變性現象 (sex change) 是指一個生物個體在其一生中能分別扮演雄性和雌性兩個角色。而一種會變性的生物其個體在它一生中可以透過變性來調控扮演雄性和雌性角色的時間以增加個體適性 (fitness)。因此變性的時機對一個個體的生殖力是相當重要的，所以為了得到最大的生殖力，變性的時機有可能會因應環境的差異而有所不同。在一個生物個體週圍的環境中可能蘊含能夠用來判斷何時變性較有利的訊息。找出這些訊息對我們瞭解變性現象的演化是相當重要的。為了瞭解生物面對不同環境時如何透過調控扮演兩性的時間以獲得最大適性，在本論文中將從個體的年齡、螺群的組成和珊瑚宿主三個不同方向來探討這些因子對紫口珊瑚螺 (Coralliophila violacea) 變性時機的影響。
在論文的第一部份，我嚐試建立判斷紫口珊瑚螺年齡的方法，並藉此估算每一隻螺的年齡。首先利用高馬氏 (Gompertz) 成長方程式估算紫口珊瑚螺的年齡，再將此估算的年齡與紫口珊瑚螺口蓋上的生長輪數相比，發現兩者為顯著的1:1關係。這結果顯示口蓋上的生長輪可用來當作年齡的指標，1輪代表1年。而不論是高馬氏成長方程式估算的年齡或生長輪的數目，都顯示紫口珊瑚螺變性的時間介於4-6歲間。此外，我還發現生長速率和個體大小有顯著的負相關，但變性中的個體成長最快。
紫口珊瑚螺會在微孔珊瑚 (Porites spp.) 上形成集聚 (patch)，因此在論文的第二部份，我探討了紫口珊瑚螺集聚內的組成對集聚內個體變性時間的影響。這些組成因子包括集聚內的性比、雄螺的隻數、雌螺的隻數和雌性的大小…等。在這些因子中只發現最大的雌螺體形大小和變性的時間有相關，集聚內最大的雌螺個體越大，集聚內個體變性的時間會越晚。
在論文的第三部份，我調查了在歧枝微孔珊瑚 (Porites nigrescens) 和團塊狀的圍塊微孔珊瑚 (P. lobata) 與鐘形微孔珊瑚 (P. lutea) 這兩類群體形態不同的宿主上的紫口珊瑚螺分布、個體大小及生殖特徵。主要的目的是探討這兩類群體形態不同的宿主對紫口珊瑚螺族群結構和生殖特徵的影響，特別是對變性時機可能的影響。
從調查的結果發現在分枝狀微孔珊瑚上的紫口珊瑚螺大部分單獨存在，而在團塊狀的微孔珊瑚上的紫口珊瑚螺會形成集聚。分枝狀微孔珊瑚上的紫口珊瑚螺個體較團塊狀的微孔珊瑚上的紫口珊瑚螺小，且分枝狀微孔珊瑚上的紫口珊瑚螺雌螺較小且總卵鞘面積較小。分枝狀微孔珊瑚上紫口珊瑚螺變性時間較早，除了受到集聚組成的影響外，珊瑚宿主本身也會影響到紫口珊瑚螺變性時間。

Sex-change (or sequential hermaphroditism) is a phenomenon whereby an organism functions first as one sex, and then as the other in the later stage of its life. Sex change is an adaptation that has evolved to allow certain organisms to increase fitness through resource allocation of male and female functions in a single lifespan. Since the timing of sex change in an organism is critical to its reproductive output, the size or age at sex change is expected to be plastic in response to different environmental conditions. Social and environmental conditions usually convey the cues about the relative advantages of functioning as one sex or the other. Identifying the factors that influence the timing at sex change is important to understand the evolution of sex change in the life history. In order to elucidate how to optimize fitness through sex allocation in response to different environment, I studied the timing at sex change of Coralliophila violacea in different approach, including the age of individual, the compositions of social groups and the difference between two types of coral hosts in this thesis.
Firstly, I present a method to determine the age of this snail and its age at sex change. The growth striae on the operculum were studied and compared to the age estimated by the Gompertz growth function based on growth data obtained from mark-recapture experiments. There is a significant correlation between the number of striae on the operculum and the age estimated from the Gompertz growth function, and the relationship is 1:1. These results suggest that the number of striae on the operculum can be used as an age index, with each stria representing 1 year of age. The age of sex change of this snail, according to my estimates by both stria number and aperture-length inferences, occurs between 4 and 6 years old. Growth rates of the snails are negatively correlated to size. Furthermore, individuals undergoing sex-change grew faster than males and females.
This snail, Coralliophila violacea, forms snail patches on the surface of the massive coral, Porites spp. Secondly, the social factors were analyzed for the timing of this sedentary snail. These factors include the sex ratio, number of male and females, and the size of females in the snail patches, all of which could be important information in determining the timing of sex change. In this study only the size of the largest female could be significantly correlated with the size at sex change in a snail patch.
Finally, I survey the distribution, size and reproductive characteristics of the snail, Coralliophila violacea (Lamarck), which inhabits the surface of both the branching coral Porites nigrescens and the massive corals, P. lobata and P. lutea. Based on these results I examined the host effect on (1) population structure and (2) reproductive characteristics, including the size at sex change of symbionts.
On branching hosts, most snails were solitary, whereas on massive hosts, most had formed multiple-snail patches. Significantly smaller snails as well as proportionally more females were found on branching than on massive hosts. Furthermore, the fecundity of the females on the branching hosts was significantly lower than that on the massive hosts. The size at sex change (male to female) of the snails was smaller on the branching hosts than on massive hosts. Patch composition differences can partly explain the smaller size at sex change for snails on branching hosts; however, there was also evidence that host morphology had a significant effect on the timing of sex change.

1. Chapter One .............................................................................................................. 1
General Introduction
1.1 Introduction ........................................................................................................ 1
1.2 Outline of thesis ................................................................................................. 6
2. Chapter Two ............................................................................................................. 8
Estimation of age in the sex-changing, coral-inhabiting snail Coralliophila violacea (Lamarck) from the growth striae on opercula and a mark-recapture experiment
2.1 Abstract .............................................................................................................. 8
2.2 Introduction ........................................................................................................ 9
2.3 Materials and methods ..................................................................................... 10
2.3.1 Collection .................................................................................................. 10
2.3.2 Striae on the operculum ............................................................................. 10
2.3.3 Length increment: mark-recapture method ............................................... 10
2.3.4 Size-at-age curve ....................................................................................... 11
2.4 Results .............................................................................................................. 11
2.4.1 Striae on the opercula ................................................................................ 11
2.4.2 Length increment: mark-recapture method ............................................... 11
2.4.3 Size-at-age curve ....................................................................................... 12
2.4.4 Relationship between the number of growth striae
and the estimated age ............................................................................... 12
2.5 Discussion ........................................................................................................ 13
3. Chapter Three ......................................................................................................... 22
Social factors influencing the timing of sex change in coral-inhabiting snail Coralliophila violacea
3.1 Abstract ............................................................................................................ 22
3.2 Introduction ...................................................................................................... 23
3.3 Materials and Methods ..................................................................................... 25
3.3.1 Specimen collection .................................................................................. 25
3.3.2 Size and sex ............................................................................................... 25
3.3.3 Data analysis ............................................................................................. 26
3.4 Results .............................................................................................................. 27
3.4.1 Sex ratio in snail patches ........................................................................... 27
3.4.2 Size composition in snail patches .............................................................. 28
3.4.3 Size, neighbor and size at sex change ....................................................... 28
3.5 Discussion ........................................................................................................ 30
4. Chapetr Four ........................................................................................................... 36
Host effect on size structure and timing of sex change in the coral-inhabiting snail Coralliophila violacea
4.1 Abstract ............................................................................................................ 36
4.2 Introduction ...................................................................................................... 37
4.3 Materials and Methods ..................................................................................... 38
4.3.1 Field survey and specimen collection ....................................................... 38
4.3.2 Size and sex ............................................................................................... 39
4.3.3 Fecundity ................................................................................................... 39
4.4 Result ................................................................................................................ 39
4.4.1 Distribution ................................................................................................ 40
4.4.2 Size and growth striae ............................................................................... 40
4.4.3 Reproductive characteristics ...................................................................... 40
4.4.3.1 Sex ratio .............................................................................................. 40
4.4.3.2 Fecundity ............................................................................................ 41
4.4.3.3 Size at sex change ............................................................................... 41
4.5 Discussion ........................................................................................................ 42
4.5.1 Size distribution ......................................................................................... 43
4.5.2 Fecundity ................................................................................................... 43
4.5.3 Size at sex change ...................................................................................... 43
5. Conclusion .............................................................................................................. 54
6. References .............................................................................................................. 56

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