Sexual dimorphism inNautilusfrom Palau

Abstract

Shells of 375 sexed specimens of Nautilus cf. N. pompilius trapped at 300–700 ft (90–215 m) depth off Mutremdiu Point and Ngemelis Island, Palau, exhibit distinctive dimorphism in shell proportion and overall size. Of 221 mature animals, males have larger shells (mean diameter = 209 mm), a broader aperture (mean maximum width = 99 mm; mean width at apertural salient = 92 mm) and greater weight (mean shell + body weight = 1426 gm). Female shells are smaller (mean diameter = 198 mm), narrower (mean maximum width = 91 mm; mean apertural salient width = 81 mm) and the overall weight is less (mean = 1157 gm). Dimorphic differences are not apparent in young shells (less than 180 mm diameter), but develop during the final ½ to ¼ whorl. This is accompanied by development of the spadix in males, indicating that shell dimorphism reflects sexual maturity. Of 375 animals trapped, 28% were female; no depth segregation was apparent. Traditional identification of larger fossil dimorphic cephalopods as females and smaller forms as males is the reverse of observed dimorphism in Nautilus and should be discontinued in favor of macroconch and microconch designations.

Comparative Population Assessments of Nautilus sp. in the Philippines, Australia, Fiji, and American Samoa Using Baited Remote Underwater Video Systems

“…On the other hand, the results obtained here are consistent with a conclusion that the fished, Philippines populations are significantly different in not only the numbers of nautiluses attracted to the bait each night, but also in terms of the age-class structure of the attracted animals. For example the number of fully mature animals seen at Osprey Reef exceeded 80% [22], and this number is consistent with other studies of age class structure of sampled nautilus populations from Palau in the 1980 s [34], [35]. Our work showed that less than 50% of the observed animals in the Philippines are mature.…”

Mature Modifications and Sexual Dimorphism

“…In Argonauta, which was already mentioned above, the size differences are just as striking as in Ocythoe tuberculata, where the female is 1 m long and so ten times as long as the male (Makowski 1962;Wells 1962Wells , 1966Mangold-Wirz 1963;Westermann 1969a;Mangold-Wirz et al 1969;Roper and Sweeney 1975). By contrast, the male is slightly larger in Recent Nautilida (Willey 1895(Willey , 1902Saunders and Spinosa 1978;Saunders and Ward 1987;Hayasaka et al 1987), but they have a different reproductive strategy. Remarkably, aptychi have been interpreted as protecting the nidamental glands (Keferstein 1866) and Siebold (1848) even suggested that the aptychi were the micromorphic males.…”

Demographic disequilibrium in living nautiloids (Nautilus and Allonautilus): Canary in the coal mines?

“…Demographic data include sex and relative maturity of each animal, following established protocols [37, 26]. Sex determination is straightforward in mature animals ( e .…”

“…Sex determination is straightforward in mature animals ( e . g ., presence of nidamental glands, lack of a spadix, the shell of females is smaller and the aperture is noticeably compressed [26]). Characteristics of mature animals (maximum growth achieved) include a sinuous aperture (ocular and hyponomic sinuses), thickened shell margin, and a blackened apertural margin in many individuals (but see comments on Osprey Reef data below).…”

“…Averaged profiles of these “equilibrium” populations, in turn, are compared to data from the intensely fished Philippines populations that were subjected to uncontrolled exploitation for more than a century. The results show that quantifying the magnitude of demographic disturbance, or “disequilibrium,” provides a basis for evaluating the need for establishing conservation measures to protect this ancient molluscan lineage, particularly given its slow growth, longevity, low fecundity and limiting factors [19, 20, 21, 22, 23, 24, 25, 26, 27]. …”