Fertilization in penaeid shrimp occurs externally immediately after spawning, when the female releases both eggs and stored sperm. Like all crustaceans, penaeid sperm are non-motile and immediately bind to eggs and undergo an acrosomal reaction for entry. Penaeid eggs are activated by Mg2+ in seawater, not sperm, resulting in cortical “rounding up”, the completion of meiosis, and formation of a hatching envelope. E mbryos undergo rapid , complete, and stereotyped cleavage divisions, in contrast to other decapod crustaceans. Equal and synchronous divisions produce a 32-cell stage blastula, when two mesendoderm cells (mesendoblasts) temporarily arrest their cell division while the remaining cells continue to divide. Cleavage produces a unique pattern of interlocking bands of cells dividing in similar directions.
Gastrulation begins when the two arrested mesendoblasts ingress at the vegetal pole and are internalized inside the blastula wall. This is followed by the oriented division and invagination of prospective naupliar mesoderm cells at the vegetal pole. Cell lineage analysis from the 4-cell stage shows that the animal pole A and B blastomeres give rise to ectoderm, the vegetal C blastomere produces ectoderm and naupliar mesoderm, and the vegetal D blastomere gives rise to ectoderm, naupliar mesoderm, endoderm, endodermal yolk cells, the teloblastic mesoderm, and the germ line. An RNA-containing granule has been identified as a possible germ line marker and can be traced from the egg to one of the mesendoblasts. Blastomere isolation and recombination experiments suggest that the mesendoblasts display both autonomous specification properties and the ability to induce morphogenesis in the rest of the embryo.
Post-embryonic growth occurs by addition and differentiation of segments to the posterior of the nauplius and subsequent larval stages. Growth occurs through the anterior-posterior division of teloblastic stem cells (ectoblasts, mesoblasts, and endoblasts). The maxillae and first two maxillipeds form in the final naupliar stages . The third maxillipeds and five pereopods form during the protozoeal larval stages, while the abdominal limbs (pleopods) form during the mysis stages. Muscle development has been followed by fluorescent phallotoxin staining, and naupliar limb muscles begin forming soon after the limbs bud. The major abdominal muscles develop in a complex interlocking pattern during the mysis stages. The development of the nervous system has been recently described in detail by fluorescent antibody and dye staining. Germ line and gonad development has been studied using the vasa probe. Studies on the development of other organ systems using molecular probes are limited.
The publication of penae id shrimp genomes and developmental transcriptomes has allowed the rapid identification of developmental genes and their temporal transcription patterns. Further progress in understanding development will require a robust, high resolution fluorescence in situ hybridization method for detecting spatial patterns of mRNAs of interest in embryos and larvae.