Lecture #14

1. Lecture #14 Phylum Chordata

2. Phylum Chordata • only 45,000 species • characteristics: • 1. bilaterally symmetrical • 2. notochord • 3. pharyngeal gill slits • 4. dorsal, hollow nerve cord • 5. post-anal tail • 6. complete digestive system • 7. thyroid gland • 8. ventral, contractile heart Numbers 2 – 5 may be in a unique combination and are found at some stage in development

3. Chordates Craniates Vertebrates Gnathostomes Osteichthyans Lobe-fins Tetrapods Amniotes Mammalia (mammals) Echinodermata (sister group to chordates) Cephalaspidomorphi (lampreys) Actinopterygii (ray-finned fishes) Chondrichthyes (sharks, rays, chimaeras) Actinistia (coelacanthus) Dipnoi (lungfishes) Amphibia (frogs, salamanders) Urochordata (tunicates) Cephalochordata (lancelets) Myxini (hagfishes) Reptilia (turtles, snakes, crocodiles, birds) • Chordate classification characteristics: • Notochord? No  echinoderms • Brain? No  Urochordate(tunicate) • Head/Cranial cavity? No  Cephalochordate (lancelet) • Vertebral column? No  Hagfish • Jaws? No  Lampreys • Bony skeleton? No  Sharks, Rays • Lobed fins? No?  Ray finned fish • Lungs? lung derivatives?  Coelocanth • Legs? No  Lungfish • Amniotic egg? No  Amphibian • Milk? No  Reptile Milk Amniotic egg Legs Lung derivatives Lobed fins Mineralized skeleton Jaws Vertebral column Head Brain Notochord Ancestral deuterostome

4. Phylum Chordata • notochord: • supportive rod that extends most of the animal’s length – extends into the tail • dorsal to the body cavity • flexible to allow for bending but resists compression • composed of large, fluid-filled cells encased in a fairly stiff fibrous tissue • will become the vertebral column in many chordates

5. 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 ventral to the vertebral column

6. Phylum Chordata • pharyngeal gill slits: • series of openings in the pharyngeal region of the embryo • develop as a series of pouches separated by grooves • in some embryos – grooves develop into slits • used in primitive chordates for filter feeding • in aquatic vertebrates – transformed these slits/pouches into gills • embryonic in terrestrial chordates

7. Phylum Chordata • SubPhyla: • Urochodata: sea squirts (tunicates) • notochord, pharyngeal gill slits, and tail present in free-swimming larvae • Cephalochordata: amphioxus • all four chordate traits persist through life • Hyperotreti: hagfishes • jawless, no paired appendages • Vertebrata: vertebrates

8. Subphylum Cephalochordata Dorsal, hollow nerve cord • known as the lancelets • 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 • filter-feeders – cilia draw water into the mouth • swim like fishes – chevron shaped muscles on either side of the notochord Muscle segments Brain Notochord Mouth Anus Pharyngeal slits or clefts Muscular, post-anal tail

9. Subphylum Urochordata Incurrent siphon to mouth • tunicates • embryonic/larval stage has the characteristics of the chordate • larva swims to a new substrate and undergoes metamorphosis – to form the adult tunicate • retain the pharyngeal gill slits in the adults • water flows in through an incurrent siphon - filtered by a net of mucus on the pharyngeal gill slits Excurrent siphon Excurrent siphon Atrium Pharynx with numerous slits Anus Intestine Tunic Esophagus Stomach

10. Early Chordate Evolution BF1 Otx Hox3 • research on lancelets had provided information on the chordate brain • the same Hox genes that organize the vertebrate brain into forebrain, midbrain and hindbrain are found in lancelets • tunicates have also been sequenced – numerous similarities with vertebrates Nerve cord of lancelet embryo BF1 Hox3 Otx Brain of vertebrate embryo (shown straightened) Hindbrain Forebrain Midbrain

11. Craniates • chordates with a head • head – consists of a brain, surrounded by a skull, and other sensory organs • living craniates all share a series of unique characteristics • one characteristic is the development of neural crest cells • aquatic craniates possess pharyngeal gill slits not clefts or pouches • most basic craniate – hagfish

12. 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 • also play roles in forming many kinds of nervous cells • certain neurons • sensory structures The cells give rise to some of the anatomical structures unique to vertebrates, including some of the bones and cartilage of the skull.

13. Vertebrates • branching off from the chordates involved innovations in the nervous system and skeleton • more extensive skull • development of the vertebral column composed of vertebrae • most vertebrates – vertebrae enclose a spinal cord (replaces the notochord) • development of fin rays in aquatic vertebrates • 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

14. Vertebrate Taxonomy • most basal vertebrate – lamprey • jawless • development of jaws marked the evolution of the gnathostomes • development of lungs marked the evolution of ray-finned fishes • development of lobed fins marked the evolution oflobe-finned fishes • development of limbs marked the development of amphibians and reptiles

15. The Jaw Gill slits Cranium • lamprey have no jaws • evolution of the jaw marked the development of the gnathostomes • gnathostomes have jaws that evolved from skeletal supports of the pharyngeal slits • gnathostome characteristics: • 1. hinged jaws with teeth • 2. duplication of Hox genes – four sets of Hox genes vs. one set in early chordates • 3. enlargened forebrain – with highly developed sensory structures • 4. lateral line system – in aquatic gnathostomes • for the detection of vibration Mouth Skeletal rods

16. 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 consideredhomeotherms(constant body temperature)

17. 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 via ATP hydrolysis • ectotherms = e.g. amphibians & reptiles • endotherms = e.g. mammals & birds

18. Thermoregulation in Vertebrates • ectotherms regulate their body temperature through behavioral mechanisms • known as behavioral thermoregulation • endotherms regulate their body temperature by altering internal metabolic heat production • can also use behavioral thermoregulation

19. 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

20. 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 involve the integumentary system • 1. Insulation – fat, feathers & fur • 2. Circulatory Adaptations • 3. Evaporative Heat Loss – sweating & panting • 4. Behavioral Responses –hibernation, basking • 5. Adjusting Metabolism

21. 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

22. 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 – transfers its heat to the cooler blood leaving via the veins • heat is exchanged along the entire length of these vessels – maximizes heat exchange • also keeps the heat localized to that specific body area

23. 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 • sweating and panting are also active processes and require expending metabolic energy • so a sweating animal generates heat when it needs to dissipate heat!!

24. 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

25. 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

26. 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

27. Metabolic Rate • physiologists can determine an animal’s metabolic rate by • 1. measuring its consumption of O2 (or production of CO2) • 2. measuring heat loss • e.g. using a calorimeter • 3. measuring the rate of food consumption and waste production • used over the long term

28. 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 • the thermoneutral zone is bounded by an upper and a lower critical environmental temperature (UCT and LCT) • when environmental temperature is within the 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

29. Metabolism • the metabolic rate of a resting endotherm in the 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

30. Metabolism • BMR correlates to body size – increased size = increased BMR • BUTincreased 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 15X 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

31. Metabolic Adaptations • when environmental temps fall below the lower critical level of the TZ – endotherms must produce heat • thermogenesis can be through: • 1. shivering heat production • 2. non-shivering heat production • most non-shivering heat production- occurs in specialized adipose tissue called brown fat • high numbers of mitochondria and blood vessels

32. Metabolic Adaptations • in the mitochondria: ATP production is uncoupled from metabolic fuel consumption – yet heat is produced (heat is 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 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

33. Metabolic Adaptations • regulated hypothermia can also be used – by many birds and mammals • hummingbirds – high metabolic rate – drop their body temps by 10 to 20C 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 1/50th of the animal’s BMR • many animals can maintain body temp’s close to freezing! • arousal from hibernation requires the hypothalamus to reset the body’s internal thermostat

34. 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 from warm surfaces • to prevent overheating – many amphibians are nocturnal or will hide in shady areas

35. 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 increased body temp through increased metabolism – brooding snakes curl around their eggs • many reptiles endure cold temps by “hibernating” in large groups

36. Thermoregulation in Ectotherms • fish • rare in most fish - most assume the temperature of the surrounding water • two kinds of thermoregulation strategies • 1. “hot” fish – tuna, mackeral, sharks • blood is oxygenated at gills – most of this cold blood is moved to the body via arteries under the skin • blood flows into muscles and is warmed by the venous blood flowing out of the muscle – counter- current exchange • so blood that returns to the heart via veins under the skin is cool • SO: this strategy keeps the heat within the muscle mass • 2. “cold” fish – most fish species • blood is oxygenated and cooled to seawater temperature at the gills • cold blood is carried into the tissues via a large aorta • veins return warmed blood to the heart • blood is warmed by the metabolism of muscles • pumped to the gills – re-cooled