A Victorian emigrant: first observation and range extension of the nudibranch Tenellia catachroma (Burn, 1963) in Western Australia (Mollusca: Gastropoda)

The southwest coast of Western Australia is heavily influenced by the south-flowing Leeuwin Current. In summer, the current shifts and the north-flowing Capes Current delivers water from the south to nearshore environments and with it a supply of larvae from cooler waters. The nudibranch Tenellia catachroma (Burn, 1963) was considered restricted to Victorian waters; however, its discovery in eastern South Australia in 2013 revealed its capacity to expand its range west. In March 2017 a single individual was observed in shallow subtidal waters at Cape Peron, Western Australia, some 2000 km to the west of its previous range limit. Moreover, its distribution has extended northwards, possibly aided by the Capes Current, into a location of warming. This observation significantly increases the range for this Victorian emigrant to encompass most of the southern Australian coast, and also represents an equatorward shift at a time when the reverse is expected.