Scanning electron microscopy of acrothoracican cypris larvae (Crustacea, Thecostraca, Cirripedia, Acrothoracica, Lithoglyptidae)

Scanning electron microscopy was used to provide a full morphological description of cypris morphology in the acrothoracican species Lithoglyptes milis and L. habei (Lithoglyptidae). Special attention was given to lattice organs, antennules, thorax, thoracopods, abdomen, and furcal rami. Cypris larvae of the Acrothoracica share some putative plesiomorphic features with the cypris-like ascothoracid larvae of the non-cirripede taxon Ascothoracida. The most notable are traces of abdominal segmentation and carapace lattice organs without pore fields. Acrothoracican cyprids also share numerous synapomorphies with those of the Thoracica and the Rhizocephala. This list includes a four-segmented antennule with a triangular first segment of two sclerites set at an angle to each other, a cylindrical second segment, a small third segment functioning as an attachment organ, and a cylindrical fourth segment bearing homologous sensory setae. Further apomorphies are a pair of frontolateral horn glands exiting anteroventrally on the headshield (carapace), a pair of multicellular cement glands exiting on the attachment organs, a single stout, serrated and non-natatory seta on the thoracopodal exopods and a highly reduced abdomen with at best traces of segmentation. These synapomorphies in cypris morphology support a monophyletic taxon Cirripedia comprising the Acrothoracica, Thoracica, and Rhizocephala but excluding the Ascothoracida.