Low diversity benthic shelly assemblages occur under conditions of both low oxygen and very soft substrates. It is therefore important to be able to distinguish between these two environmental parameters. A declining oxygen gradient causes benthic diversity (including epibionts on hard benthic islands) to decline to a zero point. A transition from firm to soft substrates is also associated with a decline in overall benthic faunal diversity but, importantly, this has little affect upon epibionts. Functional morphological studies may also provide information about substrates, for example, thin-shelled bivalves of the ‘paper pecten’ morphology may represent super-soft substrate strategists.