Pyura pachydermatina has a club-shaped body supported by a long stalk, both being covered by a tough exterior tunic.

The stalk is two thirds to three quarters the length of the whole animal which helps distinguish it from certain invasive tunicates not native to New Zealand such as Styela clava and Pyura stolonifera.

It usually co-exists with a parasitic ribbon worm, Gononemertes australiensis which lives in its digestive gland or body cavity.

[5] Cells lining the gut of Pyura pachydermatina have been found to contain an insulin-like material in two forms that are immunologically active.

[2] These spicules have a core of amorphous calcium carbonate enveloped in an insoluble layer of organic material with a thick exterior covering of calcite.