BIOL 108 Winter 2024 Echinoderms Topic 25 Deuterostomia PDF

Topic 25: Deuterostomia BIOL 108 Winter 2024 © 2024 Neil Harris The clade Bilateria contains Lophotrochozoa, Ecdysozoa, and Deuterostomia. − Clade Deuterostomia consists of three phyla: Hemichordata (acorn worms), Echinodermata (echinoderms, incl. starfish and sea urchins), and Chordata (chordates,...

Topic 25: Deuterostomia BIOL 108 Winter 2024 © 2024 Neil Harris The clade Bilateria contains Lophotrochozoa, Ecdysozoa, and Deuterostomia. − Clade Deuterostomia consists of three phyla: Hemichordata (acorn worms), Echinodermata (echinoderms, incl. starfish and sea urchins), and Chordata (chordates, incl. vertebrates). − Deuterostomia shares clade Bilateria developmental characteristics:  Bilaterally symmetrical; triploblastic embryonic development. − Deuterostomes undergo deuterostome embryonic development:  Radial, indeterminate cleavage; coelom forms when mesoderm folds from the wall of archenteron; formation of the anus from the blastopore.  However, some non-Deuterostomia animals also share some deuterostome developmental characteristics, e.g. Brachiopoda and Ectoprocta. − Clade Deuterostomia is defined by molecular phylogeny. Fig 33.2 1 Phylum Echinodermata BIOL 108 Winter 2024 © 2024 Neil Harris Echinoderms (phylum Echinodermata) include starfish (or sea stars), brittle stars, sea urchins, sea lilies, and sea cucumbers. − Greek: ekhinos = sea urchin, derma = skin − ~7,000 extant spp., all marine. − Found throughout ocean depth, from intertidal to abyssal zone. Echinoderms arose in the Cambrian and their evolution is documented in extensive fossil records. Fossil crinoids (sea lilies); WC − ~13k fossil species. − Calcareous endoskeleton suited to fossilization (endo = inside). Fossil starfish 2 Echinoderm characteristics Echinoderms are slow-moving or sessile marine animals. 1. Most adult echinoderms have radial symmetry. − Pentaradial symmetry (penta = 5) is most common, although other multiples (≠5) are also observed.  The mouth is at the center of the disc and faces downward.  Except for sea cucumbers, which are secondarily bilateral. BIOL 108 Winter 2024 © 2024 Neil Harris − Echinoderms evolved from a bilateral ancestor (Cambrian) and subsequently evolved radial symmetry.  Extant echinoderm larvae exhibit bilateral symmetry. Fig 33.50 A sea cucumber (clade Holothuroidea) Sea urchin (Pluteus) larva has bilateral symmetry (WC) 3 Echinoderm characteristics 2. A thin epidermis covers an endoskeleton of ossicles. BIOL 108 Winter 2024 © 2024 Neil Harris Sea urchin test (fused endoskeleton) (WC) − Ossicles are calcareous plates ± spines embedded in the dermis of echinoderms. − The echinoderm body wall may contain 10s to 1000s of ossicles. NH Starfish (Asterias rubens) interlocking ossicles Sea cucumber ossicles (WC) https://tinyurl.com/y27fz5oe Australasian sea cucumber (Australostichopus mollis) 4 Echinoderm characteristics NH Echinoderm tube feet BIOL 108 Winter 2024 © 2024 Neil Harris Madreporite 3. Echinoderms have a unique water vascular system. − Derived from the coelom and consists of a ring canal in the central disk and a network of hydraulic canals running down the length of each arm.  The radial canals branch into hundreds of fluid-filled, muscular tube feet, that can be adapted for locomotion, feeding, respiration, mucus production, or sensory perception. Fig 33.45 Anatomy of a sea star, an echinoderm  Tube feet are controlled by water pressure acting on the ampulla and podium, and they attach to substrates through a combination of suction and adhesive mucus production. − The water vascular system is connected to seawater via a porous ossicle, the madreporite.  Madreporite allows water to flow in and out of the water vascular system. − Water vascular system also used for circulation and respiration.  Gas exchange occurs through simple gills and diffusion through tube feet. Shape of Life – Echinoderms: https://youtu.be/9_S-dASjQ-w 5 Echinoderm characteristics BIOL 108 Winter 2024 © 2024 Neil Harris 4. Diffuse nervous system with no centralized brain. − The central disk has a nerve ring and radial nerves extending into the arms. 5. Echinoderms usually have separate sexes and are typically broadcast spawners. − Gametes (sperm and eggs) are released into open water for external fertilization, and there is no subsequent parental care of planktonic larvae. − Some species free spawn, where the male releases sperm but the female retains and broods fertilized eggs. Planktonic sea urchin larvae Female starfish brooding eggs 6 Phylum Echinodermata BIOL 108 Winter 2024 © 2024 Neil Harris − Crinoidea (sea lilies and feather stars) − Asteroidea (starfish/sea stars and sea daisies) − Ophiuroidea (brittle stars) − Echinoidea (sea urchins and sand dollars) − Holothuroidea (sea cucumbers) Diversity of adult body forms across the five clades. − Differences in the developmental regulation of a single Hox gene cluster. https://doi.org/10.1016/j.ydbio.2017.02.003 Extant echinoderms are divided into five clades that arose in the late Cambrian/early Ordovician: 7 Echinoderms Asteroidea Asteroidea includes the starfish/sea stars and sea daisies (aster = star). − They have 5 or more arms radiating from the central disk. − The undersurface of each arm bears tube feet, which grip the substrate with suction/adhesive chemicals. BIOL 108 Winter 2024 © 2024 Neil Harris Some starfish species reproduce asexually by fission of their central disc. − Many species can regenerate lost arms to regrow an entire new limb over several months or years. https://youtu.be/AaN6uRvfPLY Starfish are predators that can have a large impact on intertidal community structure. https://youtu.be/BnJ8preFDdA − They often feed on molluscs by prying them open with tube feet. − Evert stomach onto prey to engulf and digest food. https://youtu.be/l6dnmLDu6Eg 8 Echinoderms Fig 33.48 A sea urchin (clade Echinoidea) Echinoidea BIOL 108 Winter 2024 © 2024 Neil Harris Sand dollar Echinoidea includes the sea urchins and sand dollars. Overharvesting of sea urchin predators leads to sea urchin population explosions that decimate kelp forests. − No arms but have five rows of tube feet. − https://youtu.be/D3W4OCnHyCs Slow-moving. − Use spines (jointed ossicles) for locomotion and protection. Kelp forest destroyed by explosion of purple sea urchins https://www.cbc.ca/news/science/sea-urchin-explosion-1.5333232 Sea urchins are herbivorous. − Use jaw-like plates around the mouth to grasp and grind algae. WC Kina (Evechinus chloroticus, NH) 9 Echinoderms Holothuroidea Holothuroidea (sea cucumbers) are tube-shaped detritivores. BIOL 108 Winter 2024 © 2024 Neil Harris Australasian sea cucumber (Australostichopus mollis) − Deposit or filter feeders. − Appear bilaterally symmetrical.  Secondarily bilaterally symmetrical; elongated along the anterior-posterior axis.  Have rows of tube feet along the body.  In some species, tube feet around the mouth serve as feeding tentacles. NH Sea cucumber ossicles (WC) Fig 33.50 A sea cucumber (clade Holothuroidea) − Sea cucumbers lack external spines and the endoskeleton is reduced to scattered ossicles. 10