2 Different Colorations, 1 Species of Ariidae?

[Mocotun]

Hello all, I'm new to the site. I was wondering if anyone can ID these 2 Ariidae from the Rio Verde in southern Sao Paulo, Brazil. They inhabit a salty estuary very close to the ocean. A local said they are two differnt species, with the yellow growing 3 times bigger than the white. They appear to be identical except for coloration and the white one appears to have an additional anal fin. Any ideas?

[Silurus]

They both appear to be Cathorops spixii to me.

[MatsP]

Very interesting.

A couple of fish that matches the region are these:

Genidens barbus

Genidens genidens

Colouration of G. barbus matches the first picture.

Ariidae are not well covered in the Cat-eLog however, so I suspect there are several other species - in fact checking the Checklist of Freshwater fish from South and Central America, there are 46 species liste in Ariidae. The Cat-eLog covers 30 species in that family, but that includes Asian, African and North American species too. Fishbase lists 155 species from the world in Ariidea - so we have about 20% of the known species. That's not going to help us!

Oh, and I suspect you are seeing the very anterior anal fin and the paired pelvic fins - not a second anal fin (otherwise that would indeed be a key, I'd say).

Ah, I see HH (Silurus) has already answered... Well, my discussion on the coverage of Ariidae is still valid.

--
Mats

[Richard B]

Would you consider sending the photos to Jools for inclusion in cat-e-log? webmaster 'at' planetcatfish 'dot' com if you do

[Mocotun]

I can submit photos. I have several more pics of the same fish I will send. Do you have an opinion as what they are?

[Mike_Noren]

Silurus is an expert so it's a pretty safe bet both are C. spixii.

Variation in color is not all that uncommon, many species can adjust coloration short term (depending on mood) or long term (depending on environment, nutrition, or breeding status). One pretty famous example is european perch, which, depending on environment and food, can be from a light golden color via shades of green or blue or brown to almost solid black.

[Silurus]

One caveat I might add is that it is very frequently difficult to identify ariids even to genus based on external morphology alone (many ariid genera are diagnosed on the basis of internal characters, and one key feature used in identifying ariids has always been toothplate morphology).
This means that my identification of the fish as C. spixii is very tentative.

[Jools]

Silurus is really an expert, but it appears to me that the relationship of the length of the base of the adipose fin relative to the base of the anal fin (useful in determining to what group of genera an ariid belongs) is important here as I think we have two species. The grey one with the adipose fin base less than half the length of the base of the anal fin does look like C. spixii. However the golden fish is, for me, a different species likely belonging to one of Genidens, Notarius, Potamarius or Sciades.

No idea what mind you - if pushed I'd lean towards a species of Notarius.

Jools