How to Identify Two-spot Astyanax

To identify Two-spot Astyanax (Astyanax bimaculatus), look at these key features.

Physical Characteristics

Body shape

compressed

Coloration

Silvery body with a yellowish to greenish sheen dorsally. A prominent, often vertically elongated, dark humeral spot (behind the operculum) and a distinct, usually round or oval, dark caudal peduncle spot. Fins are typically hyaline (clear) to yellowish, sometimes with reddish tints, especially in males during breeding season.

Scale type

cycloid

Mouth position

terminal

Distinctive features

Two prominent dark spots: one humeral and one on the caudal peduncle., Adipose fin present., Lateral line complete., Small, conical teeth in two rows on the premaxilla.

Dorsal fin

Single dorsal fin, typically with 10-11 rays.

Tail fin

Forked caudal fin.

Pectoral fin

Paired pectoral fins, typically with 11-13 rays.

Distinctive fins

Anal fin long, with 20-28 rays. Adipose fin present between the dorsal and caudal fins.

Identification Tips

1. 1

   Humeral Spot

   Look for a distinct, often vertically elongated, dark spot immediately behind the gill cover. Its shape and intensity can vary.

2. 2

   Caudal Peduncle Spot

   Observe the second dark spot located on the caudal peduncle, just before the caudal fin. This spot is typically round or oval.

3. 3

   Body Shape

   Note the moderately compressed, fusiform body, characteristic of many Astyanax species.

4. 4

   Fin Ray Counts

   For precise identification, especially when distinguishing from closely related species, fin ray counts (e.g., anal fin rays 20-28) can be crucial, though often difficult to ascertain in the field.

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