Above a model of a pyrosome. Below: tilted to show the common exhalent opning which drives the colony
through the water by jet propulsion. In this model there are 680 zooids (assuming one per scale-like projection)
making up the colony. Each zooid has its own light-producing organ. Such an organism might be the size of a
cucumber, but they can grow to several metres in length. These remarkable organisms remind me of
spaceships! Depending on the species, the exhaust may be a simple opening as wide as the colony, or it may
be narrowed by an overhanging margin. This narrowing of the aperture probably increases the efficieny of the
propellent water jet.
|Urochordates - Sea Squirts, Salps and Larvaceans
Ascidians (Sea Squirts)
Look at one of your close relatives, shown above, human! The urochordates or tunicates are a remarkable group of
organisms and typically very beautiful and photogenic. All are marine. They comprise three classes - the ascidians, or
sea squirts, the larvaceans and the thaliaceans (salps and doliolids). The body is enclosed in a more-or-less
translucent tunic. The tunic consists of living connective tissue - a gelatinous hydrated fibre mesh which is infiltrated
by blood cells and amoebocytes and often containing a type of cellulose (tunicin) fibres. (Note: it is extremely unusual
to find cellulose in an animal!). The computer representation above is of Botryllus, a colonial ascidian. Star-like
colonies (in this case each composed of 7 individual zooids) are enclosed in a common organic secretion or test.
Sea squirts, like Botryllus, are sessile and live attached to solid objects (Botryllus is typically found attached to rocks)
or rooted in softer substrates by root-like outgrowths of the base of the body (which are covered in tunic). Botryllus
colonies occur in a dazzling variety of colours and patterns. Sea squirts are filter-feeders, sieving food particles from
the water. In each star-shaped colony (or sub-colony) each zooid has an opening toward the outside, typical of sea
squirts, called the buccal siphon. Cilia inside the pharynx draws water into the buccal siphon. This opens into the
spatial pharynx, which in sea squirts forms a sieve as it's walls are perforated by slit-like pores which let the water out
across the pharynx into the atrium (a cavity enclosing the pharynx accept along the midventral line where the pharynx
attaches to the body wall) which connects to the outside via the smaller atrial siphon (usually off to one side, but in
Botryllus each atrial siphon is an opening into a common atrial siphon or cloaca in the centre of the colony, from which
water exits in a jet. A sea squirt several centimetres in size can filter 100 to 200 litres of sea water each day.
The pharynx is lined by a mucus-sheet which acts as a sieve, straining food, even bacteria, out from the water being
pumped across it. The mesh of the porous pharynx is supported by vertical ridges, called pharyngeal-bars, with rows
of cilia that generate the ciliary currents that draw the water in through the buccal siphon.
A ciliated groove in the ventral surface of the pharynx, the endostyle, secretes the mucus-sheet which is passed
along by cilia to the dorsal surface where it collects in a gutter that transports the mucus, again by ciliary action, to the
stomach. This gutter is formed on the right-side of either a continuous ridge (the dorsal lamina) or a ridge comprised
of a series of tongue-like projections (languets). The ridge arches over to the right to form the gutter. In the gutter, the
mucus sheet is rolled into chords that are swept, by cilia, to the oesophagus.
The pharyngeal basket is made up of vertical rods, called pharyngeal bars, with pharyngeal slits (stigmata) in
between. In most forms, the bars are connected at intervals by larger horizontal or transverse bars (not depicted in the
diagram above, but see the diagram below). The horizontal bars divide the pores into single horizontal rows. The
larger vertical bars divide the rows into groups of a few pores per group. The larger horizontal or vertical bars bear
cilia that project into the lumen of the pharynx and serve to propel the mucus-sheet. When present, the vertical bars,
in particular, are raised, so lifting the mucus sheet clear of the stigmata. Cilia on the smaller bars, either side of the
slits/pores or stigmata, project across the slits. The cilia span half the width of the stigmata, so that the cilia on either
side completely occlude the slit. These lateral (on the sides of the bars) cilia beat and drive currents of water through
the stigmata, from the pharynx into the atrium (and hence out from the animal through the atrial siphon). As the water
passes through the pharyngeal basket, it crosses the very fine porous mesh of the mucus-sheet and any food
particles (from about 0.1 micrometre or larger) are trapped, sticking to the mucus, to be eventually swept to the
stomach, via the short oesophagus that connects the pharynx to the stomach.
Connecting the pharyngeal basket to the stomach is a short tube, the oesophagus, into which a digestive gland (not
visible in our diagram) may empty its contents. From the pharynx, the food (wrapped up in the mucus chords) is
transported, by cilia, into the oesophagus and hence to the stomach. From the stomach the food passes into the
intestine, which characteristically loops back on itself (the digestive loop) and emerging via the anus in part of the
atrium called the cloaca, and waste is carried out by the exhalent atrial siphon. Note that in the U-shaped part of the
gut, the descending limb is the oesophagus, the horizontal bend of the U is the stomach, and the ascending limb is the
intestine. The intestine may end in an expanded rectum immediately before the anus. A glandular system of vesicles
and tubules covers the intestine, draining by ducts into the proximal intestine (that part near to the stomach).
There is a tendency in some ascidians to increase the surface area of the pharyngeal basket for more efficient
pumping and filtering. In some forms this is achieved by throwing the wall of the pharynx into a series of folds, in others
by having spiral stigmata. The pumping of water occurs at low pressure so requires little energy (though as sea water
typically contains little organic matter, the sea squirt does not have much energy to expend!)
Some ascidians are carnivorous and the buccal siphon (now really a mouth) is equipped with lobes with which to
grapple crustaceans and small worms. In these forms the pharynx has few slits and filter-feeding is reduced if not
The structures of two of the principle types of pahrayngeal basket are illustrated below:
Above: an aggregation of solitary sea squirts. The larger buccal siphon has a filter of tentacles inside it, to prevent
particles that are unsuitable large from entering. Sea squirts are so-called, because at low tide they clamp their
siphons tightly shut, with their bodies full of sea water (which bathes the tissues) and when squeezed, the water is
forced out through the siphon valve under high pressure. (Note: this is not good for the sea squirt which needs the
water to keep its tissues bathed until the tide comes back in!). They also periodically squirt out water when submerged
Pyrosoma is essentially a colonial salp and is a very bizarre organism indeed! The name literally means 'fire-body' in
reference to the bright bioluminescence characteristically produced by these animals. The animal consists of a long
cylinder, closed at one end and open at the other. The tube may be several centimetres in length, and quite rigid, but
is up to 20 metres long in some species (!) and often 2-3 metres in length. In these longer forms, the body typically
gently undulates in the water. The wall of the tube consists of a common test in which are embedded the individual
zooids. The outer surface of the cylinder is porous, each pore being the inhalent orifice of an individual zooid, which
draws in sea water, sieving it in its pharyngeal basket, and then expels the water into the common lumen of the
cylinder. Since the cylinder is open only at one end, water jest out from this orifice, propelling the entire colony gently
through the water. The outer surface of the cylinder is often studied with hairs or scales, which are projections of the
testa. These projections will create a boundary layer of still water around the colony, which may have hydrodynamic
functions and may also make it easier for the zooids to draw in water. Other forms are more-or-less smooth, or
covered in wart-like protuberances.
Thaliaceans - Salps, Doliolids and Pyrosomes
Thaliacians ressemble ascidians, but are not sessile and are instead free-swimming (by jet propulsion). The adult has
no tail. There is an anterior oral aperture and a posterior atrial aperture (not necessarily extended into tubular
Salps are non-sessile, swimming by jet propulsion. They may be colonial or alternating solitary and colonial forms.
Colonies may be arranged in single or double chains. These chains can reach many metres in length. The range of
forms is fantastic, with salps looking rather like creatures from outer space! An individual salp resembles an ascidian
except that it is not attached and sessile, but free floating or swimming, with the inhalent orifice (generally not a tubular
siphon) at the front end and the exhalent orifice at the rear, such that they may swim by jet propulsion as the cilia
convey water through the pharynx. They rely less on cilia, however, and have incomplete hoops of muscle, like the
hoops of a barrel, which contract the animal when contracted, expelling the water. There are no pharyngeal bars to
divide-up the pharyngeal clefts. The nervous system is degenerate and replaced in large part by an epidermal
neuroid system, which controls locomotion in response to stimuli.
Salps show alternation of generations between sexual and asexual generations. The solitary sexual form produces
a single egg that develops, attached to a placenta-like organ, into the asexual form (oozoid) which asexually produces
buds (blastozoids) which are sexual forms that remain aggregated with the asexual progenitor.
In Botryllus, the test is a common tunic and the whole forms a colony that may reach 15 cm in diameter.
Within the test are several star-shaped colonies, formed by asexual reproduction by budding from a single
The pharynx of salps is not porous like the pharyngeal basket of sea squirts, instead there are only two large
pharyngeal slits. Filter-feeding does not use a mucus sheet in the same way as in ascidians, instead a mucus funnel is
used. The point of the funnel is pulled into the intestine (presumably by ciliary action and peristalsis) and apparently
not driven by pharyngeal cilia. The wide rim of the funnel is added to by new mucus secretion from the endostyle. The
flow of water through the mesh-like mucus funnel is achieved by periodic contractions of the muscle hoops rather than
by ciliary action.
Circulation and Transport
Situated near the loop of the digestive tract is the heart vesicle - a tube, one cell thick, of muscular cells that contract
in synchrony. The contraction is myogenic (initiated by the muscle cells independent of nervous innervation, as in
vertebrates) and timed by two pacemakers, one at each end of the heart. There are no structural valves. Blood is
pumped from the ventral/anterior end of the heart and is carried by a 'blood vessel' to the anterior tissues. This vessel
branches into a number of vessels. These vessels have no wall (cell lining) and so are not true blood-vessels, but are
sinuses (open channels in the tissues) One main vessel (the subendostylar vessel) runs beneath the endostyle in the
ventral wall of the pharynx and gives off branches into the pharyngeal bars. Blood returns to the heart via sinuses that
drain into its dorsal/posterior end. Occasionally, the heart reverses direction and pumps blood from the subendostylar
vessel into the dorsal/posterior tissues. The heart pumps by peristalsis - a wave of muscular constriction travelling
from one end to the other.
The blood cells contain no respiratory pigments and the blood is essentially colourless. However, individual blood cells
may contain blue, green or orange pigments due to iron-containing or vanadium-containing pigments (depending on
species). Vanadium serves a number of (not well understood) functions in sea squirts. This metal is extremely dilute in
sea water, but as sea squirts filter so much water they can accumulate high concentrations of vanadium by absorbing
it in their pharynx. The blood cells containing iron or vanadium migrate to the tunic and release heir metal-containing
enzymes which act as reducing agents in tunicin (ascidian cellulose) fibre synthesis. Other blood cells are storage
cells, accumulating waste materials. These cells are eventually taken out of circulation by accumulating in certain
regions of the body. Nutritive amoebocytes and stem cells are also presentv in the blood.
Excretion and Osmoregulation
No special excretory organs appear to be present. Nitrogenous waste is excreted as ammonia, which can easily be
carried away in the exhalent flow of water and by diffusion from the general body surface. The blood and tissues have
the same osmolarity as the surrounding sea water, so there is no need for special osmoregulation. The concentrations
of specific ions in the blood do differ from those in sea water, however.
Nervous System and Sensors
The larva possesses a neural tube running the length of its body. In the adult this disappears and the central nervous
system remains as a compacted ganglion (brain) situated at the anterior end of the body, between the two siphons.
The siphons, in particular, are equipped with sensory cells that probably serve to regulate flow and to protect the
animal from large foreign objects that may block the system by closing the inhalent siphon and temporarily
shutting-down filter-feeding. Pigment spots or ocelli (presumably light-sensitive) are reported in some forms, situated
around the buccal siphon. Ciona has eight such spots.
Sea squirts are chordates (of the urochordate type) as are vertebrates, and although soft-bodied invertebrates, they
do have some chordate features. In particular the ascidian larva is tadpole-like and possesses a notochord along its
tail, which is lost in the adult. (A notochord is a spongy, stiffening rod that runs along the back of chordates at some
stage in their life cycle. It consists of cells that contain fluid under pressure and acts as a (hydro)skeletal rod. In
vertebrates, including humans, the main supporting role is taken over by the vertebral column, but the notochord
persists inside the centra of the vertebrae.) The tail of the larva also has metamerically segmented muscle blocks.
(Metameric segmentation is the repetition of body parts in serial fashion and characteristic of many organisms such as
earthworms and vertebrates). A neural tube (a precursor to the vertebrate spinal cord) also runs along the tail of the
Pharyngeal slits are also present (as they are in the human embryo!), and as we have seen they form an essential
part of the filter-feeding apparatus. Sometimes they are called gill slits (and the pharyngeal bars, gill bars) as they
have evolved into gills in many vertebrates, such as fish. The pharyngeal basket is possibly also the main site of gas
exchange in sea squirts, though there appears to be no demonstration of this. Their primary function in sea squirts is
The endostyle, which secretes the mucus, is a precursor to the vertebrate thyroid gland and the mucus it produces
consists of mucoproteins with a lot of bound iodine. The heart, being myogenic and regulated by pacemakers, also
has some similarities to the vertebrate heart.
Reproduction and Development
Sea squirts are usually hermaphroditic, and usually equipped with a single ovary and a single testis. These open to
the outside via gonoducts (the male gonoduct or vas deferens and the female gonoduct or oviduct) that open via
gonopores in the atrium. The region of the atrium containing the gonopores and the anus is called the cloaca (a
general name in zoology for a cavity into which anus and gonopores open before their contents are discharged to the
outside). The exhalent atrial siphon is thus responsible for ejecting the gametes when they are shed. In solitary
species, the eggs are usually shed into the sea where they are fertilised. Such eggs may be equipped with flotation
membranes. In colonial species, the egg is often retained in a region of the atrium called the brood chamber, which
may contain special brood pockets to hold the eggs as they develop after internal fertilisation. In these forms, the
hatchling larva is typically released via the atrial siphon, though development may continue to a juvenile or young
adult, in the brood chamber.
The fertilised egg of ascidians undergoes complete radial cleavage to form a hollow blastula (coeloblastula).
Development is triploblastic (the embryo develops three layers of tissue). The egg undergoes determinate
development in that the fate of each part of the egg is predetermined very early on. Yolk (yellow cytoplasm)
accumulates in the vegetal pole (opposite the animal pole) and cytoplasm rich in yolk is destined to become muscle
tissue that makes up the larval tail. Such cytoplasm starts to produce the neurotransmitter acetylcholinesterase. This
developmental pattern occurs at pre-set times according to a biological clock within the egg. The egg exhibits mosaic
development, in that one can, by looking at the colour of the cytoplasm, determine which type of tissue that cytoplasm
will give rise to (following cell division) such that a map can be placed onto the egg. This is sometimes called a 'fate
map', though in this case the end result is absolutely determined so the term 'destiny map' is more accurate. At the
4-cell stage (following two cell divisions) the destiny of the cells is irreversibly determined. In many organisms,
removing one cell from a 4-cell stage embryo causes that cell to resume development into a complete embryo (albeit
reduced in size). However, in some organisms, like tunicates, if the two right-hand cells are removed and grown, then
they develop into the right-half of an embryo only. Similarly, the rear two cells develop into the back-end of the embryo
The tadpole-like larva swims by thrashing its tail, which is equipped with a stiff and elastic notochord, segmented
muscle blocks and dorsal and ventral fins that are outgrowths of the tunic which encapsulates the larva. The
non-functional mouth (which may be sealed) develops into the buccal siphon of the adult. The pharynx has a few slits,
the number increasing dramatically during metamorphosis into the adult. The pharynx is surrounded (dorsally) by an
atrium with a non-functional atrial siphon connecting it dorsally to the outside. After a period of time lasting from a few
minutes (as in Botryllus) to 36 hours, the larva attaches head-down to a solid surface via three adhesive papillae on its
front end. The tail is resorbed and the mouth rotates 180 degrees to sit on top and develops into the buccal siphon.
Many forms are solitary, but in colonial forms the colony is produced by asexual reproduction. Some forms bud zooids
from a stalk-like stolon and may resemble a leaf-less grape vine with zooid 'grapes' budding from it. In others the
stolon is short and a tuft of zooids emerge, connected to the short stolon at their bases. In others, the stolons
resemble roots and buds develop from these root-like outgrowths. In others, like Botryllus, budding occurs from the
body wall. Buds develop in a variety of often complex ways on a variety of body parts in different species. Zooids
produced asexually are called blastozooids.
Above and below: the life-cycle of a pyrosome.
Larvaceans (or appendicularians) are small, transparent and solitary and neotenous urochordates (neoteny is the
phenomenon whereby an organism retains larval characteristics when sexually mature and most likely occurs by
mutation, in the course of evolution, switching off genes for 'normal' development) - hence the name 'larvacean'.
Larvaceans are planktonic and live in a remarkable gelatinous floating house which they secrete. The house (which
does not contain cellulose) contains a pair of inhalent apertures and filter-funnels that sieve the water for food. The
house is discarded and a new one constructed when the filters become clogged. The adult resembles an ascidian
tadpole larva, except that the tail is ventral (at right-angles to the body, hanging down beneath) and the larva has a
functional gut and developed gonads (it is hermaphrodite. The two pharyngeal clefts are simple and open to the
outside either side of the midventral line. The nervous system is compacted into a ganglion or brain (as in adult
ascidians) and there is no nerve cord or tube. There is a light-sensitive ocellus and a statocyst (organ of
balance). The exhalent current assists flotation as the house floats and when outside of the house (if it is discarded
or destroyed) the larvacean swims by violent lashing of the tail. Bacteria and other very fine food particles are the
main food source, filtered from the sea water. Larvaceans are able to filter fine particles that other organisms miss.
The larvacean house is really a set of filters. The larger larvacean species may reach 50 mm in length, but the
house may be 1 metre across. The filters become clogged after about 4 to 24 hours and then the house is
abandoned, deflates and sinks to the bottom. Most species are 5 mm long. From 1 to 16 houses may be produced
in a single day. A water current is maintained through the house by beating of the tail. A new house is secreted
before the old one is shed, existing as a compact layer over the skin, and upon shedding the old one, the new one is
inflated with water!
In Fritillaria, the animal lives outside its house and attached beneath it, whereas in Oikopleura, the house is about 2
cm across and the animal lives suspended inside it. The house of Oikopleura contains a number of passageways
and an inhalent and exhalent aperture. Inhalent apertures are covered by a filter which screens out partciles that
are too large. The water then passes through a second pair of filters with a finer mesh. The filtered food is pumped
into the mouth through a mucus tube to be collected by mucus secreted by the endostyle. A typical larvacean filters
300 ml of water a day, removing thousands of tiny unicelled organisms.
Larvaceans may bloom and are common organisms in plankton. They only reproduce sexually and produce
tadpole-like larvae that are free-swimming and metamorphose into the adult in the water column.
A rear view of a pyrosome, showing the common exhalent exhaust