Protochordata Lecture 5 PDF

Protochordata Lecture 5 Mrs. K.A.M.Sudarshani Senior Lecturer Dept. of Zoology  Class 2: Thaliacea Pelagic tunicates, living in warm waters Have circular bands of muscles By that, animal moves through the water by jet propulsion Eg. Doliolum and its allies: muscle bands pass...

Protochordata Lecture 5 Mrs. K.A.M.Sudarshani Senior Lecturer Dept. of Zoology  Class 2: Thaliacea Pelagic tunicates, living in warm waters Have circular bands of muscles By that, animal moves through the water by jet propulsion Eg. Doliolum and its allies: muscle bands pass round the body Salpa: Rings are incomplete Mouth & atriopore at the opposite ends of the body Doliolum Salpa Tunic: thin & transparent Salpa can swim either forwards or backwards Life history involves alteration of generation Dolilum: - Ascidian tadpole develops into a mother/nurse zooid (oozoid) - This Oozoid: by budding produce daughter zooids Daughter zooids: three types: 1. Trophozooids: sterile, nutritive & respiratory individuals - permanently sessile on the parent 2. Phorozooids : sterile, nurse forms - eventually get free 3. Gonozooids: nursed & carried by phorozooids until sexually mature - become free  Salpa: Planktonic tunicate Have a complex life cycle The solitary life history phase (oozooid) is a single, barrel-shaped animal It reproduces asexually by producing a chain of tens to hundreds of individuals (‘aggregate’ individuals: blastozooids) Each blastozooid in the chain reproduces sexually blastozooids are sequential hermaphrodites ( first maturing as females, and are fertilized by male gametes produced by older chains)  Blastozooids (sexual forms) - produce a single egg - develops within the mother without a tadpole stage - nourished by a diffusion placenta - becomes asexual oozoid - produce a chain of blastozooids -remain until they break away by sections  Doliolum they are free-swimming plankton; have a complicated life cycle that includes sexual and asexual generations. The sexual generation consists of individuals with eight muscle bands, each having male or female gonads. These individuals are called gonozooids These asexual individuals (nurses) produces an astonishing number of mature progeny asexually  This progeny include both sexual and asexual zooids in three sequential "generations". The nurse produces buds (zooids) in its ventral stalk, but the buds grow and mature on its dorsal stalk. These spoon-shaped zooids (phorozooids) supply food for the whole colony via a common blood circulation along two blood-filled sinuses. These zooids grow into gonozooids  Gonozooids detached from the phorozooid swim free, mate, and produce fertilized eggs Next generation of asexual zooid produce and the cycle repeats. The total number of zooids produced by a single nurse colony can reach tens of thousands  Pyrosoma: - pelagic, colonial form in warm seas - number of individuals associated to form an elongated, barrel-shaped colony - all zooids are sexual - capable of budding - Yolky eggs develops within the parent - No larval stage - produce a powerful light that they shine (in photogenic organs in pharynx) for protection  Class 3: Larvacea Small neotenous tunicates & very common An important component of plankton Build a house by secretion from the oikoplastic epithelium (a special part of skin) Tail: a powerful organ - produce food currents food caught by a special filter arrangement It has “filtering windows” & a system of filter pipes Nanoplanktons (minute flagellates) are caught by these pipes - feeding filter is a cellular secretion and it becomes clogged after a few hours - then the house is abandoned & a new one is secreted Has a pharynx with two gill-slits, endostyle & peripharyngeal bands General body organization = a typical ascidian tadpole  Oikopleura  They have arisen from other tunicates by the acceleration of the rate of development of alimentary organs & gonads So, the metamorphosis & normal adult stage are eliminated Show the process of neoteny Tail: highly developed - serving for locomotion, nutrition & building of house - has a wide, continuous fin, notochord with 20 cells & hollow neural tube with ganglionic thickenings