ABSTRACT: Population-genetic studies of marine invertebrates show that planktonic larval duration of a species is not a perfect predictor of its distribution. The intertidal air-breathing slug Onchidella nigricans presents a prime example of such unpredictability. It has a broad geographic range, occurring on the coasts of Australia and New Zealand, spanning the 1500 to 2000 km wide Tasman Sea, despite having an estimated planktonic larval duration of approximately 3 wk. We used mitochondrial sequences and genome-wide AFLP fingerprint data to quantify levels of regional genetic exchange using samples from 16 locations across 3 major landmasses. We expected to find high genetic similarity between the closest populations but low similarity across large distances and ocean gaps. Instead, we detected a pattern of high genetic similarity of all study populations. We suggest that an extended planktonic larval phase is driving the high genetic connectivity.