First detected in northern Australia in late 2019 (Cairns, Port Douglas, Cooktown; Willan et al., 2021), further detection in Mourilyan Harbour in early 2021.

The black-scar oyster, Magallana bilineata, is a large, tropical oyster that can grow to up to 212 mm in shell length (Rao, 1987). It is usually found in estuaries and inlets attached to solid material such as rocks or pilings. While shells of all oyster species are extremely variable, the internal dark adductor muscle scar on both valves is distinctive of this species (Willan et al., 2021), although other species variably possess this trait. The external shell of the oyster is pale yellow or purplish in colour, and possesses thin, flaky layers (Willan et al., 2021). When freshly opened, the outer parts of the shell hinge are distinctly iridescent green (Willan et al., 2021). Other species found in the area possess internal pits, or ‘chomata’, around at least part of the shell margin; these are never seen in M. bilineata.

M. bilineata is an important aquaculture species in India (Rao and Rao, 1985) and the Philippines (Lebata-Ramos et al., 2021), and was deliberately introduced to Fiji in the 1970s for this purpose (Kinch et al., 2019a; Kinch et al., 2019b). Until recently it has only been found within disturbed habitats (marinas and harbours) in northern Queensland (Willan et al., 2021), although it has been recently discovered on rocks near unmodified anchorages near Cooktown. It is unknown whether this species poses a risk to Australian ecosystems, however as an exotic introduction it has been included as reportable ‘biosecurity matter’ in Queensland (Willan et al., 2021).