The fish that think it’s a flatworm

by | Oct 24, 2021 | Fish, Invertebrates | 0 comments

It’s been a while since I posted this image of one of the most curious little fish I’ve ever come across.  This is the Banded or Blue-edged Sole (Soleichthys heterorhinos).  It is found across the Eastern Indian Ocean and into the Western Pacific.  I’ve only ever seen them in the Red Sea. In fact I’ve only ever seen them on one particular wreck dive at night, but I imagine they’re a lot more common than my limited observations suggest.

As you can see, the fish has the typical ‘flatfish’ appearance of the Soleichthys genus and the family Soleidae in general (approx. 180 species). It also has eyes, raised above its body to allow it to settle under a surface of sand and still, periscope-like, keep abreast of any predators that might threaten.   However, it has a another weapon in its arsenal: mimicry.

This small fish, at around fifteen centimeters, is a wonderful match for a flatworm.  Not only does it mimic a flatworm in appearance, it moves with a gentle rippling motion at the edges.  Most people when seeing one of these soles, slowly gliding across the substrate assume it is a flatworm, only when the eyes are pointed out do they realize the truth.  The fish is so confident in its mimicry of the noxious flatworms that it will, at night, allow you to get quite close. During the day, I think they are typically buried, so I can’t comment further.

I’ve always suspected that the Pseudobiceros worms such as P. fulgor (below) and Pseudoceros species are the closest in appearance to the sole.

P. bedfordi (Maldives).  Quite a variable species and some are far more similar to the sole than others

These worms, according to Wikipeadia: ‘…engage(s) in penis fencing. When the “winner” touches its penis to the “skin” of the other, insemination occurs, and the “loser” has to bear the burden of motherhood.’

I think it is wise to not dwell on this fact, better perhaps to hold it in reserve, for when conversation is flagging at a polite dinner party?


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