What arthropod brains say about arthropod phylogeny.

W hat can we learn from the brain of a single member of a species? If the brain is that of a remipede, the focus of a report by Fanenbruck et al. (1) in this issue of PNAS, the answer is something new about arthropod phylogeny. The Remipedia, discovered in 1979, are crustaceans found deep in the waters of coastal caves (2, 3). To date, twelve species of remipedes are known: one from a single cave in Western Australia and the others from the Caribbean. The remipede body is divided into a head and an elongated trunk with up to 32 segments. The trunk segments feature paddle-like appendages that give the taxon its name (‘‘oar feet’’). Remipedes show adaptations to life in darkness, including the absence of pigmentation and eyes, but like other crustaceans they have two pairs of antennae. The evolutionary history of these rare arthropods is uncertain. In their study of the brain of Godzilliognomus frondosus, Fanenbruck et al. (1) provide a fresh look at remipede phylogeny. Their neuroanatomical studies suggest that the Remipedia are most likely part of a larger clade that includes the Malacostraca and the Hexapoda, two arthropod groups well known for their complex brains. This result is surprising because, at first inspection, the simplicity of the remipede body plan suggests that this group provides a glimpse of untagmatized ancestors with numerous unspecialized appendages and should be placed in a basal position in the crustacean lineage (4). This view, based on gross morphology, is contradicted by molecular analyses suggesting that the remipedes are a sister group to the hexapods (six-legged arthropods, including insects) (5, 6). These studies were based on comparisons of sequences for RNA polymerase II and elongation factors 1 and 2. The results not only elevated the Remipedia from a basal status, but also provided support for the much-discussed idea that hexapods are more closely related to crustaceans than to the other tracheated arthropods, the centipedes and millipedes (Chilopoda and Diplopoda; collectively, Myriapoda). This hypothesis unites the hexapods and crustaceans in a larger clade designated Pancrustacea (7, 8). Recent phylogenetic reconstructions based on complete mitochondrial genomes go a step further by joining crustaceans and insects but placing a non-insect hexapod group, the Collembola, outside of Pancrustacea (9). These molecular phylogenies are at odds with striking morphological synapomorphies that unite the Myriapoda and Hexapoda, such as tracheae, Malpighian tubules, and the absence of second antennae. A reexamination of these morphological characters from a detailed cellular, molecular, and developmental perspective therefore is warranted.