The urbanisation of the world’s coastlines is replacing diverse natural substrates with grey concrete. The changing shoreline colourscape may potentially influence colonisation of marine surfaces, by influencing settlement patterns and predator-prey interactions, but is overlooked in the design of marine structures for ecological co-benefits. This study assessed how manipulating the colour of concrete influenced benthic community development on intertidal seawalls in Sydney Harbour, Australia, when fin-fish predators had access to panels or were excluded by cages. Four colours were compared at high and low intertidal elevations of two sites: grey (the colour of concrete); red and yellow (the colours of natural Sydney sandstone rock) and green (the colour of algal biofilms). Red substrate developed distinct ecological communities compared to yellow, green and grey substrates, with many taxa positively associated with red. Effects of colour were greater in the low than the high intertidal, reflecting the greater number of colonising species on which colour could act.  Effects were generally similar across caged and open surfaces, consistent with effects of colour on settlement preferences rather than predation effects. Contrary to the hypothesis that effects would diminish through time with declining bare space with which colonists could interact, the colour effect persisted for the duration of the 12-month study.  Synthesis and applications. Our results show that in the low intertidal, colour can shape sessile community assembly on concrete surfaces over time scales of at least 12 months. Consequently, alongside the habitat complexity and material type of marine built structures, colour warrants consideration in the development of eco-friendly designs. The colour of marine built structures is easily manipulated, for example through the addition of non-toxic oxides to concrete as done here.

Shark bites on humans are rare but are sufficiently frequent to generate substantial public concern, which typically leads to measures to reduce their frequency. Unfortunately, we understand little about why sharks bite humans. One theory for bites occurring at the surface, e.g. on surfers, is that of mistaken identity, whereby sharks mistake humans for their typical prey (pinnipeds in the case of white sharks). This study tests the mistaken identity theory by comparing video footage of pinnipeds, humans swimming and humans paddling surfboards, from the perspective of a white shark viewing these objects from below. Videos were processed to reflect how a shark's retina would detect the visual motion and shape cues. Motion cues of humans swimming, humans paddling surfboards and pinnipeds swimming did not differ significantly. The shape of paddled surfboards and human swimmers was also similar to that of pinnipeds with their flippers abducted. The difference in shape between pinnipeds with abducted versus adducted flippers was bigger than between pinnipeds with flippers abducted and surfboards or human swimmers. From the perspective of a white shark, therefore, neither visual motion nor shape cues allow an unequivocal visual distinction between pinnipeds and humans, supporting the mistaken identity theory behind some bites.

Shark bites on humans are rare but are sufficiently frequent to generate substantial public concern, which typically leads to measures to reduce their frequency. Unfortunately, we understand little about why sharks bite humans. One theory for bites occurring at the surface, e.g. on surfers, is that of mistaken identity, whereby sharks mistake humans for their typical prey (pinnipeds in the case of white sharks). This study tests the mistaken identity theory by comparing video footage of pinnipeds, humans swimming and humans paddling surfboards, from the perspective of a white shark viewing these objects from below. Videos were processed to reflect how a shark's retina would detect the visual motion and shape cues. Motion cues of humans swimming, humans paddling surfboards and pinnipeds swimming did not differ significantly. The shape of paddled surfboards and human swimmers was also similar to that of pinnipeds with their flippers abducted. The difference in shape between pinnipeds with abducted versus adducted flippers was bigger than between pinnipeds with flippers abducted and surfboards or human swimmers. From the perspective of a white shark, therefore, neither visual motion nor shape cues allow an unequivocal visual distinction between pinnipeds and humans, supporting the mistaken identity theory behind some bites.