Evolution of the nauplius stage in malacostracan crustaceans

This article gives an overview on some aspects of malacostracan development at the levels of gene expression, cell division, and shape of embryos and larvae. The various modes of development found in malacostracans are discus sed in a phylogenetic and an evolutionary framework. It is suggested that t he plesiomorphic condition within the Malacostraca is an embryonized egg-na uplius which is characterised by a yolky egg, precocious development of the three anteriormost head segments, an antero-ventrally folded caudal papill a, and a growth zone formed by rings of 19 ecto- and eight mesoteloblasts. Several alterations of this malacostracan ground pattern occurred concernin g the arrangement and the number of teloblasts, the shape of the embryo and the timing of segmentation with respect to the naupliar segments. From the ir distribution within the phylogenetic tree and from their larval characte ristics it is concluded that the free-living nauplius larvae of euphausiids and dendrobranchiate decapods are secondarily evolved from ontogenies with out a free-living nauplius. Because the nauplius stage is lost in several m alacostracan taxa, the concept of the nauplius as a phylotypic stage of cru staceans is refuted. In addition, the nauplius as an example of the recapit ulation of ancestral characters is discussed.