The conservation status of sessile marine invertebrates is poorly understood. As a case study we present data for ascidians which share many of the problems that beset other invertebrates: i) they require a specialist to identify most of them; ii) our understanding of their biology is fragmentary; and iii) electrophoresis indicates that sibling species occur. The restricted distribution of many colonial forms and their limited vagility suggests that ascidians may be vulnerable to local environmental perturbations. We must balance new research with action to conserve diversity and ecosystem integrity. Adequate research funding and recognition of the scientific merit of taxonomy are crucial to this process.