Retrogressive Metamorphosis in Ascidian Tadpole
Retrogressive metamorphosis in
Retrogressive metamorphosis in
ascidian tadpole
ascidian tadpole
Dr. Narayan R. Mane
Department of Zoology
GKG College Kolhapur
Ascidian
•
Ascidiacea
(commonly known as the
ascidians
or
sea
squirts
) is a
in the
of sac-like
.filter feedersinvertebratemarineTunicatasubphylumclass
•
Ascidians are characterized by a tough outer "tunic"
made of the
, as compared to
other
which are less rigid.tunicatestunicinpolysaccharide
•
Ascidians are found all over the world, usually in
shallow water with
over 2.5%. While
members of the
and
swim freely
like
, sea squirts are
animals: they
remain firmly attached to
, such as rocks
and shells.substratumsessileplanktonLarvaceaThaliaceasalinities
•
There are 2,300 species of ascidians and three
main types: solitary ascidians, social ascidians
that form clumped communities by attaching at
their bases, and compound ascidians that consist
of many small individuals (each individual is
called a
zooid
) forming colonies up to several
meters in diameter.
•
Sea squirts feed by taking in water through the
oral siphon
. The water enters the mouth and
pharynx
, flows through mucus-covered gill slits
(also called
pharyngeal
stigmata
) into a water
chamber called the atrium, then exits through the
atrial siphon.
RETROGRESSIVE METAMORPHOSIS
•
Metamorphosis is a change from the juvenile to adult
stage in which larval stage is quite different from the
adult stage.
•
In retrogressive metamorphosis the larva possesses
advanced characters which are lost during the
development and the adult is either sedentary or
degenerated with primitive characters.
•
Urochordate adults, being sedentary show
degenerative characters while the free swimming
tadpole larva shows advanced chordate characters
which are lost during metamorphosis.
•
During metamorphosis the larva will loose all the
chordate characters and attains an invertebrate like
form. This type of metamorphosis, where highly
advanced larval form ends in a slowly organized adult is
called retrogressive metamorphosis
•
Retrogressive metamorphosis is characteristic of
ascidians (Urochordata or Tuncicata).
•
It is given this name because here a progressive, active
and alert larva metamorphoses into a retrograde and
sedentary adult.
•
Ascidians are marine, sedentary, bag-like creatures that
remain attached to a rocky substratum all their lives
and use a current of water passing through their large
and perforated pharynx to collect microorganisms on
which they fads.
•
The larva, on the other hand, is active, tadpole-like in
shape and has all the three chordate features, gill
clefts, notochord and dorsal tubular nerve cord.
•
Most of these characters are lost or become
degenerate as the active larva metamorphosis's.
Hence, the term retrogressive metamorphosis.
The advanced Chordate characters of the larva
•
There is a rod-like notochord in the tail to
which are attached muscle bands for
swimming.
•
There is a dorsal hollow nerve cord which is
enlarged to form brain at the anterior end. A
photoreceptor ocellus and a balancing organ,
the statocyst are attached to the brain.
•
There are only two pairs of gill slits in pharynx
but the mouth is closed by a membrane and
intestine is rudimentary.
•
Endostyle on the ventral side of pharynx is
very well developed which functions like
thyroid gland and helps in metamorphosis.
•
Heart is on the ventral side of gut but is non-
functional.
•
The larva possesses on the anterior end three
ectodermal adhesive papillae which help in
firm attachment on the substratum.
Changes during metamorphosis
•
Larva attaches to the substratum with the help of
chin warts, head downward and tail up.
•
Rapid growth takes place between the chin warts
(adhesive papillae) and mouth and almost no
growth on the opposite side of body.
•
Due to rapid growth on one side, body starts
rotating in such a way that mouth gradually
migrates to the upper side.
•
Meanwhile pharynx enlarges and stigmata
increase in numbers.
•
Intestine becomes functional and atrial
opening is formed on the opposite side of oral
aperture.
•
Both tail and notochord are gradually
absorbed in the body during metamorphosis.
•
The hollow nerve cord is reduced into a solid
nerve ganglion on the dorsal side.
•
Sense organs, namely ocellus and statocyst
are lost.
•
When the metamorphosis is over,
Herdmania
is
transformed into a bag-like sedentary animal
attached to the rock by a foot and having
branchial and atrial openings for inlet and outlet
of water respectively.
•
Pharynx becomes enormously enlarged with a
large number of stigmata for filter feeding and
digestive system becomes well developed.
•
However, other advanced chordate characters of
the larva are degenerated into simple structures,
due to which it is called retrogressive
metamorphosis.
Ascidians, also known as sea squirts, exhibit retrogressive metamorphosis where the larval stage possesses advanced features lost in adulthood. With a tough outer tunic, these marine filter feeders have three main types - solitary, social, and compound ascidians. They feed by taking in water through oral and atrial siphons. Retrogressive metamorphosis in ascidians involves the loss of chordate characteristics as the larva transitions into a sedentary or degenerated adult form. This process results in a transformation from an advanced larval form to a more primitive adult form.
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Presentation Transcript
Retrogressive metamorphosis in ascidian tadpole Dr. Narayan R. Mane Department of Zoology GKG College Kolhapur
Ascidian Ascidiacea (commonly known as the ascidians or sea squirts) is a class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid. Ascidians are found all over the world, usually in shallow water with salinities over 2.5%. While members of the Thaliacea and Larvacea swim freely like plankton, sea squirts are sessile animals: they remain firmly attached to substratum, such as rocks and shells.
There are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a zooid) forming colonies up to several meters in diameter. Sea squirts feed by taking in water through the oral siphon. The water enters the mouth and pharynx, flows through mucus-covered gill slits (also called pharyngeal stigmata) into a water chamber called the atrium, then exits through the atrial siphon.
https://upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Triphyllozoon_inornatum_%28Bryozoan%29_and_Polycarpa_aurata_%28Sea_quirt%29.jpg/300px-Triphyllozoon_inornatum_%28Bryozoan%29_and_Polycarpa_aurata_%28Sea_quirt%29.jpghttps://upload.wikimedia.org/wikipedia/commons/thumb/2/2d/Triphyllozoon_inornatum_%28Bryozoan%29_and_Polycarpa_aurata_%28Sea_quirt%29.jpg/300px-Triphyllozoon_inornatum_%28Bryozoan%29_and_Polycarpa_aurata_%28Sea_quirt%29.jpg
Metamorphosis is a change from the juvenile to adult stage in which larval stage is quite different from the adult stage. In retrogressive metamorphosis the larva possesses advanced characters which are lost during the development and the adult is either sedentary or degenerated with primitive characters. Urochordate adults, being sedentary show degenerative characters while the free swimming tadpole larva shows advanced chordate characters which are lost during metamorphosis. During metamorphosis the larva will loose all the chordate characters and attains an invertebrate like form. This type of metamorphosis, where highly advanced larval form ends in a slowly organized adult is called retrogressive metamorphosis
Retrogressive metamorphosis is characteristic of ascidians (Urochordata or Tuncicata). It is given this name because here a progressive, active and alert larva metamorphoses into a retrograde and sedentary adult. Ascidians are marine, sedentary, bag-like creatures that remain attached to a rocky substratum all their lives and use a current of water passing through their large and perforated pharynx to collect microorganisms on which they fads. The larva, on the other hand, is active, tadpole-like in shape and has all the three chordate features, gill clefts, notochord and dorsal tubular nerve cord. Most of these characters are lost or become degenerate as the active larva metamorphosis's. Hence, the term retrogressive metamorphosis.
There is a rod-like notochord in the tail to which are attached muscle bands for swimming. There is a dorsal hollow nerve cord which is enlarged to form brain at the anterior end. A photoreceptor ocellus and a balancing organ, the statocyst are attached to the brain. There are only two pairs of gill slits in pharynx but the mouth is closed by a membrane and intestine is rudimentary.
Endostyle on the ventral side of pharynx is very well developed which functions like thyroid gland and helps in metamorphosis. Heart is on the ventral side of gut but is non- functional. The larva possesses on the anterior end three ectodermal adhesive papillae which help in firm attachment on the substratum.
Changes during metamorphosis Larva attaches to the substratum with the help of chin warts, head downward and tail up. Rapid growth takes place between the chin warts (adhesive papillae) and mouth and almost no growth on the opposite side of body. Due to rapid growth on one side, body starts rotating in such a way that mouth gradually migrates to the upper side. Meanwhile pharynx enlarges and stigmata increase in numbers.
Intestine becomes functional and atrial opening is formed on the opposite side of oral aperture. Both tail and notochord are gradually absorbed in the body during metamorphosis. The hollow nerve cord is reduced into a solid nerve ganglion on the dorsal side. Sense organs, namely ocellus and statocyst are lost.
When the metamorphosis is over, Herdmania is transformed into a bag-like sedentary animal attached to the rock by a foot and having branchial and atrial openings for inlet and outlet of water respectively. Pharynx becomes enormously enlarged with a large number of stigmata for filter feeding and digestive system becomes well developed. However, other advanced chordate characters of the larva are degenerated into simple structures, due to which it is called retrogressive metamorphosis.
