Monday, 22 September 2008

Some more cool fishes

We've caught some more pretty neat fishes recently.

Below is the mouth of a ponyfish (Family Leiognathidae), which I think is Equulites elongatus. This species is much more slender than most of the other species in the family; they are also not easy to identify, as so many groups seem to be! The keys for this group also frequently seem to depend only on characters of male fish (the sort of key couplet I hate, along with relative statements like "moderately long", which might make sense if you work with the group regularly, but are more or less meaningless if you just have one specimen, or couplets that only apply to adults and you have a juvenile).

Interestingly, ponyfishes are bioluminescent. They have a layer of tissue containing bioluminescent bacteria wrapped around their gut. It is thought that they use this light, which shines out of transluscent patches on their bodies, to confuse predators, particularly at night in the shallow waters they frequently inhabit, where moonlight shining through the water would otherwise silhouette them strongly. They also seem to show differences between species and sexes, so the light is probably useful for the fish in several ways.

Ponyfishes are fairly important in many tropical countries as a food source; they apparently dry fairly easily, allowing them to be transported and traded as a good source of protein.

Most of the fishes in this group have mouths that are not only protrusible, but extremely so!

Take a look:
Mouth closed (sorry about the label - I cropped this out of a fish pic I took from genetic/stable isotope sampling - somehow, I didn't think to take a picture of the specimen with the mouth closed!).

And... Open!

As neat as that mouth is, the next fish is even more great.

We caught a fairly large flatfish postlarva/juvenile in a pelagic trawl - I noticed it had an extraordinarily long fin ray, so I felt I just had to take some photos! We tried to key it out - we suspect it's in the Family Bothidae, and it may be a Laeops of some sort. Anyway, onto the pictures!

Note the really long fin ray sticking out of the head (the whitish filament).

Here's a close-up of the head. If you look carefully, you can see the upper end of the eye that is migrating around to this side of the body (grey-ish patch above the eye you can clearly see). Yes, you can indeed see its brain and spinal cord, as well as its guts.

I first took pictures of it lying the wrong way around (as if it was a right-eyed flounder), and then took another look at it and realised my mistake! It was quite big and about 3/4 filled the petri dish I put it in to photograph it under a little water to get rid of some of the annoying glistening highlights you get off wet fish (the reason you normally put them in a tank to photograph). These are, however, the right way around. If you ever find yourself having to photograph developing flatfish and want to know which way is the right way up, check which eye is closer to the top of the head - that's going to be the "bottom". All kinds of interesting bone development will probably happen after the eye passes onto the left side of the head.

We caught these fishes about here.

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