Well guys, I finally managed to get my hands on specimens of both species of Sahelian tigerfish, Hydrocynus brevis and Hydrocynus forskahlii.
As I've been hypothesizing for quite some time, these are easily the most difficult to tell apart. They are incredibly similar in appearance even at six inchesand even with my keen eye for these guys, it was tricky to tell. There are, however, several morphological traits that anyone can spot, though admittedly they're likely going to be very hard to photograph. I certainly wasn't able to adequately photograph it with my cell phone, though I'm sure I could if I had my Nikon.
On to the pictures. I'll explain what I see as diagnostic traits at the end.
First, the FATF-
Next, the BATF-
Together-
Please reference this drawing on how to differentiate these species. This is a very reliable source that will be referenced in the next update of the ATF ID Guide.
To differentiate these two species, I find that image to be fairly spot on. In order to tell the two species apart, below are the traits I would watch for:
EDIT- added this picture: In this picture, note that the fish are scaled so that the eyes are the same size. If the heads were scaled to be the same size, the eye on the FATF would be significantly larger. As such, when scaled to the same size eye, the BATF has a much larger head. The eye on the FATF is much closer to the jaw. This is what the diagnostic image is referring to as a smaller preorbital bone. The head of the brevis is also much more massive in appearance, as per the literature.
View attachment 951281
I've not had any luck using the lateral line scale counts to tell the fish apart. I suspect that as with telling apart VATF and TATF, the fish will have to be 10" to a foot before that method becomes useful. Another thing that I've noticed that I suspect will be useful is the color of the dorsal fin and upper lobe of the tail. On a FATF, they will generally be grey, on a brevis they will be black or nearly black. This is just a general hint or guideline, because as always with ATF, except for the adipose fin, fin color is NOT a definitive diagnostic trait of species.
EDIT- After getting the new brevis to eat, I was able to quite easily see a third row of scales. See pics.
Sorry for the quality of these pics. It's actually a lot more obvious when you're looking at the fish. I just wanted to post these as proof.
View attachment 951277
View attachment 951278
View attachment 951279
Hope you all enjoyed the read!
As I've been hypothesizing for quite some time, these are easily the most difficult to tell apart. They are incredibly similar in appearance even at six inchesand even with my keen eye for these guys, it was tricky to tell. There are, however, several morphological traits that anyone can spot, though admittedly they're likely going to be very hard to photograph. I certainly wasn't able to adequately photograph it with my cell phone, though I'm sure I could if I had my Nikon.
On to the pictures. I'll explain what I see as diagnostic traits at the end.
First, the FATF-
Next, the BATF-
Together-
Please reference this drawing on how to differentiate these species. This is a very reliable source that will be referenced in the next update of the ATF ID Guide.
To differentiate these two species, I find that image to be fairly spot on. In order to tell the two species apart, below are the traits I would watch for:
The adipose fin. I suspect that this is probably the best trait to use for small fish, as the markings are apparent even in very small fish. My first brevis, the sick who died shortly after arriving, was only about 4" long and had the color. For a brevis, there will be a notably black color on the anterior and ventral part of the adipose fin. There will be a clear line of tissue on the edge from the top of the fin, following the edge of the fin all the way to the back/bottom. The FATF has a uniformly dark grey adipose fin, appearing clear over a light substrate and/or in young fish.
Shape of the head. The image above relates this observation in terms of the eye and preorbital bone. To a hobbyist, we would would describe this as the brevis has a head like a goliath and the forskahlii has a head like a TATF. The BATF has a massive, square head and the FATF has a small, pointy head. This is especially apparent when they're side by side.
Shape of the tail. As displayed above, the FATF has much wider tail than the brevis. FATF's tail is shaped rather like you'd expect an ocean fish's tail to be shaped, It's much wider, rather like the split in a capital K. The brevis' tail is narrower, rather like a less than symbol: <.
Dorsal fin location. The dorsal fins on young, tank sized specimens won't be as dramatic as illustrated above, but it is still noticeable. The FATF will be markedly in front of the pelvic fins as shown. The brevis' fin will likely only be very slightly in front of the pelvic fins.
Shape of the head. The image above relates this observation in terms of the eye and preorbital bone. To a hobbyist, we would would describe this as the brevis has a head like a goliath and the forskahlii has a head like a TATF. The BATF has a massive, square head and the FATF has a small, pointy head. This is especially apparent when they're side by side.
Shape of the tail. As displayed above, the FATF has much wider tail than the brevis. FATF's tail is shaped rather like you'd expect an ocean fish's tail to be shaped, It's much wider, rather like the split in a capital K. The brevis' tail is narrower, rather like a less than symbol: <.
Dorsal fin location. The dorsal fins on young, tank sized specimens won't be as dramatic as illustrated above, but it is still noticeable. The FATF will be markedly in front of the pelvic fins as shown. The brevis' fin will likely only be very slightly in front of the pelvic fins.
EDIT- added this picture: In this picture, note that the fish are scaled so that the eyes are the same size. If the heads were scaled to be the same size, the eye on the FATF would be significantly larger. As such, when scaled to the same size eye, the BATF has a much larger head. The eye on the FATF is much closer to the jaw. This is what the diagnostic image is referring to as a smaller preorbital bone. The head of the brevis is also much more massive in appearance, as per the literature.
View attachment 951281
I've not had any luck using the lateral line scale counts to tell the fish apart. I suspect that as with telling apart VATF and TATF, the fish will have to be 10" to a foot before that method becomes useful. Another thing that I've noticed that I suspect will be useful is the color of the dorsal fin and upper lobe of the tail. On a FATF, they will generally be grey, on a brevis they will be black or nearly black. This is just a general hint or guideline, because as always with ATF, except for the adipose fin, fin color is NOT a definitive diagnostic trait of species.
EDIT- After getting the new brevis to eat, I was able to quite easily see a third row of scales. See pics.
Sorry for the quality of these pics. It's actually a lot more obvious when you're looking at the fish. I just wanted to post these as proof.
View attachment 951277
View attachment 951278
View attachment 951279
Hope you all enjoyed the read!
