Hatching depth of Nautilus pompilius in Fiji

Analyses of oxygen and carbon isotopes in the shell and septa of Nautilus h ave shed light on the early life history of this animal. Previous studies h ave shown that delta O-18 is a reliable indicator of the temperature and, b y inference, the depth at which the shell material forms. Thus, it is clear that Nautilus hatches at 22-24 degreesC, corresponding to a depth of 100-2 00 m, depending on the location; thereafter, animals migrate to colder, dee per water (300-400 m depth). However, Davis & Mohorter (1973) reported the capture of three specimens of newly hatched Nautilus in shallow water (1.25 m deep) in Korolevu Bay, Viti Levu, Fiji. To resolve this paradox, we anal yzed the isotopic composition of the septa and apertural margin in two of t hese specimens. The specimens are approximately 25 nun in diameter and show the nepionic constriction on the outer shell and a reduction in septal spa cing, features that indicate the point of hatching. The oxygen isotope valu es of the embryonic septa in both specimens range from -0.77 to -0.55 parts per thousand, corresponding to an average temperature of 22.6 degreesC and a depth of 160 m (based on temperature-depth profiles in the area). The va lues of delta O-18 of the postembryonic septum and apertural margin are 1.3 3 and 0.99 parts per thousand, respectively, in one specimen (corresponding to 14 degreesC, 370 m depth), and 0.42 and 0.53 parts per thousand, respec tively, in the other specimen (corresponding to 17 degreesC, 300 m depth). The values of delta C-13 also show a marked difference between embryonic an d postembryonic shell material (-0.82 to 0.67 parts per thousand in embryon ic and -2.40 to -1.06 parts per thousand in postembryonic shell material). Thus, these animals did not hatch in shallow water, but exhibited the same pattern as that of other Nautilus from Fiji: hatching at a depth of 160-210 m, followed by a descent to a depth of 300-370 m. Thereafter, these animal s apparently migrated or were transported (by currents?) to shallow water w here they were captured; the geographic distance involved was 1.5 km (based on bathymetric charts). Reports of drifted shells of approximately the sam e size collected in the same area and elsewhere indicate that this post-hat ching behavior may not be uncommon and may well be fatal.