Herdimania life history

Herdimania is hermophrodite
External fertilization
Development is indirect
Tadpole larva in life history
Retrogressive meta morphosis

1. Dr.S.J.M.PrathapKumar
Lecturer in Zoology
Govt.Collegefor Women (A)
Guntur

2. HerdimaniaLife History
❖DevelopmentofHerdmaniahasbeendescribedbySebastian
(1953)andDas(1957).
❖Developmentisindirectinvolvingafreeswimmingtadpole
larva.
❖Fertilizationisexternalandcross-fertilisation.
❖Cleavageisholoblastic.
❖About8hoursafterfertilization,larvahatchesouttobecome
free-swimming.

3. Tadpole Larva
❖Thefullydevelopedascidiantadpoleisminute,about1.5mm
longandmotile.Itsbodyiscoveredbyathintunicortestsecreted
bytheectoderm.Ithasananteriorshortovalheadortrunkanda
posteriorlongtail.
Head or Trunk
❖Attheanteriorendofthetrunkarethreeadhesivepapillae
madeofectodermalcells.Thenervoussystemconsistsofan
anteriorenlargedhollowsensevesicleorbrainvesicle.Thesense
vesicleiscontinuedposteriorlyintoasolid,thickmassofnerve
cells,thevisceralganglion,itiscontinuedposteriorlyintoanerve
cordwhichishollowandcontinuedintothetail.Itliesmid-dorsally.

5. Thesensevesiclecontainsapigmentedstatocyst(otocyst)and
twopigmentedunequalocelliorsimpleeyesassenseorgans.
Thealimentarycanalbeginsfromantero-dorsalmouthleading
intonarrowbranchialsiphon,alargesac-likepharynx,short
oesophagus,stomach,intestineandasmallrectum.
Theanusopensintotheleftsideofatrium.Thepharynxhasa
ventralendostyleandalsoapairoflargegill-slitsorstigmata.
Eachgill-slitfinallysplitstoformsixstigmataoneitherlateralside
Anatrialcavityisformedaroundthepharynxlaterallyanddorsally
andopenstotheexteriorbyanatrialaperturelyingdorsally.

6. Theheartandpericardiumislocatedpostero-ventrallyinbetween
pharynxandstomach.Mesenchymecellsfoundscatteredallover
thebodybeneaththeectodermandinamassintheposterior
regionoftrunk.
Tail:Itisapowerfullocomotoryorganbeinglaterallycompressed
andpointedterminally.Tailisabout0.9mmlongandisfringed
withaverticalcontinuoustailfinformedbythetestandmarked
withobliquestriationslookinglikefinrays.Itcontainsadorsal
tubularnervecord,anotochordbeneaththenervecordand
lateralmusclebands.Theanteriorendofnotochordslightly
projectsintothetrunkregion.

7. Theascidiantadpolelarvaisunquestionablyatruechordate,
havingadorsalhollownervetube,anotochordinthetailand
pairedgill-slitsinthepharynxandafteraboutthreehoursof
active,non-feedingandfree-swimmingexistence,itsinkstothe
bottom.
HabitsandHabitatofLarva:
Justafterhatching,thelarvaisphotopositiveandgeonegative.It
cannotfeedbecauseitsmouthisstillclosedbytest.Afterashort
activefree-swimmingexistencelastingabout3to4hours,the
larvabecomesgeopositive,photonegativeandsluggish.

8. Itsinkstothebottom,attachesitselfupsidedowntoasuitable
hardsubstratumbyadhesivepapillaeandundergoesrapid
degenerationorretrogressivemetamorphosistoattainadulthood.
AccordingtoBerril(1955)theselectionofasuitablehabitatis
essentialasthelarvamaynotsurviveonanyotherhabitatormay
getsuffocatedbythebottommudanddetritus
RetrogressiveMetamorphosis:(Gr.,meta=after+morphe=form+osis=state)
Metamorphosisistheshapechangeinformduringpost-
embryonicdevelopmentandinmanycases,signalsadramatic
changeinhabitatoftheanimalsuchaspelagictobenthic
existence.

10. Metamorphosisoftheascidianlarvaisuniqueandbeginsalmost
explosively.Itinvolvestransformationofanactivenon-feeding,
pelagic,lecithotrophic(i.e.,thatfeedsonitsownyolkreserves)
andtailedlarvahavingmanyadvancedfeaturessuchasaxial
notochord,dorsalneuraltubeandspecialsenseorgans,intoan
inert,sedentaryorsessile,simple(primitive)andplankotrophic
filterfeedingadultwithonlyaphaynxwithstigmataandendostvle,
bothindicatingthechordatefeaturesofadultascidian.
Thistypeofmetamorphosiswhichshowsdegenerativeor
retrogressivechangesfromlarvatoadultiscalledretrogressive
metamorphosis.

11. Itinvolvesthefollowingthreetypesofchanges:
(i)Retrogressive,(ii)Progressiveand(iii)Molecularchanges.
(i)RetrogressiveChanges:
Thesechangesinvolvedestructionanddisappearanceofsomeof
thelarvalstructuressuchasfollows:
➢Longtailoflarvawithcaudalfinshortensandfinallydisappears.
➢Caudalmuscles,nervecordandnotochorddisappearasthey
breakdownandareconsumedbyphagocytes.
➢Larvalsenseorgans(theocellusandtheotolith)arelostand
sensoryvesicleistransformedintoanadultcerebralganglion.
➢Adhesivepapillaedisappearcompletely.

12. ii)ProgressiveChanges:
➢Somelarvalstructuresnecessaryforsurvivalbecomemore
elaboratedandspecialisedineachadult,suchasa.
➢Duetolossoftail,thetrunkbecomespear-shapedandfour
largerectodermalampullaegrowoutofitsfourcorners.
➢Theseampullaefirmlyanchorthemetamorphosingtadpoleto
thesubstratumandalsoserveforrespirationasablood-likefluid
keepscirculatingthroughthem.
➢Soontwomoresmallerectodermalampullaeappear
dorsolaterally.

13. ➢Duetolossoftail,thetrunkbecomespear-shapedandfour
largerectodermalampullaegrowoutofitsfourcorners.These
ampullaefirmlyanchorthemetamorphosingtadpoletothe
substratumandalsoserveforrespirationasablood-likefluid
keepscirculatingthroughthem.Soontwomoresmaller
ectodermalampullaeappeardorsolaterally.
➢Thebodytoorotatessothatgeneralformoftheadultsessile
organismisassumed.
➢Adultneuralglandsandnerveorcerebralganglionareformed
bytheneuraltubeandtrunkganglioncometoliemid-dorsally
betweenmouthandatriopore.Thetrunkganglionitself
persistsasvisceralnerve.

14. Withtheabsorptionofitstestcovering,themouthbecomes
functionalandfilter-modeoffeedingbyincomingciliarywater
currents.
➢Pharynxgreatlyenlarges,developsbloodvesselsandstigmata
multiplyrapidly,formingthebranchialsac.
➢Stomachenlarges,intestineelongatesandgetscurvedand
liverdevelops.
➢Atrialcavitybecomesmoreextensive.
➢Circulatorysystemwithheartandpericardiumdevelops.
➢Gonadsandgonoductsdevelopfromlarvalmesodermalcells.

15. ➢Testortunicspreadstocoverentireanimal,becomesthick,
toughandvascularandattachestheanimalbyformingafootif
necessary.
Thus,foregoingmetamorphicchangesmarkthebeginningofa
sedentary,activelyfeeding,sexualadultlifewhichsoonstarts
producinggametes,firstovaandlatersperms.
iii)MolecularChanges:
ManketandCowden(1965)studiedthemolecularchangeswhich
takeplaceduringmetamorphosis.Theystudiedthemetabolismof
proteinandnucleicacidandpointedoutthatsomeprotein
synthesisoccursthroughoutthedevelopmentbutwiththeoutset
ofmetamorphosis,

16. extensivedegradationofproteinsbeginsfollowedbyrapid
synthesisofnewproteins.
EmbryologicalSignificanceofAscidianTadpole:
ThepresenceofatadpolelarvainthelifehistoryofHerdmania
andotherascidiansissignificantinthefollowingways:
a.TaxonomicSignificance:
Thetadpolelarvapossessestruechordatecharacterssuchas
notochordanddorsaltubularnervecord,whicharelackinginthe
adult.Thus,theascidianlarvaprovidestheclueforincludingthe
ascidianunderthephylumChordata.Withouttadpolelarva,the
truenatureandtaxonomicpositionofdegeneratesedentaryadult
ascidianswouldhaveremaineduncertain.

17. b.PhylogeneticSignificance:
Onthebasisofrecapitulation,theascidianlarvapossessingthe
chordatefeaturesisconsideredastherelicofthefree-swimming
ancestralvertebrates.
c.Dispersal:
Theadultascidiansbeingsedentary,thefree-swimminghabitof
thelarvaprovidestheonlymeansofdispersalofthespecies.It
alsoprovideschancesofselectingbettersitesregardingfoodand
protection,thus,ensuringsurvivaloftherace.
d.EmbryologicalSignificance:
Ascidiansprovidebestexampleofmosaiceggswithawell-
organised,pre-patternedandwelldifferentiatedooplasmand

18. ➢highlydeterminatetypeofdevelopment.Moreover.ascidians
aretheonlychordatesexhibitingtrueretrogressive
metamorphosis.
➢Theeggcortexincaseofascidiansisthesiteofmorphogenetic
patterningrelatedtopolar,bilateralandgeneralorganisationof
developingegg.Besidesthis,cleavageinascidianstendsto
segregatecytoplasmicterritories,havingdifferentbiological,
histochemicalandbiochemicalproperties.