> As Pete points out, it's not likely to be crabs attacking the Nautiluses,
> though the latter do sometimes approach the "bottom"; they are often seen around the sides of sea mounts and walls.

Nautilus is a bottom-feeder; therefore it lives within centimeters of
the bottom and must come right down to the bottom in order to grab its
food (carcasses, live crustaceans). When it does so it is at real risk
from big crabs, which are certainly around; a crab could grab either
the soft parts or the shell edge; the rest of the shell would be
immune to crab grip or breakage, as it is convexly curved and too
large for any crab chela to fit around. Crabs are known Nautilus
predators.

Large fish could get their jaws around the shell, but very few would
be able to generate sufficient pressure to damage the shell other than
at the lip. In addition the shape of the shell is ideal to cause jaws
to slide off the shell. The fish would most likely be sharks or
groupers (what Down-Under people for some reason call "groper").
HOWEVER... teeth are harder than shells and will leave scratchmarks.
Shark teeth, with their sharp points, leave clearer and deeper
scratches than other teeth. If there are no toothmarks visible on the
shells I would immediately discount fish as the predators; because of
the slippery shape of the hard shell, a fish would almost certainly
have to try many times before managing to break the shell, unless it
is very lucky; and even then, the teeth would probably slip before
gripping and crushing.

To make the holes I'm talking
> about, however, something would need a gape of at least six inches.

 Breaks and borings are easuily distinguished. What is the diameter
and shape of these holes?  I don't think this has been mentioned.
Octopus borings are generally 0.5-2mm at the shell surface.

The
> biggest tropical crabs don't have claws anywhere near that big. In any case,
> crab claws cannot make fine punctures like this.

It's amazing just what a crab can do. They can break the side out of a
single whorl of a small gastropod shell. However no crustacean
anywhere had chelae big AND strong enough to fit around a Nautilus
shell and break it.

> On my wall I have an impressive Japanese fish print of a Napoleon Wrasse. > They couldn't find a piece of paper big enough, so they used a bed sheet. > This thing is five feet long and weighed about 160 lbs. It has a mouth that > would easily engulf a Nautilus, and only four teeth - two upper and two > lower.
That sounds like a shell-crusher, but I woukld expect it to completely
crush the shell, rather than just perforate it. At least, that would
be the fish's intention.

> In trying to solve this mystery we should keep in mind the life history of
> the chambered nautilus. It was my understanding that nautiluses are free> swimming, at rather extreme depths most of the time.

Nautilus dioesn't live at extreme depths; the shell cannot survive
pressures below about 600m. N. macromphalus comes close to the surface
at night, though other species don't seem to come up quite so far, but
certainly they come to scuba depths.

 In photos of living
> specimens I have never seen any "growths" or parasites on the shell.

Serpulid tubeworms occur, generally in the umbilicus (they are likely
to become overgrown if they settle anywhere else) in umbilicate
species. lepadomorph barnacles are not uncommon there either.

> Predation by crabs or boring mollusks or starfish seems unlikely in an
> animal that never comes in contact with the ocean bottom. Octopuses remain
> mostly near the bottom and unlikely to be able to get to nautiluses on a
> regular bases.

See above. But a Nautilus would have to be dead or moribund before a
star could get to him.

> Also, the nautilus does not have an operculum.  the soft parts are rather
> easily accessible so why would an animal bother to bore through the shell if > it didn't need to?

Nautilus has a tough leathery hood which acts as an operculum.

> Is there a possibility that the holes are bored from the inside?  Perhaps an> internal parasite needing a breathing hole?

Ha! Capulus found living in live Nautilus shell... unfortunetaly, no.

Nautilus tends to live in relatively quiet waters; its slow rocking
propulsion, plus the fact that it moves backward and can't see where
it's going, necessitates this. However if an individual finds itself
in trouble with excessive buoyancy (eg if a crab or fish breaks a big
chunk of lip off, the suddenly-too-light animal will float to the
surface, or for other reasons with intact shell) it may well find
itself being bashed against the reef by waves, as the process of
changing buoyancy is a slow process and takes hours. Some individuals
may be more inclined to frequent wave-disturbed depths. This may be
why some specimens are full of chips, evidence that the lip was over
time repeatedly and extensively chipped.

I would recommend that people interested in Nautilidae read Peter
Ward's books "In Search of Nautilus" etc. Many if not all of your
questions are answered there.

--
Regards
Andrew Grebneff
Dunedin, New Zealand
Fossil preparator
Mollusc, Toyota & VW van fanatic

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