海馬的卵到底是體內受精還是體外受精？

 

海馬既非體內受精也不是體外受精；

受精卵的發育養分來源除了卵本身，雄海馬也提供了一部分。

然而，在我的課堂上，海馬就是體內受精的動物。

本次學校段考出現下列題目：

下列有關人類試管嬰兒與海馬生殖方式的敘述，何者正確？ (A)皆為體外受精　(B)都具有臍帶　(C)前者由母親血液，後者由父親血液供應養分　(D)皆能分泌乳汁，餵哺幼兒

 參考答案A

由於上課時告訴學生海馬是行體內受精的動物，且從閱讀經驗中得知 C 比較接近事實；但此觀念並未出現在教材中，因此主張本題送分。惟與其他生物老師討論的過程中，聽到了一種奇特的定義，指體內受精是透過交配的行為使精子和卵在雌性個體內完成受精作用 ( 這是課本裡的文字，不過上課時特別要學生把雌性二字刪除，因為，不論受精處在雌體內或雄體內都不會影響它們皆有的專屬體內受精在演化上的意義，即受精機率高、配子數量少的生存策略。學習生物課程時永遠要記住一句話：Nothing in Biology Makes Sense Except in the Light of Evolution. )。由於海馬的精子與卵是在雄性個體內受精，因與定義不符，所以不是體內受精！

……

除了瞠目結舌，我不知道還有什麼文字可以表達我當下的心境。怎麼會這樣？雄性的體內就不叫體內？怎麼會這樣？

 

下列海馬繁殖的短片是由Discovery頻道節錄出來的訊息，實在無法理解這個過程為何會被認為是體外受精。

網路上可找到一些相關論文： http://sciencelinks.jp/j-east/article/200106/000020010600A0771866.php

Male members of the seaweed pipefish, Syngnathus schlegeli, incubate eggs in the brood pouch located on the tail. The eggs are directly spawned into the brood pouch by inverted copulation after intensive courtship, and the fertilization takes place in the brood pouch.During gestation, the brood pouch is filled with viscous fluid that seems to be of maternal origin, indicated by the absence of mucous secretion of the brood pouch epithelium. The spermatozoa are considered to swim in viscous ovarian fluid during fertilization. These findings indicate that the environment for fertilization is equivalent to that for internal fertilization. The pipefish spermatozoa had a bullet-shaped nucleus (3*0.6.MU.m), a spiral mitochondrion and an elongate flagellum (ca. 85.MU.m) with centrioles embedded in deep basal fossa. Based on the morphological features, the pipefish spermatozoon may be categorized in introsperm (internally fertilizing sperm). The spermatozoa swim straight by beating the entire length of theflagellum (84.1.+-.43.8.MU.m/sec, .+-.SD, n=3). The number of spermatozoa in the testis was extremely small (1-2 nuclei per whole transverse sectional area). The mode of fertilization is considered to enable the reduction of the spermatozoan density without deteriorating the success of fertilization. Apart from the typical spermatozoa, another type of spermatozoa with the head about 3 times as large as that of typical spermatozoa was observed. The atypical spermatozoa swim in circles (45 turn/min). Possible natures of the atypical spermatozoa are discussed. (author abst.)

作者研究的是與海馬同屬雄性懷孕的海龍，文中明確指出海龍的精卵在育兒袋中受精，並且說明這種受精模式能在降低精子密度(即單位體積內精子的數量)的狀況下無損其受精成功率。這項推論的意義在於，相較於體外受精，行體內受精的動物無須製造大量配子即可確保生殖的有效性。再加上海龍(與海馬)的受精卵直接在親代體內發育，意即親代擁有完善的護卵行為，這項行為更強化了生殖的成功率。就是因為海龍(與海馬)一方面行體內受精，又有極佳的護卵行為，所以這類動物的配子才會少的驚人。在下面那篇海馬的文章中顯示，海馬的睪丸中，平均只有304(±77)隻成熟的精子！更神奇的是，在那些沒有表現交配行為的海馬睪丸內，竟然沒有發現任何精子！物質與能量的使用效率簡直神乎其技！

底下這篇的主角就是海馬了。http://seahorse.fisheries.ubc.ca/Documents/Journals/2007/vanlooketal2007.pdf

Uniquely among vertebrates, seahorses and pipefishes (Family Syngnathidae) incubate their eggs within a male brood pouch. This has contributed to a widespread, but poorly founded belief, that the eggs are fertilised using spermatozoa that are deposited directly into the brood pouch via an internal sperm duct. Anatomical dissections showed, however, not only that direct sperm deposition into the pouch is physically impossible, but that spermatozoa must somehow travel a significant distance (>4·mm) outside the body of the male, to reach and fertilise eggs in the pouch. [從解剖組織上顯示，精子不但不會直接射入育兒袋，還必須在雄海馬體外移動一段對精子(約4微米)來說相當長的距離(約4毫米，即1000倍精子體長的距離)……我想就是這段話讓其他生物老師認為海馬行體外受精，但是這句英文還有後話……以便抵達並且在育兒袋內與卵受精。為何作者會這麼說？那是因為精子在海馬體外停留的時間只有6秒鐘，在這6秒內精子必須拼命向著育兒袋的方向衝刺，因為6秒鐘以後育兒袋的開口就封起來了，而精子也將被海水沖走。] Observations of ourtship and mating behaviour also revealed that the pouch closes immediately after mating, and that sperm transfer must occur within a time window of no more than 6·s. In addition to this, the yellow seahorse produces extraordinarily low quantities of dimorphic spermatozoa, but is nevertheless highly fertile and can produce broods that exceed 100 embryos. The entire fertilisation process in seahorses is therefore uniquely efficient among vertebrates, yet paradoxically involves several steps that would seem to complicate, and even appear to prevent, the interaction of the gametes. Although we are still unable to describe the exact fertilisation mechanism, we speculate that spermatozoa are ejaculated into a mixture of ovarian fluid and eggs, while the male and female are in close contact. Thereafter, this mixture must enter the pouch, whereupon the spermatozoa encounter seawater. These observations also support the view, indirectly inferred in previous publications, that sperm competition in seahorses is not only non-existent but impossible.

論文中有一段話有趣極了，請看。

During mating, the brood pouch remained open for only 6·s while egg deposition occurred. During this time, seawater entered the pouch,…... For this reason, the environment inside the pouch cannot be regarded as truly internal because it resembles seawater. [在海馬交配期間，雄海馬的育兒袋開口只會打開6秒鐘，而雌海馬就在此時排卵，同時海水也會湧入育兒袋。就因為如此，育兒袋內部環境不能被認為是體內，因為此時的育兒袋內部環境與外在海水環境雷同。這段話也許又是海馬行體外受精的論點，但我還是認為這在胡扯！你原本在餐廳裡用餐，突然從外面來了一堆新客人並且灌進大量外面的空氣，於是你變成在戶外用餐？後來有人提出異議：有個人在口袋裡放十塊錢，這十塊錢是在體內嗎？這句話的隱喻有個缺點，衣物與人體的來源完全不同，衣物永遠不會變成人體的一部份，但育兒袋可是海馬體表發育出來的一個袋子，口袋與育兒袋，二者完全不能類比；衣物永遠是體外，但育兒袋既是身體的一部份，與環境有所區隔之後，當然就是體內。以人為例，人的胚胎發育過程中，原腸胚的表面細胞向胚胎內部凹陷，最後穿過胚胎在另一端開口，這個後來出現的開口會成為人的口部，原先的凹陷處則成為肛門，兩個開口間的(內胚層)部分，則發育為消化道。換言之，消化道原本也是胚胎表面發育而來的構造，你不會認為自己吃的東西都吃到體外去吧。]Our present observations of sperm activation and motility in seawater and pouch fluid show that physiologically ‘external’ fertilisation probably occurs within a physically ‘internal’ environment, possibly after closure of the pouch.

後記：

多年之後，還有人要我相信海馬是體外受精的動物，這到底是怎麼一回事？

 
如果海馬是體外受精，請問如何解釋雌雄海馬交配的事實？交配行為不是體內受精的特徵嗎？

如果海馬是體外受精，請問如何解釋雌雄海馬的精子與卵是在育兒袋(孵卵腔)內受精的事實？將精子與卵侷限在一個相對小的空間以提升受精機率不是體內受精的特徵嗎？

如果海馬是體外受精，請問如何解釋雄海馬的精子數量是如此稀少的事實？有哪一種行體外受精的水生動物能以如此稀少的精子數來確保種族的繁衍？
 
只要承認海馬是體內受精的動物，就不用(像地心說般)做多餘的解釋了，不是嗎？


一點感觸：

一個完成研究所學位訓練的人，遇到與自己專業相關的問題時，難道沒有能力自己去判讀解答？對於那些所謂的博士、學者、教授所說的話，就應該代表權威？不可挑戰？至少我的老師不曾教過我這樣的觀念，我也不曾這樣告訴我的學生。

20161220

海馬居然能登上Nature雜誌(20161215出版)的封面，真是了不起的動物。

然後，請看海馬的眾多特出之處，還有，海馬為何會如此不像魚？它是怎麼辦到的？所以，別再理會體內受精或是體外受精了。