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). 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