The Asian sea bass, the barramundi, is a favourite food source whose popularity is expanding beyond Asia. They are farmed commercially, used for recreational fishing and, when small, are regarded as an attractive aquarium fish. In addition to their commercial value they are a favourite food of the salt water crocodile.
A monogenean parasite of the Asian sea bass
Most of the group of flatworm parasites known as theMonogenea是居住在鱼表面的小透明外寄生虫。他们的前端有一个前吸盘和一个器官，用来将它们固定在宿主身上。
Some of these attachment organs consist of elaborate arrangements of hooks and/or clamps that stop them being swept away by water currents.
单基因人的生命周期只有一个宿主，并产生了大量卵，这些卵对环境挑战有弹性。一种纤毛的幼虫形式，从鸡蛋中孵化了comiracidium，并寻找新宿主。
Neobenedeniaspp. are monogeneans that live on the surface of barramundi. They belong to a group of monogeneans that are highly virulent, causing damage to the epidermis of the fish on which they graze.
One would presume that searching for parasites that live on the outer body surface of their hosts (ectoparasites) would be easy, but not so if they are transparent.
Researchers from James Cook University, Queensland, Australia have最近出版a study ofNeobenedeniaspp .
They were interested in initial recruitment to the host and migration patterns of the post-larvae, juveniles and adults to see whether they aggregated in particular microhabitats at particular times. The problem of tracking these transparent parasites was solved by labelling them with a fluorescent dye.
A study ofNeobenedenia
Barramundi from a fish hatchery were acclimated to salt water before exposing them to newly hatched oncomiracidia of the monogeneanNeobenedenia,which had previously been held for 15 mins in seawater containing a fluorescent dye to make them visible under lighting of an appropriate wavelength.
Fifty fish were kept in individual tanks and each exposed to 50 oncomiracidium. The surface of the fish was then examined for the presence of parasites under an epifluorescence stereomicroscope at 10 time intervals between 15 mins and 16 days. The position of each parasite was mapped and the fish photographed.
Observation of infections
荧光标记使寄生虫也可以轻松检测到，即使它们在鱼尺度下被放置。标签持续了整个实验的整个16天，尽管它确实有些褪色。
观察到一些寄生虫在鱼的表面上移动。他们通过触发手段固定，从这个固定点开始，他们的前端在身体长度上敲打着他们的前端。
在不同的时间间隔计算了感染百分比成功。一些oncomiracidia在20分钟内附着在宿主上，这一数字升至每个水箱中引入数量的93％的峰值。
在同时，分析了间隔的寄生虫分布，以确定是否选择在鱼类上定位于微生境。
Fish microhabitats
Initially distribution was random; suggesting attachment may occur at whatever location on the fish’s body the oncomiracidium first arrives.
The authors observe that some parasites attached under scales. This has rarely been observed for monogeneans. The fluorescent marker assisted the detection of this behaviour, which may have evolved as a protection from cleaner organisms such as fish and crustaceans that predate on ectoparasites of fish.
Aggregation began to occur after 48 hours as the post-larvae migrated to specific microhabitats. Although they showed no preference for a single microhabitat, aggregation was particularly prominent on the fins by day 8. Concentrations also occurred on the eyes, operculum and dorsal and ventral body surface. By day 12 parasites were randomly distributed again, but were more concentrated on the fins, head and ventral surfaces.
Microhabitat选择的可能优势
One would suspect that parasites that collect in a particular sites would experience competition for grazing sites, so why aggregate?
Neobenedenia在聚集期间，在感染后第9天开始产卵。尽管这些单基因是雌雄同体，而产生可行的鸡蛋并不是必需的，但可能证明是一个优势。作者建议，寄生虫在卵子上卵之前可能会提供更多的交配机会。然后，寄生虫可以分散以觅食或寻找有利的卵子产卵地点。
他们还指出，隐藏在鱼类尺度下的新优势可能是，这些异端矿物可能受到保护，以免在水产养殖系统中消除寄生虫的尝试。可行的是，这种行为将在Barramundi的广泛耕种施加的选择压力下增加。
注释