Within the mollusks, cephalopods diverged from a monoplacophoran-like ancestor over 500 million years ago. Today, there are two living classes of cephalopods: the nautiloids (Nautilus and Allonautilus) and the coleoids (squid, cuttlefish, and octopus). The CephSeq Consortium has proposed that initial sequencing efforts be focused on a select group of species, chosen based on the curiosity of their biological features as well as the possible advantages of their practical use, and representing a broad phyogenetic distribution and ecologically diverse life histories. These species are Octopus vulgaris, Octopus bimaculoides, Hapalochlaena maculosa, Sepia officinalis, Loligo pealeii, Euprymna scolopes, Idiosepius, Architeuthis dux and Nautilus.
Octopus vulgaris has been a key species for the study of cellular and systems circuitry of learning and memory through studies of anatomy, behavior following lesions and brain stimulation and cellular neurophysiology. O. vulgaris has also served as an attractive model for neuroendocrine studies in invertebrates.
Recently, Octopus bimaculoides (California Two-spot Octopus) has emerged as a model system for cephalopod biology. The large size of O. bimaculoides eggs grants unique access to early embryonic stages, making this species a prime candidate for future genetic and developmental studies. The hardiness, ready availability in the United States, and easy husbandry of adult O. bimaculoides add to the appeal of this model species.
The deadly venom of blue ringed octopus Hapalochlaena maculosa makes this species of interest for study of the evolution and regulation of toxicity within octopods.
Sepia officinalis has been a key cephalopod for neurobiological research, and is a critical species in global fisheries. S. officinalis possesses a complex chromatophore network for countershading, camouflage and communication. Its internal calcified shell supplies buoyancy and the effect of global climate changes on this structure has become a focus of recent study. S. officinalis is emerging as a particularly versatile model organism in eco-evo-devo studies. As a practical matter, S. officinalis eggs are voluminous and easily collected, maintained and reared in the laboratory. The morphological events in S. officinalis embryogenesis are well described in the literature.
Loligo, and particularly its giant fiber system, has served as the fundamental basis for our understanding of nerve impulse conduction. The giant synapse system has recently been employed as a biomedical model of neurological disease. Loligo is one of the most important groups for cephalopod fisheries in the North Atlantic. Loligo pealeii is the premier experimental species of the genus, with not only an extensive publication base , but also annual availability at the Marine Biological Laboratory (Woods Hole, MA).
Euprymna scolopes is a unique cephalopod model organism because of its well-described symbiotic relationship with the luminescent bacterium Vibrio fischeri. This important biomedical model has been employed to study the mechanisms of host colonization and symbiont specificity, host/microbe cell-cell signaling, and innate immunity. Euprymna scolopes’ short life cycle and small egg size also make it an attractive choice for developmental studies in culture. In 2005, the V. fischeri genome was sequenced, so having access to the host genome would allow this field to advance rapidly.
The pygmy squid, Idiosepius, has one of the smallest genomes among cephalopods (2.1 Gb), making it a strong candidate for assembly and annotation. Its small body size and exceptionally short life cycle also distinguish it as a possible model organism.
The giant squid Architeuthis dux has been chosen to represent deep-sea cephalopods. Little is known about the species of Architeuthis. Architeuthis is globally distributed and a recent analysis of the complete mitogenomes of multiple giant squid worldwide showed no detectable phylogenetic structure on the mitochondrial level and an exceptionally low level of nucleotide diversity, suggesting that there is only one global species of giant squid. A nuclear reference genome for Architeuthis would clarify the population genetics of this species, and provide critical information for comparative studies across cephalopods.
Nautilus, the cephalopod “living fossil”, is a representative of a phylogenetically unique branch of the cephalopods, the nautiloids. Nautilus possesses many presumably ancestral anatomical features not shared with other cephalopods, including pinhole eyes, rhinophores for odor detection, an external shell, and numerous tentacles, all without suckers. Comparative genomic studies employing Nautilus would highlight the genetic bases of these divergent features.