| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 101 === Reproductive characteristics of the blue-green damselfish (Chromis viridis, Cuvier 1830) in the water inlet of the 3rd Nuclear Power Plant at Kenting were studied. The fish spawns year-round in the study area. Observations of its reproductive behaviors were conducted using SCUBA monthly from January 2011 to July 2012, and between November 2011 and July 2012 we conducted underwater video recordings in a total of 7 days at specific spawning sites.
To examine changes in actions of the territorial males in various spawning phases, i.e., nest-preparation, courtship and egg-caring, we analyzed the content of video recordings taken on April 7~13 and July 10~12, 2012. Firstly, actions of the territorial male were divided into 9 patterns: skimming(SK), hovering(H), signal jumping(SJ), circling(C), aggression(A), leaving(L), nipping(N), interfering(I) and fleeing(F). Then the occurring sequences of these actions were recorded.
During nest preparation, territorial males cleaned up the substratum, making the nest ready for the forthcoming courtship. In this phase, their actions followed specific trends (i.e., SK → H → N). During courtship and spawning phase, the sequence of (SJ → C) prevailed. However, the sequence of (C → I) was also noticeable, indicating that spawning could be frequently interrupted by the interference of other males and or females.
During eggs-caring phase, territorial males spent most of their time feeding in water column (for 90.02% of the actions taken). The remaining 9.08% was attributed to the egg-caring motion (i.e., SK + H). After a 3-day spawning session, eggs hatched; the territorial male left the nest and back to the feeding school.
When the male was preparing the nest, the female would stay and feed in the water column nearby. When courtship began, females would aggregate above the spawning area and the female who accepted the male’s courtship would follow the male back to and spawn on his nest.
In addition to the above spawning pattern, in which the spawning was led by the courtship of the territorial male, an alternative spawning pattern, so called ‘group spawning’, also occurred in the blue-green damselfish. Group spawning had happen in the high peak of courtship period. In this moment, without any sign of courtship invitation from the nesting male, some un-territorial males would join the females in approaching the territorial males’ nest, where they together rushed and subsequently spawned on the substratum normally defended by the nesting males. After a swift spawning session, most for less than one minute, the intruding group would leave and return to their previous school. Group spawning had occurred repeatedly on the same substratum. The male in the group was significantly smaller than the nesting males in the body size (fork length). By contrast, in the same intruding group there was no body-size difference between the male and the female.
After the intrusion, the territorial male would stay in the nest and proceed its spawning. Comparisons of the behaviors between territorial males with or without the intrusion of group spawners showed that the fish experienced intrusion had spent less time in the nest. Meanwhile both their courting activity (12.73% vs. 29.17%) and their success rate in courtship (37% vs.53%) had declined. Why did the intruded males stay and keep defending their nests? In the group spawning, the male could only fertilize a part of the eggs spawned by the companying female. This has left with an opportunity for the nest owner to fertilize and adopt the remaining eggs. This might answer the question asked. Group spawning is unusual in the damselfish spawning. For the blue-green damselfish, in the study area there were so many mature females; for those who do not have the opportunity of being courted, have to adopt an alternative strategy to insure individual’s fitness. Invading to suitable nests and spawning on them could be one that was adopted by the female. On the other hand, due to the limitation of suitable nesting substrate, those males who could not take its possession of nesting substrate might just join the female, sneak to the nests, and fertilize eggs.
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