台灣海域不同時空背景下草蝦的形態變異

Abstract

草蝦 (Penaeus monodon) 曾是台灣重要的水產養殖物種。 雖然草蝦在人工繁殖成功，但仍然仰賴天然所採的種蝦以進行養殖目的。 近年來賴以提供蝦苗的種蝦其養殖所需求之特性日益劣化，且病害時而發生，乃有再探索自然族群所潛在之基因多樣性之議。 本研究以宜蘭東北及東港西南水域之草蝦族群為對象，研析台灣附近海域草蝦之系群差異。 自2004年3月至2005年3月，共計取得321隻草蝦樣本 (雄性187隻，雌性134隻)。 將草蝦側面及背面設定13個標徵點 (landmark) 來檢視其外廓上的形狀差異。 在草蝦傳統形態測量分析方面，在雌雄別體重與頭胸甲長、全長與頭胸甲長關係，由分析結果顯示雌雄間並無顯著差異。 在草蝦幾何形態測量分析方面，結果顯示可將西南與東北水域冬季草蝦族群區分開來。 形態的變異可以反應出族群間的遺傳性 (genetic) 差異，也可反應出不同地理區間之環境 (environment) 差異。 西南四季草蝦形態形質分析方面，可以將西南族群區分為三群 (cohort)。第一群為西南冬季樣本，第二群為西南春季樣本，第三群為西南夏季和秋季樣本。 顯示出草蝦之季節群其特徵值會發生改變，而成為族群內之變異 (variation)。 此一變異造成群內之季節多型性 (polymorphism)，但由判別分析來看，其間之錯分率達到39.77~17.58，顯示族群內之多型性差異小於族群間之差異。 因此相對而言，將東北及西南的草蝦視為不同之系群 (stock) 應屬合理。 本研究發現草蝦族群內之體型大小具有雌雄二型性，即雌性個體大於雄性個體。 目前並無蝦類定年 (Age determination) 形質可供利用，因此無法得知雌雄之體型頻度差異究為雌雄異速成長所造成，或是其它選擇性因素造成差異性之死亡率。 但雌雄間之外部形質成長屬於等速 (isometry) 之關係則可預期。 因此前述之空間差異不受雌雄二型性所左右。

Giant tiger prawn (Penaeus monodon) was one of the important species for aquaculture in Taiwan. Although artificial propagation for giant tiger prawn has been developed successfully, the natural population of broodstock is essential for sustaining the genetic diversity of prawn in aquaculture. In recent years, the characters of broodstock are getting worse, and the cultured prawn infected diseases frequently. Therefore, studies on genetic diversity for natural populations of giant tiger prawn are cruicial for aquaculture. We collected the prawn samples from I-lan and Tungkung that were trawled from the Northeast and Southwest waters around Taiwan, respectively. A total of 321 prawn samples, including 187 male and 134 female, were obtained during March 2004 to March 2005. To explore the differences in morphology of prawn populations between two hydrographic areas, we selected thirteen landmarks from both lateral and dorsal side of prawns for further analysis. The results of classics morphometric analysis show no significant difference between sexes of giant tiger prawn in the relationship between body weight and carapace length and the relationship between total length and carapace length. The results of geometric morphometric analysis show significant difference between winter groups of Southwest and Northeast populations of giant tiger prawn. The variation s of morphology could be response to the genetic divergences between populations and/or environmental effects between different geographic distribution. The seasonal populations of giant tiger prawn in the Southwest Taiwan can be divided into three cohorts based on the morphometric analysis. They are winter cohort, spring cohort and summer-autumn cohort. This result suggests that the morphometric variations among seasonal cohorts of prawn could be detected and treated as the variations within population. These variations result in the seasonal polymorphism for intra-population of prawn. However, the misidentification rates are 39.77~17.58 based on the discriminant function analysis. This suggests that the polymorphism for intra-population is smaller than the polymorphism for inter-population. Thus, it is reasonable to suggest that the Northeast and Southwest populations of giant tiger prawn in Taiwan are two stocks. The results of this study show that the body size for populations of giant tiger prawn is sexual dimorphism, i.e. female larger than male. There is no suitable character can be applied to determine the age of giant tiger prawn till now. Therefore, it cannot be concluded that the difference in body size between sexes of giant tiger prawn is the result of allometric growth or selective mortalities caused by other factors. However, the isometric growth in morphology between male and female prawns is evident. Thus, the spatial difference of giant tiger prawns should not be affected by the condition of sexual dimorphism.