The morphology of marine invertebrate larvae influences population maintenance and connectivity through functional consequences for swimming, feeding, dispersal and settlement. Sea urchin echinoplutei approximate one of two forms: multi-armed larvae with arms at high elevation angles or two-armed larvae with arms at low elevation angles. According to the hydromechanical model of Grünbaum and Strathmann in J Mar Res 61:659–691 (2003), these morphologies convey either high swimming speed and weight-carrying capacity (few arms at low angles) or high stability in shear (many arms at high angles). This dichotomy, as exemplified by the multi- and two-armed larvae of Heliocidaris tuberculata and Centrostephanus rodgersii , respectively, was investigated to assess model predictions with regard to the angle of elevation of the arms. It was hypothesised that this angle differs in the two forms, multi-armed with high angles (>60°) and two-armed with low angles (<40°) and that the angle would lower as the larvae increased in weight due to growth of the arm skeleton. Arm angles differed as hypothesised and decreased with growth, 7.7° and 10.6° in the advanced larvae of H. tuberculata and C. rodgersii , respectively. The angle of elevation of the outermost arms was also determined from images of 37 additional species of echinoplutei to examine trends in arm angle and change as larvae develop. Multi-armed larvae exhibited a 3°–16° decrease in postoral arm angle from the 2–4 to 6–8 arm stage. In general, multi-armed larvae had higher arm angles than two-armed larvae and, arm angles decreased with growth in both larval forms.