You appear to have at least two species in these pictures. Based on the translucency of the tunic, the middle and bottom pictures most likely belong to the order Phlebobranchia. To identify to a lower level with any amount of certainty, the specimens would need to be dissected. With a few exceptions, it is almost impossible to definitively identify ascidians from pictures as most of the important distinguishing characters are internal. The following website has instructions for how to relax and preserve ascidians for morphological identification: http://woodshole.er.usgs.gov/project-pages/stellwagen/didemnum/htm/page41.htm
The top picture looks like it could be Didemnum vexillum, which is an invasive species in many areas of the world. However, it has been a long time since I've done any work with ascidians, so hopefully some additional researchers can confirm or reject my guess.
I doubt it is D. vexillum. Overall colony morphology is wrong, plus the water around Karachi is probably too warm. Didemnum vexillum is a temperate species.
if the first picture is taken on hard substrates is Didemnum vexillum please inspect the link http://www.rimeis.org/species/id_cards/invaders/didemnum.pdf
Do you have a close-up picture of the surface of the tunic for the specimen in the top picture? Also, are the specimens in the middle and bottom pictures completely separate individuals or do they have shared/connected tunics?
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