Lecture 14 Phylum Chordata Phylum Chordata only 65

Lecture #14 Phylum Chordata

Phylum Chordata • only 65, 000 species (3 rd largest) • deuterostomes • characteristics: – 1. bilaterally symmetrical – 2. complete digestive system – 3. thyroid gland – 4. ventral, contractile heart

Phylum Chordata • 5. notochord • 6. pharyngeal gill slits or pouches • 7. dorsal, hollow nerve cord • 8. post-anal tail Numbers 5 – 8 must be found at some stage in development

Phylum Chordata • notochord: – supportive rod that extends most of the animal’s length – extends into the tail – dorsal to the body cavity • located between the nerve cord and the digestive tract – flexible to allow for bending but resists compression • also a point of swimming muscle attachment in some species – e. g. amphioxus – composed of large, fluid-filled cells encased in a fairly stiff fibrous tissue – will become the vertebral column in many chordates • in humans, remnants of the notochord can be found in the intervertebral discs

Phylum Chordata • dorsal, hollow nerve cord: – runs along the length of the body – dorsal to the notochord – expands anteriorly as the brain – develops from ectoderm – BUT: in most vertebrates – nerve cord is solid and is surrounded by the vertebral column

Phylum Chordata • pharyngeal gill slits or grooves: – series of openings in the pharyngeal region of the embryo – first develop as a series of “ridges” = pharyngeal arches – the pharyngeal arches are also known as branchial arches in terrestrial vertebrates and gill arches in aquatic vertebrates pharyngeal arches Pharyngeal arches

Phylum Chordata • the endoderm inside the developing pharynx forms depressions called pharyngeal pouches • outside the embryo – the ectoderm forms depression called pharyngeal grooves • the pouch and groove line up with each other • in between 2 pouches – the “ridge” is the pharyngeal arch • in aquatic embryos – the grooves and pouches meet each other and become the gill slits • in terrestrial vertebrates they stay as grooves and pouches

• pharyngeal gill slits or grooves: – branchial and gill arches are supported by pieces of cartilage – e. g. 1 st branchial arch in humans = Meckel’s cartilage – these pieces of cartilage develop into specific structures • e. g. Meckel’s cartilage mandible

Phylum Chordata • pharyngeal gill slits: – used in primitive chordates for filter feeding • slits allow for the entry of food-laden water

Phylum Chordata • thyroid gland: – unique to the chordates – derived from the floor of the pharynx – from a structure called the endostyle – endostyle = ciliated groove that secretes mucus and some cells secrete iodinated proteins • also seen in protochordates • also prominent in the lamprey larvae • but develops into the thryroid gland in more advanced vertebrates

Phylum Chordata • Sub. Phyla: – Urochodata or Tunicata: sea squirts (tunicates) • 1, 600 species • free-swimming larva • adults are sessile or planktonic – Cephalochordata: amphioxus • • 45 species laterally compressed body transparent fishlike – Vertebrata: vertebrates • 45, 000 species

Subphylum Urochordata • known as tunicates – surrounded by a non-living “tunic” of cellulose • most common type – “sea squirts” • embryonic/larval stage has the 4 characteristics of the chordate • larva swims to a new substrate and undergoes metamorphosis – to form the adult tunicate – loses its tail and notochord

Subphylum Urochordata • adult tunicate retains its pharyngeal gill slits - forms a large pharynx with many slits – also known as a “pharyngeal basket” • the pharyngeal basket is located in a large chamber called an atrium • water flows in through an incurrent siphon atrium basket excurrent siphon • food particles are filtered by a net of mucus in the pharyngeal basket – mucus is secreted by the endostyle which is found on the ventral side of the basket • food is carried into digestive system by cilia • wastes are expelled out from the excurrent siphon Class Ascidiacea “the sea squirt” Incurrent siphon (ventral) to mouth Atrium Pharynx with numerous slits Tunic Excurrent siphon (dorsal) Anus Intestine Esophagus Stomach

Subphylum Urochordata • the pharyngeal basket is also used for respiration • blood is pumped into small vessels found throughout the pharyngeal basket = gas exchange • single nerve ganglion on the dorsal side functions as a brain • the axons from the ganglion form a plexus of nerves on the dorsal side • hermaphroditic – single ovary and single testes – external fertilization Class Ascidiacea the sea squirt Incurrent siphon (ventral) to mouth Atrium Pharynx with numerous slits Tunic Excurrent siphon (dorsal) Anus Intestine Esophagus Stomach

Subphylum Cephalochordata • known as the lancelets – adults – 6 cm in length – most common genus - Amphioxus • earliest diverging group of chordates • get their name (Lancelet) from their blade-like shape • embryos develop a notochord, a dorsal, hollow nerve cord, pharyngeal gill slits and a post-anal tail – adults retain these 4 chordate traits Muscle segments Notochord Dorsal, hollow nerve cord Brain Mouth Muscular, post-anal tail Anus Pharyngeal slits or clefts

• filter-feeders Amphioxus – water is drawn into the mouth by ciliary action – water flows through gill slits of the pharynx atrium out via the atriopore – net of mucus across the gill slits traps food – cilia moves the food into the intestine – food is moved through the intestine by cilia – smaller particles enter the hepatic caecum for intracellular digestion – hepatic caecum acts as a pancreas and a liver – larger particles are diverted into the intestine for external digestion and absorption hepatic caecum

• the buccal cirri or tentacles surrounding the mouth bear chemoreceptors and mechanoreceptors Amphioxus – mechanoreceptors - prevent large food particles from entering – acts like a sieve – chemoreceptors - allow the amphioxus to taste the water currents • swim like fishes – chevron or “V” shaped muscles that insert on either side of the notochord hepatic caecum

Subphylum Vertebrata: The Craniates • chordates with a head • head – consists of a brain, surrounded by a skull, and other sensory organs • lampreys, hagfishes, amphibians, reptiles, birds and mammals • monophyletic – one craniate ancestor • alternate name for Subphylum Vertebrata = Subphylum Craniata

Subphylum Vertebrata Characteristics • 1. 4 characteristics of a chordate • 2. Two layered integument: outer epidermis (ectoderm) & inner dermis (mesoderm) • 3. Endoskeleton of cartilage or bone – vertebral column & a head (made from neural crest cells) • 4. Muscular pharynx – in fishes the pharynx opens to the outside as gill slits • 5. W-shaped muscle segments called myomeres • 6. Complete, muscular digestive tract with a liver and pancreas

Subphylum Vertebrata Characteristics 7. Closed circulatory system with RBCs and hemoglobin 8. Coelom divided into pericardial and peritoneal cavities 9. Paired Kidneys 10. Endocrine system including a thyroid gland 11. Three-part brain, paired cranial nerves & special senses • 12. Greater metabolic demands • 13. Genome with duplicated genes – increased complexity • • •

Skeletal Changes • endoskeleton of cartilage or bone permits almost unlimited body size – “template” of flexible cartilage can be replaced with harder bone – some vertebrates keep their cartilage endoskeleton for more flexibility – hagfishes, lamprey and cartilagenous fishes – replaced by bone in the bony fishes and beyond – framework for the attachment of skeletal muscles

Skeletal Changes • bony endoskeletons allow for the development of larger, more powerful muscles – bones can withstand high, mechanical stress • bony endoskeletons would have been protection from predators – may have evolved as bony plates inside the skin (exoskeleton) – modified into scales • the bony endoskeleton is a source of calcium and phosphorus – high demand in animals with high metabolisms

Skeletal Changes • most obvious development – vertebral column • most vertebrates – vertebrae enclose a spinal cord (replaces the notochord)

Head, Brain and Special Senses • shift from filter-feeding to active predation required the development of more advance nervous system • end of the nerve cord expanded to form a three-part brain – forebrain, midbrain and hindbrain – protected by the cranium • paired special sensory organs associated with the cranium embryonic brain vertebrate brain

Neural Crest Cells • development of head and special senses - development of neural crest cells • collection of cells that form as bilateral bands of cells near the developing neural tube • migrate throughout the body • major roles in forming the skull & teeth • also play roles in forming many kinds of nervous cells – some cranial nerves and ganglia – Schwann cells Neural crest cells give rise to some of the anatomical structures unique to vertebrates, including some of the bones and cartilage of the skull.

The Jaw • evolution of the jaw marked the development of the gnathostomes – sharks, ray-finned fishes, amphibians, reptiles, birds and mammals • one of the most important events in vertebrate evolution • jaws evolved in aquatic vertebrates • from their branchial arches skeletal supports of the pharyngeal gill slits called Gill slits Cranium Mouth Skeletal rods

Muscular Changes • skeletal muscles transform from the V-shape of cephalochordates to the W -shaped muscles of vertebrates – increased folding and complexity – more powerful contractions and more control

Physiology “Upgrades” • adaptations in digestion – muscular pharynx – pump for moving water in aquatic vertebrates – smooth muscle within the lining of the GI tract – food movement – addition of the liver and pancreas as distinct organs • adaptations in respiration and circulation – more efficient gas exchange system – gills are modified – more efficient heart • adaptations in thermal regulation – warm blooded vs. cold blooded • adaptations in reproduction – amniotic egg – placental animals

Subphylum Vertebrata • animals with a backbone of interlocking vertebrae (cartilage or bone) plus a skull (craniate) • comprised of several classes: – Class Myxini - hagfishes – Class Hyperoartia- jawless fishes – lamprey – Class Osteichthyes - bony fishes – largest class (trout, perch, bass etc…) – Class Chondroichthyes - cartilagenous fishes – sharks, skates, rays – Class Amphibia - amphibians – Class Reptilia - reptiles – Class Aves - birds – Class Mammalia - mammals

Mammalia (mammals) crocodiles, birds) Reptilia (turtles, snakes, Amphibia (frogs, salamanders ) Dipnoi (lungfishes) Actinistia (coelacanthus) Actinopterygii (ray-finned fishes) Vertebrates Gnathostomes Osteichthyans Lobe-fins Tetrapods Amniotes Chondrichthyes (sharks, rays) Craniates Cephalaspidomorphi (lampreys) Myxini (hagfishes) Cephalochordata (lancelets) Urochordata (tunicates) Echinodermata (sister group to chordates) Chordates Milk Amniotic egg Legs Lobed fins • • Lungs or lung derivatives Jaws, mineralized skeleton Vertebral column Head Brain Notochord Ancestral deuterostome • • Chordate taxonomy involves asking several yes or no questions If YES – ask another question 1. Notochord? 2. Brain? 3. Head? 4. Vertebral column? 5. Jaws & mineralized skeleton? 6. Lungs or derivatives? 7. Lobed fins? 8. Legs? 9. Amniotic eggs? 10. Milk?

Craniates • most basic craniate – hagfish (Class Myxini) – have a skull made of cartilage – jawless and have no vertebral column but they do have a notochord – bottom dwelling scavengers of worms, molluscs, crustaceans and dead fish • burrows into the dead or dying fish – eats it from the inside out – rows of slime glands secrete copious amounts of slime – repels predators – slime coats the predator and can suffocate it • https: //www. youtube. com/ watch? v=pmaal 7 Hf 0 WA • https: //www. youtube. com/ watch? v=Bta 18 Fdk. Vc. A • https: //www. youtube. com/ watch? v=t 5 PGZRxh. Ay. U

Vertebrate Taxonomy • most basal vertebrate – lamprey – – ~20 species marine & freshwater – all spawn in freshwater most are parasitic on fishes non-parasitic lamprey do not feed – non-functional digestive tract • die after they spawn – – – jawless – agnathans (together with the hagfishes) posses a large sucker with multiple teeth attach themselves to fish and puncture the flesh suck out blood and fluids – often kills its prey very invasive – all but destroyed the Great Lakes trout population in the 50 s

Vertebrate Taxonomy • development of jaws marked the evolution of the jawed fishes and tetrapods = gnathostomes • gnathostome characteristics: – 1. hinged jaws with teeth – 2. duplication of genes – e. g. four sets of Hox genes vs. one set in early chordates – 3. enlarged forebrain – with highly developed sensory structures – 4. lateral line system – in aquatic gnathostomes • for the detection of vibration

Vertebrate Taxonomy • three groups of jawed vertebrates • 1. Placoderms – now extinct – bony plates in their epidermis – functioned as an armor • 2. Cartilagenous fishes – no dermal armor – cartilaginous endoskeleton – modern fish, sharks, skates and rays • 3. Ancestor to Bony fishes and Tetrapods – – – no dermal armor bony endoskeleton evolution into: 1. ray-finned fishes = modern fishes 2. lobe-finned fishes = lungfishes & coelocanths 3. tetrapods = amphibians, reptiles, birds & mammals

Vertebrate Taxonomy • development of limbs marked the development of amphibians and reptiles • development of wings marked the development of the birds

Vertebrates & Thermoregulation • thermoregulation in vertebrates has two sources • 1. internal metabolism – internal source of heat • 2. external environment – external source of heat • animals can be classified based on the heat that influences their body temperature – ectotherms – animals whose body temperatures are determined by external sources of heat – can also be considered to be poikilotherms (variable body temperature) – endotherms – animals whose body temperatures are determined by internal sources of heat – usually also considered homeotherms (constant body temperature)

Thermoregulation in Vertebrates • when energy from food is transformed into ATP and ATP is transferred into work – energy is lost in the form of heat – seen in both ectotherms and endotherms • endotherms produce more heat – cells are less efficient at using energy vs. ectotherms – endotherm cells are “Leaky” to ions – endotherm must spend energy to keep its ionic “balance” – this causes an increase in the production of heat as ATP is hydrolysed • caused by a mutation that resulted in leaky ion channels in the endotherm? • ectotherms = e. g. amphibians & reptiles • endotherms = e. g. mammals & birds

Balancing Heat Loss and Gain • thermoregulation depends on the animal’s ability to control the exchange of heat • essence of thermoregulation is to maintain rates of heat loss with equal rates of heat gain • animals do this by either – reducing overall heat exchange – favoring heat exchange in a particular direction • many mechanisms : – – – 1. Insulation – fat, feathers & fur 2. Circulatory Adaptations 3. Evaporative Heat Loss – sweating & panting 4. Behavioral Responses – hibernation, basking 5. Adjusting Metabolism

Thermoregulation in Vertebrates • ectotherms regulate their body temperature through behavioral mechanisms – – – known as behavioral thermoregulation orientation toward the sun when cold expanding body parts to the sun when cold retreating to cooler areas when warm social ectotherms can huddle – e. g. honey bees • endotherms regulate their body temperature by altering internal metabolic heat production – can also use behavioral thermoregulation • e. g. nesting, huddling – even putting on or taking off clothing

Thermoregulation in Vertebrates • ectotherms and endotherms can influence their body temperature using 4 ways of heat exchange: – 1. Radiation - heat transfer from a warmer medium to a cooler one via the exchange of infrared radiation – 2. Convection – heat transfer to a surrounding medium (e. g. air or water) as it flows over a surface – 3. Conduction – heat transfer directly between two objects – 4. Evaporation – heat transfer away from a surface as water evaporates

Circulatory Adaptations • heat exchange between the internal environment and the skin is through blood flow • as body temp rises – blood flow to the skin increases – heat in the blood is lost to the environment through the 4 methods described in the previous slide • ectotherms and endotherms can use blood flow to the skin to control their internal temperatures – some reptiles can divert blood to their skin when basking to warm themselves quickly – marine iguanas – furred mammals divert blood to hairless surfaces to cool

Counter Current Exchange • • seen in many birds and mammals transfer of heat between fluids flowing in opposite directions same principle as the exchange of respiratory gases seen in fish arteries and veins are adjacent to one another warm blood moves from the body core into the arteries of a limb – blood cools due to evaporative heat loss BUT the venous blood is warmed as it leaves the limb by the arterial blood – temp of the blood in the body doesn’t change dramatically heat is exchanged along the entire length of these vessels – maximizes heat exchange can keep heat localized to a specific body area – e. g. keeps heat localized in the muscle mass used for flight

Evaporative Heat Loss • endotherms must also be able to dissipate heat as environmental temperatures rise • 1. increase of blood flow to the skin • 2. evaporation of moisture off the skin’s surface through sweating or across the oral mucosa through panting • BUT water falling from the body in the form of saliva or excess sweat does not evaporate and does not cool the body • thus, when the need for heat loss is greatest – excess sweating is a waste of that water – in extremely hot and arid environments – you don’t sweat • sweating and panting are also active processes and require expending metabolic energy – so a sweating animal generates heat when it needs to dissipate heat!!

Metabolic Heat Production • heat production = thermogenesis • chemical energy is derived from food • nutrients from food are used to generate ATP • the production and use of ATP generates heat • the more ATP produced/used – the more heat generated • metabolic heat is used to establish core body temperature in endotherms

Physiologic Thermostats • regulation of body temperature in mammals is brought about by a complex system based on feedback mechanisms • sensors for thermoregulation found in the hypothalamus • functions as a thermostat • activates mechanisms that will promote heat loss – dilation of surface blood vessels – production of sweat • activates mechanisms that will promote heat gain – shivering heat production

Energy Allocation and Use • bioenergetics = overall flow and transformation of energy in an animal – determines the animal’s nutritional needs – ATP production for : cellular work + biosynthesis, growth, storage and reproduction • metabolic rate = sum of all the energy used in biochemical reactions over a given time interval – energy is measured in Joules or in calories/kilocalories – 1 kilocalorie = 4, 184 joules – calorie use by nutritionists is actually a kilocalorie

Metabolic Rate • physiologists can determine an animal’s metabolic rate by • 1. measuring its consumption of O 2 (or production of CO 2) – e. g. measuring drop in oxygen levels in the water surrounding a fish – e. g. • 2. measuring heat loss – e. g. calorimeter – closed insulated chamber that can measure temperature • 3. measuring the rate of food consumption and waste production – used over the long term – 1 gram of protein or carbs = 4. 5 to 5 kcal – 1 gram of fat – 9 kcal

male 170 lb body fat = 11. 4%

Metabolism • within a narrow range of environmental temps – the metabolic rate of an endotherm is at a low level and independent of external temperature = thermoneutral zone • thermoneutral zone is bounded by an upper and a lower critical environmental temperature (UCT and LCT) TB = basal body temperature TA = ambient temperature BMR = basal metabolic rate VO 2 = rate of oxygen consumption (measurement of metabolic rate)

Metabolism • when the temperature of the environment is within thermoneutral zone – the animal does not need to expend much energy to regulate its temp – its thermoregulatory responses are passive – e. g. fluffing fur, controlling blood flow to the skin • but outside this zone – the animal must expend metabolic energy – thermoregulatory responses are active – e. g. shivering and non-shivering heat production

Metabolism • the metabolic rate of a resting endotherm in thermoneutral zone is called the basal metabolic rate or BMR • BMR is measured when the animal is quiet but awake and not using energy for digestion, reproduction or growth • BMR = minimal amount of energy needed to carry out minimal body functions • Standard metabolic rate (SMR) is the metabolic rate of an ectotherm at rest at a specific temperature

Metabolism • BMR correlates to body size – increased size = increased BMR • BUT increased size = decreased BMR per gram of body tissue – – – BMR of an elephant – 7, 000 times greater than that of a mouse BUT per gram tissue – mouse uses energy 15 X faster than the elephant why? as an animal increases in size – its surface area to volume decreases heat dissipation relies on surface area theorized that larger animals have decreased BMRs per gram tissue to avoid overheating

Metabolic Adaptations • when environmental temps fall below the lower critical level of thermoneutral zone – endotherms must produce heat through thermogenesis • thermogenesis can be through: – 1. shivering heat production – contraction of muscles generates heat – 2. non-shivering heat production

Non-shivering heat production • most non-shivering heat production- occurs in specialized adipose tissue called brown fat – high numbers of mitochondria and blood vessels • in the mitochondria of brown fat cells: metabolic fuel consumption produces heat produced rather than ATP • brown fat is prevalent in newborn humans – decreases in adulthood – metabolic activity can be stimulated upon cold exposure – less brown fat activity in obese individuals • also present in large amounts in certain animals – cold weather animals – animals that hibernate

Other adaptations • other adaptations have evolved to help endotherms retain heat – thick layers of fur, feathers or fat – ability to decrease blood flow to the skin – counter-current exchange of heat in the appendages of many animals

Metabolic Adaptations: Hypothermia and Hibernation • regulated hypothermia can also be used – by many birds and mammals • hummingbirds – high metabolic rate – drop their body temps by 10 to 20 C when they are inactive – lowers their metabolic rate and conserves energy – called daily torpor • regulated hypothermia that lasts for days or weeks = hibernation – metabolic rate needed for hibernation may be 1/50 th of the animal’s BMR – many animals can maintain body temp’s close to freezing! – BUT hibernating animals can regulate their body temperature – arousal from hibernation requires the hypothalamus to reset the body’s internal thermostat

Thermoregulation in Ectotherms – amphibians • • assume the temperature of the water when submerged on land – the body temp can differ from the environment cooling - evaporative loss across the thin skin warming – radiation from the sun & from warm surfaces – basking in the sun after a meal is common • to prevent overheating – many amphibians are nocturnal or will hide in shady areas

Thermoregulation in Ectotherms – reptiles: wide variety of behaviors • seen best in the lizards - radiation – to heat up – bask perpendicular to sun’s rays – to cool down – parallel to rays • some reptiles can pant to regulate temp - heat loss through evaporative cooling across the mouth • some reptiles can increase body temp through increased metabolism – brooding snakes curl around their eggs • the leatherback turtle has a low surface to volume ratio and can keep themselves warmer than their environment despite being an ectotherm • many reptiles endure cold temps by “hibernating” in large groups – unlike true hibernators – reptiles cannot regulate their temp while hibernating – reptiles have very low BMRs and low activity – low food requirements • crocodiles – 1/10 th the food requirement vs. a similarly sized lion – can go without eating for 6 months – but can’t generate energy output for long chases

Thermoregulation in Ectotherms – fish • muscles of active fish produce large amounts of metabolic heat • difficulty in retaining this heat – warm blood is pumped to the gills and heat is lost to the water • two kinds of strategies • 1. “hot” fish – blood is oxygenated at gills – most of this cold blood is moved to the body via arteries under the skin – muscle metabolism warms the blood within the muscle mass – cold arterial blood flows into muscles and is warmed by the warm venous blood flowing out of the muscle – the venous blood leaving the muscle is cooled by the arterial blood – counter- current exchange – keeps the heat within the muscle mass – cold blood returns to the heart via veins just under the skin pumped to the gills

Thermoregulation in Ectotherms – fish • 2. “cold” fish – most fish species – blood is oxygenated and cooled to seawater temperature – cold blood flows through the center of the body in a large aorta – then carried into tissues where it is warmed by the metabolism of muscles – veins return warmed blood to the heart – heart pumps blood to the gills – re-cooled