Life History Paper – The Nautilus
The nautilus is an ancient and unique ocean dwelling mollusk. These animals belong to
the genus Nautilus, identical to their common name. The uniqueness of the nautilus is reflected
in its placement into the family Nautilidae as well as the order Nautilida. Nautiluses are
cephalopods and belong to the class Cephalopoda; they are considered the oldest group in the
Cephalopoda (Von Byern et al. 2012). In fact, nautiluses have been around since the Late
Cambrian and are widely considered to be living fossils (Teichert et al. 2010). Taxonomy of the
nautilus is unclear, but there are generally considered to be at least six species in the genus
Nautilus (Teichert et al. 2010). Of these species, the chambered nautilus, Nautilus pompilius, is
the most studied and well-known species.
Nautiluses are strange-looking animals that combine the physical characteristics of a snail
and an octopus. Unlike other cephalopods, they have an external spiral shell with a bold pattern
of orange to brown stripes. Next to the shell on the dorsal side is a muscular hood that covers the
rest of the animal when it is inside its shell (Sasaki et al. 2008). Numerous tentacles are found
on nautiluses; there are 38 outer (digital) tentacles and 24 male or 48 to 52 female inner (labial)
tentacles (Sasaki et al. 2008). A further unique feature of the nautilus is that its digital tentacles,
instead of containing hooks or suckers, have adhesive structures (Von Byern et al. 2012).
Although male and female nautiluses look similar, there is sexual dimorphism in size; data on
mature chambered nautiluses from Osprey Reef Seamount, Coral Sea, Australia indicated that
males are larger than females, with average shell diameters of 131.9 mm and 118.9 mm
respectively (Dunstan, Ward, and Marshall 2011a). Nautiluses have had this unchanged
appearance for hundreds of millions of years based on the fossil record.
Modern day nautiluses have a range that is restricted to tropical, deep-water habitats in
the Indo-Pacific region (Dunstan, Bradshaw, and Marshall 2011). They often live at and around
coral reef drop-offs, which provide silty or muddy substrate for optimal feeding (Dunstan, Ward,
and Marshall 2011b). Nautilus distribution is most likely clumped on a large scale, as they seem
to exist in somewhat isolated populations throughout their range. Due to their preference for
coral reef drop-offs and the likelihood that they would congregate around these favorable
habitats, nautiluses probably have a clumped distribution on a smaller scale as well. An
interesting aspect of their oceanic range is the vertical migration that nautiluses do on a daily
basis. While formerly thought to go deep during the day and ascend to shallow waters during the
night, a recent study with the chambered nautilus has shown that the daily migration patterns are
more complex (Dunstan, Ward, and Marshall 2011b). According to the study, nautiluses move
continuously during the night between depths of 130 and 700 m, and spend the day either resting
in shallower 160 to 225 m water or foraging at deeper depths of 489 to 700 m. These vertical
movements are most likely caused by factors such as optimal feeding, avoidance of daytime
predators, and the need to rest and regain neutral buoyancy (Dunstan, Ward, and Marshall
2011b).
Until recently, the details on the lifecycle and reproduction of nautiluses were virtually
unknown. Now, even though there is much to learn, scientists are starting to better understand
these processes in the nautilus. In general, everything about the nautilus’s lifecycle is slow.
When nautiluses hatch, they already have their shells, and these shells grow along with their soft
bodies (Landman and Cochran 2010). As the shell grows, it forms a logarithmic spiral and
produces new septa, or chambers, inside the shell that have a constant volume ratio (Landman
and Cochran 2010). These attributes of the nautilus shell suggest that its growth follows the
Fibonacci sequence. Young nautiluses have to move forward in their shells periodically during
growth to make room for the formation of new septa (Landman and Cochran 2010). Evidence
points toward a slow, constant growth rate for juvenile nautiluses that decreases prior to
maturation, eventually ceasing altogether (Dunstan, Ward, and Marshall 2011a). There seems to
be no difference in growth rate between males and females (Dunstan, Ward, and Marshall
2011a).
Unlike other cephalopods, the nautilus has a remarkably late maturity time. Evidence
from a chambered nautilus population at Osprey Reef Seamount suggests that a nautilus matures
at approximately15.5 years of age (Dunstan, Ward, and Marshall 2011a). At maturity, males
transfer sperm to the female mantle using a reproductive organ called a spadix, which is derived
from three modified labial tentacles (Sasaki et al. 2008). After one mating event, females lay
eggs in a unique cycle: they lay one clutch of 2.5 cm eggs, wait for another clutch of oocytes to
mature, lay those eggs, and then repeat the cycle (Arnold 2010). Each clutch contains about five
to eight eggs surrounded by a protective egg capsule (Arnold 2010). In contrast to most other
cephalopods, the nautilus is iteroparous, and it mates regularly throughout its adult life (Arnold
2010). From here, knowledge of egg laying locations and other important details is lacking
because to date there has been no direct observation of egg laying and embryonic development in
wild populations (Dunstan, Ward, and Marshall 2011a). Incredibly, nautiluses can live for more
than twenty years, which is exceptionally longer than any other cephalopod (Dunstan, Ward, and
Marshall 2011a).
Additional data from the chambered nautilus population at Osprey Reef Seamount point
out a low population density and number as well as a K-selected life history (Dunstan, Ward, and
Marshall 2011a). This study also suggested that nautilus populations are dominated by males
and mature individuals. The reason for such a low percentage of juveniles in nautilus
populations remains unknown (Dunstan, Ward, and Marshall 2011a). Nautiluses do not seem to
fit perfectly into any particular survivorship curve. Based on their lifecycle and physical
characteristics, they have traits that fit both type two and type three survivorship curves. Since
adults seem to dominate the observed populations, this may point out a type three survivorship
curve, where the smaller, younger nautiluses face more struggles early on. However, adults are
still preyed on regularly, and young nautiluses have their protective shells with them
immediately after birth, which may suggest that nautiluses also have aspects of a type two
survivorship curve. Overall, a type three survivorship curve probably predominates because
younger nautiluses are likely preyed upon more than older ones.
Reflecting its primitive nature, the nautilus eye is called a pinhole eye, which does not
have a lens or cornea and contributes to its poor vision, in contrast to the remarkable vision of its
more advanced cephalopod relatives (Ogura et al. 2013). However, nautiluses make up for this
loss by using olfaction as their primary sensory system (Basil et al. 2000). Nautiluses are
primarily scavengers on crustaceans, nematodes, and echinoids, so their olfactory organs, or
rhinophores, are essential in directing them toward food sources (Ward and Wicksten 1980, Basil
et al. 2000). In addition to the nautilus’s ability to find food through olfaction, it can also grip
food easily without suckers or hooks due to a distinctive adhesive gland system on its digital
tentacles (Von Byern et al. 2012). Secretory cells of the epithelium on the oral adhesive ridge
(facing the mouth) produce a glue of neutral mucopolysaccharides that allow the nautilus to hold
onto prey or adhere to surfaces (Von Byern et al. 2012).
Cephalopods are well known for their intelligence and complex brains, but the nautilus
has by far the most primitive brain of the group. Originally thought to be unintelligent due to its
simple brain, studies have now shown that the nautilus is capable of learning and memory in a
way comparable to the more highly evolved cephalopods (Crook and Basil 2008). The absence
of dedicated neural regions that support learning and memory do not seem to limit memory
expression in nautiluses (Crook and Basil 2008). These findings make the nautilus a smarter
animal than was previously thought.
Probably the most noticeable feature of the nautilus is its external shell; one of the most
important purposes of the shell is for defense from predators. Since the ink gland found in other
cephalopods is absent in the nautilus, the shell remains critical for survival; in fact, the
internalization of the shell in the more recent cephalopods was likely related to the development
of the ink gland (Sasaki et al. 2008). Protection is not the shell’s main function though. The
shell functions most importantly as a way to adjust buoyancy (Von Byern et al. 2012).
Buoyancy of the nautilus is influenced by how much liquid or gas exists in the shell’s inner
chambers. Differences in osmotic pressure gradients between the chambers and the external
environment can cause liquid to either enter or leave the chambers, changing the nautilus’s
individual buoyancy (Dunstan, Ward, and Marshall 2011b). Spending time at great depths
floods the shell chambers and causes negative buoyancy; nautiluses must then alleviate this and
regain neutral buoyancy by spending time at 200 m for buoyancy re-equilibration (Dunstan,
Ward, and Marshall 2011b). Near the shell is a muscular, tube-like organ called the funnel.
Although unfused and primitive in the nautilus, it is found in all cephalopods (normally as a
closed tube) and is the organ responsible for movement through water by the well-known jet
propulsion system of cephalopods (Sasaki et al. 2008).
Fortunately, nautiluses are not classified as endangered. Nevertheless, they are not
common and they do face many threats to their survival. Within their ocean habitats, they have
various physical limitations. For example, they have buoyancy requirements for re-equilibration,
they have an upper temperature limit of 25°C, and they can only survive at depths at and above
700 m, since the pressure at 800 m depths is enough to make them implode (Dunstan, Ward, and
Marshall 2011b). They also have various predators, including species of octopus that bore into
their shells (Saunders et al. 1991). The low fecundity, slow growth, late maturity, long gestation,
and long life span of nautiluses suggest that they are vulnerable to over-exploitation (Dunstan,
Bradshaw, and Marshall 2011). Because of the demand from the ornamental shell trade and
exotic food markets, many populations of nautiluses are declining rapidly (Dunstan, Bradshaw,
and Marshall 2011). As more research continues to further the understanding of nautiluses and
their populations, it is in the best interest of researchers and the public to appreciate and conserve
these ancient animals.
References
Arnold, J.M. 2010. Reproduction and embryology of Nautilus. Topics in Geobiology
6:137-162.
Basil, J.A., Hanlon, R.T., Sheikh, S.I., Atema, J. 2000. Three-dimensional odor tracking by
Nautilus pompilius. The Journal of Experimental Biology 203:1409-1414.
Crook, R., Basil, J. 2008. A biphasic memory curve in the chambered nautilus, Nautilus
pompilius L. (Cephalopoda: Nautiloidea). The Journal of Experimental Biology
211:1992-1998.
Dunstan, A., Bradshaw, C.J.A., Marshall, J. 2011. Nautilus at risk – Estimating population size
and demography of Nautilus pompilius. PLoS One 6(2):e16716.
Dunstan, A.J., Ward, P.D., Marshall, N.J. 2011a. Nautilus pompilius Life history and
demographics at the Osprey Reef Seamount, Coral Sea, Australia. PLoS One
6(2):e16312.
Dunstan, A.J., Ward, P.D., Marshall, N.J. 2011b. Vertical distribution and migration patterns of
Nautilus pompilius. PLoS One 6(2):e16311.
Landman, N.H., Cochran, J.K. 2010. Growth and longevity of Nautilus. Topics in Geobiology
6:401-420.
Ogura, A., Yoshida, M., Moritaki, T., Okuda, Y., Sese, J., Shimizu, K.K., Sousounis, K., Tsonis,
P.A. 2013. Loss of the six3/6 controlling pathway might have resulted in pinhole-eye
evolution in Nautilus. Sci Rep. 3:1432.
Sasaki, T., Shigeno, S., Tanabe, K. 2008. Anatomy of living Nautilus: Reevaluation of
primitiveness and comparison with Coleoidea. Cephalopods – Present and Past 35-66.
Saunders, W.B., Knight, R.L., Bond, P.N. 1991. Octopus predation on Nautilus: Evidence from
Papua New Guinea. Bulletin of Marine Science 49:280-287(8).
Teichert, C., Matsumoto, T. 2010. The ancestry of the genus Nautilus. Topics in Geobiology
6:25-32.
Von Byern, J., Wani, R., Schwaha, T., Grunwald, I., Cyran, N. 2012. Old and sticky – adhesive
mechanisms in the living fossil Nautilus pompilius (Mollusca, Cephalopoda). Zoology
115-884(1-3):1-11.
Ward, P.D., Wicksten, M.K. 1980. Food sources and feeding behavior of Nautilus
macromphalus. The Veliger 23:119-124.