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127. Peacock Flounder (Bothus mancus) infested with Leeches


Infested with Leeches. A Peacock Flounder (Bothus mancus) has raised eye stumps and gill slits infested with leeches. From the underwater images collected of this flounder it was impossible to name or identify the species of leeches in this underwater videoclip.

Most of the parasites encountered on fish by scuba divers belong to the subphylum of crustaceans, such as isopods and copepods. However, most reef fish suffer from internal parasites that are invisible to scuba divers. Some fish species can host up to 30 species of parasites.

Most Parasites do not kill their hosts as it would not be advantageous for the parasite to end up with a dead host. If a host-specific parasite kills its host, it may lead to the extinction of this host species and that specific parasite. However, there are exceptions to this rule.

Although many fish are plagued with marine leeches, they are not often encountered by scuba divers as many of these leeches are quite small. However, copepods are often present in great numbers, and isopods are usually too large to miss when attached to a fish.

Marine leeches are slightly flattened worm-like parasites that make a series of small cuts on the skin of their victim to suck and ingest their blood with a suction-cup-like mouth. During the process, they inject the host with their saliva, which contains an anaesthetic and an anticoagulant. The anticoagulant ensures that the blood keeps flowing by interrupting the process involved in the formation of blood clots, while the anaesthetic ensures that the whole process happens painlessly.

Leeches that feed on fish are only temporary visitors and leave their hosts after one or only a few meals. Some species of leeches feed as often as once a week, while others are satisfied with only one meal every six months or so. After feeding, they may retreat to sea grass beds, attach themselves to rocks, and some even take shelter by attaching themselves to crustaceans. They only leave to find a new potential victim to feed on or to reproduce and lay eggs. Marine leeches are poorly studied, and it is believed that most leeches are not host-specific.

Some leeches can carry pathogens from one fish to another and may spread diseases, just like mosquitoes or ticks do on land.


Making a close-up shot of a fish is always a bit tricky. Every species has its “flight distance”, which is the distance that you can approach an animal without triggering an escape response.

Always move slowly with your camera, avoid jerky movements, and show that you’re not a threat, otherwise, the fish may rapidly swim away and ruin your clip. When filming macro subjects underwater, it is extremely important to keep your camera very stable. The slightest movement of the underwater camera and housing will result in extreme movements in the footage.

Filming on or near the bottom of the seafloor makes it easier, as one can simply place their underwater camera on the substrate and by doing so create a stable base for their filming equipment.

More on this topic:

For another post about parisites (sea lice/copepods) please go to our vlog post 130 or click on this link: https://www.beyondscuba.com/post/plagued-with-sea-lice

For a more in-depth description about flight distance please go to our vlog post 70 or click on this link: https://www.beyondscuba.com/post/flight-distance

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