A NOTE ON THE AIR-SAC OF RIBBON SEALHISASHI ABE
Institute of Applied :(oology, Faculty of Agriculture, Hokkaido
University, Sapporo

YOSHIKAZU HASEGAWA
Section of Vertebrate Paleontology, National Science Museum, TokyANDKAZUO WADA
Primate Research Institute, K_yoto Unii;ersity, Inuyama

ABSTRACT

Investigations on the air-sac of seals were carried out by us, using twelve
males and three females of the ribbon seal (Histriophoca fasciata ZIMERMANN) and two males of the Okhotsk harvor seal (Phoca vitulina LINNAEUS), which were caught off Rausu of the Shiretoko Peninsula, Hokkaido,
Japan, in the middle of March and April, 1966. In the rear part of the trachea of the ribbon seal, the cartilage rings are broken to form a slit, 25-46 mm
long, which leads to an air-sac on the outer side. The two ends of the broken
rings form long and slender valvular projections. The fore part of the air-sac
is developed beneath the sternomastoid muscle, and the rear part is between
the pectoralis superfacies and the costae. In adult males, the sac is large, but
it is smaller in young individuals. Young females have a pair of valvular projections but no air-sac. Even in adult females, development of the air-sac isvery poor or entirely absent. The role played by the air-sac is not known.
The cartilage rings of harbor seal are broken at the rear part of the trachea,
but the break does not form a slit and hence air-sac is not developed.

INTRODUCTION

Studies on the morphology and ecology of the ribbon seal, Histriophoca fasciata
ZIMMERMANN, have been relatively few, and even the dental formula of this animal
had been little known until recently (SCHEFFER, 1960).
The present writers have observed that the ribbon seal has an air-sac branching out of the trachea. An outline of the observation is reported here.
Valuable specimens of the ribbon seal for this study were offered by Mr.
lchitnatsu Nedefuji, Misaki-machi, Rausu-cho, Menashi-gun, Hokkaido, who also
took care of the lodging for the writers during the period of investigation. We
wish herewith to express their sincere gratitude to Mr. Nedefuji. They are alsoindebted to many persons for helpful suggestions.
Sci. Rep. Whales Res. Inst.,No. 29, 1977, 129-135.

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ABE, HASEGAWA AND WADA

DISCOVERY OF AIR-SAC
Two of us, Wada and Hasegawa, visited Rausu-cho from March 15 to 17, 1966 forthe purpose of investigating seals. On March 15, when a male ribbon seal, captured off Rausu by Mr. Nedefuji, was being dissected, Wada noticed a bladder-likemembranous sac in the thorax, and surmised if it might be an air-sac. On the following day, four more males and one female were captured. Hasegawa examinedthem and noted that the sac seemed to occur only in males, its position being located always on the right side of the body, and that the size of the sac was considerably variable according to individuals. However, he could not get sufficient
data on this subject as the time was limited.
Later, from April 12 to 14, Abe visited Rausu and made more precise observation of seven males and two females of H. fasciata and two males of Phoca vitulina
LINNAEUS (Okhotsk harbor seal). He examined in particular the size and position of the air-sac and the difference between male and female, as well as the relationship between the length of the body and the development of the air-sac. Later,
he cut off the tracheae attached with the sac, preserved them in alcohol, andbrought them back to the laboratory for further observation of details.
POSITION AND STRUCTURE OF AIR-SAC
1) MaleIn the first half of the trachea, the tracheal cartilage forms complete rings without
any break, whereas in the latter half, the cartilage rings are broken on the dorsalside of the trachea as illustrated in Fig. 2, III-IX. On the outer side of the bothends of the broken rings, there is a more or less membranous wall, which is developed particularly on the right side, so as to form an air-sac (Fig. 1, 1-V; Fig. 2,V-VIII). Accordingly, the break in the rings serves as a passage to the air-sac.
This slitlike passage starts at about 50-60 mm from the diverging point of thebronchi and extends forward (toward the neck) (see Pls. 1 and 2). In general,
the slit becomes longer as the animal grows up (Table 1).
The sac occurs as a bulge of the relatively thick muscular membrane on theright outer side of the slit in the early stages of the development (Fig. I, I). The
sac grows larger, decreasing the thickness of membrane, and extends forward until
it reaches the neck of the animal; it also stretches rearward (toward the hind legs)
and, when fully developed, it becomes to cover the whole right ribs (Pl. 1, II; Fig.
1, V). As the sac grows larger, the membrane becomes thinner and translucent.
Also, the larger the sac, the better is its rearward development. For example, inSpecimen No. 6 the length between the foremost end of the sac and the starting
point of the slit is 120 mm, and from there to the rear end of the sac it is 70 mm
long. In Specimen No. 7 the measurement by the same method reveals that thesac is 150 mm long in its fore part and 230 mm long in the rear part.
The fore part extends forward, passing through the underside of the sternomastoid muscle. If the sac is small it is concealed by the muscle, but when it
Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

A NOTE ON THE AIR-SAC OF RIBBON SEAL

IIIll131IVFig. I. Developmental precesses of male air-sac and its opening part on the tracheaof the ribbon seal. I: No. 1 specimen in Table 1, II: No. 3, III: No. 5, IV: No.
6, V: No. 7, "0" in III to Vindicates the opening of the sac. VI: rear half oftrachea on the right side (No. 5 specimen), showing the opening and the valve(V) of the air sac which in this figure the membraneous sac has been removed.
Lower side in the figure VI is the dorsal side, the right tapering part is thebronchus.

grows larger its fore end appears ahead of the muscle. The rear part of the sac isfound between the pectoralis superfacies and the costae. Therefore, the existenceof the sac would be hardly noticed when the seal is only skinned.
On the right and left sides of the slit, part of the trachea forms a long andnarrow valve, with its rear end starting from the diverging point of the bronchi.
The valve has a slender crescent shape, being widest at the middle. The rightvalve is larger than the left in both length and width. The distance between
the two valves is largest at the slit (Table 1; Fig. 2 IX). The left valve does notseem to be functional at all, but the right valve, which is widest at the slit, is considered to conduct the important role for taking the air in and out of the sac (Pl,1, II and III). To clarify the function of these valves, further study is required
from the standpoint of histology and ecology.

2) Female
The fundamental features of the sac are identical with those of male. A valvular projection is observed at the dorsal inner side of the rear part of the trachea.
However, development of a sac is poor in female, and even in a fully grown stageof the animal the sac is of nearly the same size as that of a small male (Pl, 2, IV;
Sci. Rep. Whales Res. Inst.,
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ABE, HASEGAWA AND WADA

II.

IX
Fig. 2. Developmental processes of air sac of ribbon seals, and cross and longitudinal sections of the sac and trachia. I : No. 10 specimen; II : No. 11 ; III :cross section at the central part of the valve in No 10 female; IV, V, VI, VIIa and VIII indicate the cross sections at the central part (" a " line on IX) of valve
in specimens 11, l, 3, 5, 7, respectively; VII-band VII-c: cross sections at" b"and " c " lines in IX. Lower side in the figures is the dorsal side of trachea ;V and S: valve and air sac; IX: longitudinal section showing the dorsal innerside of the opening part of trachea.
TABLE 1.

TOTAL LENGTH OF BODY, CONDYLOBASAL LENGTH OF SKULL
AND DIMENSION OF AIR-SAC (in mm)

species name·"'i::<L> ...

s..@~<L>·08~§><~Histriophoca fasciata Z.
MlM
2M
3
4M5MM
6M
78M
M
910F
11F
Phoca vitulina L.
12
M
M
13-

air-sac~

..c:
bO

~..8
B~

right valve-«!~::I"'"'>.£-.:I Oili:: i::
0<L>

u-1260
1230
1300
1130
1370
1360
1500

177. 7
180.7
178.8
201.9

1310
1560

178.4
204.6

1450
1600

203.5
222.8

.s-.:I..c:bO.g«I._0t.@

..c:

~

40
55
70
130
190
380
105
430

515
37
35
70
75
110
55
120

70

20

i::

..!:l...

~

.....

ti::

..!:l86
82
92

..c:
~<L>

I...

c;;S

~ l=i-o~ g Jds ....<il«!

...

«!

272535404246102

14991111111340

80
841424380

left valve«!

8~t~~i::~«!

8273718255445477080

7660

768sbroken and unmeasured.

Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

A NOTE ON THE AIR-SAC OF RIBBON SEAL

133

Fig. 2, I-IV). Hasegawa examined a pregnant individual but could not confirm
the existence of the sac. In females, the sac, if any, may not be functional.
COMPARISON WITH OKHOTSK HARBOR SEAL
We dissected two males of Okhotsk harbor seal, which were captured together withthe above-mentioned ribbon seals, and examined their trachea for comparison withthat of ribbon seals. In that species, too, the cartilage rings are broken in the rearpart of the trachea, that is, immediately before the bronchil they diverge. However, the valvular projection is entirely different from that of ribbon seal, as it
occurs only on one side. One of the examined specimens had it on the right side(Fig. 3), but the other had it on the left side. Specimen No. 11 is supposedly arelatively large individual, but it had no air-sac nor its trace.

Fig. 3. The longitudinal section of the rear part of trachea of Phoca vitulina (adultmale).

CONSIDERATIONAs to the role played by the sac, we can say nothing more than a conjecture. It isevident, however, that the sac is connected with the trachea through a simple functional valve, and there remains little doubt that the sac serves as an air-sac. The
size of the sac differs markedly between male and female, and also varies with theage of individual. These facts may indicate some differences in habit andbehavior of individual seals. According to certain hunters, ribbon seals, whenchased, would stay under the water for a long time; another hunter, however, says
he has experienced no such occasion. Thus, the hunters' comments cannot berelied upon. In fact, the riddle of the sac would remain unsolved until scientificinvestigations are made on the ecology of ribbon seals.
It is of a special interest that the sac is located on the right side of the body,&i. Rep. Whales Res. Inst.,
No. 29, 1977.

ABE, HASEGAWA AND WADA

134

stretching beneath the sternomastoid muscle or between the pectoralis superfacies
and the costae, and that the sac grows larger along with the animal's growth. To
give reasonable explanation for these features, anatomical and ecological studies of
not only the ribbon seal but also other kinds of seals must be carried out in thefuture. This is why the present writers refrain possible explanations of the roleof the sac.
REFERENCESSCHEFFER, V. B., 1960.

Dentition of the ribbon seal.

Proc. Zoo!. Soc. London, 135(4): 579-585.

* We have found recently that SLEPTSOV'S paper (.(ool . .(hum. 19(3) : 379-385, 1940) had described theoutline of the air-sac of this seal.

Sci. Rep. Whales R.es. Inst.,
No.. 29, 1977.

A NOTE ON THE AIR-SAC OF RIBBON SEALEXPLANATION OF PLATESPLATE I
I: Sac of young male (8 in Table 1). Ila and llb: Sac of adult male (9 in Table1). Notice the valve closed (a) and opened (b).
PLATE II
Development of male sac (I-III) and primitive small sac of an adult female (IV).
I-IV show those of specimens 3, 5, 7, respectively. Sacs of II to IV have been dissected to show the inside and the opening.

Sci. Rep. Whales Res. Inst.,
No. 29, 1977.

135

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No.29,1977.