Immature oocytes are arrested at prophase I of the meiotic process and maturation onset is indicated by oocyte nuclear disassembly (germinal vesicle breakdown or GVBD). Signaling pathways that elevate intracellular cyclic AMP (cAMP) may either prevent or induce oocyte maturation depending on the species. In some marine invertebrates and, in particular, in ascidian oocytes, cAMP triggers GVBD rather than blocking it. In this paper, we tested different cAMP elevators in fully grown oocytes at the germinal vesicle stage (GV) of the ascidian Ciona intestinalis. We demonstrated that through the activation of adenylate cyclase or the inhibition and phosphodiesterases the oocyte remained at the GV stage. This effect was reversible as the GV-arrested oocytes, rinsed and incubated in sea water, are able to undergo spontaneous maturation and extrusion of follicle cells. In addition, oocytes acquire the ability to be fertilized and start early development. However, morphology of follicle cells, embryos and larvae from in vitro matured oocytes showed different morphology from those derived from in vivo mature oocytes. The role and the transduction mechanism of cAMP in the regulation of oocyte maturation were discussed. Finally, we indicated a variation of biological mechanisms present in the ascidian species; moreover, we sustain evidence proving that tunicates share some biological mechanisms with vertebrates. This information provided new hints on the importance of ascidians in the evolution of chordates.

The invasive ascidian Ciona intestinalis is irregularly and narrowly distributed in a few areas on the Brazilian coast. It was recorded in the States of Paraná, São Paulo, and Rio de Janeiro. In the State of Rio de Janeiro, all records refer to the Guanabara Bay, a polluted environment that probably hosts other introduced species. Ciona intestinalis was last recorded in this bay in 1991. Given the absence of records since then, researchers concluded that it had been extinct there, and even from the Brazilian coast. During our experiment, conducted with predator exclusion and light attenuation, we collected some samples of C. intestinalis, which suggests that it has either been reintroduced in the Guanabara Bay or that its presence there is a consequence of predator exclusion. Ciona intestinalis was found inside boxes with low luminosity, whose openings were turned to the bottom and protected with a net for predator exclusion. This method was successfully used 17 years ago, when numerous species samples were detected. Our conclusion is that the irregular recording of C. intestinalis on the Brazilian coast, in both time and space, probably derives from inappropriate techniques and methods to sample or observe it on natural substrate. Replicating this methodology in other regions of Brazil and in other South American countries, especially in port areas, would show that the distribution of C. intestinalis is possibly broader than previously reported.