The next fish I want to feature in my HK marine life post series is a tiny little guy, but a very interesting one to watch. Labroides dimidiatus, or the bluestreak cleaner wrasse, is only about 10cm long and is whitish or pale yellow fading to blue with a black stripe running the length of its body. It is a common sight on tropical reefs in South-East Asia — I have even had one nibble at my mask whilst on a dive in Cebu! In Hong Kong, you can see it too, though it is more timid. It is by no means rare, but you might not spot one on every dive.
As the common name suggests, the specificity of this wrasse is that it is a ‘cleaner fish’, i.e. it feeds on parasites that live on the skin of other fish. It stays in a specific area and other fish allow the wrasse to get close and have a nibble. This area of the reef thus becomes a ‘cleaning station’ where fish behave non-aggressively in order to be cleaned of bugs and parasites.
A group of rabbitfish at a cleaning station, Sung Kong. See how the rabbitfish is ‘opening up’ its fins to allow the cleaner to do its job (centre).
Observing the cleaner wrasse
The cleaner wrasse can be seen in hard coral or rocky reef areas in Hong Kong. It usually occurs singly. It’s a shy and fast-moving fish so it’s not always easy to spot but when you see one, you might notice it moving in a distinctive up-and-down darting motion (it is believed to do so to attract other fish). If so, you will have found a cleaning station. If you observe from a slight distance, you are likely to see a variety of other fish species arrive and the cleaner wrasse will nibble them. Juvenile bluestreak cleaner wrasse are black with a blue stripe.
Be aware though that this wrasse has a ‘mimic’; that is to say, another species of fish from a different family that looks almost exactly like it — a blenny known as the ‘false cleanerfish’ or ‘cleaner mimic’. This blenny imitates the wrasse and fools other fish into letting it get close, at which point it takes a cheeky bite! In Hong Kong, this mimic has not been observed in fish surveys, so if you spot a fish that looks like the ones in these pictures, it’s pretty likely to be the true cleaner fish. If you want to check the difference, although the two fish look very alike, the position of the wrasse’s mouth is ‘terminal’ (i.e. at the tip of its snout) whereas the blenny’s mouth is below the head. I don’t have a photo of the latter, but if I get one someday, I’ll post it for comparison!
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