On the Buoyancy of the Pearly Nautilus PETER WARD AND 'ARTHUR W. MARTIN Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210; *Department ofZoology, Uniuersity of Washington, Seattle, Washington 981 05

ABSTRACT Fourteen freshly captured Nautilus macromphalus were weighed in seawater, and all were found to be negatively buoyant (heavier than seawater), with a mean weight of 1.76 g. Maximum rates of cameral liquid re- moval from chambers artificially filled with seawater were 1.0 ml/chamber/day a t the surface. Specimens made artificially buoyant by the removal of cameral fluid, and maintained at 60 m or less, were unable to regain normal, slightly negative buoyancy through admission of new liquid into the emptied chambers via the siphuncle. Specimens maintained a t depths of greater than 150 m all showed new cameral liquid in the previously emptied chambers. Because of the relatively slow rate of cameral liquid removal from the chambers, and seeming inability of Nautilus to place cameral liquid into emptied chambers in shallow water, we conclude that the phragnocone-camera1 liquid system is utilized for long term buoyancy adjustments rather than short term changes which might aid vertical ascent or descent.

It is well known that the shells of Nautilus, Sepia, and Spirula provide buoyancy in the sea, and by inference we assume that the shells of many extinct cephalopods provided a similar function. Less well known is the de- gree of buoyancy control available to these chambered cephalopods. Denton and Gilpin- Brown ('61a-c; '66; '71) have shown that the volume of cameral liquid in Nautilus, Sepia, and Spirula is varied so as to produce neutral buoyancy, and that the siphuncle is the organ used to remove liquid from the chambers. These workers, however, provided no informa- tion on the rates of liquid removal, nor the pos- sibility of replacement of liquid into emptied or nearly-emptied chambers to provide a ballast function. Nevertheless, it has been suggested that vertical migrations by Nauti- lus on the fore-reef slope environment first ob- served by Willey ('021, are aided by rapid buoyancy changes during the course of a sin- gle night (Heptonstall, '70). Similar, rapid buoyancy changes brought about by addition or removal of cameral liquid have also been postulated in ammonites (Heptonstall, '70; Mutvei and Reyment, '73).

This unresolved question, whether or not Nautilus is capable of rapid buoyancy change through the addition or removal of cameral liquid, is central to any interpretation of the

J. EXP. ZOOL. (1978) 205: 5-12.

mode of life of chambered cephalopods. In order to understand better the function and capabilities of the phragmocone in Nautilus, and by inference the analogous structures in exctinct chambered cephalopods, we have con- ducted a number of observations and exper- iments on N. macromphalus Sowerby in New Caledonia, and N. pompilius Linne in the Philippine Islands. Our results and a discus- sion of their significance are presented here.

HISTORY OF INVESTIGATIONS ON THE BUOYANCY OF NAUTILUS

The way in which gas enters the chambered shell of Nautilus has been a matter of specula- tion and dispute. Lancaster (1883) visualized a close regulation of buoyancy in Nautilus by the admission of water to the closed chambers, since each new chamber was thought to be filled a t first with gas. Willey ('02) was first to recognize that a flexible organic septum is laid down before calcification of the septum begins, but he, too, thought that the space behind the flexible septum was filled with air. Bidder ('62) appears to have been the first to recognize the fact that the newly formed chamber is full of water which, being incom-

' Supported in part by a grant to Professor G. E. G. Westermann, McMaster University, by McMaster University Reeearch Board, and by Grant GB42688 of the National Science Foundation.

5

6 PETER WARD A N D ARTHUR W. MARTIN

pressible in the range of pressures encoun- tered, allows the animal a certain freedom of vertical motion until the septum is calcified. The problem now shifts to a mechanism for re- moving the water and replacing it with gas. Here Bidder (’62) was armed with knowledge of the results obtained in a study of buoyancy in Sepia by Denton and Gilpin-Brown (’61a), and so was able to suggest an osmotic mech- anism that would remove the water by way of the siphuncle and allow for slow replacement of the water with gas.

Bidder reported positive as well as negative buoyancy in newly captured Nautilus rnac- romphalus, and also observed what appeared to be rapid changes in buoyancy: “Newly cap- tured animals were sometimes of positive, sometimes of negative buoyancy, but the light animals usually became heavy in a few hours” (Bidder, ‘62, p. 6).

Denton and Gilpin-Brown (’66) were the first to measure quantitatively the buoyancy of newly captured Nautilus by weighing the animals in seawater, finding all newly cap- tured specimens to be negatively buoyant (heavier than seawater) a t all times. No obser- vations comparable to those of Bidder of rapid buoyancy changes were noted.

The experiments of Denton and Gilpin- Brown shed much light on the mechanism of buoyancy regulation in Nautilus. The siphun- cle was judged to be the organ suitably located and with the proper structure to serve as an ionic pump, probably a sodium pump, which moves salt from the closed chambers into the blood, the osmotic pressure difference then drawing the water out. The problem of decou- pling, the fact that the siphuncle is no longer bathed with the chamber fluid after the re- moval of a certain volume of water, appears to be met by the wettability of the siphuncle and chamber walls. Finally, suggestions were made about the possible addition of sea water from the blood to the cameral fluid to reduce the buoyancy of the animal and so to increase the flexibility of its responses. The hydrostatic pressure of sea water is seen as in continuous opposition to the osmotic pump. At depths a t which hydrostatic pressure is in excess, water would enter the chambers, and it is even possi- ble to visualize an animal trapped at great depth by a rate of inflow that could not be overmatched by the osmotic pump and that would prevent flight to lesser depths of water.

MATERIALS A N D METHODS

Nautilus macromphalus were captured in

New Caledonia in cubical wood and wire traps about 1 m on a side, baited with fresh fish or crab, and attached by line to a surface buoy. Traps were placed outside the barrier reef in water 50 to 100 meters deep for one to three days.

Scuba was used to capture six animals in shallow depths a t night. The animals observed in the Philippines were captured with traps similar to those used in New Caledonia.

The captured animals were kept in a 1 X 8 m outdoor tank provided with continuously flowing seawater from the nearby Bay de Citron, and with a continuously operating compressed air bubbler. The depth of the water in the tank did not exceed 1 m, a matter of importance in the buoyancies to be re- ported. Animals were fed daily with frozen fish or crab. Decomposed meat was not accept- ed as food, nor was it effective as bait in the traps.

Camera1 fluid was removed through holes drilled in the shell and the volume from each chamber was recorded. Chloride osmolarity of the cameral fluid was analyzed with a Buch- ler-Cotlove chloridometer using 0.1 ml sam- ples. To determine the weight of a Nautilus in seawater (buoyancy), the animal was aesthe- tized in a 2% solution of urethane in seawater for fifteen minutes. I t was suspended in a fine mesh bag underwater with a thin copper wire from an FPE Model LG torsion balance. A neg- atively buoyant animal has weight in sea- water but a positively buoyant animal floats. To measure positively buoyant animals a known weight was attached to the bag, the animal weighed as above, and the observed value corrected and recorded. To avoid confu- sion in this paper the numerical value (in grams) of negatively buoyant animals will be prefixed with a negative sign, and positively buoyant animals with a plus sign.

Rates of liquid removal were studied by in- jecting liquids of known volume and salinity into emptied chambers. A variety of exper- iments were performed: filling single cham- bers, placing 5 ml in a series of chambers, and filling a series of chambers. Liquid with salinities of normal seawater, 213 seawater, and 1f2 seawater were injected. The holes in the shell were sealed with a fast-setting den- tal acrylic cement (trade name Kerr-tab). Even with careful sealing leaking occasion- ally occurred. We found that roughening the shell around the hole with a carborundum stone increased the effectiveness of the seal- ing procedure. The rate of liquid removal was

-

NC75 -

- I I I I I I I I I I

7

+5 >- 0 z >- 0

a

2 0

-5

- 19

Fig. 1 Buoyancy changes over time for captive Nautilus macromphalus. Buoyancy is measured in grams; a negative buoyancy refers to the animal being heavier than seawater. Solid lines indicate that the Nautilus was kept in the aquarium at the surface, while dashed lines represent animals which were kept in a cage at 25 m.

monitored by periodically breaking the acrylic seal, carefully removing and noting the vol- ume of the cameral liquid, and replacing the liquid minus a 0.2 ml sample for salinity analysis.

To test if Nautilus can increase its specific gravity by adding water to chambers other than those in the process of formation we re- moved the cameral fluid from the latest two to six chambers of four N. macromphalus and five N. pompilius, resealed these chambers, and attached corks to the shells for added buoyancy. The Nautilus were then placed in cages a t various depths for periods ranging from two days to two weeks.

RESULTS

The buoyancies of fourteen Nautilus mac- romphalus were measured within one day of their capture. None of the animals was posi- tively buoyant; one was neutral, the others negative. A mean value of -1.76g (S.D. 1.15 g) was determined.

The buoyancies of six Nautilus were moni- tored over an extended period of time both in the aquarium and in the sea (fig. 1). Four ani- mals were kept only in the aquarium, and showed a rapid increase in buoyancy during the experiment. Two Nautilus were kept in a

screen cage at 25m. One (NC75-32) was placed in the cage for fifteen days following five days a t the aquarium. The second (NC75- 19) was placed in the same cage for the same time period after thirty days of captivity in the aquarium. Both animals were periodically observed and fed during this submersion. Air and water weights were measured immediate- ly after recovery, and then periodically during subsequent aquarium captivity. One of these animals (NC75-32) showed a slow increase in positive buoyancy during its stay a t depth, fol- lowed by a rapid increase after its return to the aquarium. The second animal underwent a slight buoyancy decrease during stay at depth, followed by a rapid increase during subse- quent surface captivity.

Camera1 fluid volumes and osmolarities for Nautilus macromphalus, and freshly captured N. pompilus are shown in table 1. All N. mac- romphalus kept in surface captivity for a week or longer appear to show smaller vol- umes of fluid with higher osmolarities than freshly captured specimens. The mean total volumes for the two groups were compared using the Student T test and found to be different a t a greater than 90% confidence interval.

The volumes and salinities measured for

8 PETER WARD AND ARTHUR W. MARTIN

TABLE 1

Volumes and osmolarities of Nautilus macromphalua (NC) and N. pompilius (P). Ch. 1 refers to most recently formed chamber, For each chamber, volume in milliliters and osmolarity (in brackets, as

percentage ofseawater value) are shown. DC refers to days in surface captivity

Animal number DC Weight CH, CH2 CH, CH, CH, CH, CH- CHa

NC75-1 NC75-2 NC75-3 NC75-4 NC75-6 NC75-9 NC75-10 NC75-22 NC75-24 NC75-25 NC75-26 NC75-27 NC75-29 NC75-30 NC75-31 P75-74 P75-149 P75-97 P75-94 P75-20 P75-36 P75-24 P75-22 P75-11 P75-31 P75-40 P75-50 P75-x P75-80 P75-167 P75-180 P75-173

1 617 1 615 1 565 1 790 1 670 7 749

23 650 4 4

21 676 36 591 33 664 20 651 8 683

28 347 279 138 338 919 795 883 894 768 967

1041 148 161 467 500 745 127

3.2 .9(31)

9.4(31) 2.2(26) 1.6(23) 2.4(56) 3.4174) 2.5(641 4.4(58) 2.0(60)

.7(63) 1.6 (60) 1.6(68) 2.2(68) 3.5(64) 5.8(23) 9.0(97) 4.6(76)

12.3(95) 5.4(2) 2.2(36) 4.0(20) 3.3(17) 2.0(32) 2.8(34) 2.6(39) 3.2(78) 3.0(92) 7.5(97)

12.4(65) 3.2(15) 5.0(91)

.5

.4(43)

.9(43) 1.8(43) .9(41) .9(61)

1.0(72) 1.6 (64) 2.1(67) 1.2(70) .2(-)

1.2 (67) .8(73)

1.6(62) 2.3(65) .8(66)

1.5(15) .4(40)

2.2(-) 1.0(35) 1.4 (42) .6(46) .5(34) .7(37)

1.8(37) .6(62)

1.5(31) .9(26)

4.9(24) .4(54) .4(21) -

.25

.25(48) 1.2(48)

.4(43)

.7(35)

.5(61)

.6(76) 1.0(67) 1.3 (69) .6(71)

TR .6(59)

TR .6(63)

0 .5(53) .3(29) .4(59) .9(41) .6(48)

1.0(41)

.4(-)

.5(40)

.8(35)

.3(61)

.1(-)

.4(27) 1.863) .8(49)

1.1(22) .4(33)

-

TR TR .8 (46) .2(-) .7 (35) ,564) .45 (7 1)

LO(-) .9(66) .8(74)

0

.6(61)

.5

.5(47)

.6(31)

.6(41)

.6(41)

.2(38)

.2(30)

.5(33)

.5(60)

.2 1.6(55) .6(41) .2(15) .2(23)

-

.4(50)

.3(35)

.2(72)

,5165) .7(73) TR

.2

.2

.4(26)

.2(36)

.4(42)

.3(63) TR

TR TR

.5(73) .4

.6(75) .4(73)

.1

.3(25)

freshly captured N. pornpilius do not appear significantly different from those of freshly captured N. rnacromphalus. Rates of cameral liquid removal were examined in animals with liquid injected into a single chamber only, and in a number of animals in which a series of chambers were drilled, and liquid added.

In the first group, with only a single cham- ber drilled (fig. 2), rates of cameral liquid re- moval varied from 1.0 ml/day (NC75-36) to 0.19 ml/day (NC75-34) with a mean rate of 0.7 ml/day. One specimen kept a t a depth of 20 m for two weeks, removed liquid at a mean rate of 0.19 ml/day during its stay at depth, but in- creased this rate to 1.0 ml/day during subse- quent surface captivity.

The reduction in salinity was monitored in NC75-35 and 36. Salinity in the filled cham- bers decreased rapidly early in the experi- ment, and then continued to decrease, but at a slower rate. The maximum salinity reduction

observed in these two animals was to 75% of seawater osmolarity, a value well above typi- cal osmolarities observed in freshly captured animals.

In the second group, with a series of holes drilled in the most recent chambers, three sep- arate experiments were run. In the first ex- periment, each animal had 5.0 ml of seawater placed in each chamber; in the second experi- ment, 5.0 ml of 1/2 normal seawater was in- jected (in one animal 2/3 normal salinity was used) and finally, every other chamber was filled with water of 1/2 normal salinity (table 2). A trend of decreasing water removal was observed in successively older chambers, but salinity reduction was approximately con- stant. Salinity levels increased in the cham- bers of those animals in which water was orig- inally less than normal salinity.

Experiments on the potential of cameral liquid addition in positively buoyant Nautilus

ON THE BUOYANCY OF THE PEARLY NAUTILUS 9

100

bNC75

100

bNC75 36

01 I I I I I I 5 10 15 2 0 2 5 30

DAYS Fig. 2 Camera1 fluid and salt removal. Solid lines refer to camera1 fluid volume as a percentage of the original

volume emplaced with a hypodermic syringe. Dashed lines refer to salinity as a percentage of the original osmolarity (100=normal seawater salinity). Original volumes emplaced: NC75-21, 17 ml; NC75-36, 20 ml; NC75-35, 20 ml; NC75- 34, 17 ml; NC75-41, 18 ml.

were conducted with both N. macromphalus and N. pompilus. At the beginning of these ex- periments all animals were strongly buoyant. In two N. macromphalus a small amount of seawater of known volume and salinity was added to the last formed chamber immediate- ly prior to placement a t depth, to test whether the animals would continue to remove liquid even when positively buoyant.

All of the Nautilus macromphalus were ob- served to be positively buoyant on recovery. Examination upon return to the laboratory re- vealed that the two animals with chambers

originally completely emptied (12, 21) con- tained no liquid; the two animals with 2 ml seawater in the last formed chamber (33, 38) had slightly reduced this volume, and added no further liquid during their stay a t depth. Analyses of the chloride contents indicates that even though positively buoyant, the Nautilus pumped salt and liquid out of the chambers, thereby further increasing buoy- ancy (table 3).

DISCUSSION

As would be expected, several of our obser- vations on the buoyancy of Nautilus are com-

10 PETER WARD A N D ARTHUR W. MARTIN

0 0 0 o m 0 0 0 0 m m m m w m m m m

m m m m m m m m *

m m m 3-3 N N C U v?z $ $ $ $ $ $ t-r-r- t-r-p’ t-t-r- v v v u v u v v v z z z z z z z z z

parable to observations of Denton et al. on captivesepia andspirula. For example, an in- crease in cameral fluid osmolarity of captive Nautilus macromphalus has also been re- ported in Sepia (Denton et al., ’611, but at much faster rates. The cameral fluid osmolar- ity in Sepia increased from 70 to 100% of sea- water within six hours of capture, whereas in Nautilus similar increases from about 50 to 80% occurred, but during a period of one to three weeks. Denton and Gilpin-Brown (’66, p. 742) observed no such change in Nautilus, but examined their experimental animals for several days a t most.

The gradual increase in buoyancy of captive N. macromphalus (fig. 1) can also be related to similar phenomena exhibited by Sepia and Spirula. Denton and Gilpin-Brown (’61a) showed that whensepia is brought to the sur- face it quickly pumps some cameral fluid out of its chambers, thereby increasing buoyancy. They also observed slight positive buoyancy in freshly captured Spirula, which was attrib- uted to rapid “pumping” of cameral fluid as the organism was brought from its living depth (600-1000 m) to the greatly reduced am- bient pressure a t the surface.

A major difference between Nautilus, and Sepia andspirula is the presence in the latter of cameral fluid in older, previously emptied chambers. In Spirula the protoconch and ini- tial ten camerae have been observed to con- tain varying amounts of liquid (Denton and Gilpin-Brown, ’711, while in Sepia all but the most recent camerae may contain cameral fluid. No cameral fluid in early chambers has been observed in Nautilus.

Keeping the posterior chambers filled with liquid in Sepia moves the animal’s center of gravity to a position beneath its center of buoyancy, thereby allowing a stable, horizon- tal living position. This apical cameral fluid thus produces the same effect as the calcar- eous cameral deposits utilized by Paleozoic orthoconic nautiloids. No adaptive advantage is obvious in the case of Spirula. There is evi- dence that the siphuncular histology changes with age in Nautilus andSpirula (Denton and Gilpin-Brown, ’73); possibly the siphuncle of Spirula can no longer maintain an osmotic gradient large enough to offset ambient pres- sure forcing liquid into these early-formed chambers, since osmolarity and liquid volume increase in Spirula directly with age of the chamber (Denton and Gilpin-Brown, ’71).

The ability ofsepia to decrease its buoyancy

ON THE BUOYANCY OF THE PEARLY NAUTILUS

TABLE 3

Liquid emplacement in positively buoyant Nautilus. Animal numbersprefwed by NC refer to N. macromphalus in New Caledonia; those prefixed by Prefer to N. pompilius in the Philippines. Salinities are listed as a percent- age of seawater normal. Fluid volumes are in milliliters. All specimens had all cameral fluid removed prior to the experiment except NC75-33 and NC75-39, in which 2 ml were emplaced in Chamber 1 prior to the experiment

11

Salinityat end exp. Volume at end exp. Animal number Duration of

Depth exp. (days) 1 2 2 4 1 2 3 4

NC75-12 NC75-21 NC75-33 NC75-39 P75-167 P75-168

P75-164 P75-180

25 m 15 25 m 15 60 m 7 60 m 7

180 rn 2 180 m 2 180 m 2 250 m 2

0 0

1.8 1.9 3.0 1.2 1.2 1.0

0 0 0 0 0 80 0 0 74

2.4 2.2 1.6 59 55 57 56 0.7 0.2 62 53 53 0.5 30 27 0.6 0.4 42 55 67

rapidly through uptake of liquid has lent sup- port to the hypothesis that other chambered cephalopods were, or are capable of overall specific gravity regulation by actively pump- ing liquid in or out of the camerae over short periods of time (Heptonstall, ’70). However, although Denton and Gilpin-Brown (’61a, p. 341) note tha t “to become denser the cuttle- fish lets (or pumps) liquid into the cuttlebone, thus decreasing its gas space,” they later sug- gest that liquid re-entry is entirely a hydro- static, rather than osmotic mechanism, when they state “The rate of uptake of liquid when the cuttlefish becomes denser is such that it is unnecessary to assume that any active process is used to pump liquid into the cuttlebone.” (Denton and Gilpin-Brown, ’61c, p. 350). Our experiments with Nautilus macromphalus and N. pompilius support this.

Based on the results of our experiments and observations, we conclude that the buoyancy system of Nautilus has evolved to produce longterm changes in buoyancy through regu- lation of cameral fluid volume. Removal of this liquid balances shell and tissue growth, so that Nautilus is nearly equal to, but always slightly heavier than seawater. Large vertical migrations, such as those that probably occur in the nighttime migrations of N. macrom- phalus first reported by Willey (‘02), are ap- parently only brought about by swimming. We observed two Nautilus ascending into waters of less than 1 0 m two hours after sunset in New Caledonia in October, 1975, and four more in September, 1977. Two of these ani- mals were weighed in seawater within two hours of their first sighting; both were neg- atively buoyant. The relatively slow rate of liquid removal in our experimental animals, and their seeming inability to admit liquid

into the chambers in shallow water suggests to us that vertical migration by the rapid re- moval of cameral fluid to cause positive buoy- ancy, followed by the rapid admission of liquid to allow descent in the same night, does not take place.

ACKNOWLEDGMENTS

We would like to thank a number of people for their aid. In New Caledonia our work would have been impossible without the aid of Doctor and Madame Catala of the Aquarium de Noumea. We were provided laboratory space a t O.R.S.T.O.M. and would like to thank Doctors Rotschi, Rual, Legand, Fourmanoir and Rancurel for their generous hospitality.

In Fiji we were given able assistance by Mr. Robert Stone, Chief Fisheries Officer of the Fiji Department of Fisheries. Grant support for Martin was provided by the National Science Foundation; grant support for Ward was most generously provided by Professor G. E. G. Westermann, Department of Geology, McMaster University.

Doctor W. Sweet critically reviewed the manuscript.

LITERATURE CITED Bidder, A. M. 1962 Use of tentacles, swimming and buoy-

ancy control in the Pearly Nautilus. Nature, 196: 451-454.

Denton, E. J., and J. B. Gilpin-Brown 1961a The buoyancy of the cuttlefish Sepia officinalis (L.). J. Mar. Biol. Ass. U.K., 41: 319-342.

1961b The effect of light on the buoyancy of the cuttlefish. J . Mar. Biol. Ass. U.K., 42: 343-350. - 1961c The distribution of gas and liquid within the cuttlebone. J. Mar. Biol. Ass. U.K., 41: 365-381.

1966 On the buoyancy of the pearly Nautilus. J. Mar. Biol. Ass. U.K., 46: 723-759.

1971 Further observations on the buoyancy of Spirula. J. Mar. Biol. Ass. U.K., 51: 363-373.

1973 Flotation mechanisms in modern and fossil cephalopds. Adv. Mar. Biol., 11: 197-268.

Denton, E. J., J. B. Gilpin-Brown and J. B. Howarth

12 PETER WARD AND ARTHUR W. MARTIN

1961 The osmotic mechanism of the cuttlebone. J. Mar. Lankester, E. R. 1883 Mollusca. The Encyclopedia Britan- Biol. Ass. U.K., 41: 351-364. nica, Charles Scribner’s Sons, New York, Ninth Ed., Vol.

1967 On the buoyancy of Spirula spirula. J. Mar. SVI, p. 671. Biol. Ass. U.K., 47: 181-191. Mutvei, H., and R. Reyment 1973 Buoyancy control and

Griffin, L. E. 1900 The anatomy of Nautilus pompilius. siphuncle function in ammonoids. Paleontology, 16(3): Mem. Natn. Acad. Sci., 8: 103-197. 623-636.

Heptonstall, W. B. 1970 Buoyancy Control in Ammonoids. Willey, A. 1902 Zoological Results. Cambridge University Lethaia, 3: 317-328. Press, Par t VI, p. 750.