With urbanisation, there is an increasing trend for artificial structures, such as seawalls, to replace natural habitats. The predatory mulberry whelk, Morula marginalba Blainville, is seldom observed on seawalls in Sydney Harbour, yet it is abundant on the rocky shores of south-eastern Australia. The Sydney rock oyster, Saccostrea glomerata Gould, is common on seawalls in Sydney Harbour, forming two types of crust, providing ‘elaborate’ or ‘simple’ habitats that differ in structure. Whelks were numerous on some seawalls with elaborate oyster crusts, but were sparse on walls with simple crusts. Thus, different types of crust, with different structure, may explain the differences in the numbers of whelks among seawalls. These different crusts may cause differences in dispersal and/or mortality. The structure of the habitat created by the oysters was manipulated on seawalls and the responses of M. marginalba were observed. Whelks emigrated more rapidly from simple than from elaborate crusts and more individuals moved into elaborate than into simple crusts. Decreases in the numbers of M. marginalba at larger scales, via mortality or emigration, did not differ between the crust types. The range of habitats that can be used by M. marginalba is extended because it can exploit the biogenic structure provided by oysters on artificial urban structures, which otherwise form unsuitable habitat.