Tuesday, April 26, 2011

Not-so-flabby flabby whalefishes

So this specimen first caught my eye because it belongs to a family of deep sea fishes known as the "flabby whalefishes" (Cetomimidae). But .... this little fish, caught off the coast of Portugal in 1959, was neither flabby, nor was it whale-y. To my eye at least. I found this pretty amusing in and of itself.

YPM ICH 4899





But THEN... I discovered that when alive, it is PINK! It so happens that I'm not really even much of a pink fan myself, but this species, Cetostoma regani, is bright pink as bright pink gets. (And its common name is the pink flabby whalefish. Fancy that.)

Photo from the MCZ

And THEN... upon further googling, I learned that in 2009, this species played a big part in an amazing story of scientific discovery. The story goes like this:

Once upon a time, there were three fish groups, the tapetails (Mirapinnidae), the bignoses (Megalomycterydae), and the whalefishes (Cetomimidae). They were all very funny-looking and lived in the deep sea. But they all looked wildly different, so --as one would expect--they were considered to be three families.

But in 1989, ichthyologist John Paxton noticed that, strangely, all of the known specimens of whalefishes were females. Similarly, biologists soon noticed that all of the known specimens of bignoses were males. And all of the known specimens of tapetails were sexually immature...

Following these clues, a team of biologists, headed by David Johnson of the Smithsonian, recently sequenced the DNA of specimens from these three families and found that the DNA from each was essentially identical.  These were not three families--they were only one! Consistent with the clues, they are simply different life stages of the same fish. The tapetails are the larvae, the bignoses are the males, and the whalefishes are the females. Pretty amazing. Here's a picture of all of the families:


Image taken from here.


The degree of morphological difference between the larval and adult stages, and between the males and females, is truly remarkable. The authors of the study in 2009 note that although significant larval transformations occur in some other deep-sea fish families and extreme sexual dimorphism is common among vertebrates, "the extraordinary combination of both ... for the whalefishes is unparalleled within Vertebrata." And if there was any doubt about their findings, the team found morphological evidence to back up the DNA evidence. They were able to identify some transitional specimens-- that is, female "tapetails" transitioning to the "whalefish" stage, and male "tapetails" transitioning to the "bignose" stage.

Note that the whalefish at the bottom of the image above looks much more whale-y than the specimen I re-curated! Perhaps our specimen is in fact a transitional specimen ... 

For more details on the story, check out this interactive Smithsonian website about the discovery.


References:

Johnson, G.J. et al. 2009. Deep-sea mystery solved: astonishing larval transformations and extreme sexual dimorphism unite three fish families. Biology Letters 5: 235-239.

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