Biol 108 Topic 26.pdf

Topic 26: Introduction to Chordates Chordates (phylum Chordata) are bilaterian animals that belong to clade Deuterostomia. − Chordates are bilaterally symmetrical coelomates with segmented bodies. − Chordates share many features of embryonic development with echinoderms (i.e. deuterostomes) but have...

Topic 26: Introduction to Chordates Chordates (phylum Chordata) are bilaterian animals that belong to clade Deuterostomia. − Chordates are bilaterally symmetrical coelomates with segmented bodies. − Chordates share many features of embryonic development with echinoderms (i.e. deuterostomes) but have evolved separately for at least 500 million years. BIOL 108 Winter 2024 © 2024 Neil Harris The majority (>95%) of chordate species have vertebral columns (backbones): the vertebrates (subphylum Vertebrata). − Chordates consist of all vertebrates and two groups of invertebrates, the urochordates and cephalochordates. 1 Chordate characteristics BIOL 108 Winter 2024 © 2024 Neil Harris All chordates share a set of derived characters. − Some traits are only evident during embryonic development for some chordate taxa. Shared, derived traits of chordates: 1. 2. 3. 4. Notochord Dorsal, hollow nerve cord Pharyngeal slits or clefts Muscular, post-anal tail Fig 34.3 Chordate characteristics 2 Chordate characteristics Fig 34.3 Shared, derived traits BIOL 108 Winter 2024 © 2024 Neil Harris 1. Notochord − The notochord is a longitudinal, flexible rod located dorsally between the digestive tract and the nerve cord.  noto = back, chord = cord  Glycoprotein core sheathed in collagen fibres. − The notochord provides flexible skeletal support throughout most of the length of invertebrate chordates. − The notochord forms during embryogenesis when organ systems begin to develop from embryonic layers.  The notochord develops from the dorsal mesoderm and is present in all chordate embryos and some adults.  In vertebrates, a more complex, jointed skeleton (vertebral column) replaces the notochord before birth.  Adult vertebrates retain only remnants of embryonic notochord as parts of the vertebral discs between vertebrae. Fig 47.14 Notochord formation in a frog embryo 3 Chordate characteristics Fig 34.3 Shared, derived traits BIOL 108 Winter 2024 © 2024 Neil Harris 2. Dorsal, hollow nerve cord − The nerve cord of chordate embryos develops from a plate of ectoderm that rolls inward forming the neural tube dorsal to the notochord.  Signalling molecules secreted by the notochord and other mesodermal cells induce the neural plate to form from the ectoderm.  The neural plate curves inward, forming the neural tube.  The neural tube develops into the central nervous system (brain and spinal cord).  A dorsal, hollow nerve cord is unique to chordates: other animal phyla have ventrally located solid nerve cords. Fig 47.14 Neural tube formation in a frog embryo 4 Chordate characteristics Fig 34.3 Shared, derived traits BIOL 108 Winter 2024 © 2024 Neil Harris 3. Pharyngeal slits or clefts − In most chordates, embryonic arches develop in the pharynx (the region just posterior to the mouth). − Pharyngeal grooves (or clefts) between the arches develop into slits that open to the outside of the body in most aquatic chordates.  Allows water entering the mouth to exit through the pharyngeal slits, preventing it from passing into the digestive tract. − Functions of pharyngeal clefts/slits:  Filter-feeding structures in invertebrate chordates.  Gas exchange in aquatic vertebrates (excl. tetrapods): gills develop along the pharyngeal arches, which are ventilated when water is forced through the gill slits.  Pharyngeal arches are only present during the embryonic development of tetrapods. They form precursors that develop into various skeletal and muscle elements of the head and neck in tetrapods. Inside of a tiger shark’s mouth 5 Chordate characteristics Fig 34.3 Shared, derived traits BIOL 108 Winter 2024 © 2024 Neil Harris 4. Muscular, post-anal tail − Most adult chordates have a muscular tail posterior to the anus.  The digestive tract of most non-chordates extends full body length. − The tail contains skeletal elements and muscles.  It provides propulsive force in many aquatic chordates, i.e. fish.  Tails function as rudders and provide balance, grip, etc. − In some chordates, the tail is greatly reduced during embryonic development. The tail provides balance to a running kangaroo Reptile prehensile tail 6 Fig 34.3 Chordate characteristics BIOL 108 Winter 2024 © 2024 Neil Harris − Most chordates have segmented body plans, at least during embryonic development.  The segments are often highly specialized into distinct body regions.  Segmentation of chordates evolved independently from segmentation of other phyla, e.g. annelids and arthropods.  Segmentation is considered a primitive character of clade Deuterostomia. Vertebral column Segmentation, e.g. vertebral column, muscle blocks. − Segmentally arranged muscle blocks are present in all adult chordates except in the Urochordata.  Muscle blocks develop from blocks of mesoderm (somites) that flank the notochord.  Muscle blocks arranged in chevron (>>>) patterns.  Alternating contractions of the muscle blocks cause the notochord/vertebral column to flex side-to-side → swimming in marine chordates! Fig 47.14 Somite formation in a frog embryo 7 Phylogeny of living chordates Hierarchical Clades BIOL 108 Winter 2024 © 2024 Neil Harris Chordates Shared, derived traits of the clades (not all shown) Fig 34.2 Phylogeny of living chordates 8 Cephalochordata BIOL 108 Winter 2024 © 2024 Neil Harris Lancelets (subphylum Cephalochordata) are named for their blade-like body shape; ~25 spp. − Resemble bony fish, but they are invertebrate chordates. Lancelets are marine sedentary suspension feeders that retain characteristics of the chordate body plan as adults. − Wriggle backward into the sand. − Water is drawn into the mouth by ciliary action and particles are extracted using mucus-covered pharyngeal slits. − Water exits the body through the atriopore. − Mucus net ingested (along with trapped food). Fig 34.4 The lancelet Branchiostoma, a cephalochordate 9 Urochordata Blue spot sea squirt (Clavelina moluccensis), a colonial tunicate Tunicates (subphylum Urochordata) are more closely related to vertebrates than they are to lancelets. − Tunicates (~2,100 spp.) are suspension-feeding marine invertebrate chordates. WC BIOL 108 Winter 2024 © 2024 Neil Harris − Adults do not look like chordates.  “Tadpole”-like larvae have a brief freeswimming phase.  Attach to the substrate by the head.  Metamorphose to adult body plan (lose chordate traits).  Become sessile suspension feeders.  Most adult tunicates are sessile, e.g. sea squirts/sea tulips that are permanently attached to rocks; other tunicates are pelagic. − Tunicate larval stages most closely resemble the chordate body plan.  The larval stage exhibits the four chordate shared, derived traits. Fig 34.5 A tunicate larva, a urochordate 10 Urochordata BIOL 108 Winter 2024 © 2024 Neil Harris Tunicates (subphylum Urochordata) − Adult tunicates are covered by a tough polysaccharide “tunic”. − They draw water in through an incurrent siphon, filtering it through the mucuscovered pharyngeal basket that collects food particles; filtered water exits out the excurrent siphon. Figure 34.5 A tunicate, a urochordate − Some tunicates reproduce asexually by budding, forming colonies. Sea tulip (Pyura spinifera) WC Some tunicate species can contract violently, squirting out water (‘sea squirt’) 11