The bigeye trevally (Caranx sexfasciatus), aka the bigeye jack, great trevally, six-banded trevally, and dusky jack, is the most common member of the Carangidae family. In this name, "Caranx" is the genus for trevallies, and "sexfasciatus" means "six-banded" in Latin - no awkward meanings here!
We often spot these trevallies in massive schools during our dives at Sail Rock, and they frequently hang out with the incredible Whalesharks. I have to say, there’s no better feeling than being enveloped in a huge cloud of trevallies, cruising through the stunning sunlit waters.
Distinguishing Features
Bigeye trevallies can grow up to about 80 cm in length, though most adults are between 40 to 60 cm. They change color as they grow. Juveniles are silver-yellow to brown with dark vertical bands. As they get older, these bands fade and their color becomes silver-blue on top and white underneath. Adults have no bands and are silvery olive to blue-green on top and white below. Their fins change too: juveniles have pale fins with dark edges, while adults have darker fins with the tail and anal fins ranging from yellow to black, and the second dorsal fin has a white tip. They also have a small dark spot near their gills.
Behavior and Color Changes
Bigeye trevallies are voracious predators, using their speed to catch small fish and crustaceans, mainly hunting at night. During the day, they often gather in groups to rest and visit cleaning stations for grooming. Their ability to change color, from silvery and shiny to dark black, may help them communicate with each other and other species.
Reproduction
Bigeye trevallies spawn in groups. They release their eggs into the water, where fertilization happens externally. The eggs hatch into larvae, which grow into juveniles. These young fish then mature into adults, ready to start the reproductive cycle again.
Predators
Bigeye trevallies are preyed upon by larger fish like groupers, snappers, and barracudas when they are young, and by sharks and larger groupers as adults, with humans also catching them for food and the aquarium trade.
Did you know?
- Bigeye trevallies are attracted to sounds in the water, including noises made by divers, as they often link these sounds to potential food sources.
- They can change color to blend into different environments, making them a striking sight underwater.
- Their large eyes and streamlined bodies help them detect prey from afar.
- These fish can get big, with the largest one being recorded at 120cm in length with a weight of 18kg.
Where to Find Bigeye trevallies on Koh Tao
While you can spot Bigeye trevallies at a few dive sites around Koh Tao, like Chumphon Pinnacle, Htms Sattakut Wreck and Southwest Pinnacle, if you're hoping to witness a massive school in action, Sail Rock is the place to go!
