Abstracts of the Oral Presentation at
the 9th Study Meeting of
the Sea of Japan Cetology Research Group

June 6-7, 1998
(In Japanese)

O1: Sighting and stranding records of whales and pinnipeds in the coast and offshore of Niigata District, since May 1997.

Yoshiharu Honma (Emer.Prof.Niigata Univ.), Yukihiro Nakamura (Johetsu Mun.Aquar.), Kazuhiro Minowa (Kashiwazaki City Mus.),^○Akira Aoyagi (Teradomari Town Aquar.) and Yoshiaki Kogawara (Sado Line Comp.)

O2： 10 years of study on Stejneger's beaked whale.

Tadasu K. Yamada（Dept. of Zoology, National Science Museum, Tokyo）

It was in 1960 when Stejneger's beakedwhale (Mesoplodon stejnegeri) was first recognized in Japan scientifically. Noticing that a whale reported as a minke whale in a newspaper article was a beaked whale, Late Prof. Masaharu Nishiwaki hurried to Akita city, dug up the carcass and confirmed it was a Mesoplodont whale. The time was just after the Gingko-toothed beaked whale (Mesoplodon ginkgodens) was described by Nishiwaki and Kamiya as a new species and people were sensitive to this group of animals. Hubbs' beaked whale (Mesoplodon carlhubbsi) was also confirmed in Ayukawa a few years before.

Stejneger's beaked whale was regrded as extremely rare and had been sporadically reported from the coasts of the Sea of Japan, and a whole skeleton was secured again in Niigata prefecture in 1985. Reports on two strandings of this species in Ishikawa prefecture lead our group, Sea of Japan Cetology Research Group into existence. I would like to summarize what we learned about this species during the last 10 years.

In the begging of our survey it was said that there would be two species of Mesoplodon in the Sea ofJapan, Ginkgo-toothed and Stejneger's. According to our survey of both then preserved specimens and the animals stranded thereafter, all the specimens we could access were Stejneger's beaked whale. Strandings were reported from the area between Tottori and Wakkanai (or Rishiri island), centering the area around Ishikawa and Niigata, with possible addition of Yamagata and Akita to the north as a larger center. Neonates were reported from Ishikawa, Niigata and Akita with body length between 200 and 220 cm. Colouration of neonates and juveniles is light gray to white ventrally and darker gray dorsally. When they grow larger than 4 m in length the colour changes into uniformly dark gray to black. There were also pregnant female strandings and it is possible that the species is calving in the Sea of Japan. Stomach contents confirmed to date are squid beaks. Squids identified were those from middle pelagic layers raging between 200 to 600 m in depth.

０3： Ecology of cetaceans in the Sea of Japan estimated from the stranding record.

Hajime Ishikawa ( Institute of Cetacean Research)

Institute of Cetacean Research (ICR) started to collect marine mammal stranding and incidental catch record on the coast of Japan from 1986. ICR Stranding Database includes nearly two thousands records until 1999. This report describes species and seasonal analysis of 532 records of whales which species were confirmed in the Sea of Japan. In the tooth whales listed 450 records of 20 species, the most frequent species in the Sea of Japan was Pacific white sided dolphin (145 records) and followed by Stejneger's beaked whale (54 records), unidentified Mesoplodon species (54 records), Dall's porpoise (47 records) and finless porpoise (29 records). In the baleen whale listed 82 records of 6 species, minke whale was the most dominant species (70 records) in the Sea of Japan. Records of Stejneger's beaked whale and Dall's porpoise suggested that these species were seldom entangled by coastal set net. The seasonal character of records in the Sea of Japan is high frequency from January to May and low from July to November. This feature was common in both stranding and incidental catch. It was considered that seasonal distribution of Pacific white sided dolphin, Dall's porpoise, minke whale and possible Stejneger's beaked whale caused such a change of frequency. Survival rate of stranded whales found to be alive was high in July-October, whereas extremely low in other months, which suggested close relation between survival rate and air and water temperature.

O4： Archaeological sites with cetacean remains in the Hokuriku districts, Central Japan.

Tetsuo Hiraguchi (Kanazawa Medical University)

O5： Cetacean fossils of the Ashiya group (Oligocene), northern Kyushu

Yoshihiko Okazaki (Kitakyushu Mus. Inst. Nat. Hist.)

The northern part of the Kyushu Island, West Japan, yields many Oligocene cetacean fossils. The assemblage is characteristic of the Japanese Oligocene Age and important from the view point of the evolutionary history of cetaceans. In the present lecture, I will introduce their materials with their classification.

Oligocene Ashiya Group

The Ashiya Group is mainly Late Oligocene marine sediments distributed in Wakamatsu, Yahatanishi in Kitakyushu City, and Shimonoseki City. It yields abundant molluscan fossils. Several species of cetaceans heve been recognized from the Ashiya Group, and they are of as much importance as those from Ashoro in Hokkaido Island, North Japan.
Among the mysticetes, primitive cetotherids and "toothed mysticetes" have been recognized. Also, an agorophid and squalodontids have been recognized among the odontcetes. So the fauna was diversed, with many (at least 14) species of sharks and rays, a sea turtle, and many species of diving birds and a dugongid.
　

Importance of the Oligocene Cetaceans　

From the archaeocetes which first occurred in the Eocene, there evolved two extant suborders, Odontoceti and Mysticeti in late Early Oligocene. In the Late Oligocene, there were many diversed cetaceans, still primitive in morphology. It is important to know when, where (both in the geography and classification), and how each morphological character occurred and changed.

For example, the most important specimen from the Ashiya Group, a primitive cetothere, is the most primitive among the true (fed by baleen plates) mysticetes. It had enough advanced features in the anterior limb for swimming, but the bone measurements of that part show quite different results. The external nare opened much anterior than seen in the present species. The animal had baleen in the rostrum, and also grooves and pits, suggesting several teeth, in the lateral margin of the upper jaw and the dorsal margin of the lower jaws.

O6： Origin of blue whales on the basis of a Pliocene dentary from Hiraizumi, Northeast Japan.

Masayuki Oishi (Iwate Prefectural Museum)

Fossil mysticetes assignable to extinct genera from Europe and North America have been found from the marine Pliocene deposit of the Kitakami Lowland, Northeast Japan. Pliocene whales have great importance in the investigation of the origin of the extant genera. Comparisons between the fossil dentary excavated from Hiraizumi of the Kitakami Lowland in 1987 and dentaries of extant and fossil balaenopterids reveals systematic close ties between extant blue whales and the Hiraizumi specimen.

The dentary (IPMM 60016) is from the Early Pliocene Tatsunokuchi Formation at a quarry of Tokushu-shigen-kogyo Co. Ltd. in Hiraizumi. The extant mysticete specimens used for comparison are in the collections of the following institutions: Nat.Hist.Mus.Tohoku Univ., Iwate Pref.Mus. (IPMM), Kushiro City Mus., Kitakyushu Mus.Nat.Hist., Marine Sci.Mus.Tokai Univ., and National Sci.Mus. Horizontal rami were laid horizontally for standardized comparison and measurement. Only literature sources were used to compare the fossil specimens.

The right dentary of IPMM 60016 is well preserved with the exception of the anterior end of the horizontal ramus and tip of the coronoid process, measuring more than 2110 mm. Small mandibular foramen is compressed dorsally. The condyle, which is also somewhat damaged, demonstrates remarkable inward inclination. The coronoid process develops dorsally and slightly laterally. The horizontal ramus shows gentle and uniform lateral curvature, having planer medial and convex lateral surfaces. The medial surface of the horizontal ramus rotates from nearly vertical orientation at just anterior to the coronoid process to slightly lateral inclination at the anterior end. The horizontal ramus tapers only slightly in vertical diameter anteriorly.

Characteristics of the above mentioned Hiraizumi specimen do not correspond with those in recent neobalaenid, balaenid, and eschrichtiid whales. Although it is difficult to compare the Hiraizumi specimen with whales of so-called "Cetotheriidae" (Late Oligocene to Late Pliocene) which have more than 30 genera and about 60 named species, the author can safely conclude that the Hiraizumi specimen differs from some genera such as Mesocetus which have large mandibular foramina and Mixocetus (Late Miocene, eastern North Pacific) which has also a large mandibular foramen and remarkable backward movement of the rostral elements of the skull. The Hiraizumi specimen shares the small mandibular foramen and the plano-convex cross-sectional shape of the horizontal ramus with balaenopterids such as minke whales.

The comparison of all nominal species in Balaenopteridae by means of actual measurement demonstrates that humpback whale has less developed a coronoid process and that blue whale differs from other species of Balaenoptera by having the condyle obviously inclined inward and having the anterior end scarcely inclined outward. The dentary of the Hiraizumi specimen corresponds with that of blue whale in these characteristics, however the condyle of the Hiraizumi specimen is more inclined inward than blue whale.

In consideration of recently revealed feeding mechanism of Balaenopteridae, dentary differences between blue whales and other rorquals as stated above tally with the maxillae morphology. The major structural change of the skull-mandible complex suggests different feeding mechanism between the two groups. The Hiraizumi specimen indicates that the change occurred, and the two groups diverged, in Early Pliocene. Consequently it may be reasonable that blue whale belongs to a separate genus rather than a member of genus Balaenoptera. The generic name Sibbaldius Flower, 1864, may be preferable to Sibbaldus Gray, 1864.

O7： Histological and genetical examination on the trials for determine the identity of the objects which were struck by the jetfoil (rapid-transit boat) operating for the Sado Line in the Sea of Japan.

^○Yoshiharu Honma, Tatsuo Ushiki, Masaei Takeda (3rd Dept.Anat.,Niigata Univ. Sch.Med.), Emiko Naito, Kohji Dewa, Haruo Yamanouchi (Dept.Foren.,Niigata Univ. Sch.Med.), Tadasu K. Yamada (Dept.Zool.,Natl.Sci.Mus.,Tokyo) and Mutsuo Goto (Genet.Ecol.Sec.,Inst.Cetac. Res.)

O8： Ovarian histology of 3 individuals of the Pacific white-sided dolphins, Lagenorynchus obliquidens Gill 1865, stranded on the coast of Niigata Prefecture, Sea of Japan, from February to April, 1997.

^○Yoshiharu Honma, Tatsuo Ushiki and Masaei Takeda (3rd Dept.Anat.,Niigata Univ. Sch.Med.) and Tadasu K.Yamada (Dept.Zool.,Natl.Sci.Mus.,Tokyo)

Ovaries of three individuals of the Pacific white-sided dolphins, Lagenorhynchus obliquidens Gill, 1865, were studied histologically to determine the maturity of gametes. The animals, 1.53-1.80m in total length and 50.0-74.5kg in weight, had been stranded on the coast of Niigata District, Sea of Japan. The ovaries were 37-40mm in length and 1.6-2.2g in weight. Macroscopically, every ovarian lobe had a smooth dorsal surface and intricately folded ventral surface, characteristic of a young immature stage. Histological examination revealed that most of the oocytes had reached the primordial and primary follicle stages, but secondary follicles were seldom seen in the deeper zone of the cortex. Many atretic follicles in various stages of degeneration were also detected. However, neither corpus luteum nor corpus albicans was encountered, the overall picture being of young immature animals, in accordance with the macroscopic diagnosis.

O９： Anatomy of the thyroid and parathyroid glands of marine mammals.

^○Daisuke Hayakawa, Huayue Chen (Dept. Anat. Gifu Univ.), Shoichi Emura, Akira Tamada (Col. Med. Sci. Gifu Univ.), Marjan Jamali, Hirotaka Taguchi, Ryuichiro Yano, Yuki Ozawa, Kishiko Yoshida (Dept. Anat. Gifu Univ.), Nobuhiro Otake, Hideo Isono (Heisei Col. Med. Tech) and Shizuko Shoumura (Dept. Anat. Gifu Univ.)

We observed the thyroid glands of false killer whales (Pseudorca crassidens), short-finned pilot whales (Globicephala macrorhynchus), Risso's dolphins (Grampus griseus), bottlenose dolphins (Tursiops truncatus), Pacific white-sided dolphins (Lagenorhynchus obliquidens), finless porpoises (Neophocaena phocaenoides), California sea lions (Zalophus californianus), northern fur seals (Callorhinus ursinus) and sea otters (Enhydra lutris), that died in aquariums or were captured by fishermen. The thyroids of all the cetaceans (Pseudoorca, Globicephala, Grampus, Tursiops, Lagenorhynchus and Neophocaena) were dark brown and situated on the rostral part of the trachea. The thyroids of Pseudoorca, Globicephala, Grampus and Tursiops had numerous indentations on their surface, while those of Lagenorhynchus and Neophocaena were relatively smooth. The thyroid glands of Zalophus and Callorhinus consisted of two smooth lobes on both sides of the trachea without an isthmus. In the sea otter, the thyroid gland consisted of two smooth lobes and an isthmus that was composed of only connective tissues without thyroid parenchyma. The greater species tend to have the higher ratio of the thyroid weight to the body weight. There was no distinct differences among the nine species regarding the height of the follicular cell.

We also examined the parathyroid glands of Risso's dolphins and bottlenose dolphins macroscopically and microscopically. They had two or four parathyroid glands on the dorsal surface of the thyroids. Considering their greater body size, each parathyroid gland was rather small. This result and electron microscopic findings of the parathyroid gland of bottlenose dolphins suggest a possibility that the activity of the parathyroid gland is suppressed to adapt to a sea habitat.

O10： Reports on dolphin watching of Kyushu.

Masayuki Nakamura (Marine World Umino-nakamichi)