A MALFORMED EMBRYO OF STENELLA COERULEOALBATOSHIRO KAMIYA
Department of Anatomy, Faculf(y of Medicine, Universif(y of Tokyo, TokyoNOBUYUKI MIYAZAKI
Ocean Research Institute, Universif(y of Tokyo, Tokyo

At the present time, the dolphin drive fishery, mainly Stenella coeruleoalba on theeast coast of lzu Peninsula is cooperatively operated at Kawana and Futo villages. At these two villages, the fishing season starts in early October and closes
in January. The mean annual catch of S. coeruleoalba for both villages has been
5,157 individuals during the past 5 seasons (1969-'70 to 1973-'74).
In 1972, 21 schools of S. coeruleoalba were driven ashore at these two villages
and 7,235 individuals were killed. In that season, we examined 2,299 individuals
and 467 foetuses taken from 20 schools. On October 17 a school of 187 individuals
was driven into the Futo Port. This was killed between the l 7th and 19th ofOctober. The composition of 115 individuals of the school is shown in Table 1.
This table indicates that 60.5 percent of the females and 33.3 percent of the males inthis school had attained sexual maturity, and the socionomic ratio was 0.28. In
the course of field examination of those 115 individuals at Fu to village, we collected
17 foetuses. One was a small malformed embryo. The mother of this embryo
was 220 cm in body length and was estimated to be 8.5 years old. The age wasdetermined by the number of opaque dentinal layers. The condition of the ovaries
was not observed. Kasuya (1972) stated that females of S. coeruleoalba attain sexual
maturity at 9 years at the length 212 cm, so that it seems that this mother was inher first or second pregnancy.
TABLE I.

SCHOOL COMPOSITION OF STENELLA COERULEOALBACAUGHT ON OCTOBER 17, 1972.

SexPregnant

LactatingRestingFemale

17(22 .4)

27(35. 5)

0MaleTotal

MatureImmatureTotal

2(2 .6)
13(33. 3)30(39. 5)26(66. 7)

76
39115

Figures in parenthesis indicate percentages.

The external characters observed in this embryo were the following: Body
length (cranio-caudal): 9 mm; greatest breadth: 7 mm; body weight: 2.0 gr.
In ventral view, the cephalic region appears to be almost normal (Fig. 1). The eye
is easily identifiable and so is the maxillary area. In the dorsal half of the head
there is a groove or depression that cannot be identified with certainty. The manSci. Rep. Whales Res. Inst.,.No. 26, 1974, 259-263

260

KAMIYA AND MIYAZAKI

dibular region is not clearly defined. The mandible is probably not developing.
The upper limb bud is nearly normal on the left side (Fig. 2). On the right side,
it is either not developing or in a state of abnormal formation (Fig. 4).
The abdominal region is not normal looking and, in particular, there is a stringlike fold that can be the umbilical cord but it is not typical. The tail, as comparedto that of a normal embryo, is too wide and placed with its tip in the oral region(Fig. 1).
In back view (Fig. 3), one can see the almost normal developing vertebrae andthe segmentation. The curvature, from this aspect, is abnormally sideward. Comparing the two lateral views, one sees that, on the left side (Fig. 2), the appearance
is almost normal (see above remark on mandible) but on the right side (Fig. 4 ),there is a mass of unidentifiable lumps of tissues.
Internal structures were observed in this embryo which was embedded in paraffin, sectioned longitudinally and serially 10 µ thick, and stained with hematoxylin
and eosin for microscopical study. Also, we used an 11 mm long normal embryo ofthe same species for longitudinal serial sections to compare with the malformed embryo.
Fig. 5 shows an approximately sagittal section of the 11 mm normal embryo
S. coeruleoalba. In this embryo, one clearly sees that the brain and spinal cord andall of the primordia of the cranial nerves are already present. Also, each visceralorgan is quite well formed: primordial thymus, thyroid gland, lung, heart and liverwith much blood, splenic primordium, gonad, primitive pancreas, mesonephrosand metanephros respectively etc. are present.
One the contrary, in the malformed embryo which is covered with simple epithelium and the inner structure of which is mainly composed of mesenchyme cells, organ
formation is extremely abnormal in comparison with the normal embryo. In thecephalic region, one cannot recognize any brain structure: there is only a non-differentiated nervous cell mass. In figure 6, the optic vesicle is cut near its centerwith the optic cup and the lens vesicle can be seen. The optic cup has inner andouter layers. The outer layer already presents the pigmented epithelium but thelayer formation is irregular. In the thoracic and abdominal regions, we can mainly
distinguish four kinds of tissue masses. Heart muscles, liver cells, neural tissue andsegmental structures (Figs. 7, 8, 9). Bundle-like heart muscles with intercalated
disks can be seen. The usual blood-cells accummulations were not observed.
Nervous tissue is degenerated but spinal ganglion-like masses were identified andglia cells are abundant.
This case is one of the malformed embryos at an early developmental stage. A
malformed embryo of S. coeruleoalba has been observed by Kawamura andKashita (1971). That case was a double monster with body length of 16.8 and17.2 mm respectively along a transverse axis. Rudimentary hind limbs in a matureindividual were also reported by Ohsumi (1969) and the external development of
normal embryos was described by Ogawa (1953). The normal cross-section of a
12 mm long embryo has been observed by Hosokawa (1955). Each of the above
three embryos was from the same species.
Sci. Rep. Whales Res. Inst.,
No. 26, 1974

MALFORMED DOLPHIN EMBRYO

261

REFERENCES
HosoKAWA, H., 1955. Cross-section of a 12 mm dolphin embryo. Sci. Rep. Whales Res. Inst., 10: 1-68.
KASUYA, T., 1972. Growth and reproduction of Stenella caeruleoalba based on the age determination bymeans of dentinal growth layer. Sci. Rep. Whales Res. Inst., 24: 57-79.
KAWAMURA, A. and K. KASHITA, 1971. A rare double monster of dolphin, Stenella caeruleoalba. Sci. Rep.
Whales Res. Inst., 23: 139-140.
OGAWA, T., 1953. On the presence and disappearance of the hind limb in the cetacean embryo. Sci.
Rep. Whales Res. Inst., 8: 127-132.
0HSUMI, S., 1965. A dolphin (Stenella caeruleoalba) with protruded rudimentary hind limbs. Sci. Rep. Whales
Res. Inst., 19: 135-136.

EXPLANATION OF FIGURES
Figs. 1-4. External form of malformed embryo of S. coeruleoalba. (ea. X 10.5).
2: Lateral view of the left side
3: Dorsal view
I : Ventral view
4: Lateral view of the right side
Fig. 5. Longitudinal median section of the 11 mm normal embryo of S. coeruleoalba(ea. X 10).
Figs. 6-9. Histological structure of malformed embryo of S. coeruleoalba.
6: Optic vesicle, L=lens (ea. x300)
7: Heart muscles (ea. x500)
8: Liver (ea. x500)
9: Segmental structure (ea. x 120)

Sci. Rep. Whales Res. Inst.,
No. 26, 1974

262

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