The minute brain of the copepod Tigriopus californicus supports a complex ancestral ground pattern of the tetraconate cerebral nervous systems.

Copepods are a diverse and ecologically crucial group of minute crustaceans that are relatively neglected in terms of studies on nervous system organization. Recently, morphological neural characters have helped clarify evolutionary relationships within Arthropoda, particularly among Tetraconata (i.e., crustaceans and hexapods), and indicate that copepods occupy an important phylogenetic position relating to both Malacostraca and Hexapoda. This taxon therefore provides the opportunity to evaluate those neural characters common to these two clades likely to be results of shared ancestry (homology) versus convergence (homoplasy). Here we present an anatomical characterization of the brain and central nervous system of the well-studied harpacticoid copepod species Tigriopus californicus. We show that this species is endowed with a complex brain possessing a central complex comprising a protocerebral bridge and central body. Deutocerebral glomeruli are supplied by the antennular nerves, and a lateral protocerebral olfactory neuropil corresponds to the malacostracan hemiellipsoid body. Glomeruli contain synaptic specializations comparable to the presynaptic "T-bars" typical of dipterous insects, including Drosophila melanogaster. Serotonin-like immunoreactivity pervades the brain and ventral nervous system, with distinctive deutocerebral distributions. The present observations suggest that a suite of morphological characters typifying the Tigriopus brain reflect a ground pattern organization of an ancestral Tetraconata, which possessed an elaborate and structurally differentiated nervous system.