Sarupa - Sarupa

A general term for marine planktonic animals belonging to the phylum Protochordata, class Urichorta, family Salpacidae. The body is cylindrical or barrel-shaped. The inlet is at the anterior end of the body, and the outlet is at the posterior or posterodorsal end. The body muscles that surround the body in a ring shape generally terminate at the ventral end, but in some species they are attached in a complex manner. The pharyngeal cavity is a cone-shaped space formed by the peripharyngeal ring that surrounds the anterior end of the pharynx that is connected to the inlet, the long endostyle that runs vertically down the ventral midline, and the slender, rod-like gill bars that extend from the anterior-dorsal midline of the pharynx to the posterior-ventral midline, and the gill slits are large holes surrounded by these three on each side. The cavity located at the posterior and posterior end of the pharyngeal cavity, separated by the gill bars, is the peribranchial cavity (cloaca), which connects to the outside world via the outlet. The esophagus opens at the posterior ventral end of the pharyngeal cavity. The digestive tract (esophagus, stomach, intestines) becomes a small mass called the nucleus, which opens into the peribranchial cavity. When alive, some species have a brightly colored nucleus, such as reddish brown, floating in the translucent body, which resembles a lit lantern, and so they are also called "lantern leeks." They move forward and backward by periodically contracting their body muscles, and as they do so, they feed by capturing small pieces of organic matter that flow into the pharyngeal cavity with a mucus net stretched across their gill slits.

Salps are famous as the first animals in which the phenomenon of alternation of generations was discovered. In some species, solitary individuals (asexual generation) that can reach a total length of 20 cm and linked individuals (sexual generation) that are much smaller and generally asymmetrical regularly alternate, but because the morphology of the two is quite different even within the same species, they were once given different species names. Linked populations in which young individuals are arranged in two rows or rings are generated one after another by segmentation of the ventral buds of solitary individuals. Each linked individual has a hermaphrodite gonad and maintains a floating chain by synchronous behavior, but later separates and becomes independent. An embryo (usually one, but several in some species) develops directly into the solitary individual while still attached to the inner wall of the parent zoon, and breaks through the parent's body wall to emerge. The embryo does not have a notochord.

They appear in a wide range of oceans, mainly in tropical and subtropical oceans, including the Antarctic region, and sometimes form large colonies. They are preyed upon by large migratory fish, various invertebrates, and birds. Some species are luminous. To date, 2 subfamilies, 13 genera, and 45 species have been identified, more than half of which live in the waters near Japan. In addition to the salps mentioned here, the taxonomic group that includes sea anemones and luminous ascidians is also called Thaliacea.

[Teruaki Nishikawa]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

原索動物門尾索綱サルパ科に属する海産浮遊動物の総称。体は円柱状または樽(たる)形。入水孔は体前端に、出水孔は後端あるいは後背端に開く。体を環状に取り巻く体筋は一般に腹方で断絶し、種類によっては複雑に接着する。入水孔に続く咽頭(いんとう)の前端部を環状に囲む囲咽帯と、腹正中を縦走する長い内柱、および咽頭の前背正中部から後腹正中部に伸びる細長い棒状の鰓桿(さいかん)gill barとがつくる円錐(えんすい)形の空間が咽頭腔(こう)で、左右各側でこれら三者に囲まれた一つの大きな穴が鰓裂である。鰓桿を隔てて、咽頭腔の後背方に位置する腔所が囲鰓腔（排出腔）で、出水孔を経て外界と連絡する。食道は咽頭腔の後腹端に開口する。消化管（食道、胃、腸）は体核nucleusとよばれる小塊となり、囲鰓腔に肛門(こうもん)を開く。生時、種類によっては、半透明の虫体のなかに赤褐色など鮮やかな色彩の体核が浮かぶさまを、火をともした提灯(ちょうちん)に見立てて、「ちょうちんにら」ともよぶ。体筋を規則的に収縮させて前進・後退し、このとき咽頭腔に流入した有機物細片を、鰓裂にかけ渡した粘液のネットでとらえて摂餌(せつじ)する。

　サルパは、世代交代現象が最初に発見された動物として名高い。種によっては全長20センチメートルにも達する単独個体（無性世代）と、それよりはるかに小形で一般に左右非相称の連鎖個体（有性世代）とが規則正しく交代するが、両者の形態が同一種でもかなり異なるため、かつてはそれぞれに別の種名があてられたこともある。単独個体の腹面にある芽茎の分節により、幼個体が2列あるいは輪状に配列した連鎖個体群が次々に生ずる。各連鎖個体は雌雄同体の生殖腺(せん)をもち、同調的な行動で浮遊群体chainを維持するが、のちに分離独立する。胚(はい)（通常1個、種によっては数個）は、親個虫の体内壁に付着したまま単独個体に直接発生し、親の体壁を破って外に出る。胚は脊索(せきさく)をもたない。

　熱帯、亜熱帯の大洋を中心に南極地方を含む広い海域に出現し、ときに大群集をなす。大形回遊魚をはじめ種々の無脊椎動物や鳥類に捕食される。発光する種がある。これまでに2亜科13属45種が知られ、この半数以上が日本近海にも生息する。なお、ここで述べたサルパのほか、ウミタル、ヒカリボヤを含む分類群をもサルパ類Thaliaceaとよぶ。

［西川輝昭］

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