Lecture #14

Phylum Chordata

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 ina unique combination andare found at some stage in development

• 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

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CraniatesVertebrates

GnathostomesOsteichthyans

Lobe-finsTetrapods

Amniotes

Milk

Legs

Amniotic egg

Mineralized skeleton

Jaws

Vertebral column

Head

Brain

Notochord

Ancestral deuterostome

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Lobed fins

Lung derivatives

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

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

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

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

Subphylum Cephalochordata

• 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

Pharyngealslits or clefts

Mouth

Brain

Dorsal,hollow

nerve cordNotochordMusclesegments

Muscular,post-anal tail

Anus

Subphylum Urochordata

• 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

Incurrentsiphonto mouth

Excurrentsiphon

Pharynxwith

numerousslits

Atrium

Tunic

Excurrentsiphon

AnusIntestine

EsophagusStomach

Early Chordate Evolution

Nerve cord of lancelet embryo

Brain of vertebrate embryo(shown straightened)

BF1

Forebrain Midbrain Hindbrain

Otx Hox3

Hox3BF1 Otx

• 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

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

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.

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

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 of lobe-finned fishes

• development of limbs marked the development of amphibians and reptiles

The Jaw

• 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

Skeletal rods

Mouth

Gill slits Cranium

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 via ATP hydrolysis

• ectotherms = e.g. amphibians & reptiles• endotherms = e.g. mammals & birds

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

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

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

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

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

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!!

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

• 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

Metabolic Rate

• 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

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

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 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

Metabolism

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

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

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

–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

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

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

Thermoregulation in Ectotherms