Cardinal Tetras secondary sexual characteristics?

[Cearbhaill]

I have 16 Cardinal Tetras in a 90g community tank (other fish also- see my "specs page"). They were sad and thin when I bought them but with care and great food have grown considerably and are now fat and happy.

But the other day I noticed "look- that Cardinal has an adipose fin!" Upon closer inspection (and a good pair of reading glasses) I see that of course all the Cardinals have adipose fins- but on two of them the fins are just more... noticable. There are no spots, sores, fungus, or anything apparently wrong- it's just that the fins are easier to see- more like real fins instead of the vestigal stubs I was used to.

The only thing I could come up with is that maybe these fat, happy mature fish are males showing secondary sexual characteristics...
Am I off the mark here?
Does that sound plausable?

[Faramir]

The fatter ones are usually females.

I've never heard of adipose fins being a sexual characteristic of characins. The shape of the anal fin and the general body shape are a better guide.

[wetmanNY]

I'm seconding faramir on that.

But I have a different, more off-the-cuff possibility. Neon Tetras when they are imported are often mixed ("contaminated") with two other species. One, I remember, is Paracheirodon simulans precisely because it's such a close mimic. Cardinal Tetras are all brought in from the Rio Negro. Is it possible that there are more than the one familiar species of "Cardinal Tetra"?

[wetmanNY]

You know who to ask? E-mail Stanley Weitzman Department of Fishes at the Smithsonian. He'll take your question seriously, and he's the characin expert.

Google "Weitzman fishes Smithsonian" for more information and his addy.

HTH!

If you get a response, post it here for all of us, eh.