Heterosaccus lunatus parasitizes the portunid crab, Charybdis callianassa, in Moreton Bay, Queensland, Australia. Larval broods, released during July and August (males only), were allowed to develop through to cyprids (lecithotrophic development). Speeds of all the developmental stages—nauplius I to nauplius IV and the cyprids were measured by timing continuously swimming larvae over 5 mm distances under a microscope. The absolute speeds of the naupliar stages were similar (3–4 mm s−1), but the cyprids were significantly faster at ≈10 mm s−1. These speeds were compared with published speeds of cirripede and copepod nauplii and other cirripede cyprids. Relative speeds (body lengths s−1) showed surprising parity, which for the cyprids probably reflects the similar efficiency of the fusiform shape and thoracopod propulsion for all sizes of this highly specialized larval form. The lecithotrophic H. lunatus nauplii have rather minimalistic appendages (compared with planktotrophic nauplii) which were examined at the light microscope and scanning electron microscope levels. The natatory setae were found to be plumo-denticulate.