Kaki (oyster) - oyster (English spelling)

Japanese: カキ（牡蠣） - かき（英語表記）oyster 英語

A general term for bivalve mollusks belonging to the family Ostrichidae in the class Bivalvia of the phylum Mollusca. Oysters attach themselves to rocks with their left shell, while their right shell is slightly smaller and less bulging, almost like a lid. There are no obvious mandibles where the two shell pieces fit together, and they are connected by a black ligament. A groove runs from the apex of the left shell to the ligament. The surface of the shell has thin-layered growth veins, which give rise to radial ribs and spine-like projections. The inside of the shell is often white, but some species are purple or yellowish brown. The soft body has one adductor muscle (adducer muscle) in the center.

Among oysters, there are oviparous species such as Magaki and viviparous species such as Itabo-gaki. Oviparous species are alternating hermaphrodites, with each individual clearly indicating their sex, and changing from male to female and back again. Viviparous species produce both eggs and sperm in the gonads, but some individuals are more female and some are more male. In general, oysters that grow poorly in unsuitable environments or are highly depleted after spawning become more male. It is difficult to distinguish between the ovaries and testes with the naked eye. In Japan, Magaki oysters spawn mainly in the summer when the water temperature is high, ranging from 15 to 30 degrees Celsius, but the fertilized eggs hatch into D-shaped floating larvae in the range of 21 to 26 degrees Celsius. After about three weeks of fertilization, they become sedimentary larvae called spat, which attach to other objects. The shell grows to about 7 centimeters and weighs about 60 grams in one year, and 10 centimeters and weighs about 140 grams in two years.

Different species live in different locations; Pacific oysters attach themselves to rocks in the intertidal zone at the back of inner bays with a salinity of 18-23, while oceanic oysters such as the Japanese oyster and the large black oyster live on reefs influenced by ocean waters with a salinity of 26-34. They feed on plankton such as diatoms, and although the back of their mantle does not have a water tube like that of clams, they take in water along with respiratory water through an entry hole created by the adhesion of part of the mantle, which they then filter through their gills. Sources suggest that the amount of seawater that passes through their gills is 0.4-1 liter per hour per day on average. When the amount of food consumed exceeds the energy consumed, it is stored as glycogen.

[Takashi Okutani]

Useful species

Oysters are called "milk of the sea" and are eaten not only in Japan but also in Europe and America, and important species are cultivated.

(1) Magaki oyster, Crassostrea gigas. An egg-laying species, it is found from Sakhalin throughout Japan, the Korean peninsula, Primorsky Krai, and the coast of China. It is the most commonly cultivated oyster in Japan, with Miyagi and Hiroshima prefectures being the main production areas, and seed oysters are also exported to the United States. Shikame, Nagagaki, and Ezogaki are names given to the ecotypes of this species. Nagagaki in particular is found in Lake Akkeshi and Lake Saroma in Hokkaido, and is large, with a shell length of 8 cm and a shell height of 35 cm, with a thick and very heavy shell.

(2) Suminoe oyster C. ariakensis It lives in the intertidal zone of inland bays in North China, mainly in the Ariake Sea. Its shell length is about 9 cm, its height about 16 cm, and large individuals can reach up to 24 cm. Compared to the Magaki, its shell is flatter and its growth ribs are shaped like cypress bark and are purple-brown in color. It is mainly cultivated in the Ariake Sea area for food. This species is mainly produced in the Kumamoto Prefecture area and elsewhere. Its Japanese name comes from Suminoe (the mouth of the Rokkaku River) in Saga Prefecture, where it is primarily produced. It is also known as Hiragaki and Saragaki. It is an oviparous species.

(3) Ostrea denselamellosa : Found in various parts of Japan south of Honshu along the coast of China, it lives at depths of about 10 meters from the low tide line in areas with slightly higher salinity than the Magaki oyster. It attaches to local vegetation but also sticks to itself to form clumps. Its shell is disc-shaped and the outer surface is covered with a layer of cypress bark-like skin. The inner surface is white. It is viviparous. It is caught with a girder net in an inland bay. Attempts were made to cultivate it before World War II, but it was not put to practical use due to the difficulty of collecting seedlings.

(4) Rock oyster C. nippona It is found in the open ocean south of Mutsu Bay, attached to subtidal reefs. It is also called "kutsugaki" because of its large size, and "natsugaki" because it is delicious in summer, whereas other oysters are in season during the cold season. It is an oviparous species.

(5) American oyster ( C. virginica ) Native to the Atlantic coast of North America. Oviparous.

(6) Olympia oyster, O. lurida , native to the Atlantic coast of North America. Viviparous.

(7) European oyster ( O. edulis) Found on the coasts of Europe. Large-scale cultivation in places such as Arcachon in southwestern France, where it is also called the French oyster. A viviparous species.

(8) Portuguese oyster , C. angulata . An edible species found in southern Europe. Oviparous.

(9) Saxostrea commercialis, found on the east coast of Australia. An oviparous species similar to the Japanese S. kegaki and S. mordax .

(10) Bombay oyster ( S. cucullata ) Found on the coast of India. Similar to the previous species. Oviparous.

[Takashi Okutani]

Aquaculture

Simple aquaculture has been practiced in China since ancient times (during the Song dynasty), and there are records of sea-planting aquaculture in Naples during the Roman period. In Japan, around 1670 (Kanbun 10) during the early Edo period, Kobayashi Gorozaemon is said to have started aquaculture in Kusatsu, Aki Province (Nishi Ward, Hiroshima City), inspired by oysters that had attached to cracks built in the sea. In 1923 (Taisho 12), Senoo Hidemi and Hori Juzo invented the raft-style hanging aquaculture method. When the larvae floating in the sea reach a size of about 0.4 mm and begin to attach to other objects, a collector made of tiles, oyster or scallop shells is inserted into the sea to allow the young oysters to attach. After 4-5 days, the attached young oysters turn into seed oysters (spats) the size of sesame seeds. In raft-style hanging culture, the rafts rise and fall with the tides, so the oysters are always underwater, and have more time to feed, which allows them to grow well. In spring, the attached organs (spaced with scallop shells or other shells according to the oysters' growth) with the spat attached are put into the sea, and by winter they will have grown to a size that can be sold. The main production area for seed oysters is Miyagi Prefecture, with other production areas including various parts of Sanriku, Hiroshima Prefecture, and the Ariake Sea. Seed oysters are also exported to North America. As with edible oysters in Europe and America, oysters are fattest and tastiest in winter, and as is often said in those countries, the "months without R" (May to August) are not only thin, but also prone to spoilage due to the high temperatures, making them unsuitable for eating raw. For eating raw, live oysters cultivated in a clean environment are best, and in the market they are divided into those for eating raw (vinegared oysters, raw oysters) and those for cooking (fried oysters, cooked in a pot, etc.). Oysters have a self-cleaning ability that allows them to expel waste materials that enter their bodies. Taking advantage of this ability, oysters are kept in sterilized running seawater for about a day and night, and then E. coli and other toxic microorganisms that were inside the body are expelled, and they are sold on the market under the product name "germ-free oysters."

[Takashi Okutani]

nutrition

It is a nutritionally excellent food, especially rich in minerals such as iron and zinc, and somewhat high in vitamin A. It has a sweet and delicious taste, which is due to glycogen. It is somewhat high in cholesterol, but the amount is not something to worry about as long as you don't eat too much.

[Tomomi Kono and Midori Otaki]

cooking

Oysters taste sweeter in winter. This is probably why months that end with the letter R are considered good. It is necessary to choose fresh oysters because they lose their freshness quickly, but the adductor muscle of new oysters is translucent, and as they get older this part becomes cloudy. The whole shellfish (soft body) is plump when it is new, but as it gets older it becomes floppy and whitish. As it loses freshness it becomes yellowish. When eating raw, always use oysters labeled for eating raw, eat them by the due date, and if they are slightly older, they must be cooked even if they are for eating raw. There are also oysters with shells that are completely purified for eating raw. There are also frozen oysters for eating raw. The flavor of oysters lies in their aroma, so use the freshest oysters for cooking. Mix the shucked meat in a mixture of grated daikon radish and an equal amount of water, then drain in a colander and rinse with salt water 2-3 times to remove the slime and shells. When eating raw, make oyster cocktail or vinegared oysters. For the cocktail, oysters are served chilled on ice and topped with a cocktail sauce made from tomato ketchup, lemon juice, grated horseradish, salt, pepper, white wine, Tabasco, etc. For vinegared oysters, nibaizu or ponzu sauce is used. For cooking, oysters can be used in a wide variety of ways, including fried, coquille, dotenabe (a hotpot), grilled bacon, and soup. For dotenabe, miso is spread around the outside of the pot, and dashi, oysters, green onions, grilled tofu, and itokonnyaku are placed inside, and the miso around the pot is gradually dissolved and the pot is simmered while eating.

[Tomomi Kono and Midori Otaki]

[Reference] | Hiroshima Oysters

Magaki (ecology)
It attaches itself to rocky reefs in the intertidal zone of inner bays, where the salinity is relatively low. It has radial ribs, and the shell shape varies depending on the environment. © Underwater Fort Enterprise, Isamu Soyama ">

Magaki (ecology)

Suminoe kid
It is a flattened oval shape without radial ribs, and the surface of the shell has plate-like growth ribs that resemble cypress bark. © Underwater Fort Enterprise, Isamu Soyama ">

Suminoe kid

Itabogaki
The shell is disc-shaped, and the surface of the shell is covered with a bark-like shell. There are some deep, rough growth rings and irregular radial ribs. © Underwater Fort Enterprise, Isamu Soyama ">

Itabogaki

Oyster farming as seen in the "Illustrated Guide to Famous Products of the Mountains and Seas of Japan"
Hiroshima Bay has been a popular place for oyster farming since ancient times. During the Edo period, oysters were farmed using the Hibi (shell) building method. "Nihon Sankai Meisan Zue" Volume 3 "Oysters " (Owned by the National Diet Library )

Oyster farming as seen in the "Illustrated Guide to Famous Products of the Mountains and Seas of Japan"

Earthen pot
Hiroshima Prefecture © Hiroshima Prefecture ">

Earthen pot

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

軟体動物門二枚貝綱イタボガキ科に属する二枚貝の総称。カキ類は左殻で岩などに付着し、右殻はやや小さく膨らみも弱く、あたかも蓋(ふた)のようである。両殻片のかみ合せには明らかな鉸歯(こうし)がなく、黒い靭帯(じんたい)で結ばれている。左殻の殻頂から靭帯にかけて溝が走る。殻表は成長脈が薄片状に発達し、放射肋(ろく)や棘(とげ)状突起が生じる。殻の内側は白い種が多いが、紫色や黄褐色を帯びる種もある。軟体部は中央に貝柱（閉殻筋）が一つある。

　カキのなかにはマガキなど卵生種とイタボガキなど胎生種がある。卵生種は各個体の雌雄性が明らかで、雄から雌、さらにまた雄という性転換をする交替性の雌雄同体である。胎生種は生殖腺(せん)に卵子と精子の両方できるが、雌性に偏った個体と雄性の強い個体がある。一般に環境が不向きで成長の悪い個体や、産卵後消耗の激しいときは雄性が強くなる。肉眼的には卵巣と精巣の区別はつきにくい。産卵は、日本のマガキでは水温の高い夏を中心に行われ、水温15～30℃の範囲であるが、受精卵が孵化(ふか)してD字状の浮遊幼生になれるのは21～26℃の範囲である。受精後およそ3週間でスパットとよばれる沈着幼生となり他物に付着する。殻は1年で7センチメートル、重量60グラムぐらい、2年で10センチメートル、140グラムぐらいになる。

　種類によってすみ場所が異なり、マガキは内湾奥の塩分18～23の潮間帯の岩に付着しているが、外洋性のケガキやオハグロガキなどは塩分26～34の外洋水の影響のある岩礁にすむ。食餌(しょくじ)は珪藻(けいそう)のようなプランクトンで、外套膜(がいとうまく)の後方はハマグリ類などのように水管にはなっていないが、外套膜の一部が接着してつくられる入水孔から呼吸水とともに取り入れ、えらで濾(こ)し取る。えらを通過する海水の量は、1日平均で1時間につき0.4～1リットルという資料がある。摂餌量が消費エネルギーを上回ったときはグリコーゲンとして蓄えられる。

［奥谷喬司］

有用種

日本のみならず、ヨーロッパやアメリカでもカキ類は「海のミルク」とよばれて食用にされ、重要な種は養殖されている。

(1)マガキCrassostrea gigas　卵生種で、樺太(からふと)（サハリン）から日本全土、朝鮮半島、沿海州、中国沿岸に分布する。日本ではもっとも普通に養殖され、宮城県、広島県などが主産地で、アメリカにも種ガキが輸出されている。シカメ、ナガガキ、エゾガキなどは本種の生態型に与えられた名である。とくにナガガキは北海道の厚岸(あっけし)湖やサロマ湖に産し、殻長8センチメートル、殻高35センチメートルにもなる長大型で、殻も厚く甚だ重い。

(2)スミノエガキC. ariakensis　有明(ありあけ)海を中心に華北などの内湾潮間帯にすむ。殻長9センチメートル、殻高16センチメートルぐらいになり、大形の個体は24センチメートルにもなる。マガキに比べて殻は平低で、成長肋が檜皮葺(ひわだぶ)き状になり、紫褐色である。食用としておもに有明海方面で養殖される。熊本県地方などで生産されるのは主として本種である。和名は主産地の佐賀県の住ノ江（六角川の河口部）にちなむ。ヒラガキ、サラガキの別名がある。卵生種。

(3)イタボガキOstrea denselamellosa　本州以南の日本各地から中国沿岸に分布し、マガキよりいくらか塩分の高い場所の低潮線から10メートルぐらいの深さにすみ、地物にもつくが互いにくっつきあって団塊状になる。殻は円板状で、殻表は檜皮葺状の殻皮で覆われる。内面は白い。胎生種。内湾の桁網(けたあみ)でとられる。第二次世界大戦前、養殖が試みられたが、採苗が困難なため実用とならなかった。

(4)イワガキC. nippona　陸奥(むつ)湾以南の外洋で潮間帯下の岩礁に固着している。大形なのでクツガキ、また、他のカキ類の旬(しゅん)が寒い時期なのに反して夏季に美味なのでナツガキの異名もある。卵生種。

(5)アメリカガキ（バージニアガキ）C. virginica　北アメリカ大西洋岸原産種。卵生種。

(6)オリンピアガキO. lurida　北アメリカ大西洋岸原産種。胎生種。

(7)ヨーロッパガキ（ヨーロッパヒラガキ）O. edulisヨーロッパ沿岸産。フランス南西部のアルカションなどで大規模に養殖され、フランスガキともよばれる。胎生種。

(8)ポルトガルガキC. angulata　南ヨーロッパの食用種。卵生種。

(9)オーストラリアガキSaxostrea commercialis　オーストラリア東岸産。日本のケガキS. kegakiやオハグロガキS. mordaxに近い卵生種。

(10)ボンベイガキ（カンムリガキ）S. cucullata　インド沿岸産。前種に近い。卵生種。

［奥谷喬司］

養殖

簡単な養殖は古くから中国（宋(そう)の時代）で行われ、ローマ時代にはナポリで地蒔式養殖(じまきしきようしょく)が行われていた記録がある。日本では、江戸初期の1670年（寛文10）ごろ安芸(あき)国草津（広島市西区）で、海中に建てたひびに付着したマガキからヒントを得て、小林五郎左衛門が養殖を始めたといわれ、1923年（大正12）に妹尾秀実(せのおひでみ)、堀重蔵(じゅうぞう)が筏式垂下養殖法(いかだしきすいかようしょくほう)を考案した。海中を浮遊する幼生が0.4ミリメートルぐらいになり他物に付着するころを見計らい、瓦(かわら)やカキやホタテガイの殻を連ねた付着器（コレクター）を海中に入れ稚貝を付着させる。付着稚貝は4～5日たつとゴマ粒のような種ガキ（スパット）になる。筏式垂下養殖は、筏が潮の干満とともに上下するためカキはつねに水中にあり、餌(えさ)をとる時間が長く成長がよい。春にスパットのついた付着器（カキの成長にあわせてホタテガイの貝殻などの間隔をあける）を海に入れると、冬季には販売可能な大きさまで成長する。種ガキの主産地は宮城県で、生産地は三陸各地や広島県、有明海などである。種ガキは北アメリカへも輸出される。マガキの場合も、ヨーロッパやアメリカの食用ガキと同様冬季がもっとも肥えていて美味で、それらの国々でよくいわれるように「Rのつかない月」（5～8月）はやせているばかりでなく、気温が高くいたみやすいので生食(なましょく)用には向かない。生食用には清潔な環境で養殖された生きのよいものがよく、市場でも生食用（酢ガキ、生ガキ用）と加熱用（フライ、土手鍋(なべ)用など）と区別されている。カキは自分の体内に入った不要物を排出する自浄作用をもつ。この作用を利用し、殺菌した海水の流水中に一昼夜程度飼って体内にあった大腸菌やその他の有毒微生物を排出させたものが「無菌ガキ」という商品名で市場に出されている。

［奥谷喬司］

栄養

無機質ではとくに鉄分や亜鉛が多く、ビタミンAもやや多く、栄養的に優れた食品である。甘味があっておいしいが、これはグリコーゲンによるものである。コレステロールはやや多いが、食べすぎなければ心配するほどの量ではない。

［河野友美・大滝　緑］

料理

冬が甘味が増しておいしい。Rのつく月がよいとされるのも、この点も含まれていると思われる。鮮度が落ちやすいから新鮮なものを選ぶ必要があるが、新しいものは、貝柱の部分が半透明で、古くなるとこの部分が白濁する。貝（軟体部）全体も新しい間はこんもりとしているが、古くなるとだらりとして白っぽくなってくる。さらに鮮度が落ちると黄色みを帯びてくる。生食にするものは、かならず生食用と表示されているものを用い、期日までに食べ、すこし古くなったものは生食用でも火を通すことが必要である。殻付きのもので、完全に浄化した生食用のものもある。冷凍の生食用もある。カキの味は香りにあり、料理にはなるべく鮮度のよいものを用いる。むき身は、だいこんおろしに同量の水を加えたものの中で混ぜたのち、ざるにあけ、塩水で2～3回振り洗いするとぬめりや貝殻がきれいに落ちる。生食ではオイスターカクテルや酢ガキにする。カクテルは、氷でよく冷やした生ガキに、トマトケチャップ、レモン汁、おろしホースラディッシュ、塩、こしょう、白ワイン、タバスコなどをあわせたカクテルソースをかける。酢ガキは二杯酢やポンス（ぽん酢）を用いる。加熱調理では、フライ、コキール、土手鍋(なべ)、ベーコン焼き、汁物など幅広く使用できる。土手鍋は、鍋の周囲にみそを塗り付け、中にだしとカキ、ネギ、焼き豆腐、糸こんにゃくなどを入れ、まわりのみそを少しずつ溶かして煮ながら食べる。

［河野友美・大滝　緑］

[参照項目] | 広島牡蠣

マガキ（生態）

スミノエガキ

イタボガキ

『日本山海名産図会』にみるカキの養殖
広島湾は古くからカキの養殖が盛んであった。江戸時代はひび（篊）建て法による養殖が行われた。『日本山海名産図会』　巻3　「牡蠣」国立国会図書館所蔵">

『日本山海名産図会』にみるカキの養殖

土手鍋
広島県©広島県">

土手鍋

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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