major evolutionary events

Precambrian cnidarian fossils

• The oldest putative cnidarian fossils are from the Ediacaran Period (635-541 million years ago)
• Ediacaran fossils, such as Haootia quadriformis and Auroralumina attenboroughii, show possible cnidarian affinities
• These early fossils suggest that cnidarians were among the first animals to evolve complex body plans and tissue organization

Paleozoic coral diversity

• Corals underwent significant diversification during the Paleozoic Era (541-252 million years ago)
• Rugose corals (Rugosa) and tabulate corals (Tabulata) were dominant reef-builders in the Silurian and Devonian periods
• The End-Permian mass extinction had a major impact on coral diversity, with the extinction of rugose and tabulate corals
• Modern scleractinian corals (Scleractinia) originated in the Triassic Period and became the primary reef-builders in the Mesozoic and Cenozoic eras

Post-Paleozoic cnidarian evolution

• Cnidarians continued to diversify and adapt to new ecological niches in the Mesozoic and Cenozoic eras
• Scleractinian corals underwent a major radiation in the Jurassic Period, coinciding with the evolution of symbiotic relationships with photosynthetic algae (zooxanthellae)
• The Cretaceous-Paleogene mass extinction had a less severe impact on cnidarians compared to other marine invertebrates
• Cnidarian diversity rebounded in the Cenozoic Era, with the rise of modern coral reef ecosystems

Cnidaria and reefs

• Cnidarians, particularly scleractinian corals, are the primary builders of coral reef ecosystems
• Coral reefs are among the most diverse and productive ecosystems on Earth, providing habitat for a wide range of marine organisms
• The evolution and distribution of coral reefs have been influenced by various biotic and abiotic factors throughout Earth's history

Coral reef formation

• Coral reefs form through the accumulation of calcium carbonate skeletons secreted by coral polyps
• Reef formation requires specific environmental conditions, including warm, clear, shallow waters with low nutrient levels
• Coral growth rates and reef accretion are influenced by factors such as temperature, light availability, and water chemistry
• Over time, coral skeletons and other calcifying organisms (coralline algae, mollusks) contribute to the buildup of reef structures

Coral reef types

• Coral reefs can be classified into different types based on their geomorphology and relationship to the surrounding environment
• Fringing reefs grow directly along the shoreline of islands or continents, with little or no lagoon separating them from the shore (Red Sea, Caribbean)
• Barrier reefs are separated from the shore by a deep lagoon and often form extensive linear structures (Great Barrier Reef, Belize Barrier Reef)
• Atolls are ring-shaped reefs that encircle a central lagoon, typically forming over subsided volcanic islands (Maldives, Tuamotu Archipelago)
• Patch reefs are small, isolated reef structures that grow within lagoons or on the continental shelf (Florida Keys, Great Barrier Reef)

Coral reefs through time

• The distribution and composition of coral reefs have varied throughout Earth's history, influenced by changes in sea level, ocean chemistry, and climate
• Ancient reef systems, such as the Permian Capitan Reef and the Devonian Great Barrier Reef, provide insights into past reef ecology and environmental conditions
• The Triassic-Jurassic transition saw the rise of scleractinian corals as the dominant reef-builders, a role they have maintained to the present day
• Quaternary coral reefs have experienced fluctuations in growth and distribution due to glacial-interglacial cycles and associated changes in sea level and climate
• Modern coral reefs face numerous threats, including climate change, ocean acidification, and human activities (overfishing, pollution)

Porifera and Cnidaria ecology

• Poriferans and cnidarians play crucial roles in marine ecosystems, contributing to nutrient cycling, habitat provision, and trophic interactions
• Both phyla have evolved diverse ecological strategies and adaptations, allowing them to thrive in a wide range of marine environments

Sponge ecological roles

• Sponges are important filter feeders, removing dissolved organic matter and particulate material from the water column
• Sponge filtration can significantly impact water clarity and nutrient dynamics in marine ecosystems
• Sponges provide habitat and shelter for various organisms, including crustaceans, polychaetes, and microbial symbionts
• Some sponge species are important bioeroders, contributing to the breakdown and recycling of calcium carbonate substrates (coral reefs, mollusk shells)

Coral ecological importance

• Corals, particularly reef-building scleractinians, are ecosystem engineers that create complex, three-dimensional habitats
• Coral reefs support high levels of biodiversity, providing food, shelter, and nursery grounds for a wide range of marine species
• Corals are primary producers, with their symbiotic algae (zooxanthellae) contributing to the overall productivity