Relative growth, nucleic acid content and cell numbers of the brain in Octopus vulgaris (Lamarck).

The brain of Octopus vulgaris, as of other cephalopoda, grows throughout life. This is apparent from even a summary inspection of the animal at different stages of its life history. When it settles to the sea floor after some 6 weeks of planktonic life, the animal weighs one-fifth of a gram. Yet the brain alone weighs twice this by the time (? one year) it is subject to the many learning studies conducted at Naples; and at a maximum body size of about 10 kg. the mass of the brain may be nearly 2 g.: i.e. more than one thousand times that of the whole Octopus at hatching (1 mg.) (see Fig. 1). Animals of no other group except fish (Geiger, 1956) show such changes in size of the organ that controls their behaviour. This continuous growth of the central nervous system in fish and cephalopods is so unlike the situation in ourselves and other placental mammals, whose brain grows rather little after birth (at most four-fold in wet weight in man and the cat) and adds relatively few cells (< 50%, Airman, 1967), that even zoologists are unused to thinking about the problems presented by an animal whose brain cells may be increasing in number by as much as one and a half orders of magnitude. We should like to report here the results of measurements made some years ago aimed at defining the growth rate of this organ and at providing an estimate of its cell numbers throughout the growth range. In order to avoid the enormous task of counting individual cells in the different lobes of many individual brains, their DNA content was measured and the amounts were calibrated by a method employing sperm counts. The results have led to a more general study still in progress on the distribution of nucleic acids in the central nervous system of O. vulgaris. A volumetric analysis of brains from a wide variety of Mediterranean cephalopods was the subject of a monograph by Wirz (1959). The DNA content of optic lobe and vertical fractions of the brain of Eledone cirrhosa has been measured by Bradford, Chain, Cory & Rose (1969). The results of the vertical lobes cannot be compared in a straightforward way with ours, however, as it is apparent from the wet weight of the tissue quoted that a large part, if not all, of the rest of the supraoesophageal lobes was contained in their slices of the vertical lobes.