Observing natural behaviours whilst underwater is one of the main reasons that I love diving so much. Whilst staying at Kasawari Resort in Lembeh Strait, Indonesia for a week in January I came across something very special and unique.
At first glance it looked like an ordinary anemonefish dancing in and out of the stinging tentacles, but as it turned towards me, I noticed an extra pair of tiny eyes checking me out from inside its mouth. They were the eyes of a very special and specialised parasite indeed.
The pin-sized beady eyes belonged to a tongue-biter cymathoid isopod parasite (Cymothoa c.f exigua). This sinister relative of the harmless woodlouse (I think they’re known as pill bugs in the US) enters the fish’s mouth as a tiny juvenile through the gills and attaches to the tongue, which eventually dies off, and the parasite grows in its place. Whilst it must be quite uncomfortable, the parasite, by definition, doesn’t kill the fish, although it sucks its blood whilst sitting motionless in the mouth.
The large cymathoid isopod visible through the mouth is in fact the female of the species, and her mate is smaller and located on the
inside of the gill arches, out of view.
Whilst in Lembeh I saw these parasites on two occasions. The first was in the mouth of a saddleback anemonefish (Amphiprion polymnus) and the second in a male spinecheek anemonefish (Premnas biaculeatus). The spinecheek was a small individual, and the size of the parasite prevented the fish from closing its mouth fully!
Keep checking back to my blog over the coming weeks and months, as I plan to make a little blog series of the behavioural observations that I’ve been lucky enough to witness. Also, feel free to sign up to my Facebook page for updates of when my blogs are posted.