Cephalochordates

Urochordates(Ascidians)

Vertebrates

Hemichordates

Echinoderms

Ambulacraria

Deuterostomia

PhylumChordata

Phylum HemichordataEnteropneusta (acorn worms)Pterobranchia (colonial)

Phylum Chordata SubPhylum Urochordata (the Tunicates)

Ascidiacea (Ascidians) Thaliacea (Salps) Appendicularia (Larvaceans)

SubPhylum Cephalochordata

SubPhylum Vertebrata

Class Enteropneusta (acorn worms, 75 spp.)

Class Pterobranchia (colonial; 25 spp.)

- wormlike body with proboscis (prosome), collar (mesosome), and trunk (metasome), each with a (coelomic compartment)

- pharyngeal gill slits and dorsal hollow nerve chord like in chordates; no notochord

- separate sexes; asexual reproduction common (in pterobranchs)

- mouth + anus form secondarily after blastopore closes

- unique excretory structure, the glomerulus

Phylum Hemichordata

Acornworm

Enteropneusts (acorn worms)

proboscis used for movement in burrows, and for deposit- feeding on organic matter on the surface outside the burrow

Enteropneusts (acorn worms)

- cilia draw water through mouth (where food particles may be captured), then water exits pharyngeal gill slits

water in

water out,thru slits

Class Pterobranchia Colonial filter-feeders with 0-1 gill slits

Bryozoan-like features: 1) tentacles move water in same way as a lophophore

2) mesocoel extends into ciliated tentacles used in filter-feeding

3) live in secreted hard tubes

4) U-shaped gut, anus opening outside the tentacles

water flow

food

Class Pterobranchia Colonial filter-feeders with 0-1 gill slits

Bryozoan-like features: 1) tentacles move water in same way as a lophophore

2) mesocoel extends into ciliated tentacles used in filter-feeding

3) live in secreted hard tubes

4) U-shaped gut, anus opening outside the tentacles

5) colony of zooids produced by asexual growth

immatureasexualbud

Cephalochordates

Urochordates(Ascidians)

Vertebrates

Hemichordates

Echinoderms

Ambulacraria

Deuterostomia

PhylumChordata

Phylum HemichordataEnteropneusta (acorn worms)Pterobranchia (colonial)

Phylum Chordata SubPhylum Urochordata (tunicates)

Ascidiacea (ascidians) Thaliacea (salps) Appendicularia (larvaceans)

SubPhylum Cephalochordata

SubPhylum Vertebrata

At some stage of development, all chordates possess:

- Pharyngeal gill slits

- Dorsal, hollow nerve chord

- Dorsal notochord (hollow flexible tube)

- Post-anal tail, tadpole stage at some point in lifecycle

- Gland in pharynx that processes iodine:- endostyle

(Urochordata, Cephalochordata)

- thyroid gland (Vertebrata)

Phylum Chordata

pharyngeal gill slits: Out-pockets in the pharynx Become gill chambers and gills in aquatic chordates Become jaws, inner ear, and tonsils in terrestrial chordates notochord: a firm, flexible rod derived from mesoderm cells; becomes the endoskeleton in vertebrates

dorsal nerve cord: a hollow tube above the notochord; becomes the spinal cord and brain in vertebrates

The combination of these characters is a synapomorphy for Chordata, but all 3 traits rarely show up in the adult stage

- numerous gill slits on pharynx used in suspension feeding

- oral + atrial siphons

- notochord and nerve chord present in tadpole larvae

- no coelom or bony tissue

- adult body covered in secreted tunic, a polysaccharide coating

SubPhylum Urochordata 3,000 spp.

Class Ascidiacea (Ascidians)- benthic; solitary or colonial

Class Thaliacea (Salps)- pelagic, colonial

Class Appendicularia (Larvaceans) - pelagic, solitary- adults retain larval characteristics

Class Ascidiacea – the ascidians, or tunicates- sessile, benthic adults; large, short-lived tadpole larvae- solitary forms, or colonial (may be social or compound)- colonies form by asexual reproduction- hermaphrodites- often colorful + chemically defended- tropical species contain a photosynthetic symbiont, the cyanobacterium Prochloron

- sheets of iodinated mucus produced by ciliated tract called the endostyle

- mucus sheets move over the gills slits in the pharynx, through which water is pumped, trapping food particles

Filter-feeding in ascidians

Solitary Ascidians

Styela montereyensis

Colonial ascidians 1: In social species, individuals are clones from asexual reproduction, but do not share a common tunic (covering)

PHOTO B: colonial species in which each individual animal retains its own inhalent and exhalent siphons

PHOTO C: colony in which individuals are packed in a ring and share a centrally placed, common exhalent siphon (yellow arrow).

Colonial ascidians 2: In compound species, individuals are clones that share a common tunic, and

may even share siphons

Tadpole larvae show the chordate features that are not seen in the adult ascidian

- all ascidian larvae are lecithotrophic (non-feeding) and most are very short lived, swimming for a few minutes to find a suitable site for attachment and metamorphosis

Ascidian Metamorphosis

Class Thaliacea, the salps

- pelagic animals living in open water- gelatinous + transparent

- atrial siphon is shifted to posterior end- contraction of circular muscles compresses the tunic, forcing water out of atrial siphon & causing jet-propulsion forwards

colonial salp

Salps can be solitary or colonial. Colonial forms make long chains, which easily break apart.

Class Appendicularia, the larvaceans- pelagic, live in a secreted mucous house- pharynx reduced, bears only 2 slits- complex mucus nets spun for filter feeding- neoteny: larval form develops gonads for reproduction, adult stage is gradually lost from the life cycle - postanal tail beats to produce water currents

- notochord + reduced dorsal nerve chord present in tail

larvacean out of its house

larvaceans live inside a 2-layer gelatinous (jelly-like) house:

(1) pre-filter web, 1 meter across, traps large particles

(2) inner mucus filter traps small particles (2-20 m) which are eaten

As webs and house become clogged with debris, they are castoff (15 times per day)

- animal spins a new house just before leaving old one, quickly inflates it

- old houses sink to deep sea

larvacean out of its house

larvaceans live inside a 2-layer gelatinous (jelly-like) house

An outstanding mystery in marine science: what supplies all necessary food to sustain animals of the deep sea?

- 30% of the carbon needed to keep the deep sea alive could not be accounted for in sediment traps

Answer: sinking larvacean webs supply the missing nutrients to feed the ocean floor (Robison et al., Science 2005, 308:1609-1611)

Also remove up to 50% of particulate material that rivers dump into the upper 5 meters of the sea

- important to keep water clear enough for light to penetrate for photosynthetic organisms

Thus, larvaceans are critical living filters that sustain communities living in near-surface waters and the deep-sea!

- Amphioxus, fishlike animals also called lancelets

- Ciliary mucus suspension feeders, much like ascidians- water moved through pharyngeal gill slits by cilia- iodinated mucous produced by endostyle traps food

- Closed circulatory system similar to that of fishes, but no heart

- Segmented muscle bands called myotomes

- notochord lasts through adult life, but never becomes vertebral column and no brain develops (i.e., definitely not a vertebrate)

SubPhylum Cephalochordata25 spp.

Expression patterns ofdevelopmental genes arealso being used to study theevolution of vertebrates

- whole nervous system of lancelet-like ancestor was essentially compressed into the vertebrate brain

SubPhylum Vertebrata: the vertebrates

vertebral column houses dorsal nerve cord

there’s a lot of interest in studying the other chordates to understand how our lineage evolved – who’s our nearest relative among the invertebrates?

Analysis of the highly conserved 18S gene suggested that the nearest relatives of vertebrates were the cephalochordates

vertebrates

New analyses of hundreds of protein-coding genes consistently support Ascidiacea as the sister group of the vertebrates, not the cephalochordates

AscidiaceaCephalochordata

Vertebrata