Larval duration and growth in two species of damselfish from the Great Barrier Reef

Light traps were used to sample presettlement larval fishes simultaneously at Lizard Island and Davies Reef, located 500 km apart on the Great Barrier Reef (GBR). At Lizard Island, three traps were moored on both the windward and leeward side, whilst at Davies Reef two traps were placed off the back reef in open water (approximately 25 m deep). On both reefs, traps were moored 200 m apart, and placed at least 100 m from the nearest reef to ensure that fish were not attracted from the reef substratum. The light traps were deployed during the nights of 19 to 23 December, 1987. All traps at both localities were set to fish for 3 hours each night (21.00 to 22.00 hrs, 12.00 to 01.00 hrs, and 03.00 to 04.00 hrs), to minimise the influence of tide on catches. Traps were cleared daily and samples were returned to the laboratory. Fish that could be identified to species immediately, were preserved in 99% ethanol. Those that had not developed sufficient pigmentation to allow identification were placed in aquaria with live coral, to initiate the development of pigmentation, which usually occurred within 12 hours. Fish were then identified and preserved. The dominant species from these samples, Chromis atripectoralis and Pomacentrus coelestis, were chosen for further study. Post-settlement individuals of these two species were collected using the fish anaesthetic quinaldine and small hand-nets. Fish of both species were collected from Lizard Island on 6-7 January 1988. Chromis atripectoralis was collected from Davies Reef on 17 January 1988, and Pomacentrus coelestis from Lodestone Reef on 20 January 1988. Fifty pre-settlement and 40 post-settlement individuals of both species, from each locality, were used for otolith examination. Otoliths were teased out the brain tissue with tungsten needles, placed on a microscope slide and dehydrated by baking at 60°C for 6 hours in an oven. The dried otoliths were mounted in Euparal, covered with a cover-slip, and left for at least one month before reading, as growth increments were noticeably clearer after this time. Analysis was confined to the lapilli of both species, as growth increments were much clearer than on the sagittae. Otoliths were viewed under a compound microscope at 250 or 400 x magnification using polarized transmitted light. A high-resolution video-camera was mounted on the microscope, which was in turn connected to a video-screen on which increments were counted. The system was also interfaced with a personal computer for measurement of otolith diameter.Live fish collected from light traps at Lizard Island were used to validate the daily periodicity of growth increments after settlement. Fish were immersed in a 0.25 g/l solution of oxytetracycline overnight, and then kept in aquaria with live coral for 12 days and 11 nights under ambient light and temperature conditions. Lapilli were dissected out of ten fish of each species after this period, and viewed under fluorescent UV light. The number of increments between the fluorescent band (corresponding to the time of tetracycline treatment) and the edge of the lapillus was then enumerated. Daily periodicity of increment formation was confirmed for both species during this time. It was not possible to validate the periodicity of increments before capture, and it was assumed that they too were daily. This research was undertaken to:1. determine if length of larval life in Chromis atripectoralis and Pomacentrus coelestis varies intra- or inter-specifically within and/or between localities on the GBR and to determine if size at settlement varies similarly2. determine if otolith dimensions are related to size in these species and if these relationships are consistent between pre- and post-settlement fishes, and between localities3. determine if any of the above characteristics are related to pre- and/or post-settlement growth rates at these localities

本研究采用灯光诱捕法，同时在大堡礁（Great Barrier Reef, GBR）上相距500公里的蜥蜴岛（Lizard Island）与戴维斯礁（Davies Reef）采集定居前仔鱼样本。在蜥蜴岛，研究人员于向风侧与背风侧各布设3台诱捕器；而在戴维斯礁，则于离岸礁后的开阔水域（水深约25米）放置2台诱捕器。两处礁体的诱捕器均以200米间距锚定布设，且与最近的礁体至少相距100米，以避免鱼类从礁体基底被诱集而来。

灯光诱捕作业于1987年12月19日至23日夜间开展。两处采样点的所有诱捕器每晚布设3小时采样（时段分别为21:00-22:00、12:00-01:00与03:00-04:00），以尽可能降低潮汐对渔获量的影响。每日清理诱捕器并将采集的样本带回实验室。可直接鉴定至物种的仔鱼，将以99%乙醇固定保存；而色素沉着不足无法完成鉴定的个体，则被放置于带有活珊瑚的水族箱中，以诱导色素沉着——该过程通常可在12小时内完成。后续完成鉴定后再次固定保存。

本研究选取样本中的优势物种——黑胸光鳃鱼（Chromis atripectoralis）与蓝雀鲷（Pomacentrus coelestis）开展后续研究。研究人员使用鱼类麻醉剂喹那定（quinaldine）与小型手抄网，采集这两个物种的定居后个体。1988年1月6日至7日，于蜥蜴岛采集两个物种的定居后个体；1988年1月17日于戴维斯礁采集黑胸光鳃鱼，1988年1月20日于磁石礁（Lodestone Reef）采集蓝雀鲷。

每个采样点的两个物种均选取50尾定居前个体与40尾定居后个体，用于耳石检测分析。使用钨针从脑组织中分离耳石，置于载玻片上，随后于60℃烘箱中烘烤6小时进行脱水处理。将干燥后的耳石用盖玻片封固于优帕腊胶（Euparal）中，静置至少1个月后再进行观测——此时耳石的生长增量会更为清晰。本研究仅针对两个物种的微耳石（lapillus）开展分析，因其生长增量相较于矢耳石（sagitta）更为清晰。

使用偏光透射光，在250倍或400倍放大倍率的复合显微镜下观测耳石。显微镜上搭载高分辨率摄像机，连接至视频显示屏以计数生长增量；该系统同时与个人计算机相连，用于测量耳石直径。

研究人员以从蜥蜴岛灯光诱捕采集的活体仔鱼，验证定居后生长增量的日周期性。将仔鱼浸泡于0.25g/L的土霉素（oxytetracycline）溶液中过夜，随后置于带有活珊瑚的水族箱中，在自然光照与温度条件下饲养12天11夜。饲养结束后，从每个物种的10尾仔鱼体内分离微耳石，在荧光紫外光下观测；随后计数对应土霉素处理时段的荧光标记带与微耳石边缘之间的增量数量。本次实验证实，两个物种的增量形成均具有日周期性。但无法验证捕获前增量的周期性，因此默认其同样为日周期性。

本研究旨在达成以下目标：
1. 探究大堡礁不同采样点内及采样点间，黑胸光鳃鱼与蓝雀鲷的仔鱼期时长是否存在种内与种间差异，并明确其定居时的体长是否存在类似变异；
2. 分析这两个物种的耳石尺寸与体长的相关性，以及该相关性在定居前、定居后个体间与不同采样点间是否保持一致；
3. 明确上述特征是否与各采样点的定居前及/或定居后生长速率相关。