Cypris larvae of acrothoracican barnacles (Thecostraca: Cirripedia: Acrothoracica)

We used SEM to investigate the morphology of the cypris larvae from a range of species of the Cirripedia Acrothoracica, representing all three families and including the first detailed account of cyprids in the highly specialized Cryptophialidae. Special attention was given to the head shield (carapace), the lattice organs, the antennules, the thoracopods, the telson and the furcal rami. The cypris larvae of the Acrothoracica fall into two morphological groups; those of the Trypetesidae and Lithoglyptidae have a well-developed carapace (head shield) that can completely enclose the body and sports fronto-lateral pores, numerous short setae and lattice organs perforated by numerous small, rounded pores and a single, conspicuous terminal pore. The fourth antennular segment has the setae arranged in subterminal and terminal groups. There is a developed thorax with natatory thoracopods and a distinct abdomen and telson. In comparison, the cyprids of the Cryptophialidae exhibit apomorphies in the morphology of the carapace, the antennules and the thorax, mostly in the form of simplifications and reductions. They have a much smaller head shield, leaving parts of the body directly exposed. The shield is conspicuously ornamented by deep pits and hexagonally arranged ridges and bears a few, very long setae but lacks fronto-lateral pores. The lattice organs have numerous elongated pores, but no large, terminal pore. The fourth antennular segment has all the setae clustered in one terminal group. The thorax and thoracopods are rudimentary and not suitable for swimming. These reductions and simplifications in morphology correlate with cryptophialid cyprids being unable to swim. They can only disperse by antennular walking resulting in small, but highly gregarious populations of adults. The variations in antennular morphology and telson structure were traced for the genera of the families Lithoglyptidae and Trypetesidae. The traditional non-cladistic taxonomy in the suborders Pygophora (Cryptophialidae+Lithoglyptidae) and Apygophora (Trypetesidae) was based largely on symplesiomorphies in adult morphology and cannot be upheld. The Lithoglyptidae and Trypetesidae may form a monophylum, but evidence remains scarce. We expect that the use of larval (cyprid) characters will in the future play an important part in more detailed phylogenetic analyses of the Acrothoracica and also shed new light on their reproductive ecology.