Sicyopterus longifilis, also known as the Threadfin Goby, is a captivating freshwater fish found in the rocky streams of New Caledonia, Vanuatu, Fiji, and Futuna. Let’s explore some fascinating details about this unique goby:

1. Appearance:

   • The Threadfin Goby gets its name from its elongated dorsal fin, which resembles a thread.
   • It features a distinctive color pattern:
     • Upper lip with no median cleft but with a lateral cleft on each side.
     • Minutely serrated margin of the upper lip between lateral clefts.
     • No tubercle behind the median cleft.
     • Feeble papillae on the ridge beneath the upper lip.
     • Tricuspid premaxillary teeth.
     • Long and filamentous first dorsal-fin spines in males.
     • In males, the posterior tip of the first dorsal fin often exceeds the base of the last soft ray of the second dorsal fin.
     • Females have somewhat elongated second to fourth spines in the first dorsal fin.
     • Caudal fin with 17-18 (usually 17) segmented rays, including 15-16 branched rays.
       

2. Habitat and Behavior:

   • Threadfin Gobies thrive in coastal streams with moderate to fast flow.
   • They are usually found in small, clear creeks, often within rainforests and approximately 50 km from the sea.
   • Their unique appearance and behavior make them a very sought after addition in the hobby. 

3. Size: The gobies reach a maximum of 9.7cm total length
   

In summary, the Threadfin Goby’s elongated dorsal fin and captivating behavior make it a delightful choice for aquarists.