Download 13-Lower Chordates

ZOO 2040
Biology of Animals
Topic 13
Lower Chordates
Phylum Chordata (“cord”)
■ 
Ancestry and Evolution
The name comes from the notochord, rodlike, semirigid tissue
enclosed in a sheath:
The earliest protochordates were soft-bodied and would not
have left many fossils.
■  Most work on living species has been conducted on early
developmental stages where early features are conserved:
■ 
• In most cases, it extends the length of the body and is between the gut
tract and the nervous system.
• The notochord mainly serves to stiffen the body or act as a skeletal axis.
• The theory that chordates evolved within the protostome lineage was
discarded due to evidence from embryo structure and development
Chordates share features with many invertebrates: bilateral
symmetry, anterioposterior axis, coelom, tube-within-a-tube body
plan, metamerism and cephalization.
■  Evolutionary position is uncertain: earlier theories were based on
a relationship with the protostome branch; this is considered
unlikely.
■  The echinoderm-hemichordate assemblage of deuterostomes are
considered the chordate sister group:
■ 
The deuterostomes are a natural grouping that has a common
origin in Precambrian seas.
■  Anatomical, developmental, and molecular evidence indicates
that chordates arose more than 570 million years ago from a
lineage related to echinoderms and hemichordates.
■  One group of fossil echinoderms, the “Calcichordata” have
pharyngeal slits and maybe other chordate attributes; their
skeleton was probably of calcium carbonate, while vertebrates
also use calcium phosphate.
■ 
• Important common features are: radial cleavage, anus derived from the
blastopore, mouth derived from a secondary opening, and a coelom
formed by fusion of enterocoelous pouches.
• Chordates have more structural unity in body plan than is found in
many other phyla.
Classification of Chordates
Classification of Chordates
Traditional and Cladistic Systems Differ
Cladistic system shows the relative time of origin of new properties and a specific
position in the hierarchical system, but it lacks a timescale or information on
ancestral lineages. In contrast, the branches of a phylogenetic tree represent real
lineages with geological information
■  Most people recognize the general traditional groups, but cladists no longer use
Agnatha and Reptilia:
■ 
• Reptiles are paraphyletic because they do not contain all of the most recent common
ancestors:
— 
Reptiles, birds and mammals compose a monophyletic clade called Amniota
• In cladistics, reptiles can only be grouped as amniotes that are not birds or mammals:
— 
There are no features that isolate reptiles and exclude birds and mammals
• Agnathans (hagfishes and lampreys) are also paraphyletic, because their most common
recent ancestor is also an ancestor of all remaining vertebrates.
■ 
Traditional classification makes certain specific distinctions:
• Protochordata (or Acraniata) are separated from the Vertebrata (or Craniata) that have a
skull.
• Vertebrates may be divided into Agnatha (jawless) and Gnathostomata (having jaws).
• Vertebrates are also divided into Amniota (having an amnion) and Anamniota (lacking an
amnion).
• Gnathostomata is subdivided into Pisces (with fins) and Tetrapoda (usually with two pair
of limbs).
• Many of these groupings are paraphyletic.
Four Chordate Characteristics
Four Chordate Characteristics
Pharyngeal Pouches and Slits
Found at some stage (even if only embryonically) in all chordates
Notochord First part of the endoskeleton to!
appear in the embryo:
■ 
• Serves as an axis for muscle attachment; it can bend!
without shortening and permits undulation.
• In protochordates and jawless vertebrates, the notochord!
persists throughout life.
• In vertebrates, a series of cartilaginous or bony vertebrae!
form from mesenchymal cells derived from blocks of!
mesodermal cells lateral to the notochord.
• In most vertebrates, the notochord is entirely!
displaced by vertebrae, although it remains!
persistent as the intervertebral discs.
Lead from the pharyngeal cavity to the!
outside of the body:
• They form by the inpocketing of the outside!
ectoderm and the evagination of the pharynx endoderm.
• In aquatic chordates, the two pockets break through to form the pharyngeal slit.
• In amniotes these pockets may not break through and only grooves are formed.
•  In tetrapods, the pharyngeal pouches give rise to a variety of structures,
including the Eustachian tubes, middle ear cavities, tonsils and parathyroid
glands.
■ 
■ 
Dorsal Tubular (Hollow) Nerve Cord In most!
invertebrate phyla, the nerve cord is ventral!
to the alimentary canal and solid:
Function as a filter-feeding apparatus in protochordates.
Fishes added a capillary network with gas-permeable walls; this
evolved to become gills.
Postanal Tail
The postanal tail, plus musculature,!
provides motility for larval tunicates!
and amphioxus to swim.
■  This was increased in fishes but became!
smaller or vestigial in later lineages.
• In chordates, the single cord is dorsal to the!
alimentary canal and is tubular.
• The anterior end enlarges to form the brain.
• The cord is produced by the infolding of!
ectodermal cells on the dorsal side of the body.
■ 
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Biology of Animals
ZOO 2040
Topic 13
Lower Chordates
Subphylum Urochordata!
Urochordate Metamorphosis
(“tail chordates”)
■ 
Tunicates: “sea squirts” (because they may discharge a!
jet of water when disturbed) and salps. About 3000 spp.
• Occur at all depths everywhere in the oceans.
• Adults are sessile and are attached to hard substrates; they!
are among the most abundant intertidal animals.
•  Some are planktonic (salps).
■ 
Their tunic is the tough, nonliving, sometime cellulose-!
containing covering that surrounds their organs.
• Colonial and solitary ascidians have their own tunics;!
compound forms share a common tunic.
• The incurrent or oral siphon is anterior; the excurrent or atrial!
siphon is dorsal. Water entering the incurrent siphon passes!
through a ciliated pharynx with an elaborate lattice. In some!
compound ascidians, each has its own incurrent siphon but!
they share the excurrent siphon.
■ 
The tadpole larvae have all four chordate characteristics.!
Adults have only one: pharyngeal gill slits
• Larvae do not feed, but swim for a while before attaching and!
developing into a sessile adult. During adult metamorphosis,!
the notochord and tail disappear; and nerve cord is reduced.
Class Thaliacea
Urochordate Classes
Known as “salps” they are pelagic!
zooplankton, with a transparent body.
■ They pump water through the body by muscular
contractions, rather than ciliary action.
■ They have alternating sexual and asexual generations and
respond rapidly to increases in food supply.
Class Ascidiacea
■ 
■ 
Feeding depends on formation!
of a mucous net that is secreted!
by the endostyle:
• Cilia on gill bars of the pharynx pull the!
mucus into a sheet; particles trapped in!
the sheet are worked into a rope and carried!
back to the esophagus and stomach.
The heart drives blood first one!
direction, then in reverse.
■  The nervous system has one nerve!
ganglion and a plexus of nerves on!
the dorsal side of the pharynx.
■  The subneural gland samples incoming water and may have an
endocrine function.
■  Sea squirts are hermaphroditic, with a single ovary and a single testis;
fertilization occurs in the water (external).
■  These organisms also concentrate very rare metals, such as vanadium,
in very high concentrations. They have also produced anti-cancer drugs.
■ 
Class Larvacea
Class Larvacea
Paedomorphic (sexually mature individuals retain the ancestral larval
form); adults resemble the larval stages of other tunicates.
■  Larvaceans build a delicate hollow sphere of mucus interlaced with
passages for water to enter. Particulate food trapped on a feeding filter
inside this sphere is drawn into the mouth through a tube.
■  After the filters become clogged with wastes, they are left behind and a
new sphere is built (about every 24 h).
■  These spheres, and the organic carbon that adheres to them, collapse and
rapidly fall to the ocean floor, where they are a major food source:
■ 
Fig. 1. In situ video frame grabs of
steps in the progression from an
actively filtering giant larvacean
house to a descending sinker. (A) An
active house occupied by
Bathochordaeus; the coarse mesh
outer filter surrounds a fine mesh
inner filter, to which the tadpoleshaped animal is attached. (B) An
abandoned and collapsed house, with
most of the outer filter condensed into
ropy strands and a small portion
domed over the inner filter. (C) As the
sinker rapidly descends, the mass
becomes more compacted, and the
inner filter is usually the last part to
collapse. Scale bars: (A) and (B), 10
cm; (C), 1 cm.
Mucus spheres of Bathochordaeus spp. can be >2 m long
B.H. Robison, K.R. Reisenbichler, and R.E. Sherlock. 2005. Giant larvacean houses:
rapid carbon transport to the deep sea floor. Science 308:1609-11.
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ZOO 2040
Biology of Animals
Topic 13
Lower Chordates
Subphylum Cephalochordata
“Lancelets” or “lancets” (British term for “scalpel”).
Slender, laterally flattened, translucent animals!
about 5-7 cm long. 25 living spp.
■  Live in sandy bottoms of coastal waters worldwide:
■ 
■ 
• Four occur in North American coastal waters.
• Asian species are eaten by humans.
■ 
■ 
• By priority, are now the genus Branchiostoma.
• ‘Amphioxus’ has been retained as a common name.
Have all 4 characteristics of chordates
■  Notochord persists throughout life.
■  A hollow nerve cord lies above the notochord:
■ 
■ 
• Water enters the mouth driven by cilia in the buccal cavity, passes through pharyngeal slits
where food is trapped in mucus.
• Food particles separated from the mucus are passed into a hepatic cecum where they are
phagocytized.
• Filtered water leaves the body by an atriopore.
Post-anal tail.
■ 
Long considered the closest living relative to the earliest vertebrates:
■ 
■ 
Pikaia gracilens was a ribbon-shaped fish-like chordate!
from the Burgess Shale (530 Mya):
Pikaia
• It dates from mid-Cambrian rocks but did not!
predate the earliest vertebrate fossils.
• It had both a notochord and muscle bands,! Yunnanozoon
and was similar to living cephalochordates.
Recent studies of homeobox-containing genes suggest the ancestor of both
amphioxus and vertebrates was cephalized:
In 1995, a 4-cm chordate-like creature!
(Yunnanozoon) was described from the!
rocks of the Chengjiang formation in!
Yunnan, China (540 Mya).
■  The Chengjiang site also contains the!
oldest fish fossil, the 3-cm long!
Haikouichthys.
■  Another new fish (Myllokunmingia)!
from the same area has simpler gills!
and is even more primitive, suggesting!
that chordates are even older.
■  It is believed vertebrates emerged from!
a lineage resembling one of the two!
protochordate groups.
■ 
• Many zoologists still consider the cephalochordates a sister group of vertebrates.
Lampreys as a Model of the Primitive Vertebrate Body Plan
■ 
Many zoologists consider living cephalochordates to be a living descendant
of ancestors that gave rise to both cephalochordates and vertebrates. This
would make them the sister group of the vertebrates.
Evolutionary History
Position of Cephalochordates
• They are not now considered a direct ancestor, although they may closely resemble the
ancestor.
• They lack a brain and the specialized sensory equipment of vertebrates.
• There are no gills in the pharynx and no mouth for pumping water.
■ 
Cephalochordates also have vertebrate features:
• A cecum is a diverticulum resembling the vertebrate pancreas that secretes digestive
enzymes.
• The trunk muscles suggest future vertebrate patterns.
• They possess the basic circulatory plan that advanced chordates elaborate upon.
Pharynx with pharyngeal gill slits:
■ 
Sexes are separate:
• Sex cells are set free in the atrial cavity and pass out the atriopore; external fertilization.
• Cleavage is holoblastic and a gastrula forms by invagination.
• The larvae soon hatch and gradually become the shape of adults.
• Pairs of spinal nerve roots emerge at each trunk!
segment.
• Sense organs are simple, unpaired bipolar receptors located in various parts of the body.
■ 
Closed circulatory system: complex but there is no heart:
• Blood is pumped by peristaltic contractions in the ventral aorta, passes upward through
branchial arteries in the pharyngeal bars to paired dorsal aortas.
• Blood moves by microcirculation through tissues and returns to the ventral aorta.
• Blood has no erythrocytes or hemoglobin and mainly transports nutrients.
Originally placed in the genus Amphioxus:
Lampreys have a larval stage called an ammocoete that closely resembles the
amphioxus, and were originally considered!
cephalochordate adults. They have the!
most primitive set of vertebrate structures:
• Their mouth is similar, but draws!
water in by muscular pumping.
• Their endostyle, mucus, body muscles,!
notochord and circulatory system!
closely resemble amphioxus.
• In contrast to amphioxus, ammocoetes have a two-chambered heart, a three-part brain,!
a median nostril, auditory vesicles, a thyroid and pituitary gland.
• More extensive pharyngeal filaments serve in respiration.
• The ammocoete has a true liver, gallbladder and pancreatic tissue.
Haikouichthys
Myllokunmingia
Living Endoskeleton
Subphylum Vertebrata
The internal location of the endoskeleton departs from the invertebrate pattern.
By growing with the body, it is not confining and does not have to be shed.
■  The endoskeleton has a greater economy in building materials:
■ 
■ 
• Provides jointed scaffolding for muscles, and muscles then cushion it from impact.
• Like an exoskeleton, the skull and thoracic rib cage enclose and protect vulnerable organs.
■ 
Cartilage was probably the first endoskeletal material; it is superior to bone for fast
growth:
• In agnathans and sharks, cartilage remains the primary endoskeletal material.
Bone offers the advantages of storing phosphate and mechanical strength for
terrestrial life.
■  The tough integument serves to protect and often bears nonliving structures.
■ 
Pharynx and Efficient Respiration
As protovertebrates shifted from filter-feeding to a!
predatory life habit, the pharynx modified into a!
muscular feeding apparatus that could pump water.
■  Circulation in internal gills was improved by!
addition of capillary beds and muscular!
aortic arches.
■  These changes supported an increased!
metabolic rate.
■ 
■ 
■ 
This monophyletic group has a number of novel homologies that the others do not
share.
The alternative name Craniata derives from the fact that they all enclose the brain
in a case (cranium).
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ZOO 2040
Biology of Animals
Topic 13
Lower Chordates
Advanced Nervous System
Garstang’s Hypothesis of Chordate Larval Evolution
A switch to active predation favored new!
sensory, motor and integrative controls.
■  This selected for paired eyes with lenses!
and inverted retinas, pressure receptors,!
paired ears, electroreceptors, and!
chemical receptors.
■  The neural crest is formed from!
ectodermal cells along the neural tube;!
it forms the cranium, cranial nerves,!
branchial skeleton and aortic arches.
■  The epidermal placoides are plate-like ectodermal thickenings anterior
to either side of the neural tube; they give rise to the nose, eyes, ears,
taste receptors and lateral line mechanoreceptors.
■  This development of completely new cell types is a rare event in animal
evolution.
At the time of its discovery in 1869, the tadpole larva was considered a descendant of
an ancient free-swimming chordate ancestor:
■ 
• Adults were then considered degenerate sessile descendants of the free-swimming form.
In 1928, Walter Garstang turned this sequence around, suggesting that the sessile
adults were ancestral:
• He theorized that the larva failed to metamorphose into an adult and developed gonads and
reproduced in the larval stage.
• He termed this paedomorphosis, the evolutionary retention of larval traits in an adult body
• His suggestion that evolution may occur at larval stages matches embryological evidence.
• Paedomorphosis occurs in some amphibians (also called neoteny).
• Humans have also sometimes been described as paedomorphic, because our features are
more similar to baby apes than they are to adults.
Paired Limbs
Pectoral and pelvic appendages originated as swimming stabilizers.
■  Jointed limbs that developed later are suited for life on land and
permit fine movement.
■ 
Ostracoderms: the Earliest Vertebrates
Ostracoderms: the Earliest Vertebrates
Earliest fossils showing articulated vertebrae.
Found in the late Cambrian deposits in the United States, Bolivia and Australia.
■  They were small, heavily armored, jawless, and lacked paired fins.
■  Most had heavy armor of dermal bone on the outside of their bodies.
■  The earliest Paleozoic fossils of jawless ostracoderms!
show vertebrate organ systems.
Anaspids were streamlined!
and more closely resembled!
modern lampreys.
■ Paleozoic sediments were!
once dated using!
microscopic, tooth-like!
fossils called conodonts.
■ Complete conodont!
animals have been!
discovered; we do!
not yet know how to!
classify them.
■ Agnathans, defined by the absence of jaws, may be
paraphyletic:
■ 
■ 
■ 
Heterostracans
Probably filtered particles from the bottom.
Unlike ciliary filter-feeding protochordates,!
sucked in water by muscular pumping.
■  May have been able to feed on soft-bodied animals.
■  The Devonian saw a major radiation of!
heterostracans that never evolved jaws or paired fins.
■ 
■ 
Osteostracans
Coexisted with heterostracans.
Developed paired fins that stabilized their movement.
Toothless, jawless mouth.
■  They had a sensory lateral line, paired!
eyes, and inner ears with semicircular canals.
■  Although the head was well-armored, there was no!
axial skeleton or vertebrae.
■ 
■ 
■ 
• Living agnathans (lampreys and hagfishes), often called cyclostomes
because of their circular mouths, do not appear to be closely related
to each other.
Early Jawed Vertebrates
■ 
Superclass Agnatha: Jawless Fishes
In contrast to agnathans, all living and extinct jawed!
vertebrates are called gnathostomes:
Living jawless fishes are!
represented by hagfishes!
and lampreys (both are!
referred to as cyclostomes!
because of their circular!
mouths).
■  About 43 spp. of hagfishes!
and 41 spp. of lampreys!
have been described.
■  Members of both groups lack jaws, internal ossification, scales or
paired fins.
■  Both groups share porelike gill openings and an eel-like body.
■  Hagfishes are the least derived; lampreys!
are much closer to gnathostomes. The!
two diverged 100s of Mya.
■  The grouping ‘Agnatha’ is!
thus a paraphyletic!
assemblage of jawless ‘fishes’.
■ 
• Evidence indicates that jaws arose by modification of the first two!
cartilaginous gill arches.
• Both gill arches and jaws form from upper and lower bars that bend!
forward and are hinged.
• Both are derived from neural crest cells rather than from mesodermal!
tissue as are most bones.
• Jaw muscles are homologous to the muscles that originally supported!
gills.
■ 
Gnathostomes are monophyletic; all derived organisms share!
their features:
• Placoderms appeared in the!
early Devonian; most were!
large and heavily armored.
• Acanthodians were!
contemporaries of placoderms;!
may have given rise to bony!
fishes.
■ 
Compared to the more abundant fossils that establish more recent bird and
mammal ancestry, the ancestry of modern fishes is uncertain:
• From examination of living forms, vertebrates are clearly a monophyletic group, however,
we still do not know from which chordate group the vertebrates originated.
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ZOO 2040
Biology of Animals
Topic 13
Lower Chordates
Class Myxini: Hagfishes
■ 
■ 
Class Cephaloaspidomorphi (Petromyzontes): Lampreys
Entirely marine scavengers and predators of annelids and molluscs.
Hagfishes enter a dead or dying animal through a body orifice or by
digging inside. They use keratinized plates on the tongue to rasp
away bits of flesh:
■ 
All lampreys in the N. Hemisphere belong to the family Petromyzontidae.
■ 
• Nearly or completely blind, but can locate food by an acute sense of smell
and touch.
• To provide leverage, the hagfish ties a knot in its tail and passes it forward to
press against the prey.
•  The sea lamprey Petromyzon marinus occurs on both Atlantic coastline; grows to 1 m.
•  There are 22 species of lampreys in North America; half belong to nonparasitic brookdwelling species (one of these species lives in southern Illinois and is endangered).
■ 
Special glands along the body secrete fluid that becomes slimy in
contact with seawater:
Most lampreys ascend freshwater streams to breed (some live out their
lives in streams):
•  Marine forms are anadromous, leaving the sea where they were adults to spawn!
upstream.
• This fluid contains fibers of keratin and has antimicrobial properties.
■ 
The hagfish circulatory system includes three accessory hearts in
addition to a heart behind the gills. The body fluids are in osmotic
equilibrium with seawater (they are the only vertebrate for which
this is true).
■  The Copenhagen Academy of Science offered a prize over a century
ago for information on hagfish breeding habits. It remains
unclaimed:
In North America, all lampreys spawn in winter or spring:
•  Males build a nest by lifting stones with their oral discs and using body vibrations.
•  Female anchors to a rock and male attaches to her head.
•  As eggs are shed into the nest, the male fertilizes!
them; adults die soon after.
•  Eggs hatch in two week into unique (ammocoetes)!
larvae.
•  The larva lives first on its yolk supply and then drifts!
downstream to burrow into sandy areas.
•  The larva is a suspension-feeder until it!
metamorphoses into an adult.
•  Change to an adult involves eruption of eyes,!
keratinized teeth replacing the hood
■ 
• In some species, females outnumber males by 100 to one.
• Females produce small numbers of surprisingly large, yolky eggs 2-7 cm in
diameter.
Sea Lampreys in the Great Lakes
Lampreys (cont.)
Nonparasitic lampreys do not feed as adults;!
their alimentary canal degenerates, and!
they die after spawning.
■  Parasitic lampreys attach to a fish by a!
sucker-like mouth and sharp teeth on their!
tongues rasp away flesh.
■  They suck tissue fluids and cells; anticoagulants!
in saliva promote flow of blood.
■  When engorged, the lamprey drops off, but!
the wound to the fish may be fatal.
■  Parasitic freshwater adults live 1-2 years!
before spawning and dying; anadromous!
forms live 2-3 years.
No lampreys were in the Laurentian Great Lakes west of Niagara Falls
until the Welland Ship Canal was opened in 1829.
■  A century later, lampreys were first seen in Lake Erie.
■  They had spread to all of the Great Lakes by!
the 1940s.
■ 
■ 
• Lampreys preferred lake trout and destroyed!
this commercial species.
• They then turned to rainbow trout, whitefish,!
burbot, yellow perch and lake herring.
• After decimating these populations, they!
resorted to chubs and suckers.
■ 
Lamprey populations declined due both to!
depletion of food and control measures:
• Larvicides were used in spawning streams.
• Release of sterile males is also used.
• Great Lakes were restocked, but new species of predatory fishes!
were introduced (e.g., coho and Atlantic salmon) that are not native and do not
spawn in the Lakes.
• Maintaining these stocks costs about $80 million/year.
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