If you like nature documentaries, you’ve probably seen the following clip (from the BBC’s “Planet Earth“):
Nautiluses are really cool – they’re misfits among cephalopods, having many tentacles and external shells while their fellow squids and octopodes are squishy and eight- or ten-armed. In this clip, at least, they come across as sort of mysterious, spending most of their time floating about in the abyss and only coming up to where it’s easy for us to see them at night. In fact, daily vertical migrations are common in the ocean, with a truly enormous number of organisms moving from shallow to deep waters and back again each day. The nautilus takes part in this migration, probably for its own safety – it’s harder for predators to see you (and eat you) if you stay where it’s dark.
Now I love learning new things about cool animals, but there’s something that I’m more interested in: how do we get that information? Presumably, biological facts are not true just because Sir David Attenborough says they are. How do the producers of that clip (or anybody, for that matter) know that this is what Nautiluses do?
It turns out that the answer is pretty simple: we monitor some of them for a period of time and see what they do. We can’t actually follow them underwater and visually watch them (because it would be difficult and expensive to do this for very long), so we have to use some sort of remote communications device (a radio or ultrasonic transmitter) to see what they do.
This is just what a group of researchers from the University of Queensland, have been doing for some years. Following up on reports about the possibility of daily vertical migrations in Nautilus, they attached ultrasonic transmitters to eleven nautiluses they caught around Osprey Reef in Australia. They kept track of each animal’s vertical movement, and described what they saw.
As far back as 1899, a naturalist by the name of Arthur Wiley made note of the depths at which Nautilus live, both because it made them hard to study, and it meant that they were only be caught in shallow-water traps at night (although to be fair, he provides us no proof of this, only his word.) There are several other reports that support the idea that Nautiluses make vertical migrations (for example, a report published in Nature about the daily migrations of Nautiluses in Palau by Peter Ward et al. in 1984, and a report by Bruce Carlson et al. published in the journal Pacific Science about the same population of Natuiluses.)
Enter Dunstan and co-workers, the group of scientists at the University of Queensland that I mentioned earlier. They followed eleven individuals for up to around 80 days each and also used information from remote-operated vehicle dives to get a clearer picture of what Nautiluses spend their time doing. It’s easy to get an impression of their data from the following graph (click on it to see a bigger version):
Each line is one recording – there are multiple recordings from each animal. The “depth” of the line at the time of day shown at the top of the graph indicates the depth of the recording.
So what do we learn from this? Basically, we learn that it’s not as simple as “shallow at night, deep during the day” – just look at that graph! It doesn’t look like the movement of nautiluses is coordinated very well, either between animals or with times of the day. The authors note that the nautiluses often spend the daytime resting in relatively shallow water, and at night are found in all sorts of depths. In summary, it looks like there are several factors that influence where a nautilus ends up hanging out; purely physically, they can’t go deeper than about 800 meters (2600 feet) due to the high pressure (their shells implode at depths greater than this) or shallower than about 100 meters (320 feet) due to higher temperatures near the surface. During the day, nautilus appear to rest in shallow water or forage in deep water, while at night they are active and move through a whole variety of depths – this is probably influenced by things like food availability, the type and number of predators around, the conditions of the water, and the size and age of the animal. These results don’t contradict earlier results, although the authors note that different populations of Nautilus have been observed to have different patterns of behavior, and chalk this up to differences in their environments.
On a final note, I’d like to mention a study by Kanie and coworkers in which they determined what depth of water would cause a nautilus’s shell to break. To do this, they put a nautilus in a tank of water, and pressurized the tank until the nautilus imploded, all the while measuring its ventilation rate (how quickly it was “breathing”). From this, they came up with a maximum depth for nautilus of 785 meters. At first, this sounded like a cruel experiment to me, but on second thought, it was done very well. They used a single animal (although one might criticize them for that, it means that they only caused harm to a single animal) and in doing so adequately and rather elegantly answered a basic question in cephalopod biology that has implications not only for the ecology of living nautiluses but also for the study of nautiloid fossils and their distribution.
Thanks for reading!
NOTE: This post has been edited from its original form in response to Dr. Peter Ward’s comment (see below). As I did not have access to the research he mentions when I wrote this post (and still don’t), it was inappropriate of me to comment on the state of the field as I did. I’d like to sincerely apologize to Dr. Ward and his coworkers for any disrespect I may have shown to them and their work. I’ll do my best to bring myself up to speed on the research he mentions and include a more detailed discussion of it in a future post, so that I’m accurately representing the field. The text of this post before it was revised is available at this link.
Yasumitsu Kanie, Yoshio Fukuda, Hideaki Nakayama, Kunihiro Seki, Mutsuo Hattori (1980). Implosion of Living Nautilus under increased pressure Paleobiology, 6 (1)
Dunstan AJ, Ward PD, & Marshall NJ (2011). Vertical Distribution and Migration Patterns of Nautilus pompilius. PloS one, 6 (2) PMID: 21364981
Arthur Willey (1899). On a Zoological Expedition to the South Seas Proceedings of the general meetings for scientific business of the Zoological Society of London
Ward, P., Carlson, B., Weekly, M., & Brumbaugh, B. (1984). Remote telemetry of daily vertical and horizontal movement of Nautilus in Palau Nature, 309 (5965), 248-250 DOI: 10.1038/309248a0