藤壺是所有生物中陰莖長度比例最大的，通常可達體長的數倍。為何會需要如此長的陰莖，這當然是因為藤壺本身不會移動，需要靠陰莖來接近配偶。可想見，陰莖愈長配偶數可能愈多，但另一方面在陰莖上的投資也愈高，藤壺是如何在配偶數和陰莖投資上得到平衡的？是不是有環境因子的影響？本研究探討個體陰莖長度是否可能因密度而調整，藉此來解釋種內陰莖長度的高度變異。研究對象紋藤壺（Amphibalanus amphitrite)、黑潮笠藤壺（Tetraclita kuroshioensis）及中華小笠藤壺(Tetraclitella chinensis)來自台南七股、東北角、墾丁和澎湖，首先測量每隻最近的鄰居距離(NND, Nearest Neighbor Distance)及形態指標（MI, Morphological Index）作為密度的指標，採集後測量第三與第六蔓足內肢長度作為個體大小的指標，並與藤壺陰莖長度做迴歸分析；另一方面，利用程式模擬藤壺在不同個體密度下及花費下的最適陰莖長度。實測結果發現，陰莖長度除了與體長有正相關之外，與MI也有正相關，也就是密度高時，陰莖也較長。程式模擬顯示增加陰莖長度所能增加的配偶數與所處密度無關，但顯示在個體間沒有精子競爭情況下，增加投資在陰莖長度以獲得更多的配偶數是有好處的；在有精子競爭情況下，陰莖長度越長對於個體而言是沒有好處的。此外，將不同的鄰居數當做處理組的實驗也發現，陰莖隨著附近個體數的增加而增長。在高密度下藤壺受擠壓，使得外型呈現底盤較小的圓柱狀；此時因空間限制(底盤較小)孵卵數目，減少在雌性功能的投資，使得雌雄同體的藤壺有多出的能量，可用來投資在雄性功\\能上，由於雄性個體間精子競爭機會不大，因此利用增加陰莖長度來增加可能配偶數，成為有效投資的唯一選項。

Barnacles have the longest penis length, relative to body length, among all organisms. It is common for their penis length to be several times that of their bodies’. One reason is that barnacles are immobile and they rely on penis to reach mates. Thus the longer the penis the more mates there may be. How do barnacles balance mate numbers and penis cost? In this investigation, we studied the possible relationship between local densities and penis lengths of barnacles. Our goal is to explain the high intraspecific variation in penis lengths of barnacles. First, we measured the Nearest Neighbor Distance(NND) and Morphological Index (MI), both indices of local barnacle densities, of individuals of Amphibalanus amphitrite at Tainan, Tetraclita kuroshioensis at Keelung and Kenting, and Tetraclitella chinensis at Penghu. The cirrus lengths were used as indices of body sizes. A high percentage of penis length variation could not be explained by body sizes in these barnacles. In A. amphitrite and T. kuroshioensis, a positive correlation between residue penis length, i.e., after deducting the effect of body size, and MI was found. Assuming no sperm competition, we found, through simulation, that the optimal penis length, is not related to local densities, whereas in the presence of sperm competition, the longer penis length the lower fitness.In lab experiment, however, penis length of A. amphitrite was found to increase with increasing number of neighbors.Under high densities, the shell base become relatively small (high MI) and the shell becomes elongated. Investment in eggs may be limited by space available for brooding, thus more energy is available to invest in male functions. Under the assumption of no sperm competition, penis length remains the only option for effective investment. The preliminary result here suggests that the penis length of barnacles is plastic and is influenced by the local density. In Tetraclitella chinensis, no correlation between local densities and penis lengths was found; the very flat shell morphology of the species may have contributed to this phenomenon.

壹、前言………………………1
貳、材料與方法………………5
參、結果………………………11
肆、討論………………………14
參考文獻………………………18
附錄……………………………45

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