Radiometric determination of the growth rate of Nautilus in nature

Nautilus is the sole surviving genus of externally shelled cephalopods and therefore, determination of its rate of growth is important for understanding the life histories of a host of related fossil forms. Direct observations of its growth rate are difficult, however, so we have used here naturally occurring radionuclides. We used the ²¹⁰Po/²¹⁰Pb activity ratio as a chronometer to estimate the growth rates of seven specimens of Nautilus belauensis Saunders. In three immature animals the age of the last septum ranged from 10 to 52 days and, based on the age difference between the last two septa, the rate of septal formation was >120 to ∼230 days. In four animals near or at maturity the age of the last septum was 110-240 days and the rate of septal formation 100- >290 days. Based on these rates the time to maturity is more than 10 yr. Rates of septal formation translate approximately into rates of <0.12 and <0.08 mm of ventral circumference per day for immature and mature animals, respectively, and agree well with mark-recapture observations of this species in Palau¹.