Appearance
Like all tunicates, "P. nigra" has a thick leathery envelope containing cellulosic material. Like all solitary ascidians, the tunic encloses a sac-shaped body with separate water entrance and exit tubes. It lives on plankton that it filters from seawater with a mucous net. This tunicate is often host to the small symbiotic pea crab "Tunicotheres moseri" which takes up residence in its atrial chamber.An adult "P. nigra" may be 10 cm long. The tunic is usually velvet black or dark brown, but may be gray in specimens that are younger or live in shaded areas. Its original range is unclear; the tropical Western Atlantic Ocean, the Red Sea, and the Indian Ocean have been proposed.
The tunic of "P. nigra" contains many vesicles filled with a strong acid, containing mostly sulphate SO42− and chloride anions. The vesicles are concentrated towards the outer surface and are easily ruptured by contact; they are believed to protect the animal from predation and fouling.
Substances extracted from the dried tunic with methanol have been found to have cytotoxic, antibacterial, antipyretic, analgesic, and histamine-like activity.
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