Biol 108 - Topic 21.pdf

Topic 21: Porifera and Cnidaria Phylum Porifera (sponges) BIOL 108 Winter 2024 © 2024 Neil Harris Phylum Porifera are basal animals that lack true tissues. − Phylum Porifera (por = pore, fer = to bear) are informally known as sponges. − Sponges include ~5,500 extant species, predominantly found in m...

Topic 21: Porifera and Cnidaria Phylum Porifera (sponges) BIOL 108 Winter 2024 © 2024 Neil Harris Phylum Porifera are basal animals that lack true tissues. − Phylum Porifera (por = pore, fer = to bear) are informally known as sponges. − Sponges include ~5,500 extant species, predominantly found in marine environments, ranging from a few millimetres to several meters in height. − Sponges are sessile (attached to one spot), except for a motile larval stage. WC Fig 33.2 1 Sponge morphology BIOL 108 Winter 2024 © 2024 Neil Harris Sponges lack true tissues. − e.g. no internal organs, muscles, nervous system, or circulatory system. − Sponges only have a few specialized cell types. Structural support provided by: Diversity of sponge spicules (WC) Calcareous sponge: calcium carbonate (WC) − Spicules, tiny needles or rods of silica or calcium carbonate that maintain the shape and structure of the sponge. − Spongin, a fibrous collagen-protein network for support. Glass sponge: silica (WC) Bath sponge: spongin (WC) 2 Sponge morphology BIOL 108 Winter 2024 © 2024 Neil Harris Sponges consist of four simple cell types: Choanocytes (collar cells) line the inner cavity of the sponge (spongocoel). They generate water currents by beating their flagella and capture suspended food particles. They also deliver oxygen and nutrients to the sponge while also removing waste and carbon dioxide. Porocytes are tubular cells that make up the pores of a sponge, allowing water to flow through. Epidermal cells form the outer layer of tightly packed cells. Fig 33.4 Anatomy of a sponge Mobile amoebocytes are found in the mesohyl where they transport nutrients to other cells of the sponge body, produce materials for skeletal fibres (spicules), and can differentiate into other cell types as needed. The body of a sponge consists of two layers of cells (epidermis and the choanocyte layer) separated by a gel-like, non-cellular matrix, the mesohyl (“middle matter”), which provides support and structure to the sponge. The mesohyl contains mobile amebocytes and skeletal elements (spicules or spongin) deposited by the amebocytes. 3 Sponge suspension feeding BIOL 108 Winter 2024 © 2024 Neil Harris Sponges are suspension feeders, capturing food particles suspended in water passing through their bodies. − Water is drawn by beating choanocyte flagella through pores (ostia) into a cavity called spongocoel and out through an opening (osculum). https://youtu.be/m8a0oNsDEx8?t=5m26s − Collar cells extract food particles (protist plankton) using mucus-covered microvilli, which are then engulfed through phagocytosis and digested (endocellular digestion) or transferred to amoebocytes. − Amoebocytes transport nutrients to other cells or utilize them for structural materials like spicules. 4 Sexual reproduction of sponges BIOL 108 Winter 2024 © 2024 Neil Harris Most sponges are hermaphrodites, possessing both male and female reproductive functions. − Sponges do not have gonads (reproductive organs).  Sperm develop from modified choanocytes.  Eggs develop from modified amoebocytes. − Most sponges are sequential hermaphrodites, so they are not able to self-fertilize. Spawning barrel sponge Sponge fertilization can be external (rare) or internal (common), with zygotes developing into motile ciliated larvae within the mesohyl. − Sponge embryonic development is highly variable. Sponges can reproduce asexually by fragmentation and by budding. Transverse sections of generalized sponge larvae: A. Solid parenchymella larva B. Hollow amphiblastula larva 5 Sexual reproduction of sponges BIOL 108 Winter 2024 © 2024 Neil Harris Choanocytes of “female” sponge trap sperm cells, which are transferred to amoebocytes and delivered to the eggs retained in the mesohyl Eggs produced by modified amoebocytes in the “female” sponge Sperm released into the aquatic environment through the osculum Sperm produced by modified choanocytes in the “male” sponge Internal fertilization and embryo development within the mesohyl Motile ciliated larvae are released and disperse from the parent sponge Larvae develop in the water, becoming structurally more similar to adult sponges, eventually becoming non-motile The larvae settles and attaches to a solid substrate where they complete development to the adult stage Note: except for the ciliated larval stage, adults are sessile (attached to one spot). 6 Sponge ecology and importance Sponges contribute to marine ecosystems by cleaning water through suspension feeding. − A single sponge filters thousands of kilograms of water to grow 1 kg of mass. Many sponges form symbiotic mutualisms with unicellular algae. BIOL 108 Winter 2024 © 2024 Neil Harris Most sponges produce toxic chemicals, making them unpalatable to predators. − Many organisms live inside sponges. Soft sponges, particularly those with spongin, are harvested for bath and art purposes, − Limits the distribution of these sponges to the photic zone (zone to which light can penetrate the water column). 7 Eumetazoa BIOL 108 Winter 2024 © 2024 Neil Harris Clade Eumetazoa includes all animals, excluding sponges and a few other groups, characterized by the presence of true tissues. The most basal Eumetazoan phyla are Ctenophora and Cnidaria. − Despite sharing similar body plans, these phyla are not closely related. − Most basal Eumetazoans exhibit radial symmetry and are diploblastic, comprising ectoderm and endoderm layers. − Basal eumetazoans possess muscle and nerve tissues, but no centralized nervous system or cephalization. Fig 33.2 8 Topic 21: Porifera and Cnidaria Phylum Cnidaria BIOL 108 Winter 2024 © 2024 Neil Harris Phylum Cnidaria is one of the earliest descendants of Eumetazoa. Phylum Cnidaria includes a diverse array of sessile and motile forms, including jellyfish, sea anemones, and corals. − ~10,000 extant species, the vast majority of which inhabit marine environments. Cnidarians have a simple radially symmetrical, diploblastic body plan. Fig 33.2 9 Cnidarian body plan BIOL 108 Winter 2024 © 2024 Neil Harris Cnidarians have a sac-like body plan with a central gastrovascular cavity (digestive compartment). − A single opening to the gastrovascular cavity functions as a mouth and anus. − Tentacles equipped with stinging cells, known as cnidocytes, surround this opening for prey capture and defence. − The body comprises an outer epidermis derived from ectoderm and an inner gastrodermis derived from endoderm that lines the gastrovascular cavity. − The epidermis and gastrodermis are separated by the mesoglea (‘middle jelly’), a gelatinous, non-cellular matrix. − Gas exchange occurs via diffusion across the epidermis. Fig 33.5 Polyp and medusa forms of cnidarians 10 Cnidarian body plan BIOL 108 Winter 2024 © 2024 Neil Harris Two variations of the body plan: − The sessile polyp, which attaches to a substrate, oral end upwards, e.g. sea anemone. − The bell-shaped, motile medusa that moves freely through the water via a hydrostatic skeleton, oral end downwards, e.g. jellyfish. Medusa form Polyp form Sea anemone Coral Jellyfish 11 Cnidarian feeding Cnidarians are predators that use tentacles to capture prey. − Tentacles are armed with cnidocytes, explosive cells that function in defence and capture of prey. Fig 33.6 A cnidocyte of a hydra Cnidarians are named for cnidocytes (cnid = nettle) BIOL 108 Winter 2024 © 2024 Neil Harris Cnidocytes contain complex organelles called nematocysts that deliver stinging toxins to immobilize prey. − The nematocyst consists of a bulbshaped capsule containing a coiled tubular thread.  When an external hair-like “trigger” (a mechano- and chemo-receptor) is activated, the tubular thread is ejected forcefully into the target organism and toxins are injected to immobilize the prey.  Tentacles move the immobilized prey to the gastrovascular cavity for digestion. Jellyfish feeding: https://youtu.be/BF6lNP0Uiss 12 Phylum Cnidaria Cnidarians diverged into two major clades early in their evolutionary history. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 33.7 Cnidarians 1. Clade Medusozoa includes all cnidarians that produce a medusa stage in their life cycle.  Hydrozoans  Scyphozoans (jellyfish)  Cubozoans (box jellyfish) 2. Clade Anthozoa includes cnidarians that occur only as polyps, such as corals and sea anemones.  Solitary or colonial forms. 13 Phylum Cnidaria Clade Medusozoa – Hydrozoans BIOL 108 Winter 2024 © 2024 Neil Harris Most hydrozoans alternate between polyp and medusa forms. − The polyp is sessile and often colonial, reproducing asexually by budding.  e.g. freshwater cnidarian Hydra exists solely in polyp form (only reproduces asexually). Medusae are produced asexually through budding from polyps but they reproduce sexually. − Motile larvae settle and transform into polyps. − The life cycle alternates between sexual and asexual forms, both of which are diploid. Fig 33.8 The life cycle of the hydrozoan Obelia 14 Phylum Cnidaria Clade Medusozoa – Scyphozoans/cubozoans BIOL 108 Winter 2024 © 2024 Neil Harris Medusa is the predominant stage in the life cycle of most scyphozoans (“true jellyfish”) and cubozoans (box jellyfish). − Coastal scyphozoans may have a brief polyp stage, whereas oceanic species generally have no polyp stage. − Jellyfish (scyphozoans) use their hydrostatic skeleton to move through contraction-pulsation of the bell-shaped body. WC Moon jellyfish (Aurelia aurita) WC  Some species actively swim, while others float passively.  The majority of the medusa’s mass consists of gelatinous ‘middle jelly’ (mesoglea), which is mostly water. − Box jellyfish (cubozoans) often have highly toxic cnidocytes.  e.g. the sting of various cubozoan species found in Australian tropical waters can lead to respiratory failure, cardiac arrest, and death within minutes. Australian box jelly (Chironex fleckeri) 15 Phylum Cnidaria Clade Anthozoa BIOL 108 Winter 2024 © 2024 Neil Harris Anthozoans include corals and sea anemones (antho = flower). Anthozoans occur only as polyps; they lack a medusa stage. − Some live as single individuals, such as most sea anemones. − Others are colonial, like most corals. Symbiotic photosynthetic algae inside coral polyps https://ocean.si.edu/  Colonies form asexually through budding or fission.  Many corals gain nutrition from symbiotic algae.  Corals that secrete a hard exoskeleton (external skeleton) of calcium carbonate are crucial reef-building organisms. Exoskeleton https://coral.org/coral-reefs-101/ Underwater time-lapse shows secret life of a coral reef: https://youtu.be/_mijYXcSCS4 16 Cnidarian ecology and importance BIOL 108 Winter 2024 © 2024 Neil Harris Reefs constructed by corals provide shelter and food for numerous marine species. − Coral reefs are economically important (tourism, fishing). − Runoff from agriculture and climate change are two major threats to coral reefs.  Coral bleaching due to the loss of symbiotic algae is symptomatic of warming oceans. Bleached coral (theguardian.com) Jellyfish blooms indicate ecosystem stress. Toxins produced by cubozoan species, like box jellyfish, can be dangerous to humans and are sometimes fatal. https://www.doctorportal.com.au − Blooms are often caused by overfishing of jellyfish predators and eutrophication (nutrient addition), which increase zooplankton abundance and decrease oxygen levels. 17