Echinoderms & Hemichordates PDF

Echinoderms & Hemichordates Clade Ambulacraria Phylum Echinodermata Characteristics found in no other phylum: Their endoskeleton are made of calcareous plates and spines; They have an unique water vascular system, which connects to the environment via sucke...

Echinoderms & Hemichordates Clade Ambulacraria Phylum Echinodermata Characteristics found in no other phylum: Their endoskeleton are made of calcareous plates and spines; They have an unique water vascular system, which connects to the environment via sucker like tube feet, this system is responsible for locomotion, prey capture and gas exchange; Pedicellariae; Dermal branchiae: Exhibit pentaradial (five-hold) symmetry, *only the larvae exhibit bilateral symmetry. They are deuterostomes, blastopore forms anus first, mouth second; They have radial and regulative cleavage, enterocoelous coelom development. Echinoderm Evolution Origins in early Cambrian. Likely descended from bilateral ancestor bc larvae are bilateral but become radially symmetrical later in development; Early fossils were sessile with radial symmetry; Sessile forms (Class Crinoidea) still survive, but most living forms are free moving; Ecological Relationships All marine —> cannot osmoregulate (move out of marine environment) Intertidal to abyssal regions Most are benthic as adults (live on the sea floor) Diverse feeding modes: – suspension feeders, deposit feeders, scavengers, predators, browsers or grazers Some brittle stars are commensal on sponges Keystone species 5 main taxa: 1. Class Asteroidea —> sea stars 2. Class Ophiuroidea —> bristle stars 3. Class Echinoidea —> sea urchins 4. Class Holothuroidea —> sea cucumbers 5. Class Crinoidea —> feather stars, sea lilies Class Asteroidea — Sea Stars Live in shorelines, rocks, muddy or sandy bottoms, coral reefs; They don’t have bones, they have —> Ossicles embedded in skin, softish body. Decentralized nervous system—> no brain or cephalization Asteroid Aboral Features Madreporite is a light-colored, calcerous opening used to filter water into the water vascular system of echinoderms. Papulae —> dermal branchiae or skin gills are used in gas exchange, serve in respiration and waste removal. Echinoderms use diffusion to take up and get rid of gases. Pedicellarie are the spines like jawed pincers that function as remover of particles from the surface of the animal and occasionally capture small prey. Madreporite Cleaning Anus Protection Papulae (dermal branchiae) Food capture Pedicellariae Spines Asteroid Oral Features Tube feet is used mainly for grabbing and locomotion. Ambulacral grooves extend from tip of each arm to the middle of the central disk. A starfish can open and close its underside along grooves located on each arm. Mouth Ambulacral grooves Tube feet and moveable spines Mesodermal Endoskeleton Ossicles: calcareous, mesodermal plates – joined by connective tissue (“catch collagen”) Catch collagen → mutable collagenous tissues– quickly & reversibly change from soft ↔ rigid Stereom: meshwork that penetrates ossicles Sea Star Internal Anatomy Coelom and Gas Exchange Coelom has ciliated peritoneal lining – circulates coelomic fluid around body & into papulae; Respiratory gases diffuse across the papulae & tube feet — nitrogenous waste = ammonia Water Vascular System Coelomic compartment; Unique system of canals and tube feet; Water vascular system + ossicles = hydraulic system The entrance to the water vascular system is via the madreporite. Water passes through the madreporite into the stone canal and into the ring canal. Five radial canals carry water down each arm, and into the tube feet via lateral canals. Functions—> feeding, locomotion, respiration and excretion; Feeding and Digestive System Two part stomach 1- Cardiac stomach (eversible) 2- Pyloric stomach connected to pyloric ceca (digestive glands) in arms digestion mostly extracellular (in pyloric ceca) – Food broken down outside of cells, smaller nutrients absorbed Each arm has its own set of digestive glands (pyloric ceca)  Keystone Species: a species whose impacts on its community or ecosystem are large that would be expected from its abundance. Sea star Pisaster spp., is a keystone species on Pacific coast Sea stars present: eats mussels → high species diversity → low competition Sea stars absent: mussels dominate → low species diversity → high competition (mussels outcompete others) Nervous system Decentralized nervous system —> no brain or cephalization 3 subsystems that each have: -Central nerve ring -Radial nerve Systems connected by nerve net Sense organs Eyespot at tip of each arm Sensory cells scattered on skin Regeneration and Autonomy Regenerate lost parts Autonomy: detach injured arm and regenerate Cast-off arm can regenerate new sea star 1/5 of central disc needed Reproduction Separate sexes (most), some with simultaneous hermaphroditism Pair of gonads in each inter radial space Broadcast spawners (external fertilization) Brooding in some species (parental case) — Direct development Free swimming larvae (most) Class Ophiuroidea: Bristtle Stars and Basket Stars Largest class (>2000 species) Distinct central disk Arms: slender, flexible (jointed) and brittle Anatomy No pedicellariae or papulae (skin gills) Ambulacral grooves: closed & covered with ossicles Tube feet lack suckers → feeding – locomotion by arm movement Madreporite on oral surface Five plates → jaws No anus/no intestine Organs in central disc Reproduction and Gas Exchange 5 bursae (paired) in arm pits → open to oral surface Water circulation into bursae – gas exchange – discharge of gametes (external fertilization) Separate sexes (most), few hermaphroditic Ecology Live on hard substrates where no light penetrates Many negatively phototropic / nocturnal Suspension or deposit feeders May use mucus to catch food Very fragile → regeneration & autonomy (more than sea stars)  Class Echinoidea - Sea Urchins, Sand Dollar and Heart Urchins; Test —> made of ossicles that form plates Sea urchins are a great model species for research on development bc their development is similar to humans. They are easy to raise in the lab & produce lots of eggs (important for medical research). Lack arms —> test shows five part symmetry -ambulacral rows folded up to any\us (periproct) Two types: 1- Regular: most species 2- Irregular: sand dollars and heart urchins Regular Urchins Hemispherical shape Pentaradial symmetry -> divided into five equal parts Long spines Hard-rocky substrate Move with tube feet, spines and podia Eat algae and other organic matter (urchin barrens) Irregular Urchins Secondarily bilateral Short spines Soft-sandy substrates Move mostly with spines Collect food particles on ciliated tracts or with podia Echinoid Reproduction Separate sexes Broadcast spawners (external fertilization) Sea urchins (stronglylocentrotus spp.) Three main urchins species in BC: Green Purple Red (ups to 200 years) Echinoid Features Anus, madreporite & genital pores on aboral surface (“dorsal”) Pedicellariae: several types, some with venom glands Movement: spines & podia Aristotle’s lantern Aristotle’s lantern: Complex set of chewing structures Mouth surrounded by 5 teeth Ciliates siphon connect esophagus to intestine  Class Holothuroeida- Sea cucumbers; Elongated—> oral-aboral axis Ossicles reduced and in skin —> body wall is leathery Lack spines 5 rows of tube feet and muscles Pentaradial symmetry Holothuroid Form Typically lie on one side Tube feet only well developed in three strips of ambulacra Oral tentacles → 10-30 modified tube feet surrounding mouth Secondary bilaterality is present Digestive system: Oral tentacles: Muscular cloaca Modified tube feet Coelomic cavity: Hydrostatic skeleton Madreporite: Inside body cavity Respiratory tree: # Respiration and excretion Gas exchange also Single gonad through tube feet and skin Ecological importance: Suspension/deposit feeders: consume suspended particles or detritus off sea floor Evisceration: escape response Cuvierian tubules —> entangle predators  Class Crinoidea: Sea Lilies and Feather Stars “Primitive” Fossils —> numerous in the past Attached for most of life (unique!) Many are deep water species Appearance and Features Leathery- calcareous plates Five arms branch out to form more arms – each with lateral pinnules Sessile forms have a stalk Holdfast present Lack: – Madreporite – Spines – pedicellariae Phylum Hemichordata Formerly considered a subphylum of chordates – gill slits & rudimentary notochord (= chordate characteristic) “notochord” not homologous with chordates → evagination of mouth cavity Wormlike bottom-dwellers, widely distributed Two main classes: Enteropneusta and Pterobranchia Acorn Worms Mucus-covered body has three parts – proboscis + collar + trunk Proboscis catches food in mucous strands No gills → gas exchange across skin